How Important are Post-Tropical Cyclones to European Windstorm Risk?

Elliott Sainsbury, e.sainsbury@pgr.reading.ac.uk

To date, the 2020 North Atlantic hurricane season has been the most active on record, producing 20 named storms, 7 hurricanes, and a major hurricane which caused $9 billion in damages across the southern United States. With the potential for such destructive storms, it is understandable that a large amount of attention is paid to the North Atlantic basin at this time of year. Whilst hurricanes have been known to cause devastation in the tropics for centuries, until recently there was little appreciation for the destructive potential of these systems across Europe.

As tropical cyclones such as hurricanes move poleward – away from the tropics and into regions of lower sea surface temperatures and higher vertical wind shear, they undergo a process called extratropical transition (Klein et al., 2000): Over a period of time, the cyclones change from symmetric, warm cored systems into asymmetric cold core systems fuelled by horizontal temperature gradients, as opposed to latent heat fluxes (Evans et al., 2017). These systems, so-called post-tropical cyclones (PTCs), often reintensify in the mid-latitude Atlantic with consequences for land masses downstream – often Europe. This was highlighted in 2017, when ex-hurricane Ophelia impacted Ireland, bringing with it the strongest winds Ireland had seen in 50 years (Stewart, 2018). 3 people were killed, and 360,000 homes were without power.

In a recent paper, we quantify the risk associated with PTCs across Europe relative to mid-latitude cyclones (MLCs) for the first time – in terms of both the absolute risk (i.e. what fraction of high impact wind events across Europe are caused by PTCs?) and also the relative risk (for a given PTC, how likely is it to be associated with high-impact winds, and how does this compare to a given MLC?). By tracking all cyclones impacting a European domain (36-70N, 10W-30E) in the ERA5 reanalysis (1979-2017) using a feature tracking algorithm (Hodges, 1994, 1995, 1999), we identify the post-tropical cyclones using spatiotemporal matching (Hodges et al., 2017) with the observational record, IBTrACS (Knapp et al., 2010).

Figure 1: Distributions of the maximum intensity (maximum wind speed, minimum MSLP) attained by each PTC and MLC inside (a-c) the whole European domain (36-70N, 10W-30E), (d-f) the Northern Europe domain (48-70N, 10W-30E) and (g-i) the Southern Europe domain (36-48N, 10W-30E), using cyclones tracked through the ERA5 reanalysis all year round, 1979-2017. [Figure 2 in Sainsbury et al. 2020].

Figure 1 shows the distributions of maximum intensity for PTCs and MLCs across the entire European domain (top), Northern Europe (48-70°N, 10°W-30°E; middle) and Southern Europe (36-48°N, 10°W-30°E; bottom), using all cyclone tracks all year round. The distribution of PTC maximum intensities is higher (in terms of both wind speed and MSLP) than MLCs, particularly across Northern Europe. The difference between the maximum intensity distributions of PTCs and MLCs across Northern Europe is statistically significant (99%). PTCs are also present in the highest of intensity bins, indicating that the strongest PTCs have intensities comparable to strong wintertime MLCs.

Whilst Figure 1 shows that PTCs are stronger than MLCs even when considering MLCs forming all year round (including the often much stronger wintertime MLCs), it is also useful to compare the risks posed by PTCs relative to MLCs forming at the same time of the year – during the North Atlantic hurricane season (June 1st-November 30th).

Figure 2 shows the fraction of all storms, binned by their maximum intensity in their respective domains, which are PTCs. For storms with weak-moderate maximum winds (first three bins in the figure), <1% of such events are caused by PTCs (with the remaining 99% caused by MLCs). For stronger storms, particularly those of storm force (>25 ms-1 on the Beaufort scale), this percentage is much higher. Despite less than 1% of all storms impacting Northern Europe during hurricane season being PTCs, almost 9% of all storms with storm-force winds which impact the region are PTCs, suggesting that a disproportionate fraction of high-impact windstorms are PTCs. 8.2% of all Northern Europe impacting PTCs which form during hurricane season impact the region with storm-force winds. This fraction is only 0.8% for MLCs, suggesting that the fraction of PTCs impacting Northern Europe with storm-force winds is approximately 10 times greater than MLCs.

Figure 2: The fraction of cyclones impacting Europe which are PTCs as a function of their maximum 10m wind speed in their respective domain. Lower bound of wind speed is shown on the x axis, bin width = 3. Error bars show the 95% confidence interval. All cyclone tracks forming during the North Atlantic hurricane season are used. [Figure 4 in Sainsbury et al. 2020].

Here we have shown that PTCs, at their maximum intensity over Northern Europe, are stronger than MLCs. However, the question still remains as to why this is the case. Warm-seclusion storms post-extratropical transition have been shown to have the fastest rates of reintensification (Kofron et al., 2010) and typically have the lowest pressures upon impacting Europe (Dekker et al., 2018). Given the climatological track that PTCs often take over the warm waters of the Gulf stream, along with the contribution of both baroclinic instability and latent heat release for warm-seclusion development (Baatsen et al., 2015), one hypothesis may be that PTCs are more likely to develop into warm seclusion storms than the broader class of mid-latitude cyclones, potentially explaining the disproportionate impacts they cause across Europe. This will be the topic of future work.

Despite PTCs disproportionately impacting Europe with high intensities, they are a relatively small component of the total cyclone risk in the current climate. However, only small changes are expected in MLC number and intensity under RCP 4.5 (Zappa et al., 2013). Conversely, the number of hurricane-force (>32.6 ms-1) storms impacting Norway, the North Sea and the Gulf of Biscay has been projected to increases by a factor of 6.5, virtually all of which originate in the tropics (Haarsma et al., 2013). Whilst the absolute contribution of PTCs to hurricane season windstorm risk is currently low, PTCs may make an increasingly significant contribution to European windstorm risk in a future climate.

Interested to read more? Read our paper, published in Geophysical Research Letters.

Sainsbury, E. M., R. K. H. Schiemann, K. I. Hodges, L. C. Shaffrey, A. J. Baker, K. T. Bhatia, 2020: How Important Are Post‐Tropical Cyclones for European Windstorm Risk? Geophysical Research Letters, 47(18), e2020GL089853, https://doi.org/10.1029/2020GL089853

References

Baatsen, M., Haarsma, R. J., Van Delden, A. J., & de Vries, H. (2015). Severe Autumn storms in future Western Europe with a warmer Atlantic Ocean. Climate Dynamics, 45, 949–964. https://doi.org/10.1007/s00382-014-2329-8

Dekker, M. M., Haarsma, R. J., Vries, H. de, Baatsen, M., & Delden, A. J. va. (2018). Characteristics and development of European cyclones with tropical origin in reanalysis data. Climate Dynamics, 50(1), 445–455. https://doi.org/10.1007/s00382-017-3619-8

Evans, C., Wood, K. M., Aberson, S. D., Archambault, H. M., Milrad, S. M., Bosart, L. F., et al. (2017). The extratropical transition of tropical cyclones. Part I: Cyclone evolution and direct impacts. Monthly Weather Review, 145(11), 4317–4344. https://doi.org/10.1175/MWR-D-17-0027.1

Haarsma, R. J., Hazeleger, W., Severijns, C., De Vries, H., Sterl, A., Bintanja, R., et al. (2013). More hurricanes to hit western Europe due to global warming. Geophysical Research Letters, 40(9), 1783–1788. https://doi.org/10.1002/grl.50360

Hodges, K., Cobb, A., & Vidale, P. L. (2017). How well are tropical cyclones represented in reanalysis datasets? Journal of Climate, 30(14), 5243–5264. https://doi.org/10.1175/JCLI-D-16-0557.1

