The PRESTO project was funded by the Natural Environment Research Council (NERC) with the grant references - NE/I024984/1 and NE/I026545/1 - led by Professor Suzanne Gray (University of Reading) and Professor David Schultz (University of Manchester).

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.

PRESTO provided a leap forward in the understanding and prediction of quasi-stationary orographic convection in the UK and beyond. This was achieved through an intensive climatological analysis over several regions of the globe where continuous radar data was available, which identified the environmental conditions that support the bands and their characteristic locations and morphologies. Complementary high-resolution numerical simulations pinpointed the underlying mechanisms behind the bands and their predictability in numerical weather prediction models. This work provides positive impacts for the forecasting community, general public, and other academics in the field. Forecasters 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 were used to directly engage with forecasters to effectively disseminate our findings. The public benefit from this improved forecasting of potentially hazardous precipitation events. The academic community benefit from the advanced physical understanding (which was disseminated through conferences, workshops, and peer-reviewed publications) and the numerous international collaborations associated with this project.

Terrain-locked convective bands are gaining recognition for their potential to focus precipitation over localized regions and enhance flash-flooding risks in vulnerable watersheds. Despite these hydro-meteorological hazards, the prediction of these features is compromised by insufficient understanding and inherent limitations of NWP models. This project aimed to take a leap forward in the understanding and prediction of these features through a synthesis of observations and high-resolution numerical simulations.

Specific objectives included:
1. To construct and synthesize a multi-region climatology of banded orographic convection using a combination of high-resolution operational observations and model reanalyses.
2. To identify from the climatology the general environmental conditions that support quasi-stationary convective bands and control their precise locations and persistence.
3. To isolate the physical mechanisms behind quasi-stationary convective bands through high-resolution numerical case studies and sensitivity experiments.
4. To quantify the predictability of this convection in numerical weather prediction models.
5. To assist operational forecasters in the identification of potentially hazardous banded events from model forecasts and upstream soundings.

The experimental approach was motivated by four hypotheses that were explicitly tested in this research:
1. Terrain-locked convective bands form under similar conditions over different parts of the earth.
2. The bands owe their existence to a range of different physical mechanisms, including forced orographic lifting, lee-side convergence, and the localized generation of slantwise convection.
3. The exact band locations, and hence the potential for flooding in specific watersheds, are sensitive to small-amplitude uncertainties in the large-scale atmospheric flow.
4. The organization and steadiness of these bands is delicate and can be disrupted by turbulent eddies that propagate through the convective regions.