Wednesday, 9 July 2014
A set of high-resolution radar observations of convective storms has been collected to evaluate such storms in the UK Met Office Unified Model during the DYMECS project (Dynamical and Microphysical Evolution of Convective Storms). The 3-GHz Chilbolton Advanced Meteorological Radar was set up with a scan-scheduling algorithm to automatically track convective storms identified in real-time from the operational rainfal radar network. More than 1,000 storm volumes scanned over fifteen days in 2011 and 2012 are used to evaluate the model under various synoptic conditions supporting convection. The model is run over the southern UK and evaluated at different resolutions, from 1500-m down to 100-m grid length, and configurations including prognostic graupel. Results suggest that model storms are a factor 1.5--2 too broad in the 1500-m grid length simulations, but slightly too narrow at the highest resolutions. Simulations with prognostic graupel show little improvement in storm morphology, but the vertical distribution of reflectivity in columns of heavy precipitation compares better with observations.
Vertical scans were performed through locations of maximum surface rainfall rate in the observations to estimate updraft size and strength using mass continuity. A similar method was used in the model for a like-with-like evaluation, but also tested against the model vertical velocities to test the retrieval method. Retrieved updrafts are too broad and weak in the 1500-m grid-length simulations, whereas in the high-resolution simulations they are approximately 2 km wide and peaked in a triangular shape, similar to those observed. A study of storm morphology in terms of reflectivity and updraft size is performed to understand the discrepancy in model performance in terms of microphysics and dynamics.
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