The Climatic Impacts of Precipitating Ice and Snow

Introducing their study, authors Waliser et al. (2011) write that "key to the proper use of satellite retrievals in the evaluation of modeled cloud ice and liquid is that many global climate model representations ignore or diagnostically treat the falling hydrometeor components (e.g., rain, snow) and only consider -- for the purposes of radiation calculations -- the 'suspended' component of water that the model deems 'clouds'." And they state that "the variations in the annual mean integrated ice water path and liquid water path between global climate models contributing to the IPCC AR4 range over two orders of magnitude," citing Li et al. (2008) and Waliser et al. (2009).

Employing estimates of cloud and precipitating ice mass and characterizations of its vertical structure supplied by CloudSat retrievals, Waliser et al. performed radiative transfer calculations "to examine the impact of excluding precipitating ice on atmospheric radiative fluxes and heating rates."

The four researchers report that exclusion of precipitating ice "can result in underestimates of the reflective shortwave flux at the top of the atmosphere (TOA) and overestimates of the down-welling surface shortwave and emitted TOA longwave flux, with the differences being about 5-10 Wm^-2 in the most convective and rainfall intensive areas." In addition, they say "there are also considerable differences (~10-25%) in the vertical profiles of shortwave and longwave heating, resulting in an overestimation (~up to 10%) of the integrated column cooling." And they state that "the magnitude of these potential errors is on the order of the radiative heating changes associated with a doubling of atmospheric carbon dioxide."

Waliser et al. say that "when the above results are considered in the context of a climate model simulation, the changes would not only impact the radiative heating of the atmosphere but would be expected to impact the circulation, and possibly even the manner the model adjusts to external forcings such as increasing greenhouse gases." In addition, they note that since the "models are tuned to get the right TOA radiation balance, the implications here are that without considering the ice in precipitating hydrometeors explicitly, the models will be getting the right result (i.e., TOA balance) for the wrong reasons," and that "in doing so, there are likely to be compensating errors in other quantities such as cloud cover, cloud particle effective radius and/or cloud mass." Consequently, it would appear that climate modelers still have a long way to go in their quest to adequately represent the intricate complexity of earth's climatic system.

Additional References
Li, F.-F., Waliser, D.E., Woods, C., Teixeira, J., Bacmeister, J., Chern, J., Shen, B.W., Tompkins, A. and Kohler, M. 2008. Comparisons of satellites liquid water estimates with ECMWF and GMAO analyses, 20th century IPCC AR4 climate simulations, and GCM simulations. Geophysical Research Letters 35: 10.1029/2008GL035427.

Waliser, D.E., Li, J.-L.F., Woods, C.P., Austin, R.T., Bacmeister, J., Chern, J., Del Genio, A., Jiang, J.H., Kuang, Z., Meng, H., Minnis, P., Platnick, S., Rossow, W.B., Stephens, G.L., Sun-Mack, S., Tao, W.-K., Tompkins, A.M., Vane, D.G., Walker, C. and Wu, D. 2009. Cloud ice: A climate model challenge with signs and expectations of progress. Journal of Geophysical Research 114: 10.1029/2008JD010015.