Simulations versus observations of a sheared convective boundary layer

A comparison was undertaken between observations and simulations of the convective boundary layer (CBL) diurnal evolution on a day of the International H2O Project (IHOP) experiment that was characterized by strong winds across part of the Oklahoma and Texas panhandles. The initial numerical setup was based on observational data obtained from IHOP measurement platforms and supplementary data sets from surrounding locations. The goals of the study were: (i) numerical investigation of the structure and evolution of the CBL by means of large eddy simulation (LES); (ii) evaluation of LES predictions of the sheared CBL growth against lidar observations of the CBL depth evolution; and (iii) comparison of the simulated turbulence structures with those observed by lidar and vertically pointing radar during the CBL evolution.

For a period during the mid- to late morning hours, the simulated CBL evolution was found to be in a fair agreement with atmospheric lidar and radar observations, and the simulated entrainment dynamics are consistent with those from previous studies. However, CBL depths, determined from lidar data, increased at a faster rate than in the simulations during the afternoon, and the wind direction veered in the simulations more than in the observations. The CBL depth discrepancy can be explained by a solenoidal circulation between two distinct CBLs with different mixed layer virtual potential temperature and CBL depth. The discrepancy in winds can be explained by time variation of the large-scale pressure gradient, which was not included in LES. Additional simulations were performed to include the time variation of the large scale pressure gradient and the large scale advection, and these new results will be compared with the initial LES results.