The Impact of Precipitating Ice and Snow on the Radiation Balance in Global Climate Models

Climate models often ignore the radiative impact of precipitating hydrometeors (e.g., rain, snow) due in part to the perception that the combination of their limited spatial and temporal extent and large particle radii are insufficient to have a tangible radiative impact on the atmosphere and because there has been limited observations on the amount of precipitating hydrometeor mass in the atmosphere. CloudSat retrievals of ice water content provide one of the first comprehensive means to estimate the amount of precipitating ice mass in the atmosphere and characterize its vertical structure. With this information, atmospheric radiative transfer calculations are performed to examine the impact of excluding ice associated with precipitating hydrometeors on atmospheric radiative fluxes and heating rates. The results show that exclusion of precipitating ice can result in underestimates of the surface shortwave by 5-10 Wm-2 in the most convective and rainfall intensive areas. Similar errors are found at the top of the atmosphere (TOA) with underestimates of the reflected shortwave and overestimates of the emitted longwave. There are considerable differences (up to ~25%) in the vertical distribution of radiative heating (O[0.1 K day-1]) with about a 10% overestimation of the integrated column cooling. The implications of these results are that models that exclude these ice components in their radiation calculations are achieving TOA radiation balance through compensating errors as well as introducing biases in atmospheric circulations.