Roles of Wind Shear at Different Vertical Levels, Part I: Cloud System Organization and Properties

Friday, 19 December 2014
Samson M Hagos1, Qian Chen2, Jiwen Fan1, William I Gustafson Jr1 and Larry K Berg3, (1)Pacific Northwest National Laboratory, Richland, WA, United States, (2)Nanjing University of Information Science and Technology, Key Laboratory for Aerosol-Cloud-Precipitation of China Meteorological Administration, Nanjing, China, (3)Pacific Northwest National Lab, Richland, WA, United States
Understanding critical processes that contribute to the organization of mesoscale convective systems (MCSs) is important for accurate weather forecasts and climate model parameterization development. In this study, we investigate the effects of wind shear at different vertical levels on the organization and properties of cloud systems using the Weather Research and Forecasting (WRF) model with spectral-bin microphysics. Based on a control run for an MCS with weak wind shear, we find that increasing wind shear at the both lower (0-5 km) and middle vertical levels (5-10 km) reduces accumulated precipitation and occurrence of heavy rain, while increasing wind shear at the upper levels (> 10 km) leads to small changes in precipitation. Although increasing wind shear at the lower-levels is favorable for a more organized quasi-line system, the precipitation is still reduced by 18.6% compared with the control run due to stronger rain evaporation. Strong wind shear in the middle vertical levels produces a strong super-cell over a narrow area, leading to 67.3% reduction of domain mean precipitation. Increasing wind shear at the upper levels only, does not significantly change the organization of the convection, but it increases cloudiness at the upper-levels which in turn leads to stronger surface cooling, which then stabilizes the atmosphere and weakens convection. When strong wind shear exists over the entire vertical profile, a deep dry layer (from 2 to 9 km) is produced and convection is severely suppressed, leading to fewer high and deep clouds, and the precipitation is reduced by up to 90% in comparison to the control run. The changes in cloud microphysical properties further explain the reduction of surface rain by strong wind shear especially at the lower- and middle-levels. The insights obtained from this study help us better understand the cloud system organization and provide a foundation for improved parameterization of the effect of organized MCS on large-scale circulation.