How the Earth ‘breathes’ on a daily timescale

Email: J.Gristey@pgr.reading.ac.uk
Web: http://www.met.reading.ac.uk/~fn008822/

As the Earth rotates, each location on its surface is periodically exposed to incoming sunlight. For example, over London at the beginning of September, the intensity of incoming sunlight ranges from zero overnight, when the sun is below the horizon, to almost 1000 W m–2 at noon, when the sun is highest in the sky (Fig. 1).

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Fig. 1. Top-of-atmosphere incoming sunlight over London (51.5° N, 0° E) for the first three days in September 2010. Data is from the Met Office numerical weather prediction model.

Earth’s atmosphere and surface respond to this repeating daily cycle of incoming sunlight in ways that can change the amount of energy that is emitted or reflected back to space. For example, the increased amount of sunlight in the afternoon can heat up the surface and cause more thermal energy to be emitted to space. Meanwhile, the surface heating can also cause the air near the surface to warm up and rise to form clouds that will, in turn, reflect sunlight back to space. The resulting daily cycle of the top-of-atmosphere outgoing energy flows is therefore intricate and represents one of the most fundamental cycles of our weather and climate. It is essential that we can properly represent the physical processes controlling this daily variability to obtain accurate weather and climate forecasts. However, the daily variability in Earth’s outgoing energy flows is not currently well observed across the entire globe, and current weather and climate models can struggle to reproduce realistic daily variability, highlighting a lack of understanding.

To improve understanding, dominant patterns of the daily cycle in outgoing energy flows are extracted from Met Office model output using a mathematical technique known as “principal component analysis”.

The daily cycle of reflected sunlight is found to be dominated by the height of the sun in the sky, or the “solar zenith” angle, because the atmosphere and surface are more reflective when the sun is low in the sky. There is a lesser importance from low-level clouds over the ocean, known as “marine stratocumulus” clouds, which burn off during the afternoon, reducing the amount of reflected sunlight, and tall and thick clouds, known as “deep convective” clouds, which develop later in the afternoon over land and increase the amount of reflected sunlight. On the other hand, the daily cycle of emitted thermal energy is dominated by surface heating, which increases the emitted energy at noon, but also by deep convective clouds that have very high and cold tops, reducing the emitted energy later in the afternoon. These dominant processes controlling the daily cycle of Earth’s outgoing energy flows and their relative importance (summarised in Fig. 2) have not been revealed previously at the global scale.

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Fig. 2. A schematic diagram showing the first (top) and second (bottom) most important processes controlling the daily cycle in emitted thermal energy (left) and reflected sunlight (right).

The physical processes discussed above are consistent with the daily cycle in other relevant model variables such as the surface temperature and cloud amount, further supporting the findings. Interestingly, a time lag is identified in the response of the emitted thermal energy to cloud variations, which is thought to be related to changes in the humidity of the upper atmosphere once the clouds evaporate.

The new results highlight an important gap in the current observing system, which can be utilized to evaluate and improve deficiencies in weather and climate models.

Gristey, J. J., Chiu, J. C., Gurney, R. J., Morcrette, C. J., Hill, P. G., Russell, J. E., and Brindley, H. E.: Insights into the diurnal cycle of global Earth outgoing radiation using a numerical weather prediction model, Atmos. Chem. Phys., 18, 5129-5145, https://doi.org/10.5194/acp-18-5129-2018, 2018.

Estimating the Effects of Vertical Wind Shear on Orographic Gravity Wave Drag

Orographic gravity waves occur when air flows over mountains in stably stratified conditions. The flow of air creates a pressure imbalance across the mountain, so a force is exerted on the mountain in the same direction as the flow. An equal and opposite force is exerted back on the atmosphere, and this is gravity wave drag (GWD).

GWD must be parametrized in Global Circulation Models (GCMs), as it is important for large-scale flow. The first parametrization was formulated by Palmer et al. (1986) to reduce a systematic westerly bias. The current parametrization was formulated by Lott and Miller (1997) and partitions the calculation into 2 parts (see figure 1):

  1. The mountain waves. This is calculated by averaging the wind, Brunt-Väisälä frequency and fluid density in a layer between 1 and 2 standard deviations of the subgrid-scale orography above the mean orography.
  2. The blocked flow. This is based on an interpretation of the non-dimensional mountain height.
Fig 1
Figure from Lott and Miller (1997).

