Global climatology simulated from an upgraded multiscale modeling framework model

Description

Multiscale modeling framework (MMF), which replaces traditional cloud parameterizations with a 2D cloud-resolving model (CRM) in each atmospheric column, is a promising approach to climate modeling. This approach represents convective processes well but not the boundary-layer turbulence and clouds. An upgrade on the CRM component with an advanced third-order turbulence closure has been made. In this study, two simulations are performed with a grid size of 1.9° x 2.5°, but they differ in the vertical resolution. The number of model levels below 700 hPa increases from 6 in one simulation (6L) to 12 in another (12L) to better resolve the vertically thin stratus clouds. This reconfiguration was first tested in 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 12L MMF simulation is better not only in the global-mean low cloud amount that is 4.4% higher than the 6L simulation and is within 3.1% of that of CloudSat-CALIPSO observations, but also in the horizontal distributions and vertical structures that are more realistic in several ocean basins. Another significantly improved result of both simulations is the spatial patterns of tropical precipitation—in particular, a single intertropical convergence zone (ITCZ) in the Pacific, instead of double ITCZs in an earlier simulation with coarser horizontal resolution—and realistic intensity of South Pacific convergence zone and the ITCZ in the Atlantic. Many aspects of the global climatology from the ten-year 12L simulation agree with observations very well—in particular, a nearly balanced TOA radiative energy budget. In terms of spatial correlations and patterns in the tropical/subtropical regions, most surface/vertically integrated properties show greater improvement for increased horizontal resolution than for increased vertical resolution. The relationships among low cloud amount, lower-tropospheric stability, surface relative humidity, and planetary boundary-layer height are consistent with those observed in five low-cloud deck regions. An aspect for future improvement is related to the imbalance in surface energy budget.