High-resolution Simulation of Low Clouds with a Multiscale Modeling Framework with a Third-order Turbulence Closure in its Cloud-Resolving Model Component

This study presents results from a high-resolution multiscale modeling framework model (MMF) with an advanced third-order turbulence closure in its cloud-resolving model (CRM) component, which are compared with those from an earlier low-resolution simulation. The high-resolution simulation uses the finite-volume dynamic core with a grid size of 1.9° x 2.5° while the low-resolution simulation uses the semi-Lagrangian dynamic core at T21 resolution. The number of model levels below 700 hPa increases from 6 to 12 in order to better resolve the vertically thin stratus clouds. This vertical reconfiguration was first tested in an offline CRM testing using large-eddy simulation as the benchmark. The testing confirmed that low-cloud simulation could be sufficiently improved with the doubling of the vertical resolution in the lower troposphere. The high-resolution MMF simulation produces not only a global-mean low cloud amount that is 5% higher than the low-resolution simulation and is much closer to that of CloudSat-CALIPSO observations, but also the spatial distributions that are more realistic in several ocean basins. Another significantly improved aspect of the simulation is the spatial patterns of tropical precipitation, in particular, a single intertropical convergence zone (ITCZ) in the Pacific, instead of double ITCZs in the low-resolution simulation, and realistic intensity of South Pacific convergence zone and the ITCZ in the Atlantic, although both simulations slightly underestimate the global-mean precipitation. Further improved top-of-the-atmosphere (TOA) radiative fluxes result in a nearly balanced TOA energy balance although the surface energy balance is not achieved in either simulation. In terms of spatial correlation and pattern, most simulated properties show improvement to various degrees, in particular, surface pressure, surface precipitation and shortwave cloud radiative forcing.