18th Conference on Weather and Forecasting, 14th Conference on Numerical Weather Prediction, and Ninth Conference on Mesoscale Processes

Tuesday, 31 July 2001: 8:01 AM
Mesoscale variability in boundary layer development over the southern Great Plains
Ankur R. Desai, Pen State Univ., Universtiy Park, PA; and K. J. Davis, D. R. Stauffer, B. P. Reen, R. J. Dobosy, and S. Ismail
Poster PDF (80.4 kB)
The land surface is highly heterogeneous. Atmospheric mixing, however, rapidly blends small-scale variations in surface forcing. A quantitative understanding of the influence of micro- and mesoscale surface forcing on the overlying atmosphere is still unavailable. Model studies have suggested that landscape heterogeneity can lead to significant mesoscale flows. Observations of lake breezes and surface layer flows at scales of a few to a few tens of kilometers support these model results, but typically the atmospheric heterogeneity is limited to turbulent fields within the atmospheric boundary layer. Observational evidence of landscape-driven heterogeneity in the mean properties of the boundary layer (e.g. depth, water vapor content) at scales of ten to hundreds of kilometers is rare.

We present observational methods for systematic study of the linkage between the land surface and atmospheric heterogeneity, and results of a case study using these methods. Surface flux observations, combined with remote sensing of the land surface, provide estimates of the spatial pattern of surface sensible and latent heat fluxes. Airborne water vapor lidar observations provide observations of boundary layer depth and water vapor content along the flight track. Rawinsondes and wind profiles provide thermodynamic initial conditions and boundary layer wind profiles.

Observations were collected over the 50km x 250km domain of the 1997 Southern Great Plains Experiment (http://daac.gsfc.nasa.gov/CAMPAIGN_DOCS/SGP97/sgp97.html). Two days of data show that surface fluxes are highly correlated with soil moisture shortly after rainfall, and that surface fluxes are very heterogeneous at a wide variety of spatial scales. Correlation between surface fluxes and soil moisture appears to degrade with time past rainfall, suggesting the surface vegetation and root-zone moisture becomes more important.

The potential for heterogeneous boundary layer forcing is examined and found to be large. Airborne boundary layer soundings show heterogeneity in boundary-layer growth at two sites 30 km to 50 km apart that are consistent with airborne surface flux measurements in these regions. Lidar observations, however, only show significant boundary layer heterogeneity at the scale of the entire 250km experimental domain, suggesting that smaller-scale surface features were homogenized in the atmosphere by horizontal advection and mixing. These observational results are presented along with MM5 simulations of surface fluxes and boundary-layer development to help resolve the role of surface heterogeneity on day of the case study.

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