Assessment of Atmospheric Boundary-Layer Processes Represented in the Numerical Model MM5 for a Clear Sky Day Using LASPEX Observations

Data collected during the Land Surface Processes Experiment (LASPEX) in a semi-arid region of the state of Gujarat in north-west India for a clear sky day (16 May 1997) are used to assess the performance of the atmospheric boundary-layer (ABL) and land-surface parameterizations in the fifth-generation Pennsylvania State University-National Center for Atmospheric Research (PSU-NCAR) Mesoscale Model (MM5). The ABL turbulence parameterizations examined are the Blackadar scheme coupled to a simple soil slab model (SSM), and the Troen-Mahrt scheme coupled to SSM or to the more sophisticated Noah land-surface model (NSM). The comparison of several two-way nested high resolution (9-km) MM5 short term 24-h simulations indicate that, although the model is able to capture the trend in the observed data, the computed results deviate from observations. The NSM with a modest treatment of vegetation outperforms the SSM in capturing the observed daily variations in surface heat fluxes and aspects of ABL structure over the tropical land surface at local scales. Detailed analysis showed that, with the incorporation of observed local vegetation and soil characteristics, the NSM reproduced a realistic surface energy balance and near-surface temperature. It is further found that the coupling of the NSM with the Troen-Mahrt ABL scheme leads to excessive ABL mixing and a dry bias in the model simulations.