The Effect of Land Surface Heterogeneity and Background Wind on Large-Eddy Simulations of Shallow Cumulus Clouds and the Transition to Deep Convection

Idealized large-eddy simulations of the shallow cumulus clouds (ShCu) over the heterogeneous land surface are presented emphasizing the impact of heterogeneity length scale and the background wind speed in land-planetary boundary layer (PBL)-cloud interaction. The surface heterogeneity is represented as the chess-board pattern of alternating ‘WET’ and ‘DRY’ patches with the prescribed surface heat fluxes. The shallow-to-deep convection transition over the heterogeneous surface is characterized with the larger and taller ShCu that are attached to the organized moisture pool near PBL top over DRY patches. This feature is attributable to the surface induced secondary circulation that shows dependency on the surface heterogeneity size and the background wind speed. The secondary circulation over the large patch ( ≥ 5km) under zero background wind promotes the vertical transport of moisture at mesoscale forming the moisture pool over DRY patches. On the other hand, the circulation over the small patch ( < 5km) effectively homogenizes the entire PBL and only the ShCu develops widespread across the domain. The influence of the background wind speed on the secondary circulation is strong, and vertically constant, pure zonal 2 m/s ambient wind can effectively disconnect the surface of 14.4km patches from the overlying PBL with no secondary circulation. This is because that the secondary circulation tries to minimize the thermal contrast in PBL imposed by the surface and induces negative feedback to slow down the circulation. Combining these features, the non-dimensional parameter F_hetero , which is the ratio of the flow advective time scale to the convective time scale, is introduced to determine the shallow-to-deep convection transition for a given patch size and background wind speed. The patch size ≥ 5 km and F_hetero ≥ 20 are the required conditions for the shallow-to-deep convection transition. Additionally, we show that the cloud rolls form over the same heterogeneous surface when the background wind is misaligned with the orientation of the surface heterogeneity. The distance between cloud rolls is determined by the relative angle between the wind direction and the surface heterogeneity. The effect of the vertical wind shear on the ShCu and its transition into deep convection is also explored.

This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-746024.