Double, double, toil and trouble

By Geoff Wadge

Iceland has been forcing itself on our consciousness in recent years, culminating recently in the humiliation of the English football team. There may be another unpleasant surprise up Iceland’s sleeve. Since 2010 there have been three volcanic eruptions in Iceland that have caused concern in two cases and consternation and alarm in the other. The 2010 southward dispersal of ash from the explosive phase of the Eyjafjallajokull eruption caused the panicked shutdown of half of Europe’s airports for six days. This eruption was unusual in that it lasted quite a long time – 39 days, and produced ash, some of which was transported south to Europe.

At the end of July 2016 the Icelandic Met Office issued a statement noting increased earthquake activity at Katla volcano, the neighbouring, larger, “double” of Eyjafjallajokull. This is not that unusual, but is nevertheless a concern. Katla (meaning kettle or cauldron in Icelandic) has been one of Iceland’s most active volcanoes (20 eruptions in the last 1100 years), capable of eruptions that dwarf those of its neighbour.  The last eruption was in 1918 and produced five times more ash than Eyjafjallajokull in 2010. The next eruption of Katla will almost certainly start with a swarm of earthquakes merging to a tremor signal. Such an eruption today could cause widespread disruption to aircraft, but what is the likelihood?

There is a chain of contingent factors at play. Firstly, what is the likelihood of a Katla eruption in the near future? Ten years ago this was modelled probabilistically at 20% in the next 10 years. Secondly, what magnitude and type of eruption will it be? The larger the magma mass flux, the higher the eruption column and the further the likely transport distance. Katla (like Eyjafjallajokull) has both low viscosity, basaltic magma (common, 93%) and high viscosity, silicic (dacitic) magma (rarer, 7%). The 1918 eruption was a basaltic eruption. Also the way in which these liquids are converted into fine fragments (<50 microns) in the eruption column and are advected 1000 km or more as “ash clouds” depends on the dynamics of the magma’s interaction with several hundred metres of overlying icecap, and is difficult to predict. The third main factor is the weather.  As you can see from the deposition axes of past eruptions more northerly directions are favoured by the silicic ash plumes (Figure 1a), but there have been plenty of basaltic plumes heading south to Europe from Katla (Figure 1b). The pressure chart on the first days of the 1918 eruption is shown in Figure 2. These deposition patterns are created by the proximal fallout of larger fragments from the plume, the far-travelled fraction often having more complex trajectories. The longevity of explosive lofting of ash is important here, as the conditions for southerly transport will eventually occur.

2016 09 08 - Geoff WadgeFig.1

Figure 1 (a) – Axes of the deposition of basaltic ash (tephra) from eruptions of Katla over the past 1100 years. Arrows proportional to the volume of ash. Figure 1 (b) – As in (1a) but for silicic ash deposits from Katla between 10,200 BCE and 400 CE. Figures from Futurevolc’s Catalogue of Icelandic Volcanoes (futurevolc.vedur.is).

2016 09 08 - Geoff WadgeFig.2

Fig.2  Sea-level pressure chart for the North Atlantic on the second day of the 1918 eruption of Katla, modified from www.netweather.tv/index.cgi?action=gfs;sess=b50fb0a8f44739b36e854be54e8dbf5d

Since 2010 our ability to respond to a potential Katla eruption in a less panicked manner has improved.  The UK Civil Aviation Authority relaxed the “any ash-no fly” tolerance of planes to volcanic ash, by defining low (<2 mg m-3 of ash), medium (2 -4 mg m-3) and high (>4 mg m-3) ash zones to manage flight safety. Also the Icelandic scientific community is well primed to respond (futurevolc.vedur.is).

 

 

 

 

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Earth System Modelling in the UK

By Till Kuhlbrodt

UKESMMaking climate projections for the next couple of decades is a pressing and complex task for the global climate modelling community. One of the most important purposes of modelling the future climate is to provide society and Government with projections of climate change – for instance, how will temperature, precipitation and sea level change in the UK?

