The Dynamical and Radiative Effects of Shallow Cumulus below, in and Above the Grey Zone

The presence of shallow cumulus over land triggers a chain of processes occurring at a wide range of scales. In addition to its direct dynamic and radiative effects, shallow cumulus lead to surface dynamic heterogeneities that reduce the energy and shift its surface partitioning, perturbing the turbulent intensity and structure within the atmospheric boundary layer. These surface, dynamic and radiative effects influence processes at the mesoscale, such as the transition of shallow to deep convection. Due to the small scales required to resolve shallow convection dynamics and the large scales needed to model the mesoscale motions, we find ourselves in the so-called grey zone: a domain too large to be explicitly resolved by models with current computational power, but where some assumptions underlying in mesoscale parameterizations no longer hold. To study the impact of solving explicitly or parameterizing shallow cumulus we design an idealized numerical experiment of shallow cumulus over land. We first test the resolution-dependent ability of a mesoscale model (including an interactive land surface scheme responding to, among others, radiation fluctuations) to reproduce the case by designing systematic experiments. To this end we perform independent identical experiments with increasing horizontal resolution: at the mesoscale (27 km), on the grey zone edge (9 km) and within the grey zone (3 km). The domains are 5400x5400 km², 1800x1800 km², and 600x600 km² respectively. For each suite of experiments we analyse results obtained by using two state of the art convection parameterizations . Very relevant for our study is to compare these surface, cloud dynamic and radiative effects obtained at different grid resolution against the explicit simulations by two different LES models. Here, we perform identical experiments of a cloudy boundary layer coupled to a land surface , but with a horizontal resolution of 50 meters on a 48x48 km² domain.

We first discuss the domain average thermodynamic variables for all experiments. Here it is already evident that, while both LES simulations coincide almost completely, all the mesoscale experiments start deviating at the onset of first clouds. We further study the evolution of cloud cover, mean and most frequent cloud base and cloud top and the ice and water content of clouds and its variability. We find that none of the mesoscale experiments is able to adequately represent the cloud structure and some of them overestimate cloud fraction. Our analysis quantifies large differences between the mesoscale and LES results on basic variables such as cloud base and top height. We then analyse the spatial distribution of clouds and how they affect differently the direct and diffuse components of shortwave radiation at surface, as this partition affects differently the vegetation at surface and, thus, the surface fluxes. The LES results show a dynamic evolution of the cloud layer with varying location, magnitude and height along the day, whereas all the mesoscale experiments show very little cloud variability on time.