LES simulation of the shallow cumulus clouds over the chessboard land surface: Influence of the heterogeneity length scale and the background wind speed

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Description

We present the idealized LES simulation of the shallow cumulus clouds (ShCu) over the heterogeneous land surface emphasizing the impact of heterogeneity length scale and the background wind speed in land-PBL clouds interaction. Our simulation set up follows the pure ShCu case based on the multi-year observations at ARM SGP sites by Zhang et al. (2017, JAS). However, we adopt the chessboard pattern land surface with the alternating ‘WET’ and ‘DRY’ patches where the diurnally varying surface heat fluxes are prescribed. The patch length ranges in 14.4, 7.2, 4.8, 2.4 and 1.2km over 28.8 x 28.8 km2 surface domain and the background wind speed is set to be 0, 1, 2, 3 or 10m/s with no shear. In our LES simulations, the cases with the patch size larger than 5km under zero background wind speed develop deep convection around local noon over the DRY patches. The transition cases are characterized with the larger and taller ShCu that are attached to the moisture pool near PBL top before the transition. Over the smaller patches, the horizontal mixing induced at the patch boundary is more influential and leaves the entire PBL well-mixed over the heterogeneous land surface. On the other hand, over the larger patches, such mixing is only limited to the region close to the patch boundary, and hence, the moisture pool near PBL top over the DRY patch is better protected. We also find that even the 1-2m/s background wind speed is powerful enough to dampen the surface heterogeneity influence. Our analysis reveals that the land breeze circulation over the 14.4km patch case can withstand only up to 1.5 m/s background wind. Thus, the 14.4km patch becomes non-transition case with the 2m/s background wind while the same size patch becomes the transition case under 0 and 1m/s background wind. The reason for such small threshold is because the land breeze circulation tries to minimize the thermal contrast across the patches and induces negative feedback to the circulation. Lastly, we introduce the non-dimensional parameter, Fhetero as a way to predict the shallow-to-deep convection transition for a given heterogeneity length scale and the background wind speed. 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-744122.