The ongoing global climate change is largely driven by gases and particles that human societies emit when burning fossil fuels – carbon dioxide (CO2) stands out in this role. The physical effect of CO2 has long been understood and represented in models – this is the greenhouse effect that warms up large parts of the lower atmosphere, where we live. But carbon in the Earth System (the atmosphere, the ocean, the ice sheets and the biosphere) undergoes a complex set of chemical reactions and biological processing too. Understanding and representing these is equally important since they determine how much of the additional (emitted) carbon the Earth System processes, and how much stays in the atmosphere leading to further warming. And there are many aspects to global climate change that are not carbon-related, but equally important, that require a comprehensive view of the Earth System. What is the fate of the tropical rainforests under changing temperature and precipitation patterns? Could the Amazon rainforest die back, as some simulations suggest? How will the air quality change globally and regionally? Can we bring down the (currently large) uncertainty margins around projections of regional sea level rise?

EarthSystemIn a close-knit partnership between National Environmental Research Council (NERC) institutions, universities and the Met Office, the UK scientific community is currently building a next-generation Earth System Model: UKESM. The Department of Meteorology, in close cooperation with the National Centre for Atmospheric Science (NCAS), is contributing expertise in three fields:

  • Coupled ocean-atmosphere modelling: developing and running sophisticated computer models (more than a million lines of code) of the atmospheric and oceanic flow and dynamics.
  • Dynamic ice sheet modelling: The melting rate of the ice sheets on Greenland and Antarctica (both several km thick) currently poses the largest uncertainty for projections of sea level rise.
  • Computational Support: Running these very large computer models, along with processing and analysing their output (millions of megabytes), crucially requires support from computational scientists.

Work on UKESM started three years ago. Currently, we’re conducting and analysing test runs of the coupled Earth System Model (atmosphere, ocean, atmospheric chemistry and oceanic chemistry) and we’re planning to start production runs (for full scientific exploitation) in a couple of months. Ice sheet modelling is a much younger field and needs a little more development, but it will be integrated into UKESM next year. For more information – just have a look at our website.

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Atmospheric effects of solar eclipses

2016 08 30 - Phil Trans TA2077 banner

Solar eclipses rarely cross populated regions, but provide great opportunities both for science and science outreach when they do. The recent 20 March 2015 solar eclipse tracked across the Atlantic, giving substantial solar radiation changes in the UK and Iceland, and totality in the Faeroes. This Theme Issue of the Philosophical Transactions of the Royal Society brings together a unique series of studies on effects of eclipses on the weather, placed in the context of societal responses to eclipses. Professor Giles Harrison from this department was joint lead editor on the issue, and the journal includes contributions from many members and ex-members of the Department of Meteorology at the University of Reading on a very wide variety of topics.

Investigating effects of eclipse-induced weather changes (e.g. in surface air temperatures, wind and cloud amount) has a long history, usually exploiting observations made during the eclipse for comparison with comparable non-eclipse conditions the day before or after. New approaches to study the weather-related changes are now possible, employing high resolution numerical models of the atmosphere in which an eclipse can be turned on or off at will, combined with the extensive coverage of good quality amateur and professional weather data available. This issue includes work analysing surface, balloon and satellite observations, alongside high resolution numerical modelling studies. In doing so it defines a new interdisciplinary research area in eclipse weather, closely focussed in scope, but diverse in the work it contains.

The contents of this special issue, available online and shortly in print, is given below:

Introduction: The solar eclipse: a natural meteorological experiment: RG Harrison, E Hanna
Symbolism and discovery: eclipses in art: I Blatchford
Atmospheric changes from solar eclipses: KL Aplin, CJ Scott, SL Gray
Meteorological effects of the solar eclipse of 20 March 2015: E Hanna, J Penman, T Jónsson, GR Bigg, H Björnsson, S Sjúrðarson, MA Hansen, J Cappelen, RG Bryant
On the variability of near-surface screen temperature anomalies in the 20 March 2015 solar eclipse: MR Clark
Satellite observations of surface temperature during the March 2015 total solar eclipse:
E Good
Meteorological responses in the atmospheric boundary layer over southern England to the deep partial eclipse of 20 March 2015: S Burt
Effects of the March 2015 solar eclipse on near-surface atmospheric electricity: AJ Bennett
Terrestrial atmospheric responses on Svalbard to the 20 March 2015 Arctic total solar eclipse under extreme conditions: JM Pasachoff, MA Peñaloza-Murillo, AL Carter, MT Roman
Coordinated weather balloon solar radiation measurements during a solar eclipse: RG Harrison, GJ Marlton, PD Williams, KA Nicoll
On the detection and attribution of gravity waves generated by the 20 March 2015 solar eclipse: GJ Marlton, PD Williams, KA Nicoll
Using the ionospheric response to the solar eclipse on 20 March 2015 to detect spatial structure in the solar corona: CJ Scott, J Bradford, SA Bell, J Wilkinson, L Barnard, D Smith, S Tudor
Eclipse-induced wind changes over the British Isles on 20 March 2015: SL Gray, RG Harrison
Numerical simulations of the impact of the 20 March 2015 eclipse on UK weather: PA Clark
The National Eclipse Weather Experiment: use and evaluation of a citizen science tool for schools outreach: AM Portas, L Barnard, C Scott, RG Harrison
The National Eclipse Weather Experiment: an assessment of citizen scientist weather observations: L Barnard, AM Portas, SL Gray, RG Harrison

