3.2 Boundary layer properties in a numerical weather forecast model

Monday, 9 June 2008: 1:45 PM
Aula Magna (Aula Magna)
Margaret A. LeMone, NCAR, Boulder, CO; and F. Chen and M. Tewari

Observations of surface heat and moisture fluxes and PBL structure for four fair-weather days during the 2002 International H2O Project (IHOP_2002) are compared to simulations using the Weather Research and Forecasting (WRF) model, which uses the YSU planetary boundary layer (PBL) scheme coupled to the Noah land surface model. The model had three nested domains with the innermost domain having horizontal 1-km grid spacing. The simulations had successes and shortcomings, the latter traceable to the input land-characteristics data and the model itself. The surface sensible heat (H) was overestimated and latent heat flux (LE) was underestimated, with model values emphasizing a pronounced east-west gradient at larger scale while neglecting smaller-scale variability. From the observations, the missing smaller-scale variability appears to be from horizontal vegetation variability not represented in the input land-use data. The high H and low LE values resulted in an overestimate in PBL depth; if we normalize the model PBL heights to account for the difference in buoyancy forcing during the morning hours, the heights match much better. Boundary layer structure, as revealed by cloud patterns in the satellite images, is reproduced remarkably well, with two days showing cloud streets (evidence for PBL roll vortices), and one day showing longitudinal variation suggestive of modulation of boundary-layer motions by tropospheric gravity waves. The resolved PBL convection has vertical velocities reaching ~0.5-1 m s-1, implying a significant contribution to the fluxes already represented by the PBL scheme, a matter of concern that needs to be addressed.
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