Modifications to WRF's dynamical core to improve the treatment of moisture for large-eddy simulations: WRF DY-CORE MOISTURE TREATMENT

Journal Article · Thu Oct 29 00:00:00 EDT 2015 · Journal of Advances in Modeling Earth Systems

DOI:https://doi.org/10.1002/2015MS000532· OSTI ID:1243286

Xiao, Heng ^[1]; Endo, Satoshi ^[2]; Wong, May ^[1]; Skamarock, William C. ^[3]; Klemp, Joseph B. ^[3]; Fast, Jerome D. ^[1]; Gustafson, William I. ^[1]; Vogelmann, Andrew M. ^[2]; Wang, Hailong ^[1]; Liu, Yangang ^[2]; Lin, Wuyin ^[2]

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1. Atmospheric Science and Global Change Division, Pacific Northwest National Laboratory, Richland Wash.
2. Brookhaven National Laboratory, Upton N. Y.
3. National Center for Atmospheric Research, Boulder Colo.

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. 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 sub­stepping 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. This modification improves the applicability of WRF for LES applications, and possibly other models using similar dynamical core formulations, and also permits the use of longer time steps than in the original code.

• Research Organization: Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 1243286
• Report Number(s): PNNL-SA-112067; KP1701000
• Journal Information: Journal of Advances in Modeling Earth Systems, Vol. 7, Issue 4; ISSN 1942-2466
• Publisher: American Geophysical Union (AGU)
• Country of Publication: United States
• Language: English