Phil. Trans. R. Soc. A 2016 374: access content online, or purchase the print issue at the reduced price of £35 (usual price £59.50) by visiting the above web page and entering the promotional code TA 2077 when prompted, or contact:
Turpin Distribution T +44 1767 604951 E royalsociety@turpin-distribution.com
For more information, contact:
The Royal Society
6 – 9 Carlton House Terrace, London, SW1Y 5AG
T +44 20 7451 2500

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UK drought monitoring and forecasting

By Laura Baker

After what feels like a pretty wet start to the year, it may seem strange to be talking about drought (although admittedly the warm weather over the last couple of weeks should help!). But in spring 2012, just four years ago, much of the south of England was experiencing severe drought conditions, following two years in which almost all months had lower than average rainfall (Figure 1). In this case it then proceeded to rain considerably more than average for much of the summer, preventing the drought from developing further; however had this not happened, parts of the country could have experienced severe water shortages. There are many other notable drought events that have occurred in the UK in recent decades, the most extreme being the 1976 drought where water restrictions became so severe in some areas that standpipes were introduced in the streets.

2016 07 28 Laura Baker precip_Apr10_Mar12

Figure 1: Percentage of average rainfall for April 2010 to March 2012. Source: http://www.metoffice.gov.uk/climate/uk/interesting/2012-drought

Drought in the UK can have severe social, economic and environmental impacts.  Although little can be done to prevent a drought, monitoring and early warning systems can reduce the vulnerability of society to these events. At the recent RMetS/NCAS conference on high-impact weather and climate, we held a workshop on the topic of UK drought monitoring and forecasting science. This was in association with two ongoing research projects: IMPETUS (improving predictions of drought for user decision-making; part of the NERC UK Droughts and Water Scarcity programme) and DrIVER (drought impacts: vulnerability thresholds in monitoring and early-warning research). Speakers from these two projects gave an overview of recent scientific advances in this field, and a presenter from the Environment Agency gave a decision-maker’s viewpoint. These presentations were followed by a lively group discussion exercise in which the following three questions were discussed (Figure 2):

  1. What are the biggest challenges in drought monitoring and forecasting at present?
  2. How should we tackle these in order to improve drought monitoring/forecasting?
  3. How can users of drought monitoring/forecast systems get more from them?

2016 07 28 Laura Baker flipboard_pics

Figure 2: Flipcharts summarizing group discussion at the workshop

Workshop attendees were from a range of backgrounds, including meteorologists, hydrologists, social scientists and industry practitioners, so the discussion was very varied.

Challenges in drought monitoring that were highlighted include issues with sparse data in some areas, meaning that accurate drought predictions and modelling of drought in these regions are difficult. Drought monitoring could be improved by using different instruments and methods to monitor rainfall and drought development, including potentially the use of drones. Better use could be made of existing data, including satellite data for monitoring soil moisture, and data that are collected on the socioeconomic impacts of drought could be more widely used.

Forecasting drought on timescales of months to seasons ahead is a challenging task. One key factor is the need to be able to forecast skilfully the atmospheric drivers of drought on these timescales. Recent advances in forecasting systems mean that there is promising skill in forecasting atmospheric circulation patterns that influence UK drought, such as the wintertime North Atlantic Oscillation. However, forecasting smaller scale features such as summer precipitation remains considerably more challenging.

In terms of users, the key issues raised were to do with communication, including the importance of consistency between sources of information, communicating the uncertainty in drought forecasts and being aware of how statements may be interpreted, or potentially misinterpreted.

The outcomes from this workshop will feed into ongoing work research UK drought, and hopefully when faced with the next 2012-type drought event we will be better placed to deal with it.