Hodges, K. I. (1994). A general method for tracking analysis and its application to meteorological data. Monthly Weather Review, 122(11), 2573–2586. https://doi.org/10.1175/1520-0493(1994)122<2573:AGMFTA>2.0.CO;2

Hodges, K. I. (1995). Feature Tracking on the Unit Sphere. Monthly Weather Review, 123(12), 3458–3465. https://doi.org/10.1175/1520-0493(1995)123<3458:ftotus>2.0.co;2

Hodges, K. I. (1999). Adaptive constraints for feature tracking. Monthly Weather Review, 127(6), 1362–1373. https://doi.org/10.1175/1520-0493(1999)127<1362:acfft>2.0.co;2

Klein, P. M., Harr, P. A., & Elsberry, R. L. (2000). Extratropical transition of western North Pacific tropical cyclones: An overview and conceptual model of the transformation stage. Weather and Forecasting, 15(4), 373–395. https://doi.org/10.1175/1520-0434(2000)015<0373:ETOWNP>2.0.CO;2

Knapp, K. R., Kruk, M. C., Levinson, D. H., Diamond, H. J., & Neumann, C. J. (2010). The international best track archive for climate stewardship (IBTrACS). Bulletin of the American Meteorological Society, 91(3), 363–376. https://doi.org/10.1175/2009BAMS2755.1

Kofron, D. E., Ritchie, E. A., & Tyo, J. S. (2010). Determination of a consistent time for the extratropical transition of tropical cyclones. Part I: Examination of existing methods for finding “ET Time.” Monthly Weather Review, 138(12), 4328–4343. https://doi.org/10.1175/2010MWR3180.1

Stewart, S. R. (2018). Tropical Cyclone Report: Hurricane Ophelia. National Hurricane Center, (February), 1–32. https://doi.org/AL142016

Zappa, G., Shaffrey, L. C., Hodges, K. I., Sansom, P. G., & Stephenson, D. B. (2013). A multimodel assessment of future projections of North Atlantic and European extratropical cyclones in the CMIP5 climate models. Journal of Climate, 26(16), 5846–5862. https://doi.org/10.1175/JCLI-D-12-00573.1

A Journey through Hot British Summers

Email: s.h.lee@pgr.reading.ac.uk

The phrase “British summer” tends to evoke images of disorganised family barbecues being interrupted by heavy rain, or the covers coming on at Wimbledon, or the saying “three fine days and a thunderstorm”. Yet in recent years, hot weather has become an increasingly regular occurrence. Let me take you on a brief tour of notably hot summers in the UK. I’ll largely draw on the Met Office HadUK-Grid dataset, shown in Figure 1.

Figure 1: Nationally-averaged daily maximum temperatures for June-July-August from HadUK-Grid. In red is a 30-year centred running mean, which has risen by 1°C since the mid-20th century.

HadUK-Grid begins in 1884, but thanks to the Central England Temperature dataset (which extends back to 1659), we do have records of earlier heatwaves.  These include the hot summer of 1666, which set the scene for the Great Fire of London in September. The summers of 1781, 1826 and 1868 were also particularly hot. The first hot summer in the HadUK-Grid series is 1899, which was the warmest summer by average maxima in that series until 1976!

But our journey properly begins in 1911, when the temperature reached 36.7°C on August 9th. At the time, this was the highest reliably recorded temperature measured in the UK. It is hard to imagine how this summer must have felt at the time – not least in the cooler average climate, but also with the less developed infrastructure and clothing customs of the time. As with any heatwave, its impacts were large with increased death, drought, and agricultural impacts. The summer of 1911 was followed by the summer of 1912, which was the 2nd wettest on record for the UK. Such a turnaround must have been equally hard to believe and does highlight that extreme swings in the British weather are not, in themselves, new. In a series from 1884, the summer of 1911 is the 8th warmest in terms of the UK average maximum temperature (at the time, it would have been 2nd, with only 1899 warmer).

Stopping briefly in 1933 (which eclipsed 1911, but pales in comparison with the dustbowl conditions being experienced in the US at the time) and then again in August 1947 (which remains 2nd warmest for UK average maxima and the nation’s driest, and was in huge contrast to the tremendously snowy and cold February), our next destination is 1975.

1975 currently ranks as the 11th warmest for UK average maxima but is also the 19th driest. This, when combined with the dry winter that followed, set the scene for the infamous summer of 1976. Both these summers followed a spell of very cool summers, with no particularly remarkable summers in the 1960s, while the UK did not see a temperature above 28°C in 1974 (almost unthinkable nowadays). I won’t go into huge detail about the 1976 summer, but it is engrained in the minds of a generation thanks not only to its remarkable June heatwave (which has never been matched) but also the cool climate in which it occurred. It ranks as the 2nd driest summer for the UK and remains the warmest on record in terms of average maxima – though no individual month holds the number 1 spot.

Let us next whizz off to July 1983, which at the time had the warmest nationally averaged maxima for the month (it now ranks 3rd). Oddly enough, while the UK baked in heat, the temperature at Vostok, Antarctica dropped to -89.2°C on the 21st – the lowest surface-based temperature ever recorded. I am keeping the topic of this blog to hot summers, but I want to give 1985 a special mention – the most recent summer when the UK-average maxima were less than 17°C, a formerly frequent occurrence.

As we hot-foot it toward the end of the 20th century (pun intended), we arrive at 1990. Liverpool had just won the First Division (sound familiar?) and on August 3rd the temperature at Cheltenham, Gloucestershire reached 37.1°C – beating the record set in 1911 after 79 years. That night, the temperature fell to only 23.9°C in Brighton – the warmest night on record. However, the heatwave was rather brief but intense (3 consecutive days exceeded 35°C, the only other occurrences were in 1976). For a prolonged heatwave, we jump to August 1995. With a UK average maximum of 22.8°C, it remains the UK’s warmest August by that metric, and the 2nd driest. The summer ranks 2nd warmest by maxima. Soon after, the August of 1997 (4th warmest) added to growing evidence of a change to the British climate.

But it was in the August of 2003 when things really kicked off. In the earliest heatwave I remember, the temperature hit 38.5°C on the 10th at Faversham, Kent (satellite image in Figure 2) – the first time the UK had surpassed 37.8°C (100°F) and breaking the record from 1990 after only 23 years. 30°C was exceeded somewhere for 10 consecutive days. The summer of 2003 ranks nowadays as 6th warmest by average maxima; across Europe conditions were more extreme with a huge estimated death toll.

Figure 2: Terra-MODIS imagery from 10 August 2003, when the UK first surpassed 100°F and most of Europe was experiencing an intense heatwave (via https://worldview.earthdata.nasa.gov/)

Only 3 years later, July 2006 set the record for the hottest month for the UK-average maxima (23.3°C), and set – at the time – a record for the highest-recorded July temperature (36.5°C at Wisley on the 19th). Ranking 4th warmest by average maxima, the summer was even more extreme across mainland Europe.

What followed from 2007 through 2012 was a spell of wet summers, but we shrug off all that Glastonbury mud to arrive at July 2013, which currently ranks as 4th warmest by average maxima and saw the longest spell of >28°C weather since 1997.

Skipping through in increasingly short steps, we arrive for a brief blast on July 1st, 2015 – when the July record from 2006 fell, with 36.7°C at Heathrow in an otherwise cool month. We hop over now to 2018…

The summer of 2018, memorable for England’s performance in the World Cup, saw very warm temperatures in June and July. By nationally averaged maxima, June 2018 ranks 2nd behind 1940, and July sits 2nd behind 2006. The summer ranks 3rd, but by mean temperature is the warmest. Though not reaching the dizzying highs of 2003 (“only” 35.3°C was reached on July 26th), the prolonged dry conditions which began in May across England led to parched grasses (Figure 3), wildfires, and low river levels. I may have also had a viral tweet.

