“High-Resolution Ensemble Modelling of Rainbands Downwind of Mountainous Terrain

In this study we investigate rainbands that form downwind of mountainous terrain using observational data, output from operational forecasts, and convection-permitting ensemble model forecasts. Although relatively small in scale (a few tens of km across by up to ~100 km in length), these often poorly forecast bands can cause localised flooding as they can be associated with intense precipitation over several hours due to the anchoring effect of orography.

A case study is performed of a rainband that was observed over Scotland on the 29 December 2012. This case study was chosen due to the occurrence of heavy continuous precipitation near orography. Radar images during the event showed a clear band along The Great Glen Fault, a deep valley between the Northern Highlands and the Grampian Mountains. The operational deterministic convection-permitting Met Office model forecast, using the United Kingdom Variable resolution (UKV) model (1.5 km horizontal grid spacing), failed to represent this band. However, some members of the operational ensemble convection-permitting (MOGREPS-UK, 2.2 km horizontal grid spacing) Met Office forecast produced a similar band to that observed. Localised convergence and weak convective available potential energy (CAPE) along the fault supported the formation of the valley band.

To determine the effect of model resolution on the model's representation of the rainband, a forecast was performed with the horizontal gird spacing decreased to 500 m. In this forecast a rainband formed in the correct location which generated precipitation accumulations close to those observed, but with a time displacement. The robustness of this forecast skill improvement is being assessed by performing an ensemble of these convection-permitting simulations by nesting down from members of the operational regional MOGREPS-R (18 km grid spacing) ensemble. Results suggest that accurate representation of these mesoscale rainbands requires resolutions higher than those used operationally by national weather centres.