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Home is where the chart is …

By Andrew Gabey

Increasingly detailed weather forecasts will need data from far more weather stations than at present. Filling this gap is especially important for urban areas, where the built environment and the heat emitted by human activities have a complex interaction with the local climate.

Recently there’s been talk of using internet-connected home weather stations (HWS) signed up to the Met Office WOW network to help with forecasting. Greater London alone contains about 1,500 HWS compared with around 10 Met Office and Airport weather stations. So what could HWS data tell us and what are the challenges for researchers in using it?

As an example, here’s a map of air temperature recorded by HWS throughout Greater London on 19 July 2016 (smoothed data from NetAtmo and Weather Underground). This was the hottest day of the year so far with temperatures exceeding 33°C. Blotches on the map reflect HWS locations and a simple calculation fills the space in between. Outlandish readings are filtered out beforehand.

2017 07 22 Andy Gabey - Annotated

Having so many stations immediately tells us about the structure of heat through the city, which wouldn’t be obvious from using a handful of stations:

  • An urban heat island is present (and shrinking) before sunrise. This vanishes once heating gets underway and the wind picks up slightly.
    2017 07 22 Andy Gabey - heat_island
  • A slight easterly wind in the evening cools this half of the city, and the heat island is blown off to the south-west.

2017 07 22 Andy Gabey - cool_east

The big picture and temperatures here are what we’d expect given the city layout and weather conditions, but we need more information before using these numbers confidently in calculations:

  1. The environment that each station represents: Unlike the networks used here, WOW registration includes a questionnaire for contributors to describe their station’s surroundings.
  2. The accuracy and bias of each measurement: Do the blotches on the map tell us something scientific, or should they be weeded out?
  3. How to distinguish a measurement flaw from something surprising but real. Clever methods exist to weed out nonsensical measurements.

Observing stations currently used in forecasting and research are well-understood and maintained regularly, and there’s clearly new information for forecasting and urban meteorology in HWS data. The challenge for researchers is to agree how it complements existing stations, and how to rate the quality of the measurements.

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Spotlight on aviation CO2 emissions

By Emma Irvine

Climate change, resulting from emissions of CO2 amongst other factors, is a major topic of research here at Reading. This blog focusses on the emissions from one particular sector, aviation, and progress on tackling them.

I write this as the 2016 Farnborough Airshow is taking place, with major aircraft manufacturers such as Boeing and Airbus showcasing their latest technological innovations, and eco-efficiency is the buzzword. Just this week at Farnborough, General Electric tweeted that their technological developments to the engines on Boeing-737s make them 15% more fuel efficient. Not to be outdone by their US rivals, Airbus have been showing-off their A350-XWB which claims to be 25% more fuel efficient than its (unnamed) nearest competitor (although probably not when it’s doing this near vertical take-off).

Back in 2009, when 2020 seemed a long way off and 2050 the distant future, the International Air Transport Association (IATA) set itself a set of environmental targets which included: to achieve carbon neutral growth by 2020, and to reduce CO2 emissions by 50% by 2050 (relative to 2005 levels). Other international aviation organisations have similar pledges. So how is the industry doing? The European Environment Agency’s latest annual greenhouse gas report is not particularly encouraging. In 2014, emissions from international aviation rose by 1.4% while those from domestic aviation fell by 0.8%.

So while the technological developments from the manufacturers are encouraging, they aren’t enough by themselves. Amongst other initiatives to reduce CO2 emissions are increasing the use of biofuels (disappointingly, British Airways project to turn London’s rubbish into biofuel for their planes was recently scrapped) and improvements to air traffic management. In Europe, the big air traffic initiative takes the form of the Single European Sky Air Traffic Management Research (SESAR), which is aiming at a 2.8% reduction per flight in environmental impact as well as a 40% reduction in accident risk and 27% increase in capacity. Here at Reading University we are part of one of the new SESAR projects investigating the potential of reducing the overall environmental impact of European flights through optimising the routing of aircraft over Europe.  Having proven the feasibility of climate-optimised routing over the relatively unconstrained airspace of the north Atlantic, we are applying this novel concept to some of the busiest airspace in the world. With over 28,000 flights a day occurring in or passing through European airspace, optimising the routes to minimise their environmental impact will be quite a challenge.