Figure 3: Brown grass during summer 2018 at the University of Reading, as seen in Google Earth.

With the present day in sight, our journey is not yet over. Stepping into 2019, an otherwise unremarkable summer was characterised with huge bursts of heat – setting records across Europe – which on July 25th saw the temperature reach 38.7°C at Cambridge Botanic Gardens. This eclipsed the 2003 record and became only the 2nd day – at the time – when 100°F or more had been reached in the UK.

But that is still not the end of the story! After a record-setting sunny spring followed by a mixed first half of summer, on July 31st 2020 the temperature at Heathrow hit 37.8°C – becoming the UK’s third warmest day on record and the third time 100°F had been recorded. The following Friday, 36.4°C was reached at Heathrow and Kew – the UK’s 9th warmest day on record, and highest temperature in August since 2003. Figure 4 shows the view at the University atmospheric observatory shortly after 34.8°C was reached, Reading’s 4th highest in August since records began in 1908.

Figure 4: The University of Reading Atmospheric Observatory on the afternoon of August 7th, shortly after 34.8°C had been recorded by the automatic sensor.

Forecasts suggest a continuation of hot weather through the next week or so, with many records up for grabs. However, we should be mindful that heatwaves cause suffering and excess deaths, too. And, with the evidently increasing frequency with which these hot extremes are occurring (note how so many of the stops on my tour were clustered in the last 30 years), they are not good news, but another sign that our climate is changing.

Now that we have blasted through the 100°F barrier, our attention turns to 40°C. Research suggests this is already becoming much more likely thanks to climate change and will continue to do so. Reaching such extremes in the UK requires a unique combination of factors – but when these do come together, expect yet more records to fall.

Thanks to Stephen Burt for useful discussions.

Further Reading:

McCarthy, M., et al. 2019: Drivers of the UK summer heatwave of 2018. Weather, https://doi.org/10.1002/wea.3628.

Black, E., et al. 2006: Factors contributing to the summer 2003 European heatwave. Weather, https://doi.org/10.1256/wea.74.04

Burt, 2006: The August 2003 heatwave in the United Kingdom: Part 1 – Maximum temperatures and historical precedents. Weather, https://doi.org/10.1256/wea.10.04A

Burt and Eden, 2007: The August 2003 heatwave in the United Kingdom: Part 2 – The hottest sites. Weather, https://doi.org/10.1256/wea.10.04B

Brugge, 1991: The record-breaking heatwave of 1-4 August 1990 over England and Wales. Weather, https://doi.org/10.1002/j.1477-8696.1991.tb05667.x

How do ocean and atmospheric heat transports affect sea-ice extent?

Email: j.r.aylmer@pgr.reading.ac.uk

Downward trends in Arctic sea-ice extent in recent decades are a striking signal of our warming planet. Loss of sea ice has major implications for future climate because it strongly influences the Earth’s energy budget and plays a dynamic role in the atmosphere and ocean circulation.

Comprehensive numerical models are used to make long-term projections of the future climate state under different greenhouse gas emission scenarios. They estimate that the Arctic ocean will become seasonally ice free by the end of the 21st century, but there is a large uncertainty on the timing due to the spread of estimates across models (Fig. 1).

Figure 1: Projections of Arctic sea-ice extent under ‘moderate’ emissions in 20 recent-generation climate models. Model data: CMIP6 multi-model ensemble; observational data: National Snow & Ice Data Center.

What causes this spread, and how might it be reduced to better constrain future projections? There are various factors (Notz et al. 2016), but of interest to our work is the large-scale forcing of the atmosphere and ocean. The mean atmospheric circulation transports about 3 PW of heat from lower latitudes into the Arctic, and the oceans transport about a tenth of that (e.g. Trenberth and Fasullo, 2017; 1 PW = 1015 W). Our goal is to understand the relative roles of Ocean and Atmospheric Heat Transports (OHT, AHT) on long timescales. Specifically, how sensitive is the sea-ice cover to deviations in OHT and AHT, and what underlying mechanisms determine the sensitivities?

We developed a highly simplified Energy-Balance Model (EBM) of the climate system (Fig. 2)—it has only latitudinal variations and is described by a few simple equations relating energy transfer between the atmosphere, ocean, and sea ice (Aylmer et al. 2020). The latitude of the sea-ice edge is an analogue for ice extent in the real world. The simplicity of the EBM allows us to isolate the basic physics of the problem, which would not be possible going directly with the complex output of a full climate model.

Figure 2: Simplified schematic of our Energy-Balance Model (EBM; see Aylmer et al. 2020 for technical details). Arrows represent energy fluxes, each varying with latitude, between the atmosphere, ocean, and sea ice.

We generated a set of simulations in which OHT varies and checked the response of the ice edge. This is a measure of the effective sensitivity of the ice cover to OHT (Fig. 3a)—it is not the actual sensitivity because AHT decreases (Fig. 3b), and we are really seeing in Fig. 3a the net response of the ice edge to changes in both OHT and AHT.

Figure 3: (a) Effective sensitivity of the (annual-mean) sea-ice edge to varying OHT (expressed as the mean convergence over the ice pack). (b) AHT convergence reduces at the same time, which partially cancels the true impact of increasing OHT on sea ice.

This reduction in AHT with increasing OHT is called Bjerknes compensation, and it occurs in full climate models too (Outten et al. 2018). Here, it has a moderating effect on the true impact of increasing OHT. With further analysis, we determined the actual sensitivity to be about 1.5 times the effective sensitivity. The actual sensitivity of the ice edge to AHT turns out to be about half that to the OHT.

What sets the difference in OHT and AHT sensitivities? This is easily answered within the EBM framework. We derived a general expression for the ratio of (actual) ice-edge sensitivities to OHT (so) and AHT (sa):

A higher-order term has been neglected for simplicity here, but the basic point remains: the ratio of sensitivities mainly depends on the parameters BOLR and Bdown. These are bulk representations of atmospheric feedbacks and determine the efficiency of outgoing and downwelling longwave radiation, respectively. They are always positive, so the ice edge is always more sensitive to OHT than AHT.

The interpretation of this equation is simple. AHT converging over the ice pack can either be transferred to the underlying sea ice, or radiated to space, having no impact on the ice, and the partitioning is controlled by Bdown and BOLR. The same amount of OHT converging under the ice pack can only go through the ice and is thus the more efficient driver.

Climate models with larger OHTs tend to have less sea ice (Mahlstein and Knutti, 2011). We have also found strong correlations between OHT and the sea-ice edge in several of the models listed in Fig. 1 individually. Ice-edge sensitivities and B values can be determined per model, and our equation predicts how these should be related. Our work thus provides a way to investigate how much physical biases in OHT and AHT contribute to sea-ice-projection uncertainties.

Methane’s Shortwave Radiative Forcing

Email: Rachael.Byrom@pgr.reading.ac.uk

Methane (CH4) is a potent greenhouse gas. Its ability to effectively alter fluxes of longwave (thermal-infrared) radiation emitted and absorbed by the Earth’s surface and atmosphere has been well studied. As a result, methane’s thermal-infrared impact on the climate system has been quantified in detail. According to the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5), methane has the second largest radiative forcing (0.48 W m-2) of the well-mixed greenhouse gases after carbon dioxide (CO2) (Myhre et al. 2013, See Figure 1).  This means that due to its change in atmospheric concentration since the pre-industrial era (from 1750 – 2011), methane has directly perturbed the tropopause net (incoming minus outgoing) stratospheric temperature-adjusted radiative flux by 0.48 W m-2, causing the climate system to warm.