The parametrization does not include the effects of wind shear. Wind shear is a change in the wind with height and it alters the vertical wave length of gravity waves and so alters the drag. It has been shown (Teixeira et al., 2004; Teixeira and Miranda, 2006) that a uniform shear profile (i.e. a change in the magnitude of the wind with height) decreases the drag whereas a profile in which the wind turns with height increases the drag. This effect was seen by Miranda et al. (2009) to have the greatest impact over Antarctica, where drag enhancement was seen to occur all year with a peak of ~50% during JJA. Figure 2 shows this.

Fig 2
Figure 2: Annual mean linear GWD stress (1992-2001). Vectors show the surface stress with shear. Shading indicates the anomaly of the modulus of the surface stress due to shear. Computed from ERA-40 data. Taken from Miranda et al. (2009).

The aim of this work is to test the impact of the inclusion of shear effects on the parametrization. The first stage of this is to test the sensitivity of the shear correction to the height in the atmosphere at which the necessary derivatives are approximated. We carry out calculations using 2 different reference heights:

  1. The top of the boundary layer (BLH). This allows us to avoid the effects of boundary layer turbulence, which are not important in this case as they are unrelated to the dynamics of mountain waves.
  2. The middle of the layer between 1 and 2 standard deviations of the sub-grid scale orography (SDH). This is the nominal height used in previous studies and in the parametrization.

All figures shown below focus on Antarctica and are averaged over all JJAs for the decade 2006-2015. We are interested in Antarctica and the JJA season for the reasons highlighted above. All calculations are carried out using ERA-Interim reanalysis data.

We first consider the enhancement assuming axisymmetric orography. The advantage of this is that it considerably simplifies the correction due to terms related to the anisotropy becoming constant (see Teixeira et al, 2004). Figure 3 shows this correction calculated using both reference heights. We can see that the enhancement is greater when the SDH is used.

Fig 3
Figure 3: Drag enhancement over Antarctica with shear corrections computed at the BLH (left) and SDH (right), during JJAs for the decade 2006-2015, using axisymmetric orography.

We now consider the enhancement using mountains with an elliptical horizontal cross-section. This is how the real orography is represented in the parametrization. Again, we see that the enhancement is greater when the SDH is used (figure 4).

Fig 4
Figure 4: Drag enhancement (left) and enhancement of drag stress (in Pa) (right) over Antarctica with shear corrections calculated at the BLH (top) and SDH (bottom), during JJAs for the decade 2006-2015, using orography with an elliptical horizontal cross-section.

It is interesting to note that at both heights the enhancement is greater when axisymmetric orography is used. This occurs because, in the case of elliptical mountains, the shear vector is predominantly aligned along the orography, resulting is weaker enhancement (see figure 5).

Fig 5
Figure 5: Histograms of the orientation of the shear vector relative to the short axis of the orography over Antarctica for JJAs during the decade 2006-2015, using the BLH (left) and SDH (right).

We also investigate the fraction of times at which the terms related to wind profile curvature (i.e. those containing second derivatives) dominate the drag correction. This tells us the fraction of time for which curvature matters for the drag. We see that second derivatives dominate over much of Antarctica for a high proportion of the time (see figure 6).

Fig 6
Figure 6: Fraction of the time at which terms with second derivatives dominate the drag correction relative to terms with first derivatives over orography with an elliptical horizontal cross-section, for JJAs during the decade 2006-2015, calculated using the BLH (left) and SDH (right).

In summary, the main findings are as follows:

  • The drag is quantitatively robust to changes in calculation height, with the geographical distribution, seasonality and sign essentially the same.
  • The drag is considerably enhanced when the SDH is used rather than the BLH.
  • Investigation of the relative magnitudes of terms containing first and second derivatives in the drag correction indicates that second derivatives (i.e. curvature terms) dominate in a large proportion of Antarctica for a large fraction of time. This leads to an average enhancement of the drag which is larger over shorter time intervals.
  • Use of an axisymmetric orography profile causes considerable overestimation of the shear effects. This is due to the shear vector being predominantly aligned along the mountains in the case of the orography with an elliptical horizontal cross-section.