This brings me to the ultimate in climate-optimal flight: Solar Impulse. This innovative aircraft produces zero CO2 emissions (or emissions of any kind) as it flies, being powered purely by solar energy it receives through the 17,000 solar cells in its wings. It’s currently about to embark on the final leg of its around the world tour, from Cairo to Abu Dhabi (you can follow its progress here). Setting records along the way, it made the first trans-Atlantic crossing without using fuel, flying from New York to Seville in 70 hours (at the same time achieving the more dubious accolade of ‘selfie of the year’). Although solar power is unlikely to prove the answer to aviation’s CO2 problems – at least with current technology – solar impulse is an inspiring demonstration project harnessing the power of ‘green’ energy. #futureisclean

 

 

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An artefactually introduced monthly cycle in the ensemble fields constrained by HadISST2

By Xiangbo Feng

ECMWF has recently developed two major ensemble products for 20th century climate, i.e. ERA-20CM and CERA-20C, within ERA-CLIM and ERA-CLIM2 projects. ERA-20CM is in ECMWF’s public datasets now, and CERA-20C has been scheduled to disseminate in the near future. There is no doubt that these two exciting climate datasets will become hugely popular in the research fields and also in other relevant communities. It is worth being aware of some uncertainty in their data before implementing them.

The former is a 10 member ensemble of atmosphere model integrations, using observationally based reanalysis HadISST2 to describe SST and sea ice and using CMIP5 radiative forcing to force model. There is no data assimilation applied. CERA-20C is a 10 member ensemble of coupled ocean-atmosphere reanalysis, also using HadISST2 to constrain SST of model via heat relaxation scheme, with data assimilation applied in ocean and atmosphere individually. They both provide daily atmospheric and SST fields (and ocean in CERA-20C) at 3 hours resolution. For more details on these two products, read Hersbach etal. (2015) and Laloyaux etal. (2016) respectively. One key point within these two products is that SST is strongly constrained by HadISST2, a 10 member ensemble of realizations produced by UK Met Office within ERA-CLIM project. Thanks to the involvement in ERA-CLIM2 project, I have limited internal access to CERA-20C data.

Interestingly, we recently found an unexpected monthly cycle in ensemble spread of daily SST field in both ERA-20CM and CERA-20C. The shape of the cycle is similar in both products, but phase is slightly lagged in CERA-20C (suspected to be due to the relaxation scheme applied in the data assimilation schemes). For demonstration, the monthly cycle in January (2005-2007) seen in CERA-20C is shown in Figure 1. It can be characterised by following features:

  • A month cycle consistently exists in SST ensemble spread at all latitudes and all months in all years from 1900 to 2010! In January 2005-2007, global average of the amplitude is 0.015 deg.C (Figure 1 top left), which is about 15% of spread mean.
  • The amplitude is larger in summer hemisphere and smaller winter hemisphere. This follows the pattern of SST spread mean (not shown).
  • In regions with strong western boundary currents, such as the Gulf Stream and Kuroshio, where SST uncertainty is usually the largest, the monthly cycle is not more significant than other regions. This indicates this signal is more likely produced at large scales.
  • This cycle generally has the lowest and highest values around 5th and 20th in each month, but with noticeable seasonal variations (5-10 days) at mid-high latitudes (Figure 1 top right).
  • This signal also exists in forecasting fields (Figure 1 bottom). The amplitude gradually becomes smaller when forecasting time steps are longer. This means that this signal is presumably propagating into atmosphere through ocean-atmosphere coupling.

2016 07 08 Xiangbo Feng - Picture1

Figure 1. Amplitude (top left) and phase lag (top right) of monthly cycle statistically fitted in time series of ensemble spread (standard deviation) of daily SST analysis, in January 2005-2007, and time series of global average (60°N-60°S) of ensemble spread of daily SST at different forecast leadtimes (0-24 h) on each day of January 2005-2007 (bottom). Note that the monthly cycle shown in maps is significant at 95% confidence level. Data are obtained from CERA-20C.

It turns out this is being imposed from the SST reconstruction method as an artefact of the HadISST2 data processing, which is briefly reported in section 3.1.1 of Hersbach et al. (2015). HadISST2 is constructed as a 10 member ensemble of realizations with a monthly window, based on methods separately considering large-scale variability and small-scale perturbations. Daily fields were then obtained by temporal interpolation of monthly analysis fields from adjacent months with weights such that the average of all daily fields in one analysis window equals the monthly analysis again. However, because of locally strong small-scale perturbations dominating the area-averaged ensemble spread at monthly window, this interpolation method leads to interference between the small-scale perturbations. As a result, the ensemble spread appears smaller at start dates of each month and larger at middle of month. In other words, a monthly cycle, which is supposed not to exist, is artefactually introduced by the interpolation process.