Figure 1: Radiative forcing of the climate system between the years 1750 – 2011 for different forcing agents. Hatched bars represent estimates of stratospheric-temperature adjusted radiative forcing (RF), solid bars represent estimates of effective radiative forcing (ERF) with uncertainties (5 to 95% confidence range) given for RF (dotted lines) and ERF (solid lines). Taken from Chapter 8, IPCC AR5 Figure 8.15 (IPCC 2013).

However, an important effect is missing from the current IPCC AR5 estimate of methane’s climate impact – its absorption of solar radiation. In addition to longwave radiation, methane also alters fluxes of incoming solar shortwave radiation at wavelengths between 0.7 – 10 µm.

Until recently this shortwave effect had not been thoroughly quantified and as such was largely overlooked.  My PhD work focuses on quantifying methane’s shortwave effect in detail and aims to build upon the significant, initial findings of Etminan et al. (2016) and a more recent study by Collins et al. (2018).

Etminan et al. (2016) analysed methane’s absorption of solar near-infrared radiation (at wavelengths between 0.2 – 5 µm) and found that it exerted a direct instantaneous, positive forcing on the climate system, largely due to the absorption of upward scattered, reflected near-infrared radiation by clouds in the troposphere. Essentially, this processes results in photons taking multiple passes throughout the troposphere, which in turn results in increased absorption by CH4. Figure 2 shows the net (downwards minus upwards) spectral variation of this forcing at the tropopause under all-sky (i.e. cloudy) conditions. Here it is clear to see methane’s three key absorption bands across the near-infrared region at 1.7, 2.3 and 3.3 µm.

As Etminan et al. (2016) explain, following a perturbation in methane concentrations all of these bands decrease the downwards shortwave radiative flux at the tropopause, due to increased absorption in the stratosphere. However, the net sign of the forcing depends on whether this negative contribution compensates over increased absorption by these bands in the troposphere (which constitutes a positive forcing). As Figure 2 shows, whilst the 3.3 µm band has a strongly net negative forcing due to the absorption of downwelling solar radiation in the stratosphere, both the 1.7 µm and 2.3 µm bands have a net positive forcing due to increased CH4 absorption in an all-sky troposphere. When summed across the entire spectral range, the positive forcing at 1.7 µm and 2.3 µm dominates over the negative forcing at 3.3 µm – resulting in a net positive forcing. Etminan et al. (2016) also found that the nature of this positive forcing is partly explained by methane’s spectral overlap with water vapour (H2O). The 3.3 µm band overlaps with a region of relatively strong H2O absorption, which reduces its ability to absorb shortwave radiation in the troposphere, where high concentrations of H2O are present. However, both the 1.7 µm and 2.3 µm bands overlap much less with H2O, and so are able to absorb more shortwave radiation in the troposphere.

Figure 2: Upper: Spectral variation of near-infrared tropopause RF (global mean, all sky). Lower: Sum of absorption line strengths for CH4 and water vapour (H2O). Taken from Etminan et al. (2016).

In addition to this, Etminan et al. (2016) also found that the shortwave effect serves to impact methane’s stratospheric temperature-adjusted longwave radiative forcing (the process whereby stratospheric temperatures readjust to radiative equilibrium before the change in net radiative flux is calculated at the tropopause; Myhre et al. (2013)). Here, absorption of solar radiation in the stratosphere results in a warmer stratosphere, and hence increased emission of longwave radiation by methane downwards to the troposphere. This process results in a positive tropopause longwave radiative forcing. Combing both the direct, instantaneous shortwave forcing and its impact on the stratospheric-temperature adjusted longwave forcing, Etminan et al. (2016) found that the inclusion of the shortwave effect enhances methane’s radiative forcing by a total of 15%. The results presented in this study are significant and indicate the importance of methane’s shortwave absorption. However, Etminan et al. (2016) note several areas of uncertainty surrounding their estimate and highlight the need for a more detailed analysis of the subject.

My work aims to address these uncertainties by investigating the impact of factors such as updates to the HITRAN spectroscopic database (which provides key spectroscopic parameters for climate models to simulate the transmission of radiation through the atmosphere), the inclusion of the solar mid-infrared (7.7 µm) band in calculations of the shortwave effect and potential sensitivities, such as the vertical representation of CH4 concentrations throughout the atmosphere and the specification of land surface albedo. My work also extends Etminan et al. (2016) by investigating the shortwave effect at a global spatial resolution, since a two-atmosphere approach (using tropical and extra-tropical profiles) was employed by the latter. To do this I use the model SOCRATES-RF (Checa-Garcia et al. 2018) which computes monthly-mean radiative forcings at a global 5° x 5° spatial resolution using a high resolution 260-band shortwave spectrum (from 0.2 – 10 µm) and a standard 9-band longwave spectrum.

Initial results calculated by SOCRATES-RF confirm that methane’s all-sky tropopause shortwave radiative forcing is positive and that the inclusion of shortwave bands serves to increase the stratospheric-temperature adjusted longwave radiative forcing. In total my calculations estimate that the shortwave effect increases methane’s total radiative forcing by 10%. Whilst this estimate is lower than the 15% proposed by Etminan et al. (2016) it’s important to point out that this SOCRATES-RF estimate is not a final figure and investigations into several key forcing sensitivities are currently underway. For example, methane’s shortwave forcing is highly sensitive to the vertical representation of concentrations throughout the atmosphere. Tests conducted using SOCRATES-RF reveal that when vertically-varying profiles of CH4 concentrations are perturbed, the shortwave forcing almost doubles in magnitude (from 0.014 W m-2 to 0.027 W m-2) when compared to the same calculation conducted using constant vertical profiles of CH4 concentrations. Since observational studies show that concentrations of methane decrease with height above the tropopause (e.g. Koo et al. 2017), the use of realistic vertically-varying profiles in forcing calculations are essential. Highly idealised vertically-varying CH4 profiles are currently employed in SOCRATES-RF, which vary with latitude but not with season. Therefore, the realism of this representation needs to be validated against observational datasets and possibly updated accordingly.

Another key sensitivity currently under investigation is the specification of land surface albedo – a potentially important factor controlling the amount of reflected shortwave radiation absorbed by methane. Since the radiative properties of surface features are highly wavelength-dependent, it is plausible that realistic, spectrally-varying land surface albedo values will be required to accurately simulate methane’s shortwave forcing. For example, vegetation and soils typically tend to reflect much more strongly in the near-infrared than in the visible region of the solar spectrum, whilst snow surfaces reflect much more strongly in the visible (see Roesch et al. 2002). Currently in SOCRATES-RF, globally-varying, spectrally-constant land-surface albedo values are used, derived from ERA-Interim reanalysis data.

Figure 3: Left: Annual mean all-sky tropopause shortwave CH4 radiative forcing calculated by SOCRATES-RF (units W m-2). Right: Annual mean all-sky tropopause near-infrared CH4 radiative forcing from Collins et al. (2018)

Figure 3 compares the spatial distribution of methane’s annual-mean all-sky tropopause shortwave forcing as calculated by SOCRATES-RF and Collins et al. (2018). Both calculations exhibit the same regions of maxima across, for example, the Sahara, the Arabian Peninsula, and the Tibetan Plateau. However, it is interesting to note that the poleward amplification shown by SOCRATES-RF is not evident in Collins et al. (2018). The current leading hypothesis for this difference is the fact that the land surface albedo is specified differently in each calculation. Collins et al. (2018) employ spectrally-varying surface albedo values derived from satellite observations. These are arguably more realistic than the spectrally-constant values currently specified in SOCRATES-RF. The next step in my PhD is to further explore the interdependence between methane’s shortwave forcing and land-surface albedo, and to work towards implementing spectrally-varying albedo values into SOCRATES-RF calculations. Along with the ongoing investigation into the vertical representation of CH4 concentrations, I aim to finally deliver a more definitive estimate of methane’s shortwave effect.