These results highlight the need to ‘tune’ the calculation by identifying the optimum height in the atmosphere at which to approximate the derivatives. This work is ongoing. We expect the optimum height to be that at which the shear has the greatest impact on the surface drag.

References:

Lott F. and Miller M., 1997, A new subgrid-scale orographic drag parametrization: Its formulation and testing, Quart. J. Roy. Meteor. Soc., 123: 101–127.

Miranda P., Martins J. and Teixeira M., 2009, Assessing wind profile effects on the global atmospheric torque, Quart. J. Roy. Meteor. Soc., 135: 807–814.

Teixeira M. and Miranda P., 2006, A linear model of gravity wave drag for hydrostatic sheared flow over elliptical mountains, Quart. J. Roy. Meteor. Soc., 132: 2439–2458.

Teixeira M., Miranda P. and Valente M., 2004, An analytical model of mountain wave drag for wind profiles with shear and curvature, J. Atmos. Sci., 61: 1040–1054.

Visiting Scientist 2018

With thanks to Kaja Milczewska

Every year the PhD students in the Meteorology Department invite a distinguished scientist to spend a few days with us. This year, the students voted for the Visiting Scientist to be Prof. Olivia Romppainnen-Martius, who came to the Department from 4th-7th June 2018.

Prof. Romppainen-Martius is based at the University of Bern, in Switzerland, as an Associate Professor researching climate impacts.

Olivia’s research interests broadly covers mid-latitude atmospheric dynamics, with topics from how blocking events are precursors Sudden Stratospheric Warming events, to more impact based work on heavy Alpine precipitation and extreme hail in and around Switzerland. Her main research areas can be summarised as dynamics of short-term climate variation, forecasting and statistics of high-impact weather events and mid-latitude weather systems. More about her research and publications can be found here.

As is usual for the start of our distinguished visitor’s stay, Prof. Romppainen-Martius’s visit began with an introduction from Prof. Sue Gray during the coffee reception. This was immediately followed by a special seminar, titled “Recent hail research in Switzerland – the challenges and delights of complex orography and crowd-sourced data”. Her talk covered various probabilistic measures for predicting hail in the mountainous region that is Switzerland and how the climatology of these identified events is strongly linked with these mountainous areas. Verification of these predictions has recently been achieved through observer reports via the MeteoSwiss app, where observers record the time, location, and size of the hail they have observed.

The day was rounded off with a social at Zero Degrees, with Olivia and many PhD students engaging in fruitful conversation over pizza and beer.

After a busy first day, the second day of her visit included individual meetings with both research staff and students, and attending the Mesoscale and HHH (Hoskins-Half-Hour) research groups. On Wednesday 5th July, some PhD students presented their research to Olivia to showcase the breadth of topics covered in the Meteorology department. Interestingly, one of the talks ‘reliably’ informed us that Arctic sea-ice melting meant it was now possible to go on holiday cruises to see penguins. Clearly these penguins are on holiday too…

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At the weekly PhD Group meeting, Prof. Romppainen-Martius gave some useful advice on careers in academic research and the pathway to her current position – which of course includes lots of skiing. Additionally, she advertised some post-doctoral funding opportunities in Switzerland and Germany, which was sure to encourage the keen skiers in the crowd. This was an engaging open discussion about the realities of research life, and attendance was made all the better by biscuits from the group leaders Beth and Liam.

On the last day of her visit (Thursday 8th June), Olivia gave her second departmental seminar titled “Periods of recurrent synoptic-scale Rossby waves and associated persistent moderate temperature extremes”. The seminar was followed by a well-attended leaving reception, which concluded Olivia’s visit to our department. The students prepared a photo frame and other England themed items as a gift, to thank our distinguished scientist for accepting the invitation to spend an inspiring week with us.  Unfortunately, Olivia could not stay for the ‘world-renowned’ annual Met BBQ and Barn Dance on the Friday, but nonetheless we hope that she enjoyed her visit as much as we did!