So, the next question is to what extent could this monthly spread variability modulate an atmosphere response through air-sea interactions?  It is expected that any changes in SST uncertainty will be reflected in the lower atmospheric fields, depending of course on the time scales and regions considered. This is especially expected in the case of CERA-20C which uses fully coupled ocean-atmosphere models.

The answer is that, at large scales, unfortunately we have not found a clear indication of this artefactual signal in the air so far. It is not surprising as at daily time scales the atmosphere usually has higher-frequency variations and much larger ensemble uncertainty than SST does, and this increases the difficulty of statistically distinguishing a cycle that has a relatively small amplitude. For example, in CERA-20C the global mean of ensemble spread of daily 2 m temperature (T2m) in January 2005-2007 is about 0.3 degC, which is 3 times that of SST. This is true even for the case where the atmosphere is forcing SST, like the Western Tropical Pacific with strong deep convection.

However, in the dry and calm regions of ocean, where the sensible heat flux is thought to be more responsible for atmospheric heating, a monthly cycle in T2m was found, although the indication is much less significant than in SST. Figure 2 shows the regional average of ensemble spread of SST at different forecast leadtimes (0-24 h) in the south-east Pacific, with corresponding T2m ensemble spread (note that the ensemble spread in T2m is increasing with forecast leadtimes). Despite the strong high-frequency variations, T2m does show a hint of a monthly cycle which well matches the phase and amplitude observed in SST. In all, we believe that this monthly variability does have an impact on the atmosphere, but it might need better ways to extract it from a background with a lot noise.

2016 07 08 Xiangbo Feng - Picture2

Figure 2. Time series of regional average (the south-east Pacific, 10°S-40°S and 120°W-80°W) of ensemble spread of SST (top), and T2m (bottom) at different forecast leadtimes (0-24 h) on each day of January 2005-2007. Note that the time series of T2m spread is detrended. Data are obtained from CERA-20C.

Suggestions:

  • Be aware of this monthly cycle that is artefactually introduced into the ensembles of daily SST field in both ERA-20CM and CERA-20C, and of its potential impact on the atmosphere. If the daily fields of these two datasets are used in your work, please keep the possibility in your mind that the ensemble uncertainty is systemically varying with dates. This adds more uncertainty than originally designed.
  • This problem only exists in data with daily scale, and is not expected to influence the assessments at long term.
  • This problem can only be solved by improving the daily data processing scheme in HadISST2.

REFERENCES:

Hersbach, H., Peubey, C., Simmons, A., Berrisford, P., Poli, P. and Dee, D., 2015. ERA-20CM: a twentieth-century atmospheric model ensemble. Q.J.R. Meteorol. Soc., 141: 2350–2375. doi: 10.1002/qj.2528

Laloyaux, P., Balmaseda, M., Dee, D., Mogensen, K. and Janssen, P., 2016. A coupled data assimilation system for climate reanalysis. Q.J.R. Meteorol. Soc., 142: 65–78. doi: 10.1002/qj.2629

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A month’s worth of rain …

By Ben Harvey

Phrases like a month’s worth of rain fell in just one day are often seen in media reports of extreme precipitation. But what does this statistic actually mean? How rare is it to see a month’s worth of rain fall in a day? Are certain locations or seasons more susceptible than others to such events? This blog post takes a brief look at some UK raingauge observations to find out.

To achieve the status of a month’s worth of rain in a day, a daily accumulation (by convention, the 09-09 UTC total) should exceed the corresponding climatological mean monthly precipitation value. The blue bars in Figure 1 show the climatology of mean monthly precipitation at the Reading University Atmospheric Observatory for the period 1981-2010. During the last one hundred years (1916-2015), the monthly climatology has been exceeded by a daily accumulation on only ten occasions (as indicated by the solid line). The most recent events were 9 August 1999 and 18 August 2011. Interestingly, all ten events occurred during July-September, so were presumably associated with intense convective storms rather than large-scale frontal systems.

