Sensitivity of WRF to boundary layer parameterizations in simulating a heavy rainfall event using different microphysical schemes. Effect on large-scale processes

Abstract

In this study, the sensitivity of the Weather Research and Forecasting (WRF) model rainfall predictions to the choice of two commonly used boundary layer schemes, is examined through the simulation of an exceptionally heavy rainfall event over Chalkidiki peninsula in northern Greece. This major precipitation event, associated with a case of cyclogenesis over the Aegean Sea, occurred on 8 October 2006 affecting northern and central Greece and causing severe flooding and damage in Chalkidiki peninsula. Simulations with the Yonsei University (YSU) and Mellor–Yamada–Janjic (MYJ) boundary layer parameterizations using three bulk microphysical schemes, showed that MYJ runs had significantly lower predicted rain rates, 24h accumulations and rain volume regardless of the microphysical scheme used. YSU runs produce more localized areas of intense precipitation especially when they are used in conjunction with the Purdue Lin and WRF Single Moment-6 class microphysics. The general verification results from the comparison of model predictions with available raingauge data over the complex topography of Chalkidiki indicate that configurations using YSU scheme provide better statistical scores for heavy precipitation with ETA microphysics better simulating high precipitation rates and Purdue Lin the 24h accumulations. It was shown that as a local closure scheme, MYJ produced insufficient vertical mixing confining moisture to lower levels, greatly decreasing condensates and corresponding latent heating that resulted in surface precipitation reduction, compared to YSU runs. Sensitivity tests revealed that condensational heating from the microphysical processes shows a pronounced contribution to the synoptic scale environment by increasing the intensity of larger-scale baroclinicity. Therefore, diabatic heating seems to be one of the most important factors affecting cyclogenesis and controlling the differences in the simulations between the local and non-local BL scheme in this case forced by their ability in transferring moisture to upper levels.