Dynamical and microphysical evolution of convective storms (DYMECS)

Lead Research Organisation: University of Reading
Department Name: Meteorology

Abstract

A series of wet summers in the UK have reminded us of the devastating floods that can be caused by thunderstorms, from the iconic images of the Boscastle flood in August 2004 through to the repeated flooding events in 2007 that led to towns in Gloucestershire without mains water for up to 17 days and according to the subsequent Pitt Review, caused in excess of 3 billion pounds worth of damage. There is clearly an urgent need to improve our ability to forecast these storms from a few hours to a few days ahead. Most computer models used for weather forecasting worldwide divide the atmosphere up into boxes several tens of kilometers across. Since convective shower clouds and thunderstorms are typically only between 2 and 10 km across, these models have no chance to simulate individual clouds; rather they must try to estimate the effect of an ensemble of clouds within each box on surface rainfall. This is known as a 'convection parameterization' and is very error prone. However, there has been a continued increase in computer power in recent years, and the Met Office currently runs one of the highest resolution national weather forecast model operationally, its model having a horizontal grid-box size of 1.5 km over the whole of the UK. At this resolution it is just about able to simulate the air flows in individual clouds, and a convection parameterization is not needed. While there is evidence that this has improved the accuracy of forecasts, it is clear that there are still very significant shortcomings in the nature of convective shower clouds and thunderstorms simulated at this resolution. What we need are very detailed observations of a large number of shower clouds and thunderstorms over the UK, and how each cloud grows and decays, with which to test and improve these high resolution models. In this project we will obtain this information using the Chilbolton weather radar in Hampshire, which is able to measure all kinds of useful properties of storms at very high resolution (3D structure, surface rain rate, the occurrence of hail, the amount of ice in the upper parts of the cloud, the airflows within the storm and the levels of turbulence). A particularly innovative aspect to the project is that we will develop software to control the radar automatically, so that it can track individual thunderstorms as they evolve. Detailed information on potentially hundreds of storms will be obtained by operating the radar on 40 suitable days over an 18-month period, including the summers of 2011 and 2012. This unique dataset will be used to evaluate the evolution of storms in the forecast model in a level of detail and a range of conditions that has never been achieved before. We will rerun the model with different model configurations (e.g. different ways to describe the ways that cloud ice particles and liquid droplets grow and interact to eventually form rain), in order to determine what it is that limits the realism of shower clouds and thunderstorms in this model, and hence how to improve forecasts. These findings will be applicable to other models worldwide. We will examine the detailed way that storms evolve in the model and in reality, particularly how the airflows in one storm conspire to initiate another storm, and what it is that causes storms to rain persistently in one place, since it is often this behaviour that is responsible for flooding. A further aim of this project will be to test a number of the assumptions that are made in convection parameterizations. Although convection parameterization is not as accurate as simulating individual clouds explicitly, for making 100-year climate forecasts we do not have the computer power to do this with the 1.5-km boxes that would be required over the entire world. Our detailed dataset will help us to ensure that the assumptions about the size and properties of clouds in these parameterizations are realistic, potentially improving the accuracy of climate forecasts.
 
Description Summer flooding is a serious problem in the UK and there is an urgent need to evaluate and improve the 1.5-km resolution model used by the Met Office to make weather forecasts over the UK.

We have developed an algorithm that automatically scans the Chilbolton weather radar to observe showers and thunderstorms as they evolve, and with this have gathered statistics on over a thousand storms on 40 separate days. In parallel, the Met Office cloud-resolving forecast model has been run with resolutions of 1.5 km, 500 m, 200 m and 100 m, as well as changes to its treatment of cloud processes and small-scale mixing. We have used the radar observations to rigorously evaluate the model in terms of the intensity, scale and duration of rainfall features from thunderstorms, as well the intensity and scale of the rising motions ("updrafts") within these storms.

We find that the operational 1.5-km resolution of the model produces storms that are typically too large and too long lived, and their updrafts are too wide. The character of thunderstorms is improved as the model resolution is reduced to 200 m, although the storms tend to become too small and too frequent if the resolution is reduced further.

An important control on the size of storms in the model is the "mixing length", which is ordinarily set to be proportional to the gridbox size. We have shown varying this independently of model resolution allows the typical size of storms to be adjusted and potentially improved.