Olivia

Top websites for weather enthusiasts!

If you’re searching for some weather-related procrastination, then look no further – we’ve got just what you need! Here’s our top picks for your coffee break-browsing:

  • Want a cool animated globe that shows you wind, temperature and aerosols, amongst other things? Null School is for you!

nullschool

  • Severe Weather Europe has photos and videos of awesome hailstorms, supercells and more.
  • If you’re wanting wind maps – then Windy.com is the place to go.

windy

  • Space weather more your thing? Then help with some research and find Solar Storms. Read more about the science in Shannon’s blog.
  • If you’ve been following all the recent thunderstorms, then check out the locations of all the lightning, updated in near real-time at Blitzortung.

lightning

  • The Met Office website has forecasts and pollen counts, but also cool things like podcasts about the weather.
  • Real-time satellite imagery is available at sat24 for the UK and Europe.

Sat24

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  • For articles on climate change and environmental science, Carbon Brief  is the answer.

 

Polar Prediction School 2018

From 17th-27th April three Reading students trekked to the to the far north to attend the APECS Polar Prediction School at the Abisko Research Station. The aims of this course were to provide a general education in the Polar climates, from ocean and ice to atmosphere to help the participants understand the issues of prediction in polar regions and contribute to the current academic push to improve our understanding and forecasting skill of these regions.

 

 

 

Abisko research station is situated 68°N on the banks of lake Torneträsk, the sixth longest lake in Sweden. Frozen from approximately December to June the lake provided a great base for experiencing taking observations of the poles. On the first full day we put up a met mast which we then used data from to explore boundary layer turbulence. Drilling the holes for the guy ropes to find the ice was still a metre thick was rather reassuring after people had stripped down to t-shirts in the sun.

Throughout the week we also launched multiple radiosondes which was another excellent excuse to spend some time drinking in the scenery. This caused a stir when there was an ice-fishing competition on the lake, so several local school children ended up assisting with the launch.

 

 

 

 

After a week packed full of lectures, from sea-ice dynamics to observations from an ice breaker, on the Sunday in the middle of the school we had the day off. Most people took this as a chance to explore a bit further afield. A few of us rented snowshoes which turned out to be an excellent idea as there were plenty of places where the snow was still a meter thick. However difficult the terrain the scenery was 100% worth it, and the kanelbullar in our packed lunches certainly helped keep us going.

 

 

This was followed by another week of lectures, covering boundary layers, clouds and much more. We also spent time working on our science communication, both to other scientists and the general public. This culminated with everyone giving a 1 minute “Frostbyte” presentation of their work.

The course was a great chance to learn about the polar climate more broadly which has been helpful in putting my PhD work in context. It’s also great to be able to say I have been to the Arctic when people ask in the future!

PPS2018_group

A big thank you to APECS, APPLICATE and the Polar Prediction Project for supporting the course as well as all the staff who gave their time to speak. More details about the course can be found here.

The Test MIST Special Meeting, Southampton, March 26-28th, 2018

Email: t.bloch@pgr.reading.ac.uk

MIST Logo_small_0
In its own words MIST is “the community of Magnetosphere, Ionosphere and Solar-Terrestrial researchers working in the United Kingdom. We represent the interests of MIST scientists and hold meetings to showcase MIST science twice a year”.

It is a group which focuses on Space Science, both theoretically and empirically, covering everything from Solar physics [see Shannon’s work] to Planetary Atmospheres, incorporating data from many space-missions, ground-based measurements (for Earth) and models which underpin our current understanding.

MIST holds two meetings a year: Autumn MIST, a one-day meeting; and Spring MIST, a longer 2.5 day meeting (unfortunately these names lead to a lot of pictures of dewy mornings when using Google…).

Each Spring MIST meeting is given a name based on a local geographical feature, this year we were at Southampton University near the River Test. The “Special” is to honour MIST’s 50th anniversary. Less obvious to me was why people kept laughing when the name was mentioned. The only clue was “Any similarity to low frequency emissions on 198 kHz is purely coincidental”. If you like cricket, this may be obvious… but for the rest of us, it’s a reference to the cricket ‘Test Match Special’ radio broadcast. Good thing we’re scientists and not comedians.

