A Comparison of Simulated Cloud Radar Output from the Multiscale Modeling Framework Global Climate Model with CloudSat Cloud Radar Observations

Over the last few years a new type of global climate model (GCM) has emerged in which a cloud-resolving model (CRM) is embedded into each grid cell of a GCM. This new approach is frequently called a Multiscale Modeling Framework (MMF) or superparameterization. In this article we present a comparison of MMF output with radar observations from the NASA CloudSat mission, which uses a near nadir-pointing millimeter-wavelength radar to probe the vertical structure of clouds and precipitation. We account for radar detection limits by simulating the 94 GHz radar reflectivity that CloudSat would observe from the high-resolution cloud-resolving model output produced by the MMF. Overall the MMF model does a good job of reproducing the broad pattern of tropical convergence zones, subtropical belts, and mid-latitude storm tracks, as well as, their changes in position with the annual solar cycle. Nonetheless, the comparison also reveals a number of model shortfalls including: (1) Excessive hydrometeor coverage at all altitudes over many convectively active regions. (2) A lack of low-level hydrometeors over all subtropical oceanic basins. (3) Excessive low-level hydrometeor coverage (principally precipitating hydrometeors) in the mid-latitude storm tracks of both hemispheres during the summer season (in each hemisphere). (4) The MMF also has a thin band of low-level hydrometeors in the southern hemisphere of the central (and at times eastern and western) Pacific, which is not observed by CloudSat. This band resembles a second much weaker ITCZ, but is restricted to low levels. Marchand et al. 2008, Comparison of MMF with CloudSat – Accepted JGR 05/2008 p. 3