A Comparison of Seven WRF Planetary Boundary Layer/Surface Layer Physics Schemes over Complex Terrain

The Weather Research and Forecasting model (WRF) model includes a wide array of physics packages that can be selected by the user, parameterizing sub grid scale processes. WRF v.3.8.1 includes twelve planetary boundary layer (PBL), eight surface layer (SL), and a number of radiation, microphysics, and cumulus parameterization options selectable by the user. Spatially, the model physics are capable of resolving large scale synoptic weather systems, mesoscale phenomena such as severe storms, and sub-km scale phenomena down to larger boundary layer eddies. Using an adequate resolution grid, WRF is capable of providing robust forecasts of atmospheric conditions over fine scale complex topography.

The present study examines seven of the PBL/SL combinations since surface and near surface properties are of primary importance for military operations, including ground forces operations, artillery meteorology impacts, and land mobility. The model domain chosen for these analyses is over Southern California domain, approximately centered near San Diego, California. The domain was chosen primarily because of the availability of high density surface observations, and variety of complex terrain and land use. The inner model domain extends from the coastal plain to the Peninsular Range of Southern California. A triple nest configuration (9-3-1km) of WRF v3.8.1 was run, with the focus on a single day in which conditions were quiescent in order to observe the evolution of the PBL in the absence of rapidly changing synoptic scale weather influences.

The seven PBL schemes we tested included represented both local and non-local physics, and a hybrid approach. In the case of the daytime convective boundary layer (CBL), the differences between the schemes are relatively small, but are notably greater for the largely stable nocturnal boundary layer. The increased model spread during the nighttime can be attributed to differences in the formulations in the nocturnal boundary layer formulations and assumptions. At nighttime, the best performing scheme is dependent upon the meteorological parameter. Consistent among all schemes is the model’s difficulty in capturing the transition between daytime and nighttime boundary layers. In addition, the variation of PBL depth was most variable between schemes over the higher terrain. The surface variables, vertical profiles, and boundary layer depth are important for our application, and for the prediction of weather effects on Army operations using weather impacts decision aids (WIDAs). Both the YSU and MYJ schemes, used in the common WRE-N configurations and in operational configurations, appear to perform reasonably well. The scale-aware scheme (Shin and Hong, 2013) addresses the scale dependency problem, however, for our 1 km inner grid, we see no notable improvement. Statistical analysis shows that the PBL scheme accounts for about 3-4% of the total variance (Smith and Penc, 2016), and we conclude that any physics based ensemble must consider more than PBL/SL options. This is addressed in the Design of Experiments approach described by Smith and Penc (2016) and Smith et al (2017).

References:

Shin, H., and S. Hong, 2013: Analysis of Resolved and Parameterized Vertical Transports in Convective Boundary Layers at Gray-Zone Resolutions. J. Atmos. Sci., 70, 3248-3261.

Smith, J.A. and R.S. Penc, 2016: A Design of Experiments Approach to Evaluating Parameterization Schemes for Numerical Weather Prediction: Problem Definition and Proposed Solution Approach. Joint Statistical Meetings Proceedings, Section on Statistics in Defense and National Security, Conference on Applied Statistics in Defense, p. 4183-4192.

Smith, J.A., R. Penc, and J.W. Raby, 2018: Statistical Design of Experiments in Numerical Weather Prediction: Emerging Results, in 98th Annual AMS Meeting, Joint with 25th Conference on Probability and Statistics. American Meteorological Society: Austin, TX. Paper 6.1.