The impact of Boundary Layer Processes on Simulation of a Severe Weather Event over the City Mecca, Saudi Arabia

We report results from various sensitivity experiments to study the impact of planetary boundary layer (PBL) processes on the prediction of a severe weather event that occurred in city of Mecca, Kingdom of Saudi Arabia. On 11 September 2015, severe winds with heavy rainfall triggered the collapse of highly elevated crane mounted for construction activities over Mecca grand mosque, causing nearly 115 deaths.

We used the Weather Research and Forecasting (WRF) with two nested interactive domains of 3 km and 1 km with four different local, 1.5-order closure turbulent kinetic energy (TKE) PBL schemes. We also performed two simulations with non-local, first order closure and another two simulations with hybrid, local-nonlocal closure PBL schemes. The simulations were conducted based on global forecast fields available at 0.25 degree from the National Centers for Environmental Prediction.

Analysis of surface and upper air observations reveals that this event is initiated by synoptic scale conditions and gets intensified by interaction with the local topography, triggering high convective rainfall and strong winds. The model sensitivity analysis with different PBL schemes suggests that the local, 1.5 order TKE closure schemes are most suitable for predicting the characteristics of rainfall and gale winds in this region. The TKE schemes predicted maximum wind speeds of 20 - 25 m.s-1, which sustained for about 2 hours. Whereas, other non-local first-order closure and hybrid schemes were not able to generate the gale winds and rainfall because of misrepresentation of vertical mixing. The TKE schemes properly resolved the vertical transport of heat and momentum in the lower troposphere, enables to realistic prediction of severe weather event over the Mecca city.