Atmospheric blocking: why is it so hard to predict?

Atmospheric blocks are nearly stationary large-scale flow features that effectively block the prevailing westerly winds and redirect mobile cyclones. They are typically characterised by a synoptic-scale, quasi-stationary high pressure system in the midlatitudes that can remain over a region for several weeks. Blocking events can cause extreme weather: heat waves in summer and cold spells in winter, and the impacts associated with these events can escalate due to a block’s persistence. Because of this, it is important that we can forecast blocking accurately. However, atmospheric blocking has been shown to be the cause of some of the poorest forecasts in recent years. Looking at all occasions when the ECMWF model experienced a period of very low forecast skill, Rodwell et al. (2013) found that the average flow pattern for which these forecasts verified was an easily-distinguishable atmospheric blocking pattern (Figure 1). But why are blocks so hard to forecast?

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Figure 1:  Average verifying 500 hPa geopotential height (Z500) field for occasions when the ECMWF model experienced very low skill. From Rodwell et al. (2013).

There are several reasons why forecasting blocking is a challenge. Firstly, there is no universally accepted definition of what constitutes a block. Several different flow configurations that could be referred to as blocks are shown in Figure 2. The variety in flow patterns used to define blocking brings with it a variety of mechanisms that are dynamically important for blocks developing in a forecast (Woollings et al. 2018). Firstly, many phenomena must be well represented in a model for it to forecast all blocking events accurately. Secondly, there is no complete dynamical theory for block onset and maintenance- we do not know if a process key for blocking dynamics is missing from the equation set solved by numerical weather prediction models and is contributing to the forecast error. Finally, many of the known mechanisms associated with block onset and maintenance are also know sources of model uncertainty. For example, diabatic processes within extratropical cyclones have been shown to contribute substantially to blocking events (Pfahl et al. 2015), the parameterisation of which has been shown to affect medium-range forecasts of ridge building events (Martínez-Alvarado et al. 2015).

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Figure 2: Different flow patterns, shown using Z500 (contours), that have been defined as blocks. From Woollings et al. (2018).

We do, however, know some ways to improve the representation of blocking: increase the horizontal resolution of the model (Schiemann et al. 2017); improve the parameterisation of subgrid physical processes (Jung et al. 2010); remove underlying model biases (Scaife et al. 2010); and in my PhD we found that improvements to a model’s dynamical core (the part of the model used to solved the governing equations) can also improve the medium-range forecast of blocking. In Figure 3, the frequency of blocking that occurred during two northern hemisphere winters is shown for the ERA-Interim reanalysis and three operational weather forecast centres (the ECMWF, Met Office (UKMO) and the Korean Meteorological Administration (KMA)). Both KMA and UKMO use the Met Office Unified Model – however, before the winter of 2014/15 the UKMO updated the model to use a new dynamical core whilst KMA continued to use the original. This means that for the 2013/14 the UKMO and KMA forecasts are from the same model with the same dynamical core whilst for the 2014/15 winter the UKMO and KMA forecasts are from the same model but with different dynamical cores. The clear improvement in forecast from the UKMO in 2014/15 can hence be attributed to the new dynamical core. For a full analysis of this improvement see Martínez-Alvarado et al. (2018).

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Figure 3: The frequency of blocking during winter in the northern hemisphere in ERA-Interim (grey shading) and in seven-day forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF), the Met Office (UKMO) and the Korean Meteorological Administration (KMA). Box plots show the spread in the ensemble forecast from each centre.

In the remainder of my PhD I aim to investigate the link between errors in forecasts of blocking with the representation of upstream cyclones. I am particularly interested to see if the parameterisation of diabatic processes (a known source of model uncertainty) could be causing the downstream error in Rossby wave amplification and blocking.

Email: j.maddison@pgr.reading.ac.uk.

References:

Rodwell, M. J., and Coauthors, 2013: Characteristics of occasional poor medium-range weather  forecasts for Europe. Bulletin of the American Meteorological Society, 94 (9), 1393–1405.

Woollings, T., and Coauthors, 2018: Blocking and its response to climate change. Current Climate Change Reports, 4 (3), 287–300.

Pfahl, S., C. Schwierz, M. Croci-Maspoli, C. Grams, and H. Wernli, 2015: Importance of latent  heat release in ascending air streams for atmospheric blocking. Nature Geoscience, 8 (8), 610– 614.

