Extreme precipitation and east coast low events: the impact of physical parametrization.

On the eastern seaboard, East coast lows (ECLs) can lead to very high rainfall totals in short time periods. We investigate the underlying physics of this observation by examining the impact of different physics parameterizations on extreme precipitation in ECLs. A number of ECLs are simulated with the Weather Research and Forecasting model, and we focus on the strongest simulated event here: the 2007 Newcastle ECL. We examine the sensitivity of precipitation extremes to microphysical schemes, radiation schemes, boundary and surface layer physics, and cumulus parameterizations. We show how the cumulus parameterization, and to a lesser extent the boundary layer, can have a significant impact on the most extreme precipitation accumulations on the sub-daily time scale. Extreme accumulations on daily and longer time scales are less sensitive to the choice of physical parameterization. The most likely explanation for this behaviour involves changes in moisture transport when different cumulus parameterizations and boundary layer schemes are employed.