Parameterizing the Large Scale Impact of Land Surface Heterogeneity Induced Circulations on Convective Cloud Development

Understanding the role of surface heterogeneity of surface fluxes in the development of convection is a critical topic in land-atmosphere interactions. This is especially relevant in the context of Earth System Models (ESMs), where simulated sub-grid surface heterogeneity over the land surface is mostly ignored by the overlying modeled atmosphere. Indeed, previous studies using Large Eddy Simulation (LES) have shown that heterogeneities in the surface field below ESM spatial resolution (~100 km) can cause appreciable secondary circulations and, at times, a significant increase in convective cloud development. These large scale changes initiated by small scale heterogeneity have yet to be adequately parameterized in ESMs. To address this particular weakness, this presentation presents a parameterization scheme for a near surface density driven circulation between two lower atmosphere columns with variable surface heating for use within ESMs.

The secondary circulation parameterization is fit to data from 184 LES runs over 92 days, one run each day with a homogeneous surface and one run each day with a heterogeneous surface derived from land surface model output, over a 100 km square domain centered around the ARM site in the US Southern Great Plains (SGP) in Oklahoma. It is then tested over those 92 simulation days at the SGP site, as well as shallow convective days over four heterogeneous sites where differential heating is common: Wisconsin (lake-land), Florida (ocean-land), Missouri (urban-rural) and Appalachia (elevation). 

To test the circulation scheme, we use standalone columns of Cloud Layers Unified by Binormals (CLUBB) a boundary layer, cloud and shallow convection scheme used in multiple modern ESMs. CLUBB is run for three cases on each simulation day at each site: 1) as a single homogeneous column model over the domain, 2) as two separate columns over high and low sensible heat portions of the domain, and 3) following 2) with the addition of the circulation parameterization scheme. The homogeneous CLUBB simulations and those with secondary circulations are compared to evaluate the impact of the secondary circulation on cloud development, turbulent kinetic energy, and profiles of the means and variances of heat and moisture. Results over the SGP site show that the parameterized circulation yields similar changes in cloud development and profiles of heat and moisture to LES.