Cloud-Precipitation-Radiation-Dynamics Interaction in Global Climate Models: A Snow and Radiation Interaction Sensitivity Experiment

Clouds continue to be one of the largest uncertainties in the climate system. For example, radiation interactions with clouds and associated cloud-feedbacks remains a pressing challenge in both observations and modeling efforts reported in the IPCC AR5 (i.e., CMIP5). Key issues include, though are not limited to, the representation of convection and cloud microphysical processes as well as their interactions with coupled atmosphere-ocean-land processes. This conclusion is generally reached by considering a plausible range of unknown contributions by processes yet to be accounted for in current climate models.

Conventional global climate models (GCMs) often consider radiation interactions only with small-particle/suspended cloud mass, ignoring large-particle/falling and convective core cloud mass. We characterize the radiation and atmospheric circulation impacts of frozen precipitating hydrometeors (i.e., snow) focusing on regions over the Pacific Ocean using the National Center for Atmospheric Research (NCAR)-coupled GCM and conducting sensitivity experiments that turn off the radiation interaction with snow. The changes associated with the exclusion of precipitating hydrometeors exhibit a number differences that are notably consistent with those systematic biases in CMIP3 and CMIP5. Neglecting the radiation interaction of snow leads to moisture/warm air convergence south of the ITCZ and north of the SPCZ in conjunction with the model's northeastward overextension of the SPCZ. Other associated effects include changes in upper-level ocean temperatures as well as the vertical structures of humidity and temperatures over the regions. Broader dynamical impacts include a stronger local meridional overturning circulation over the central and east Pacific, and commensurate changes in low- and upper-level winds, in addition to differences in the large-scale ascending motion across the tropical Pacific.