The Environments of Convective Storms: Challenging Conventional Wisdom

Large thunderstorms are one of the most damaging of weather phenomena. Hail can devastate crops, flash flooding can inundate towns and homes, lightning can threaten people and ignite fires, and strong gusts can damage transport and infrastructure. Convective storms and associated phenomena cause 5-8 billion euro per year in damage across Europe. Such storms have the potential to be forecast and the public warned beforehand, but forecasting becomes increasingly difficult as the length of a forecast increases. In the near-term, observations and high-resolution computer modelling can provide adequate warning of impending storms, but for periods longer than three days ahead the outbreak of thunderstorms has to be deduced indirectly from the computer forecast even if the large-scale flow is well forecasted. The aim of this project is to improve our understanding of the relationship between thunderstorms (also called convective storms) and the larger-scale environment in the atmosphere, to provide better understanding of the physical processes responsible to aid forecasters in interpreting the model predictions.

Convective storms require three ingredients: sufficient moisture to condense and fuel the storm, instability or the rate at which temperature decreases with height (temperature dropping quickly with height is better), and something to lift air to release the instability. In this proposal, we focus on the instability ingredient.

In the United States, environments with large instability are believed to occur because of heating over the elevated terrain of the western United States, resulting in the elevated mixed-layer (EML). In Europe, EMLs are attributed to passage over the elevated terrain of central Spain, resulting in the Spanish plume. Such sensible heating of lower-tropospheric air (3-5 km above sea level) by an elevated heat source such as the Rockies or Spanish plateau is a natural explanation for the steep lapse rates in the EML.

How much of a contribution is the elevated heating to the formation of instability? The smaller scale of the Spanish high terrain compared to the Rocky Mountains makes it difficult to imagine that the Spanish high terrain creates such large instability. One hypothesis for the origin of the steep lapse rates is the Sahara Desert, where a well-mixed boundary layer forms steep lapse rates that can be advected away from northern Africa (known as the Saharan Air Layer). Yet, this hypothesis has not been tested, either for the Spanish plume or other regions downstream of high heated terrain.

A different factor said to explain the occurrence of instability is the differential transport of air with low temperature or low moisture aloft. Although such explanations have been used in the literature, other studies have questioned the applicability of this factor.

Our proposed research asks what processes produce the environment for midlatitude convective storms around the globe. What environments are favourable for instability, and how does this differ around the globe? What are the physical processes that create instability? Is instability - in Europe generally and the UK specifically - attributed to elevated heating, as in the EML of the central United States or by long-range transport?

Despite conventional wisdom stating that the elevated mixed layer is responsible for creating the instability downstream of high terrain, it remains untested. Our aim in this proposal is to develop a better understanding of the relationship between high terrain, large-scale processes, and instability for midlatitude convective storms. These concerns motivate a multifaceted research project to answer these questions.
Q1: What are the physical processes responsible for creating instability?
Q2: How does topography create a favourable environment for deep moist convection?
Q3: How important is differential temperature and moisture advection to creating instability?

Two groups will benefit from this proposal: forecasters and the general public.

1. Forecasters: Although much conventional wisdom underlies forecaster knowledge, forecasters benefit from our research by learning the physical processes for convection, leading to improved conceptual models. We target the forecasting community in three ways.

a. The PI or PDRA will attend the European Severe Storms Laboratory Testbed for one week in Year 2 or 3. Operational forecasters from national hydrometeorological centres and commercial companies across Europe are among the attendees. Daily presentations by research meteorologists and forecasters provide an opportunity for interaction and collaborations, as well as testing the new ideas being developed as part of this proposal.

b. Training within EUMETSAT. We will also take our findings to the European operational forecasters through EUMETSAT. We will develop a training program around our research and deliver online training for European forecasters through their facilities.

c. Publication accessible to forecasters. By the end of these training experiences, we will have defined a suite of operational forecasting tools and recommendations for improvements to training programmes. A synthesis article on instability and convection initiation will be written for forecasters and the scientific community. The article will be submitted to the Bulletin of the American Meteorological Society. The aim will be to distil our results into a framework aimed at forecasters and to disseminate our research to the international forecasting community.

2. General public.
The UK public has a great fascination with the weather, and events such as the 2005 Birmingham tornado and winter 2013-2014 floods make the research within this project relevant to them. Our engagement with the public consists of three strands.

a. Sponsorship of two Barometer Podcasts. The Barometer is broadcast from the Centre of Atmospheric Science, University of Manchester, and communicates atmospheric science to an interested lay audience. The Barometer will work with our project team to develop two episodes on convective storms and numerical weather prediction.

b. Sponsorship of two articles by theWeatherClub. theWeatherClub newsletter is published by the Royal Meteorological Society as an effort to tap the potential audience interested in the weather. We will sponsor two articles that feature our research. In addition, video and web chat will be employed on their Web site and social media to attract attention to this research.

c. Wiki Editing Day. Two online wikis provide information about convective storms to the general public: Glossary of Meteorology and Wikipedia. Entries in these two wikis are out of date or have never been written. Some links are broken or direct to inappropriate pages written by nonexperts. During Wiki Editing Day, we will organise the global severe storms community to write, edit, and update wiki pages and the Glossary. Wiki Editing Day will serve several purposes: update/revise/create new entries, call public attention to the research project through press releases and social media, and call the attention of other scientists to the sometimes low quality of online scientific resources and the need to contribute their expertise to improve them.

d. Other public engagement activities. We envision talks on convective storms at the Manchester Science Festival, Manchester Museum, Royal Meteorological Society public meetings, and other opportunities as they occur. Schultz has been involved with the Manchester Science Festival and the Manchester Museum, as well as given school and public presentations. Schultz is in discussions with the Museum about creating a workshop on weather forecasting. We will look for opportunities to highlight our research through other radio appearances, public lectures and through the media via NERC and university press releases.