Identification of distinct types of surface-water-flood-producing convection in the UK

A set of 103 cases of convection resulting in substantial surface-water flooding (SWF) impacts in the UK between 2015 and 2021 has been constructed. The morphologies and environments of these storms have been explored using observations, model data, and ERA5 proximity soundings. Most SWF-producing storms were found to comprise multicellular systems; however, these exhibited a very wide range of morphologies and spatial scales across the event set. Whilst the environments of these storms were typically characterised by rather small CAPE and weak vertical wind shear (median CAPE of ~560 J kg^-1 and median effective bulk wind difference of ~13 knots), a wide range of environmental conditions was noted, ranging from cases with meagre CAPE, low equilibrium levels and strong boundary layer convergence (e.g., along well-marked surface fronts) to cases with large CAPE and high equilibrium levels. Cases also ranged from those with very strong flow in the boundary layer and/or the convective layer as a whole, to those with very weak flow in these layers. Furthermore, examples of both elevated and surface-based convection are represented within the event set.

Faced with this variability, we have employed clustering analysis to objectively define different types of SWF-producing storms in the UK. In this presentation, we will outline the clustering methodology and describe the key characteristics of the several identified event types. A longer-term goal is to understand whether storm environmental characteristics, morphologies and/or physical processes (e.g., those potentially having a bearing on the likelihood and/or location of triggering of secondary cells) vary systematically between types; if so, this information could potentially be used to alert forecasters to mechanisms that are likely to be most relevant on any given day that SWF impacts are possible (alternatively, it may be that there are no clear systematic differences in these respects between types, e.g., if such processes depend on storm-scale factors that cannot be represented in the proximity sounding data or be resolved by available observations). We also seek to explore how the performance of two recently developed observations-based nowcasting tools (the PLUVIA mesoanalysis and PLUVIA cell tracker) varies between types, in order to provide more nuanced guidance to Operational Meteorologists in their use of these new tools.