Simulated boundary layer structure and evolution in an Appalachian cold-air damming event with the WRF model

The atmospheric boundary layer (ABL), the interfacial layer between the earth surface and the free atmosphere transporting momentum, energy, trace gases and pollutants, is directly influenced by its interaction with the underlying surface, and usually responds to changes in surface forcing in an hour or less. Accurate representation of the ABL and its interaction with the land surface in numerical models is of great importance for weather forecast and for applications as air pollutant transport. It is particularly crucial to be able to simulate ABL structure and evolution (regime change). The state-of-science WRF model has several ABL parameterization schemes including schemes specializing for stable boundary layer, and the unified Noah LSM. In this study, The WRF model along with an advanced four-dimensional data assimilation system (Liu et al., 2009) is employed to investigate the ABL structure and evolution in a cold air damming event in Appalachian Mountains. Cold air damming, a frequent winter weather phenomenon happens to the east side of the Appalachian mountains when cold air is trapped by the mountains and a anti-cyclonic pressure system to the northeast. Freezing temperature combined with snow and rain brings severe social and economic impacts. During cold air damming event, the ABL is evolving from unstable condition as the cold air dam builds up to a very stable one when the dam dissipates. Interaction between the boundary layer and the land plays an influential role in the ABL evolution. Simulating such ABL regime change and structure in cold air damming event still presents challenges to numerical weather prediction nowadays.

With the modeling system and available observational data, a number of experiments have been conducted with different boundary layer parameterizations aiming to understand the ABL structure and evolution during the cold air damming event, and the role of land-ABL interaction, and to examine uncertainty in the representation of the boundary layer in the model. The experiments demonstrate the importance of the land-ABL interaction in the cold air damming evolution. The results indicate that all ABL schemes in WRF model have certain difficulty capturing boundary layer regime change. The near surface model errors are largely attributed to the discrepancies between the simulated and observed ABL. Applying the 4D data assimilation in the model can alleviate the inability of ABL scheme to some extent and certainly reduce the model systematic errors. The case study results with detailed diagnostics on ABL formation, evolution along with flux exchange within the ABL will be presented in this talk.