This year, it’s safe to say that the Reading delegation took over the meeting. With 9 of us attending (6 presenting!) the Reading Space group was definitely very well represented. Mike Lockwood gave an excellent speech at the conference dinner on the importance of MIST to the community and how he has seen it evolve over the years. See below to read about what everyone presented.

The bi-annual meetings are excellent for keeping up with the current state of the UK’s space-science research as well as maintaining a more informal atmosphere due to the small nature of the community (there were around 60 people at Spring MIST ’18). I find that this all comes together to form a very inviting platform for those of us just starting out in research.

SpringMist18
This year’s spring MIST group photo

Shannon Jones presented a poster, “Solar Stormwatch: Using citizen science to investigate CME distortions”.

Coronal mass ejections (CMEs) are the main drivers of hazardous space weather. We are using a novel dataset, created with the help of many citizen scientists through the Solar Stormwatch project, to investigate the effect of the solar wind on these storms. Participants track the shape of CMEs in images from the heliospheric imagers on board the twin STEREO spacecraft, providing an unprecedented level of detail (Barnard et al., 2017). We intend to use this data to extend the work of Savani et al. (2010), looking at how CMEs are distorted under varying solar wind conditions. 


Sarah Bentley gave a talk, “A solar wind-parameterised, probabilistic model of ground-measured ULF waves in Earth’s magnetosphere”.

Large scale ultra-low frequency (1-15 mHz) plasma waves in the magnetosphere are involved in the energisation and transport of radiation belt electrons, a hazardous environment for the satellites underpinning our everyday life. We can construct a statistical model predicting when and where we see these waves in the magnetosphere solely using causally correlated solar wind properties [Bentley et al., 2018]. Unlike existing models, this can be used probabilistically, so that instead of outputting a single value for the power in these waves at each location we can use a probability distribution. Sampling from these distributions turns out to be the best way of predicting total power over a longer event while using the mean values is the best way of predicting the power in the oncoming hour. Using these predicted power values we will eventually be able to predict the effect of these waves on radiation belt particles more precisely over a larger range of the magnetosphere.

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Oliver Allanson gave a talk, “Particle-in-cell models of diffusion due to whistler mode waves: comparing quasi-monochromatic to broadband waves”.

The momentum space diffusion of electrons due to whistler mode waves is a cornerstone of our current theoretical framework of acceleration (and loss) in Earth’s outer radiation belt. The quasilinear theory of wave-particle interactions provides us with a tractable method to estimate the amount of momentum space diffusion that occurs for a range of wave and ambient plasma conditions. Underlying quasilinear theory is the assumption that waves are broadband, incoherent, and of small amplitude. The right-handed whistler mode manifests in different ways throughout the outer belt: structured chorus, incoherent hiss, near monochromatic transmitter waves, lightning generated whistlers, and large amplitude nonlinear wave packets. It is possible that incoherent hiss is the only example that satisfies all of the formal requirements of quasilinear theory. We use particle-in-cell simulations (the EPOCH code) to model different cases, i.e. from near monochromatic to unstructured broadband, and from small amplitude to large. Through the use of various diagnostics, we explore whether the quasilinear diffusion description is a reasonable description of each case.


Clare Watt gave a talk, “The origin of the whistler-mode spectral “gap” at half electron gyro-frequency in the magnetosphere”.