References:

Checa-Garcia, R., Hegglin, M. I., Kinnison, D., Plummer, D. A., and Shine, K. P. 2018: Historical tropospheric and stratospheric ozone radiative forcing using the CMIP6 database. Geophys. Res. Lett., 45, 3264–3273, https://doi.org/10.1002/2017GL076770

Collins, W. D. et al, 2018: Large regional shortwave forcing by anthropogenic methane informed by Jovian observations, Sci. Adv. 4, https://doi.org/10.1126/sciadv.aas9593

Etminan, M., G. Myhre, E. Highwood, K. P. Shine. 2016: Radiative forcing of carbon dioxide, methane and nitrous oxide: a significant revision of methane radiative forcing, Geophys. Res. Lett., 43, https://doi.org/10.1002/2016/GL071930

IPCC, 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, 1535 pp

Myhre, G., et al. 2013: Anthropogenic and natural radiative forcing, in Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, edited by T. F. Stocker et al., pp. 659–740, Cambridge Univ. Press, Cambridge, U. K., and New York.

Roesch, A., M. Wild, R. Pinker, and A. Ohmura, 2002: Comparison of spectral surface albedos and their impact on the general circulation model simulated surface climate, J. Geophys. Res., 107, 13-1 – 13-18, https://doi.org/10.1029/2001JD000809

The philosophy of climate science

Email: m.prosser@pgr.reading.ac.uk

On the recommendation of my supervisor, I along with Javier Amezcua, Vicky Lucas and Benedict Hyland represented Reading Meteorology at the Institute of Physics (IOP) “Studying the Climate: A Challenge of Complexity” conference on February 6th, 2020. The programme and speaker list can be viewed here. It was a fantastic set of speakers delivering many a killer point in front of an engaged audience.

While some may consider the philosophy of science a complicating distraction, I think I ignore it at my peril. Certainly climate science is not without its philosophical issues; one might even say it is riddled with them…

David Stainforth (LSE), the keynote speaker stated it thus:
“The study of anthropogenic climate change presents a range of fundamental challenges for scientific and wider academic inquiry. The essential nature of these challenges are often not well appreciated.”

So how does climate science compare with other natural sciences? Opinions abounded, but here are just some I can recall:
1 – We can’t really conduct controlled experiments in the way that other natural scientists can, as we have just the one Earth and can’t turn back time (we have to beware of post-hoc explanations, and some of our predictions may never be verifiable/falsifiable).
2 – We therefore rely heavily on numerical models.
3 – We’re also doing our science while the climate is changing around us, and thus there is a strong sense of urgency.
4 – There is therefore a pressure to be multidisciplinary.

On a more practical side, David’s talk left me with a novel way of thinking about ‘climate’. Thinking about a climate metric, such as temperature, I would have thought hitherto of simply a mean and a standard deviation (a very Gaussian way of looking at it!). But David argued that climate is often best conceived of as a more generalised distribution. While a bell curve is symmetric, unimodal, a distribution need not be (and this can be true in the climate system). Studying and predicting a stable climate distribution may already be difficult but studying and predicting a changing one is even harder!

A visualisation of global sea surface temperatures. (Credit: Los Alamos National Laboratory)

Now for a bit of a whirlwind tour of other arguments/points. There was Reading’s own Ted Shepherd arguing that in climate science we often over focus on avoiding false positives (type I errors) at the expense of incurring false negatives (type II). In other words, we get reliability at the price of informativeness, especially at the regional level where policy makers are somewhat eager to be informed.

Then there was Geoff Vallis (University of Exeter) who posed the question “If models were perfect, would we care how they worked?”. Perhaps a pertinent question, as there appears to be a trade-off, an inverse correlation between complexity of models and our ability to understand them. If the models became so complex that they were beyond the abilities of any human past or future to comprehend, what would we do then? If they become as complicated as the Earth system itself, surely we would have long since lost any grasp on them? Indeed, models already appear to be predicting phenomena without us understanding why. Complexity is not necessarily accuracy (How do we assess accuracy in climate science?) and Erica Thompson (LSE) highlighted the importance of ‘getting out of model land’, and staying with the real world, something some of us may need occasional reminding of.

What even are models? Two expressions given were ‘book-keeping devices’ (Wendy Parker) and ‘Prosthesis of your brain (Erica Thompson). No doubt there were others.

Marina Baldissera Pacchetti (University of Leeds) talked about her work on climate information for adaptation that gives us: “guidelines on when quantitative statements about future climate are warranted and potentially decision-relevant, when these statements would be more valuable taking other forms (for example, qualitative statements), and when statements about future climate are not warranted at all.”

In the afternoon, there were breakout ‘lightning’ discussions. We could choose to join 1 of the following 8 groups:

1. Should we aim to estimate the mean/expectation behaviour of the climate or focus on the worst-case?
2. Is the way we go about climate science now the only way of doing it?
3. If our computers were infinitely fast, what science would we do with them?
4. If our models were infinitely good, what science would be left to do?
5. What fact, if only we knew it, would have the biggest impact on climate change?
6. How should climate science approach the question of geoengineering?
7. What is the benefit to society of general circulation models?
8. What is the public needing to know, and are we working enough on these questions?

My group was 3, but we ended up accidentally merging with 4 and made for a very interesting and varied discussion!

Which group would you have been most pulled towards had you been there? What philosophical thoughts on climate science have you had? What do you think is the most under-appreciated? I would be interested to hear your thoughts.

Many thanks to the event organiser Goodwin Gibbins (Imperial) and all involved for a thoroughly enjoyable and stimulating day.

If anyone would like to get more into the Philosophy of Science, I would recommend this thoroughly engaging 10-hour course of lecture by the Uni of Toronto on YouTube, the trailer of which can be viewed here.

The impact of atmospheric model resolution on the Arctic

Email: sally.woodhouse@pgr.reading.ac.uk

The Arctic region is rapidly changing, with surface temperatures warming at around twice the global average and sea ice extent is rapidly declining, particularly in the summer. These changes affect the local ecosystems and people as well as the rest of the global climate. The decline in sea ice has corresponded with cold winters over the Northern Hemisphere mid-latitudes and an increase in other extreme weather events (Cohen et al., 2014). There are many suggested mechanisms linking changes in the sea ice to changes in the stratospheric jet, midlatitude jet and storm tracks; however this is an area of active research, with much ongoing debate.

Stroeve_et_al-2012-fig2a
Figure 1. Time-series of September sea ice extent from 20 CMIP5 models (colored lines), individual ensemble members are dotted lines and the individual model mean is solid. Multi-model ensemble mean from a subset of the models is shown in solid black with +/- 1 standard deviation in dotted black. The red line shows observations. From Stroeve et al. (2012)

It is therefore important that we are able to understand and predict the changes in the Arctic, however there is still a lot of uncertainty. Stroeve et al. (2012) calculated time series of September sea ice extent for different CMIP5 models, shown in Figure 1. In general the models do a reasonable job of reproducing the recent trends in sea ice decline, although there is a large inter-model spread and and even larger spread in future projections. One area of model development is increasing the horizontal resolution – where the size of the grid cells used to calculate the model equations is reduced.