2016 06 29 Ben Harvey dept_blog_jul2016_RdgByMonthFigure 1. The climatology of mean monthly precipitation in Reading (blue bars) for 1981-2010 and the number of occurrences of each of the three thresholds discussed in the text (lines) during the 100 year period 1916-2015.

The other two lines show similar but less severe thresholds also seen in media headlines: the number of daily accumulations exceeding just half the monthly climatology (dashed) and the number of two-day accumulations exceeding the full monthly climatology (dot-dashed). As for the solid line, both are largest in summer. The number of occurrences in the 100 year period are 134 and 38 respectively: whilst a month’s worth of rain falls in a day typically only once a decade in Reading, half a month’s worth of rain falls in a day typically more than once a year.

Do these numbers vary much across the UK? The total occurrences for each threshold from stations across the UK are shown in Figure 2 (each threshold is based on the local climatology). These data are from the MIDAS database and only cover the 30 year period 1981-2010. The 47 stations are those climate network stations which consistently reported daily rainfall amounts during the period. The number of occurrences of a month’s worth of rain falling in a day vary from 0 to 3 (except for one outlier station which recorded 8 such events) – Reading had 2 events in that time, and the number of occurrences of half a month’s worth of rain falling in a day vary from 0 to 53 – Reading had 27. Crucially then, how rare a given month’s worth of rain event is depends strongly on location.

2016 06 29 Ben Harvey dept_blog_jul2016_map

Figure 2. The total number of occurrences of each of the three thresholds discussed in the text during the 30 year period 1981-2010. Data from 47 climate data stations are shown. The numbers in the top right corners are the number of days where the threshold is met at at least one station.

What factors influence the spatial variations? Scotland provides an interesting case study: there is a striking east-west difference in the occurrence of all three thresholds. A closer look at the data reveals that this difference is due predominantly to the monthly climatologies being much smaller in the east than the west, rather than any given daily event being larger there.

Finally, can we tell from this data how often a month’s worth of rain falls in a day somewhere in the UK? In other words, how often can we expect to see headlines like the first sentence of this post, even if only for a small area? Figure 2 also shows the number of days on which each threshold was exceeded at at least one station. On average, a months worth of rain in a day was received at at least one station 1.3 times a year, and there are 16 days a year where at least one station received half a month’s rain in a day. However, care is needed with these numbers: since many of the events are localised to small areas it is likely that many events have been missed here. Using a higher density of observations would increase these numbers substantially.

Posted in Climate, Environmental hazards, Hydrology, Measurements and instrumentation, Weather | Tagged | Leave a comment

Why does it always rain on me?

By Helen Dacre

Last Monday morning I got so wet on my cycle to work that I had to spend 10 minutes under the hand dryer in the toilets to stop myself looking like a drowned rat. Being the keen meteorologist that I am, however, my next steps took me to the coffee room to look at the synoptic charts to find out exactly why I’d got so wet. A fairly cursory glance at the chart for 00 UTC on Monday 20 June (Figure 1) showed me an occluding low-pressure system sitting to the north-west of the UK with a long trailing front extending over the entire length of the country (so I doubt I was the only person standing under the hand dryer that morning).

2016 06 23 Helen Dacre blog Fig 2

You don’t need to have studied meteorology to know that fronts mean clouds: and clouds, more often than not, mean rain – particularly those associated with an active low-pressure system like that passing through on Monday morning. For most of the morning we sat under low cloud in the warm sector (that’s the bit between the warm front and the cold front) and I was glad for once to be stuck at my desk with no need to trek through the wilderness to a meeting on the other side of campus.

Having experienced the passage of a low-pressure system over the UK many times over the last 30+ years (and taught Introduction to Weather Systems often enough), I knew things were about to change and sure enough around 12 o’clock the cloud began to lift, the rain stopped, the sunshine broke through and by the time I left work to cycle home (wearing my soggy shoes from the morning) there were glorious blue skies overhead with no trace of a cloud to be seen.

A quick look at the Reading atmospheric observatory measurements in the foyer as I left the Department confirmed the passage of the cold front at 12:00 (Figure 2), marked by an increase in pressure (known as a pressure kick), a wind shift from southerly to westerly (known as a wind veer), lifting of the cloud base (measured by our ground-based lidar) but no expected decrease in temperature. Why not? Probably due to the decrease in cloud cover allowing the solar radiation to reach the surface and warm the air above.  Other than that, a pretty classic frontal passage.