Near-Earth space contains high-energy electrons, high-energy protons, and a host of different electromagnetic waves that exist over a wide-range of frequencies. Because of the Earth’s magnetic field, and the presence of the high-energy charged particles, the electromagnetic waves do not behave exactly like light in a vacuum, rather they are guided along and across the magnetic field, and interact with the electrons and protons to transport energy and momentum throughout the system. One type of waves are known as whistler-mode waves. They have frequencies of roughly 100-1000Hz and interact with the high-energy electrons in the outer radiation belt. These interactions are thought to be responsible for the energisation of the outer radiation belt. But the waves themselves have many fascinating and mysterious features. Decades of in-situ observations of the waves reveal a persistent frequency gap. Many theories have been put forward to explain the gap, but most rely upon special circumstances that are not guaranteed throughout space. Our recent physics-based simulations reveal a ubiquitous process that can explain the frequency gap, and what’s more, we have identified an independent observational test for the process. Our simulations revealed this new process because advances in computing and simulations allow us to use higher resolution than before – previous work had missed the important fine details of the interaction. At MIST, we reported not only on our simulation results, but also on the recently-published evidence from NASA Van Allen Probes and NASA Magnetosphere Multi-Scale that confirms our independent observational test.


Mike Lockwood gave a talk, “A homogenous aa index”.

Originally complied for 1868-1968 by Mayaud, and extended to the present day by ISGI (International Service for Geomagnetic Indices), the aa geomagnetic index has been a vital resource for studying space climate change over the past 150 years. However, there have been debates about the intercalibration of data from the different measuring stations. In addition, the effect of drift in geomagnetic latitude of the stations, caused by the secular change in the Earth’s field, has not been allowed for. As a result, the components of the aa index for the southern and northern hemispheres have drifted apart. We have corrected for these effects and also for the time-of-day and time-of-year sensitivity of the stations. The resulting indices for the northern and southern hemisphere now agree very closely and the aa index for all years shows a time-of-day and time-of-year “equinoctial” response pattern, as seen in the am index which has been compiled by ISGI from a much larger network of stations since 1959.


Chris Scott gave a talk, “The ionospheric response to intense bombing during World War II”.

There is an increasing number of case studies that demonstrate that the ionosphere can be perturbed from below. The explosion of the chemical plant at Flixborough in 1974 was sufficiently energetic that its effects were detected in the ionosphere (Jones and Spracklen, 1974), lightning has been shown to enhance ionospheric sporadic-E layer electron concentrations (Davis and Johnson, 2005) and there is much interest in the impact of earthquakes on the ionosphere (e.g. Astafyeva et al, 2013). The influence of the troposphere was also cited as the source of unknown variability in modelling work by Rishbeth and Muller-Wodarg (2006). Throughout the second-world war, routine measurements of the Earth’s ionosphere were made at Slough, UK. In this study we will use these data to investigate the impact on theionosphere of various bombing campaigns in order to determine the threshold above which such explosions can be detected in the upper atmosphere.

Accurate spectral measurements of solar radiation – why we need improved understanding in the near-infrared

Weather and climate processes are fundamentally driven by energy flows within the Earth-atmosphere system. Incoming solar radiation is absorbed and scattered by gases and aerosols within the atmosphere and absorbed and re-emitted by the Earth’s surface. We therefore need to know how much energy is absorbed by the atmosphere and the height at which this radiation is absorbed.

Currently, we know to reasonable accuracy and precision where most of this energy is accounted for (what we call the global energy budget).

Slide1
Fig. 1 (Stephens et al. 2012, Nature Geoscience), values in W m-2

Some of the values on the above figure (Figure 1) are highlighted in purple – this indicates that the relative uncertainty (i.e. the range in which this value might plausibly be) on these values is rather high. Reducing the uncertainty on these values is important: this will improve the accuracy of models we use to determine weather and climate. This is achieved by advances in modelling techniques, or in the case of my PhD improvements in available measurements of processes in the atmosphere.

My PhD work focuses on the components circled above, the short-wave atmospheric absorption (i.e. solar energy which is absorbed by the atmosphere as it travels from the Sun toward the surface), and on the incoming solar radiation. The latter of these has a small uncertainty, but this does not quite tell the whole story. The spectral distribution (i.e. at what wavelengths this radiation is emitted) of this energy is also extremely important, since the atmosphere is more transparent at some wavelengths than others.

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Fig. 2: Model run of water vapour absorption in the near-infrared for a mid-latitude Summer atmosphere. Everywhere (roughly) above the blue line is considered completely opaque, with varying degrees of transparency below this.