The aim of my PhD is to investigate the impact that climate model resolution has on the representation of the Arctic climate. This will help us understand the benefits that we can get from increasing model resolution. The first part of the project was investigating the impact of atmospheric resolution. We looked at three experiments (using HadGEM3-GC2), each at a different atmospheric resolutions: 135km (N512), 60km (N216) and 25km (N96).

sea_ice_concentration_obs_GC2
Figure 2. Annual mean sea ice concentration for observations (HadISST) and the bias of each different experiment from the observations N96: low resolution, N216: medium resolution, N512: high resolution.

The annual mean sea ice concentration for observations and the biases of the 3 experiments are shown in Figure 2. The low resolution experiment does a good job of producing the sea extent seen in observations with only small biases in the marginal sea ice regions. However, in the higher resolution experiments we find that the sea ice concentration is much lower than the observations, particularly in the Barents Sea (north of Norway). These changes in sea ice are consistent with warmer temperatures in the high resolution experiments compared to the low resolution.

To understand where these changes have come from we looked at the energy transported into the ocean by the atmosphere and the ocean. We found that there is an increase in the total energy being transported into the Arctic which is consistent with the reduced sea ice and warmer temperatures. Interestingly, the increase in energy is being transported into the Arctic by the ocean (Figure 3), even though it is the atmospheric resolution that is changing between the experiments. In the high resolution experiments the ocean energy transport into the Arctic, 0.15 petawatts (PW), is in better agreement with observational estimates, 0.154 PW, from Tsubouchi et al. (2018). Interestingly, this is in contrast to the worse representation of sea ice concentration in the high resolution experiments. (It is important to note that the model was tuned at the low resolution and as little as possible was changed when running the high resolution experiments which may contribute to the better sea ice concentration in the low resolution experiment.)

strait_locations
Location of ocean gateways into the Arctic. Red: Bering Strait, Green: Davis Strait, Blue: Fram Strait, Magenta: Barents Sea

ocean_heat_transport_GC2
Figure 3. Ocean energy transport for each resolution experiment through the four ocean gateways into the Arctic. The four gateways form a closed boundary into the Arctic.

We find that the ocean is very sensitive to the differences in the surface winds between the high and low resolution experiments. In different regions the differences in winds arise from different processes. In the Davis Strait the effect of coastal tiling is important, where at higher resolution a smaller area is covered by atmospheric grid cells that cover both land and ocean. In a cell covering both land and ocean the model usually produces wind speeds to low for over the ocean. Therefore in the higher resolution experiment we find that there are higher wind speeds over the ocean near the coast. Whereas over the Fram Strait and the Barents Sea instead we find that there are large scale atmospheric circulation changes that give the differences in surface winds between the experiments.

References

Cohen, J., Screen, J. A., Furtado, J. C., Barlow, M., Whittleston, D., Coumou, D., Francis, J., Dethloff, K., Entekhabi, D., Overland, J. & Jones, J. 2014: Recent Arctic amplification and extreme mid-latitude weather. Nature Geoscience, 7(9), 627–637, http://dx.doi.org/10.1038/ngeo2234

Stroeve, J. C., Kattsov, V., Barrett, A., Serreze, M., Pavlova, T., Holland, M., & Meier, W. N., 2012: Trends in Arctic sea ice extent from CMIP5, CMIP3 and observations. Geophysical Research Letters, 39(16), 1–7, https://doi.org/10.1029/2012GL052676

Tsubouchi, T., Bacon, S., Naveira Garabato, A. C., Aksenov, Y., Laxon, S. W., Fahrbach, E., Beszczynska-Möller, A., Hansen, E., Lee, C.M., Ingvaldsen, R. B. 2018: The Arctic Ocean Seasonal Cycles of Heat and Freshwater Fluxes: Observation-Based Inverse Estimates. Journal of Physical Oceanography, 48(9), 2029–2055, http://journals.ametsoc.org/doi/10.1175/JPO-D-17-0239.1

The Colour of Climate

Email: Jake.J.Gristey@noaa.gov
Web: https://cires.colorado.edu/researcher/jake-j-gristey

Gristey, J.J., J.C. Chiu, R.J. Gurney, K.P. Shine, S. Havemann, J. Thelen, and P.G. Hill, 2019: Shortwave Spectral Radiative Signatures and Their Physical Controls. J. Climate, 32, 4805–4828, https://doi.org/10.1175/JCLI-D-18-0815.1

Sunlight reaching the Earth is comprised of many different colours, or wavelengths. Some of these wavelengths cannot be detected by the human eye, such as the ultraviolet (UV) wavelengths which famously cause sunburn. Fortunately for us, the most intense sunlight is found at harmless visible wavelengths and reaches the surface with relative ease, allowing us to see during the daytime. Sometimes nature aligns to dramatically separate these wavelengths, producing beautiful optical phenomena such as rainbows. More often, however, the properties of the atmosphere and surface lead to intricate differences in the wavelengths of sunlight that get reflected back to space (Fig. 1).

Fig. 1. Schematic showing how the spectral structure of reflected sunlight at the top of the atmosphere can emerge via interactions with various atmospheric/surface properties*.

Satellites have observed specific wavelengths of reflected sunlight to infer the properties and evolution of our climate system for decades. Satellites have also independently measured the total amount of reflected sunlight across all wavelengths to track energy flows into and out of the Earth system. It has been less common to make spectrally resolved measurements at many contiguous wavelengths throughout the solar spectrum. In theory, these measurements would simultaneously provide the total energy flow – by integrating over the wavelengths – and the “spectral signature” associated with all atmospheric and surface properties that determined this energy flow. Our recent study puts this theory to the test.

Almost 100,000 spectra of reflected sunlight were computed at the top-of-atmosphere under a diverse variety of conditions. Applying a clustering technique to the computed spectra (which identifies “clusters” in a dataset with similar characteristics) revealed distinct spectral signatures. When we examined the atmospheric and surface properties that were used to compute the spectra belonging to each spectral signature, a remarkable separation of physical properties was found (Fig. 2).

Fig. 2. (top row) Three of the extracted “spectral signatures” of reflected sunlight. (bottom row) Their relationship to the underlying atmospheric/surface properties. Seven others are shown in the published article.

Surprisingly, the separation of physical properties by distinct spectral signatures, as shown in Fig. 2, was found to be robust up to the largest spatial scales tested of 240 km. This is similar to the footprint size of one of the only previous satellite instruments to measure contiguous spectrally resolved reflected sunlight, the SCIAMACHY**, providing an exciting opportunity to investigate spectral signature variability in real observations. We found that the frequency of spectral signatures in real SCIAMACHY observations followed the expected behaviour during the West African monsoon very closely (Fig. 3).

Fig. 3. (left) The annual cycle of precipitation [mm/day] associated with the West African monsoon, and (right) frequency of the three “spectral signatures” shown in Fig. 2 from real satellite observations during 2010 over West Africa.

Overall, the separation of physical properties by distinct spectral signatures demonstrates great promise for monitoring evolution of the Earth system directly from spectral reflected sunlight in the future.