Other findings are that on showery and thundery days, the cloud-top height in the model tends to be too low by 0-2 km, on average. Furthermore, we have shown that the treatment of ice in the model as low-density slow-falling aggregates, which while suitable for cirrus and frontal ice clouds, could be improved in the simulations of thunderstorms. The introduction of a "graupel" scheme for representing higher density ice leads to improvements in this aspect overall.
Exploitation Route Our research is focused on ways to improve cloud resolving models in an operational weather forecasting context, and so there is potential for our findings to improve not only the Met Office forecast model, but also other cloud resolving forecast models worldwide. This project involved very strong collaboration with the Met Office, with weekly meetings with the head of the Met Office Mesoscale Modelling Group (Humphrey Lean) and two of his scientists (Carol Halliwell and Kirsty Hanley). Moreover, University and Met Office scientists involved in this project both made use of the MONSOON supercomputer, enabling easy exchange of model configurations. Follow-up projects are ensuring that this collaboration is continuing. The dataset generated in DYMECS is currently being used in the ongoing development of the high resolution forecast model used operationally at the Met Office.
Sectors Environment,Other

URL http://www.met.reading.ac.uk/~dymecs/home/
 
Title DYMECS approach to evaluating high resolution atmospheric models 
Description We have developed a rigorous statistical approach to evaluating the size, intensity and other properties of convective storms in high resolution weather forecast models. 
Type Of Material Data analysis technique 
Year Produced 2012 
Provided To Others? Yes  
Impact The method is being taken up by the Met Office and others for ongoing evaluation of new versions of its high resolution model weather forecast model. It is being applied in other areas of the UK using the UK weather radar network, as well as in Darwin, Australia, for evaluating tropical convection. International collaborations have been initiated to perform similar analysis of updrafts and cloud sizes for recent field campaigns over the Southern Great Plains (USA) and Darwin, Australia. 
 
Description DYMECS collaboration on convective storm evaluation in the Met Office model 
Organisation Meteorological Office UK
Country United Kingdom 
Sector Public 
PI Contribution New scanning capability for Chilbolton radar, and subsequent analysis of data leading to valuable information on the performance of the Met Office model in simulating convective storms. Simulations of the Met Office model at various resolutions on the MONSOON supercomputer.
Collaborator Contribution Simulations of the Met Office model at various resolutions on the MONSOON supercomputer. Regular scientific meetings to discuss progress and plans.
Impact The DYMECS dataset and methodology is now used routinely in ongoing development of the Met Office high resolution forecast model.
Start Year 2011
 
Description GASS Grey-zone workshop 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Thorwald Stein gave a talk on the innovative statistical approach for evaluating convective storms in high-resolution NWP using radar observations.
Year(s) Of Engagement Activity 2014
 
Description Meeting on convection in Met Office models at high resolution 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact Organised a meeting around current issues with forecasting convection, including discussion groups providing suggestions for targeted further research.
Year(s) Of Engagement Activity 2013
 
Description NCAS workshop on flood forecasting 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact Thorwald Stein gave an invited talk on initial results of tracking convective storms with radar.
Year(s) Of Engagement Activity 2012
 
Description Podcast for "Planet Earth" on the aims of the DYMECS project 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? Yes
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact NERC's "Planet Earth" recorded a podcast with Professor Robin Hogan, describing the motivation, aims and novelty of the DYMECS project.
Year(s) Of Engagement Activity 2012
URL http://www.thenakedscientists.com/HTML/interviews/interview/1937/
 
Description Presentation to Year 3 of St Columba's college (St Albans) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? Yes
Geographic Reach Local
Primary Audience Schools
Results and Impact A 3-hour presentation was given to year 3 (7 and 8 year olds) of St Columba's college, a boys' school in St Albans, on the subject of weather forecasting and thunderstorms. This included a weather forecasting game to demonstrate how numerical weather forecasts are performed, and five experiments to demonstrate various aspects of thunderstorms. The same presentation was given to year 3 in the same school in 2015, 2016 and 2017.
Year(s) Of Engagement Activity 2013
URL http://www.met.reading.ac.uk/~swrhgnrj/schools/
 
Description RMetS national meeting 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact We designed a meeting on the current status of our understanding of convection over the UK, this will include a talk by Thorwald Stein.
Year(s) Of Engagement Activity 2016
 
Description Radar evaluation of the structure of thunderstorms in high resolution forecasts 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact Talk at: RMetS National Meeting, 20th March, London, UK.
Year(s) Of Engagement Activity 2013