My work focuses on the “near-infrared” spectral region, between about 1-5 μm (or 2000-10000 cm-1) . This region, as can be seen in the above figure, has a “band-window” structure, where parts of the spectrum are completely opaque to radiation, but other parts are almost entirely transparent. Solar radiation with the same wavelength as these band regions where the absorption is strongest will therefore be deposited in the upper atmosphere, while radiation within the windows will be absorbed throughout the atmosphere and by the surface. This structure is almost entirely due to absorption by water vapour.

It is therefore extremely important to characterise the absorption in these windows as much as possible, since any additional absorption will affect where in the atmosphere solar energy is absorbed (unlike additional absorption in the band regions which will barely affect where in the atmosphere this absorption takes place).

Figure 2 also shows the water vapour continuum; a component of absorption which is not currently fully understood. This absorption is a phenomenon not fully accounted for by the theory of water vapour absorption; currently we model it using the MT_CKD model (named such after its creators). The strength of this absorption may be significantly stronger than this model however; laboratory measurements show differences of up to a factor of 100 in the strength at about 1.6 μm!

It is believed (e.g. Radel et al. (2013)) that an increased continuum could contribute about 3 W m-2 to the overall shortwave atmospheric absorption; a significant portion of the 10 W m-2 uncertainty in Figure 1.

My work attempts to resolve this using direct measurements of solar radiation in this 2000-10000 cm-1 region using a Fourier Transform spectrometer, made by the National Physical Laboratory at a site at Camborne, Cornwall, UK. There are a number of challenges making such measurements in the atmosphere; the instrument needs to be properly calibrated, and the conditions in the atmosphere (specifically temperature, pressure, humidity and aerosols) need to be well characterised. This is done using contemporaneous measurements using a radiosonde (to measure the atmospheric profile) and a sunphotometer (to measure aerosol optical depth). These radiosonde measurements are then put into a line-by-line radiation code to calculate the atmospheric optical depth, and these two contributions are subtracted from the total optical depth to get the continuum optical depth.

To derive the continuum it is necessary to know what the incoming solar radiation is. It turns out this is also a significant uncertainty in the literature in the 2000-10000 cm-1 region. While the total incoming solar irradiance is well-known, the distribution of that energy with the electromagnetic spectrum is not so well known. In the spectral region I’m looking at, that uncertainty is about ~10% between different sets of observations.

Since we have direct measurements of the Sun with absolute calibration, we can determine this from our own measurements, and found that the irradiance in this region may be significantly lower (16 W m-2 integrated over the whole spectral region) than expected, which must be made up by contributions elsewhere in the spectrum to account for the small uncertainty in the incoming solar radiation from Figure 1.

Slide3
Figure 3: Solar spectral irradiance in the 4000-10000 cm-1 region from CAVIAR2 (my work) compared with different sets of observations from other groups (ATLAS3 and Solar2). My work agreed significantly better with the Solar2 work in the 4000-7000 cm-1 region, with good agreement with both in the 7000-10000 cm-1 region. (From Elsey et al. [2017])
Following this, more work was put in to deriving the continuum. This is a more difficult task than simply measuring the incoming solar radiation, since we need to know the different components of the absorption in detail rather than filtering out the effect of the entire atmosphere. Figure 4 shows our best estimate of the continuum, showing a much stronger absorption than MT_CKD. There are large uncertainties however, due to the difficulty in attributing each component of the absorption. Thus, it cannot be ruled out entirely that MT_CKD is representative of the continuum, merely that is likely to be too weak.

Slide4
Fig. 4: Derived continuum absorption from my observations vs MT_CKD. Dark blue regions indicate k = 1 (67% confidence interval) uncertainties, cyan indicates k = 2 (95% confidence interval) uncertainties.

In the last part of my PhD, I hope to look at what effect these two results might have on the Earth’s energy budget, and look at how much of this 10 W m-2 uncertainty might be accounted for by them. This ties in well with a new project (ASPIC, Advanced Spectroscopy for improved characterisation of the near-Infrared water vapour Continuum) starting up at Reading and the Rutherford Appleton Laboratory in June, which hopes to look at new laboratory measurements of the continuum and assess the effect a strengthened continuum may have on radiation models.