AN EVALUATION OF THE LARGE-EDDY SIMULATION OPTION OF THE REGIONAL ATMOSPHERIC MODELING SYSTEM IN SIMULATING A CONVECTIVE BOUNDARY-LAYER - AFIFE CASE-STUDY

A large-eddy simulation (LES) model was used to simulate the convectiv e boundary layer (CBL) that developed on 1 July 1987, over the domain of the First International Satellite Land Surface Climatology Project Field Experiment (FIFE). Three simulations were produced using differe nt boundary conditions at the ground surface, namely, (i) spatial dist ribution of topography and spatial distribution of surface heat fluxes ; (ii) spatial distribution of topography but mean surface heat fluxes ; and (iii) no topography and mean surface heat fluxes. The diurnal va riation of mean surface fluxes and their spatial distribution were der ived from the FIFE network of observations. In all cases, the model wa s initialized with the atmospheric sounding observed in this domain al 0700, and run until 1500 local time. The resulting mean profiles of t emperature and specific humidity were compared to those observed with atmospheric soundings at 0900, 1030, and 1230 local time. The simulate d structure of turbulence was qualitatively compared with that obtaine d from a volume-imaging lidar (VIL) scanning the CBL over the simulate d domain during that day. Power spectra and autocorrelations of mixing ratio were calculated from the model outputs and were compared to tho se obtained from the VIL. Overall, the model performed quite well. Obs erved atmospheric soundings were within 1 K and Ig ka-l of the simulat ed mean profiles of temperature and specific humidity, respectively, a nd indicated that the model correctly predicts the CBL height. Similar ities in the structure of the eddies obtained from the model and the V IL were clearly identified. Spectral analysis indicated that resolved eddies (i.e., eddies larger than 200 mi are relatively well simulated with the model, but that the energy cascade is not well represented by the Deardorff 1.5-order-of-closure subgrid-scale parameterization. Au tocorrelation analysis indicated that the model correctly simulates th e characteristic size of the eddies, but that their mean lifetime is l onger than that observed with the VIL, indicating a too weak dissipati on of the eddies by the subgrid-scale scheme. Thus, this study emphasi zed the need to develop better subgrid-scale parameterizations for LES models. The different simulations also indicated that topographical f eatures of the order of 100 m and micro beta-scale heterogeneity of su rface heal fluxes had only a minor to modest impact on the CBL develop ing over a relatively humid surface.