Funding acknowledgement: This work was supported by the Natural Environment Research Council (NERC) SCience of the Environment: Natural and Anthropogenic pRocesses, Impacts and Opportunities (SCENARIO) Doctoral Training Partnership (DTP), Grant NE/L002566/ 1, and from the European Union 7th Framework Programme under Grant Agreement 603502 [EU project Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa (DACCIWA)]

*Note several key simplifications in Fig. 1 for the purposes of visual effect: atmospheric properties are separated, but often occur simultaneously and throughout the atmosphere; the depicted path of sunlight is one option, but sunlight emerging at the top of the atmosphere will come from many different paths; sunlight reflected by the surface will need to travel back through the same gases (and likely other properties) on its way back to the top of the atmosphere, which is not shown. The spectra in Fig. 1 are generated with SBDART using a set of arbitrary but realistic atmospheric and surface properties.

** SCIAMACHY = Scanning Imaging Absorption Spectrometer for Atmospheric Chartography.

Jake completed his PhD at Reading in 2018 and now works at the NOAA Earth System Research Laboratory (ESRL) in Boulder, Colorado.

Island convection and its many shapes and forms: a closer look at cloud trails

Despite decades of research, convection continues to be one of the major sources of uncertainty in weather and climate models. This is because convection occurs across scales that are smaller than the numerical grids used to integrate these models – in other words, the convection is not resolved in the model. However, its role in the vertical transport of heat, moisture, and momentum could still be important for phenomena that are resolved so the impact of convection is estimated from a set of diagnosed parameters (i.e. a parameterisation scheme).

As the community moves toward modelling with smaller numerical grids, convection can be partially resolved. This numerical regime consisting of partially resolved convection is sometimes called the ‘Convection Grey Zone’. New parameterisations for convection are required for the convection grey zone as the underlying assumptions for existing parameterisations are no longer valid.

With smaller grid spacing, other important processes are better represented – for example, the interaction with the surface. In some coarse climate models, many islands are so small that they are neglected altogether. We know that islands regularly force different kinds of convection and so they offer a real-world opportunity to study the kind of locally driven convection that can now be resolved in operational weather models. My thesis aims to take existing research on small islands a step further by considering the problem from the perspective of convection parameterisation.

Bermuda_DEM
Figure 1. Topographic map of Bermuda showing the coastline in blue, elevation above sea level in grey shading, and the highest elevation is marked by a red triangle.

Bermuda (where I’m from) is a small, relatively flat island located in the western North Atlantic Ocean (e.g. Fig. 1). Cloud trails (CT) here have been unwittingly incorporated into a local legend surrounding an 18th century heist during the American Revolution. This plot to steal British gunpowder to help the American revolutionaries involved the American merchant ‘Captain Morgan’, whose ghost is said to haunt Bermuda on hot, humid summer evenings when dark cloud looms over the east end of the island. This legend is where the local name for the cloud trail “Morgan’s Cloud” comes from (BWS Glossary, 2019).

This story highlights what a CT might look like from a ground observer – a dark cloud which hangs over one end of the island. In fact, CT could only be observed from the ground until research aircraft became feasible in the 1940s and 50s. Aircraft measurements revealed the internal structure of the CT including an associated plume of warmer, drier air immediately downwind of the island.

In the coming decades, the combination of publicly available high-quality satellite imagery and computing advances introduced new avenues for research. This allowed case studies of one-off events and short satellite climatologies constructed by hand (e.g. Nordeen et al., 2001).

Observed from space, CTs look like bands of cloud that stream downwind of, and appear anchored to, small islands. They can be found downwind of small islands around the world, mainly in the tropics and subtropics.

fig1
Figure 2. (Johnston et al., 2018) Observations from visible satellite imagery showing (a) an example CT, (b) an example NT, and (c) an example obscured scene. Imagery from GOES-13 0.64 micron visible channel. In each instance a wind barb indicating the wind speed (knots) and direction. Full feathers on the wind barbs represent 10 kts, and half feathers 5 kts.

In my thesis, we design an algorithm to automate the objective classification of satellite imagery into one of three categories (Fig. 2): CT, NT (Non-Trail), and OB (Obscured). We find that the algorithm results are comparable to manually classified satellite imagery and can construct a much longer climatology of CT occurrence quickly and objectively (Johnston et al., 2018). The algorithm is applied to satellite imagery of Bermuda for May through October of 2012-2016.

We find that CT occurrence peaks in the afternoon and in July. This highlights the strong link to the solar cycle. Furthermore, radiosonde measurements taken via weather balloon by the Bermuda Weather Service show that cloud base height (which is controlled by the low-level humidity) is too high for NT days. This reduces cloud formation in general and prevents the CT cloud band forming. Meanwhile, large-scale disturbances result in widespread cloud cover on OB days (Johnston et al., 2018).

These observations and measurements can only tell us so much. A case CT day is then used to design numerical experiments to consider poorly observed features of the phenomenon. For example, the interplay between the warm plume, CT circulation, and the clouds themselves. These experiments are completed with very small grid spacing (i.e. 100 m vs. the ~10 km in weather models, and ~50 km in climate models). This allows us to confidently simulate both convection and a small island without the use of parameterisations.

Within the boundary layer which buffers the impacts from surface on the free atmosphere, a circulation forms downwind of the heated island. We show that this circulation consists of near-surface convergence, which leads to a band of ascent, and a region of divergence near the top of the boundary layer. This circulation acts as a coherent structure tying the boundary layer to convection in the free atmosphere above.

Further experiments which target the relationship between the island heating, low-level humidity, and wind speed have been completed. These experiments reveal a range of circulation responses. For instance, responses associated with no cloud, mostly passive cloud, and strongly precipitating cloud can result.

We are now using the set of CT experiments to develop a set of expectations upon which existing and future convection parameterisation schemes can be tested and evaluated. We plan to use a selection of the CT experiments with grid spacing increased to values consistent with current operational grey zone models. We believe that this will help to highlight deficiencies in existing parameterisation schemes and focus efforts for the improvement of future schemes.

Further Reading:

Bermuda Weather Service (BWS) Glossary, accessed 2019: Morgan’s Cloud/Morgan’s Cloud (Story). https://www.weather.bm/glossary/glossary.asp

Johnston, M. C., C. E. Holloway, and R. S. Plant, 2018: Cloud Trails Past Bermuda: A Five-Year Climatology from 2012-2016. Mon. Wea. Rev., 146, 4039-4055, https://doi.org/10.1175/MWR-D-18-0141.1

Matthews, S., J. M. Hacker, J. Cole, J. Hare, C. N. Long, and R. M. Reynolds, 2007: Modification of the atmospheric boundary layer by a small island: Observations from Nauru. Mon. Wea. Rev., 135, 891-905, https://doi.org/10.1175/MWR3319.1

Nordeen, M. K., P. Minnis, D. R. Doelling, D. Pethick, and L. Nguyen, 2001: Satellite observations of cloud plumes generated by Nauru. Geophys. Res. Lett., 28, 631-634, https://doi.org/10.1029/2000GL012409

APPLICATE General Assembly and Early Career Science event

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On 28th January to 1st February I attended the APPLICATE (Advanced Prediction in Polar regions and beyond: modelling, observing system design and LInkages associated with a Changing Arctic climaTE (bold choice)) General Assembly and Early Career Science event at ECMWF in Reading. APPLICATE is one of the EU Horizon 2020 projects with the aim of improving weather and climate prediction in the polar regions. The Arctic is a region of rapid change, with decreases in sea ice extent (Stroeve et al., 2012) and changes to ecosystems (Post et al., 2009). These changes are leading to increased interest in the Arctic for business opportunities such as the opening of shipping routes (Aksenov et al., 2017). There is also a lot of current work being done on the link between changes in the Arctic and mid-latitude weather (Cohen et al., 2014), however there is still much uncertainty. These changes could have large impacts on human life, therefore there needs to be a concerted scientific effort to develop our understanding of Arctic processes and how this links to the mid-latitudes. This is the gap that APPLICATE aims to fill.

