Statistical Properties of Cumulus Ensembles in High-Resolution Radiative–Convective Equilibrium Simulations

Several studies have shown that deep convection spontaneously aggregates in idealized radiative-convective equilibrium state simulated by cloud-resolving models. While the aggregation of convection is considered to be caused by physical interactions among cloud, radiation, surface energy flux, moisture, and circulation, it also largely depends on numerical configurations such as spatial resolution and domain size. Thus, this study investigates the resolution dependence of the statistical properties of cumulus cloud ensembles.

Numerical experiments were conducted with the use of the non-hydrostatic Weather Research and Forecasting (WRF) model version 3.8.1 at horizontal grid spacing varied from 1600 m to 200 m in an area of 200 km by 200 km square domain. The experiment with the lowest resolution shows inhomogeneity in the moisture field, which is known as a dry patch, a signature of convective self-aggregation. With the decrease in the horizontal resolution, intermittent deep convection accompanied by the strong updraft and rainfall becomes dominant. The middle-to-upper troposphere appears to become stabler and drier with the decrease in the horizontal resolution.

We further analyzed the cloud structures by using a three-dimensional cloud detection method. With the decrease in the horizontal resolution, the horizontal size of the cumulus cloud becomes larger and vertical declination of in-cloud moist static energy becomes smaller. In addition, while a total number of cumulus cloud becomes less, the proportion of deep cumulus clouds, which reaches to the mid-to-upper troposphere within a cumulus ensemble, becomes more. In the next step, it should be investigated the relationship among the horizontal distribution of cumulus ensemble and that of radiation, surface energy flux, moisture, and circulation.