A21D-0165
Modifications to WRF’s dynamical core to improve the treatment of moisture for large-eddy simulations
Tuesday, 15 December 2015
Poster Hall (Moscone South)
Heng Xiao1, Satoshi Endo2, May Wong3, William C Skamarock4, Joseph Klemp3, Jerome D Fast1, William I Gustafson Jr1, Andrew Mark Vogelmann5, Hailong Wang1, Yangang Liu5 and Wuyin Lin2, (1)Pacific Northwest National Laboratory, Richland, WA, United States, (2)Brookhaven National Laboratory, Upton, NY, United States, (3)National Center for Atmospheric Research, Boulder, CO, United States, (4)Organization Not Listed, Washington, DC, United States, (5)Brookhaven Natl Lab, Upton, NY, United States
Abstract:
Yamaguchi and Feingold (2012) note that the cloud fields in their Weather Research and Forecasting (WRF) large-eddy simulations (LESs) of marine stratocumulus exhibit a strong sensitivity to time stepping choices. In this study, we reproduce and analyze this sensitivity issue using two stratocumulus cases, one marine and one continental. Our results show that (1) the sensitivity is associated with spurious motions near the moisture jump between the boundary layer and the free atmosphere, and (2) these spurious motions appear to arise from neglecting small variations in water vapor mixing ratio (qv) in the pressure gradient calculation in the acoustic sub-stepping portion of the integration procedure. We show that this issue is remedied in the WRF dynamical core by replacing the prognostic equation for the potential temperature θ with one for the moist potential temperature θm=θ(1+1.61qv), which allows consistent treatment of moisture in the calculation of pressure during the acoustic substeps. With this modification, the spurious motions and the sensitivity to the time stepping settings (i.e., the dynamic time step length and number of acoustic substeps) are eliminated in both of the example stratocumulus cases.