The overarching goal of APPLICATE is to develop enhanced predictive capacity for weather and climate in the Arctic and beyond, and to determine the influence of Arctic climate change on Northern Hemisphere mid-latitudes, for the benefit of policy makers, businesses and society.

APPLICATE Goals & Objectives

Attending the General Assembly was a great opportunity to get an insight into how large scientific projects work. The project is made up of different work packages each with a different focus. Within these work packages there are then a set of specific tasks and deliverables spread out throughout the project. At the GA there were a number of breakout sessions where the progress of the working groups was discussed. It was interesting to see how these discussions worked and how issues, such as the delay in CMIP6 experiments, are handled. The General Assembly also allows the different work packages to communicate with each other to plan ahead, and for results to be shared.

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An overview of APPLICATE’s management structure take from: https://applicate.eu/about-the-project/project-structure-and-governance

One of the big questions APPLICATE is trying to address is the link between Arctic sea-ice and the Northern Hemisphere mid-latitudes. Many of the presentations covered different aspects of this, such as how including Arctic observations in forecasts affects their skill over Eurasia. There were also initial results from some of the Polar Amplification (PA)MIP experiments, a project that APPLICATE has helped coordinate.

1
Attendees of the Early Career Science event co-organised with APECS

At the end of the week there was the Early Career Science Event which consisted of a number of talks on more soft skills. One of the most interesting activities was based around engaging with stakeholders. To try and understand the different needs of a variety of stakeholders in the Arctic (from local communities to shipping companies) we had to try and lobby for different policies on their behalf. This was also a great chance to meet other early career scientists working in the field and get to know each other a bit more.

What a difference a day makes, heavy snow getting the ECMWF’s ducks in the polar spirit.

Email: sally.woodhouse@pgr.reading.ac.uk

References

Aksenov, Y. et al., 2017. On the future navigability of Arctic sea routes: High-resolution projections of the Arctic Ocean and sea ice. Marine Policy, 75, pp.300–317.

Cohen, J. et al., 2014. Recent Arctic amplification and extreme mid-latitude weather. Nature Geoscience, 7(9), pp.627–637.

Post, E. & Others, 24, 2009. Ecological Dynamics Across the Arctic Associated with Recent Climate Change. Science, 325(September), pp.1355–1358.

Stroeve, J.C. et al., 2012. Trends in Arctic sea ice extent from CMIP5, CMIP3 and observations. Geophysical Research Letters, 39(16), pp.1–7.

The Role of the Cloud Radiative Effect in the Sensitivity of the Intertropical Convergence Zone to Convective Mixing

Email: j.f.talib@pgr.reading.ac.uk

Talib, J., S.J. Woolnough, N.P. Klingaman, and C.E. Holloway, 2018: The Role of the Cloud Radiative Effect in the Sensitivity of the Intertropical Convergence Zone to Convective Mixing. J. Climate, 31, 6821–6838, https://doi.org/10.1175/JCLI-D-17-0794.1

Rainfall in the tropics is commonly associated with the Intertropical Convergence Zone (ITCZ), a discontinuous line of convergence collocated at the ascending branch of the Hadley circulation, where strong moist convection leads to high rainfall. What controls the location and intensity of the ITCZ remains a fundamental question in climate science.

ensemble_precip_neat_thesis
Figure 1: Annual-mean, zonal-mean tropical precipitation (mm day-1) from Global Precipitation Climatology Project (GPCP, observations, solid black line) and CMIP5 (current coupled models) output. Dashed line indicates CMIP5 ensemble mean.

In current and previous generations of climate models, the ITCZ is too intense in the Southern Hemisphere, resulting in two annual-mean, zonal-mean tropical precipitation maxima, one in each hemisphere (Figure 1).  Even if we take the same atmospheric models and couple them to a world with only an ocean surface (aquaplanets) with prescribed sea surface temperatues (SSTs), different models simulate different ITCZs (Blackburn et al., 2013).

Within a climate model parameterisations are used to replace processes that are too small-scale or complex to be physically represented in the model. Parameterisation schemes are used to simulate a variety of processes including processes within the boundary layer, radiative fluxes and atmospheric chemistry. However my work, along with a plethora of others, shows that the representation of the ITCZ is sensitive to the convective parameterisation scheme (Figure 2a). The convective parameterisation scheme simulates the life cycle of clouds within a model grid-box.

Our method of showing that the simulated ITCZ is sensitive to the convective parameterisation scheme is by altering the convective mixing rate in prescribed-SST aquaplanet simulations. The convective mixing rate determines the amount of mixing a convective parcel has with the environmental air, therefore the greater the convective mixing rate, the quicker a convective parcel will become similar to the environmental air, given fixed convective parcel properties.

AEIprecipCREon
Figure 2: Zonal-mean, time-mean (a) precipitation rates (mm day-1}$) and (b) AEI (W m-2) in simulations where the convective mixing rate is varied.

In our study, the structure of the simulated ITCZ is sensitive to the convective mixing rate. Low convective mixing rates simulate a double ITCZ (two precipitation maxima, orange and red lines in Figure 2a), and high convective mixing rates simulate a single ITCZ (blue and black lines).

We then associate these ITCZ structures to the atmospheric energy input (AEI). The AEI is the amount of energy left in the atmosphere once considering the top of the atmosphere and surface energy budgets. We conclude, similar to Bischoff and Schneider, 2016, that when the AEI is positive (negative) at the equator, a single (double) ITCZ is simulated (Figure 2b). When the AEI is negative at the equator, energy is needed to be transported towards the equator for equilibrium. From a mean circulation perspective, this take place in a double ITCZ scenario (Figure 3). A positive AEI at the equator, is associated with poleward energy transport and a single ITCZ.

blog_figure_ITCZ_simulation
Figure 3: Schematic of a single (left) and double ITCZ (right). Blue arrows denote energy transport. In a single ITCZ scenario more energy is transported in the upper branches of the Hadley circulation, resulting in a net-poleward energy transport. In a double ITCZ scenario, more energy is transport equatorward than poleward at low latitudes, leading to an equatorward energy transport.

In our paper, we use this association between the AEI and ITCZ to hypothesize that without the cloud radiative effect (CRE), atmospheric heating due to cloud-radiation interactions, a double ITCZ will be simulated. We also hypothesize that prescribing the CRE will reduce the sensitivity of the ITCZ to convective mixing, as simulated AEI changes are predominately due to CRE changes.

In the rest of the paper we perform simulations with the CRE removed and prescribed to explore further the role of the CRE in the sensitivity of the ITCZ. We conclude that when removing the CRE a double ITCZ becomes more favourable and in both sets of simulations the ITCZ is less sensitive to convective mixing. The remaining sensitivity is associated with latent heat flux alterations.

My future work following this publication explores the role of coupling in the sensitivity of the ITCZ to the convective parameterisation scheme. Prescribing the SSTs implies an arbitary ocean heat transport, however in the real world the ocean heat transport is sensitive to the atmospheric circulation. Does this sensitivity between the ocean heat transport and atmospheric circulation affect the sensitivity of the ITCZ to convective mixing?

Thanks to my funders, SCENARIO NERC DTP, and supervisors for their support for this project.

References:

Blackburn, M. et al., (2013). The Aqua-planet Experiment (APE): Control SST simulation. J. Meteo. Soc. Japan. Ser. II, 91, 17–56.

Bischoff, T. and Schneider, T. (2016). The Equatorial Energy Balance, ITCZ Position, and Double-ITCZ Bifurcations. J. Climate., 29(8), 2997–3013, and Corrigendum, 29(19), 7167–7167.