Mart´ınez-Alvarado, O., E. Madonna, S. Gray, and H. Joos, 2015: A route to systematic error in forecasts of Rossby waves. Quart. J. Roy. Meteor. Soc., 142, 196–210.

Mart´ınez-Alvarado, O., and R. Plant, 2014: Parametrized diabatic processes in numerical simulations of an extratropical cyclone. Quart. J. Roy. Meteor. Soc., 140 (682), 1742–1755.

Scaife, A. A., T. Woollings, J. Knight, G. Martin, and T. Hinton, 2010: Atmospheric blocking and mean biases in climate models. Journal of Climate, 23 (23), 6143–6152.

Schiemann, R., and Coauthors, 2017: The resolution sensitivity of northern hemisphere blocking in four 25-km atmospheric global circulation models. Journal of Climate, 30 (1), 337–358.

Jung, T., and Coauthors, 2010: The ECMWF model climate: Recent progress through improved physical parametrizations. Quart. J. Roy. Meteor. Soc., 136 (650), 1145–1160.

NAWDEX Campaign – Experiencing the Jet Stream

Email:  j.maddison@pgr.reading.ac.uk

NAWDEX (North Atlantic Wave and Downstream impact Experiment) was an International field campaign led by Ludwig-Maximilians-Universität (LMU) Munich and the Deutsches Zentrum für Luft- und Raumfahrt (DLR) Oberpfaffenhofen in cooperation with the Eidgenössische Technische Hochschule (ETH) Zurich and the Office of Naval Research in the USA, with many other international collaborators. Multiple aircraft were deployed from Iceland (the HALO aircraft and the DLR and Safire Falcons) and the UK (the FAAM aircraft) to take meteorological measurments with the aim of providing knowledge of mid-latitude dynamics and predictability. There was involvement from across the UK, including the University of Reading, the University of Manchester, and the Met Office as well as from the FAAM.

The NAWDEX operations centre was based in Keflavik, Iceland (number 27 in Figure 1), which I visited for a week to join the campaign as one of the representatives from the University of Reading, UK. I was tasked with being the ground-based observation coordinator.

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Figure 1: Radiosonde launch locations for the campaign.

A Europe-wide network of radiosonde launch locations (Figure 1) had been readied for additional launches during the NAWDEX period. Our role was to choose sites to launch sondes from that would complement measurements taken by the aircraft and/or support one of the NAWDEX objectives. Of particular interest was downstream high impact weather events over Europe. It was great to be given real responsibility and be able to actually contribute to the NAWDEX project.

Below is a typical daily schedule I would have in Iceland:

Daily schedule:

UK call: 8:30am Icelandic. Conference call between UK parties discussing plans for the coming days and any updates from Iceland or the UK.

General meeting: 12pm Icelandic. Go over brief weather summary, instrument status reports, flight plans for the coming days and reports of previous flights.

Weather meeting: 4pm Icelandic. Detailed look at the weather situation for the short and medium-ranges, highlighting key features that would be of interest to fly into, e.g. extratropical transitions of tropical cyclones (which we were fortunate to observe more than once). Radiosonde launch updates.

In between: assessing forecasts and flight plans for the coming days and meeting with scientists for their input to decide where we want to launch radiosondes from. Along with preparing slides to present to the group proposed launch locations and emailing various meteorological services to request the launches (the most time consuming).

My time in Iceland was a great learning experience. Working with some of the pre-eminent scientists in the fields of dynamics and predictability (and spending most of the day discussing the weather!) really helped improve my understanding of the development of mid-latitude weather systems and better understand their predictability.

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Figure 2: On-board the FAAM aircraft.

After returning from Iceland I got the opportunity to fly on the FAAM aircraft (Figure 2) whilst it was on a mission for another project. The flight aim was to perform a radiometer inter-comparison by taking coordinated measurements of deep-frontal cloud to the north of Scotland with the HALO and Safire aircraft. The flight was remarkably turbulent free (I‘d been hoping for more of a roller coaster ride), although we did perform a profile right through the cloud to an altitude of less than 50 ft, which was pretty fun! Whilst on the aircraft we were also able to plot measurements being taken in real time on an on-board computer.

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Figure 3: Flying at an altitude of 35 ft.

NAWDEX was a great opportunity to get first-hand experience of a major international field campaign (and see some of Iceland).

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