2016 06 23 Helen Dacre blog Fig 1

This all got me thinking on my cycle home about the demise of the synoptic chart.  In an age where text based postcode forecasts are growing in popularity, my phone can tell me, hour by hour, the chance of rain in my backyard. But it doesn’t tell me at a glance why it’s raining or why it’s going to stop, or if it’s raining in Reading is it also raining in Liverpool? It’s like the Indian proverb of trying to describe an elephant blindfold whilst only touching its leg, trunk or tail.  It’s very difficult to explain the weather in my backyard without knowing what’s going on elsewhere.

So, whilst I continue to use my phone to find out whether to pack my waterproofs, please lets keep the tried and tested synoptic chart so we can understand at a glance why the weather is doing what it’s doing.  Forget the cloud appreciation society (sorry Gavin Pretor-Pinney), how about a synoptic chart appreciation society – because a picture really does tell 1000 words (well 571 words according to my word count).

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Understanding Summer Flash Flooding

By Adrian Champion

‘Flash flooding’ is flooding that only lasts between a few hours and a day and typically has very little warning. There are many causes of flash flooding, from the meteorological conditions that lead to the rainfall that cause the flooding, to the ground situation that results in flooding. Flash flooding is generally very localised, but can be very costly and result in significant disruptions.

Flash flooding is due to intense rainfall that only lasts a short period – from less than an hour to a few hours. The amount of rain recorded over the course of the day may be low in comparison to rainy winter days, however the difference is that this amount of rain falls in perhaps only a few hours (Figure 1). The difficulty in forecasting such rain events is that the meteorological conditions that lead to intense rainfall are very small in scale. The predominant cause of hourly extreme rain is a convective storm, or a feature with convective elements. These are only around a few kilometres in size, smaller than the forecast resolution of any national weather centre’s weather forecast model. There may also be other processes, or other factors from the prevailing wind conditions to the orography, that will act to enhance the convective system. It is due to the small size of the meteorological processes that cause intense rainfall that make it so difficult to forecast.

2016 06 16 Adrian Champion Department Blog Hourly Record Totals Met Office

Figure 1. Short-period depth-duration extremes of rainfall in the United Kingdom. Source: Met Office climate extremes

Once the rain reaches the ground there are also significant difficulties in predicting what will happen to all of the water. Outside of these hourly extreme rain episodes, we’re able to model how much of the water will be absorbed by the ground via infiltration and how much will run off the ground into rivers and drains (Figure 2). We’re also able to model the resulting changes in river flow from this over-land run off and water release from the ground. The natural (and man-made) systems are also able to respond to ‘normal’ rainfall intensities. During extreme rainfall it is a lot harder to model what will happen to the water. The ground is not able to absorb the water as quickly as it is falling, and other factors such as how wet the ground already is play a significant role. Therefore, it can be expected that the majority of the water will flow over the surface.

2016 06 16 Adrian Champion Example Hydrology Model SHETRAN Newcastle University

Figure 2. An example of a hydrology model as used by Newcastle University, their SHETRAN model

This surface run-off is difficult to model and is highly dependent on the type of land use – in towns and cities tarmac and concrete surfaces will result in fast run-off speeds resulting in a rapid accumulation of run-off water in low-lying areas, e.g. under a railway bridge when the road dips (Figure 3). It may take only tens of minutes for the water to collect and exceed the drainage capacity. In rural areas there will be natural barriers, e.g. trees and hedgerows, however intense rainfall can still result in rapid increases in local river levels causing localised river flooding typically of natural floodplains, as the river is unable to get rid of the excess water quick enough.

2016 06 16 Adrian Champion Department Blog Bridge Flooding London Fire Brigade

Figure 3. A recent example of flooding underneath a railway bridge in an urban area that would have accumulated quickly and took drivers by surprise – south London, 7 June 2016. Source: BBC News website, photograph credited to the London Fire Brigade.

Due to the rainfall events lasting only a few hours, the flooding also only lasts a few hours as drainage systems, either natural (rivers) or man-made (drains), recover and move water further downstream. However, the speed at which the flooding occurs can often have large consequences due to the lack of warning or the speed and volume of water. We usually only see such flooding in winter as the convective processes that dominate the hourly extreme rain dominate in summer due to the stronger incoming solar radiation (it’s summer, it’s warmer). Such convective processes cause “summery showers” that last only a few hours, or sometimes minutes.

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