PRESTO (PREcipitation STructures over Orography)

Flash floods cause loss of life and billions of pounds of damage each year within the UK, and take an additional toll on society through lasting impacts including a four-fold enhancement in the risk of depression. Because of the acute hazards and long-term consequences of these events, it is essential that they be accurately understood and predicted. Two of the three principal mechanisms behind UK flash-flooding events are convective storms and orographic precipitation (the other being frontal systems). Their impact has been reinforced in recent years by a series of devastating events. The Boscastle flood of 2004 and the Ottery St Mary's hailstorm of 2008 were both caused by quasi-stationary convective storms, and the Carlisle flood of 2005 and Cockermouth flood of 2009 were both caused by orographically enhanced rainfall. Although convection and orography may act independently to produce extreme rainfall, they are often closely linked over the complex UK terrain. The mechanical ascent upstream, over, and downwind of steep terrain and the thermally-driven ascent due to elevated heating are primary convection-initiation mechanisms in conditionally unstable flows. Because orography is fixed in space, these storms may anchor to specific terrain features and focus their precipitation over preferred areas. In particular, quasi-stationary precipitation bands are a manifestation of orographic convection that greatly increases flood risks because they focus heavy precipitation over specific regions. Such events are of particular concern over orographic watersheds, which, due to their steep gorges and confined basins, are highly susceptible to floods.

Thanks to the high resolution radar systems, quasi-stationary convective bands have been observed over numerous mountain regions including Japan, the Mediterranean region, Rocky Mountains, Pacific Northwest United States, and Caribbean islands. The hydro-meteorological importance of these bands is reflected by the planned installation of a dedicated observational network for banded orographic convection over the French Massif Central during the upcoming Hydrological Cycle in the Mediterranean (HyMEX) programme. Although these bands also develop regularly over the UK, they have received little previous attention. Moreover, the majority of previous studies have focused on specific cases and have not generally identified the environmental conditions that favour their formation, the mechanisms that cause them to develop, or their predictability in numerical models.

The proposed work will provide a leap forward in the understanding and prediction of quasi-stationary orographic convection in the UK and beyond. This will be achieved through an intensive climatological analysis over several regions of the globe where continuous radar data is available, which will identify the environmental conditions that support the bands and their characteristic locations and morphologies. Complementary high-resolution numerical simulations will pinpoint the underlying mechanisms behind the bands and their predictability in numerical weather prediction models. This work will provide positive impacts for the forecasting community, general public, and other academics in the field. Forecasters will benefit from the identification of simple diagnostics that can be used operationally to predict these events based on available model forecasts and/or upstream soundings. A series of activities are proposed to directly engage with forecasters to effectively disseminate our findings. The public will benefit from improved forecasting of potentially hazardous precipitation events. The academic community will benefit from the advanced physical understanding (which will be disseminated through conferences, workshops, and peer-reviewed publications) and the numerous international collaborations associated with this project.

Terrain-locked convective rainbands are important flood-producing features that give rise to heavy precipitation over many different parts of the globe including the UK. Their accurate prediction is essential because of the flooding and hazardous road conditions that they can produce. Because of their sensitivity to uncertainties in the initial flow field and to small-scale turbulent eddies that are unresolved even in today's highest-resolution operational weather forecasts, the bands represent a fundamental forecasting challenge. The ability of state-of-the-art operational forecast models (such as the Met Office's Unified Model with a grid-length of 1.5 km) to forecast these storms has not been evaluated; we will quantify the predictability of these bands in these models. Their climatological characteristics and thus importance relative to other types of precipitation are also presently unknown; we will construct and synthesize a multi-region climatology of these bands. We will also identify the general environmental conditions and physical mechanisms that lead to bands and control their evolution. This will enable us to provide guidance to forecasters on predicting the formation of these rainbands from low-resolution model forecasts and/or upstream soundings. This will be valuable information for operational forecast centres engaging in short-range forecasting using a range of resolutions and models. We intend to visit twice with Met Office operational forecasters over the course of this project to learn about their current approach for forecasting these events and then to provide tangible guidelines that will help them to refine their practice in the future. Additional dissemination activities include a forecaster-focused workshop and a scientific paper that targets the forecasting community.

Because of the potential socio-economic impacts of the hazardous weather associated with this mountain convection, the findings of this project will also be of interest to users with the responsibility for the mitigation of and response to these impacts. These include DEFRA, the environment agency, utilities providers such as power companies, and the insurance industry. The general public is also interested in the accurate forecasting of orographic convection. Events such as the Carlisle flood in 2005 and Cumbria floods in 2009 highlight the risks of extreme orographic precipitation events.