Scale-dependence of precipitation impacts on radiative effects in low cloud regimes

Abstract:

In low cloud regimes, precipitation is coupled to cloud radiative effects through its relationship with cloud fraction and cloud liquid water content. For example, precipitation is thought to play an important role in the development of open cells, which significantly reduces cloud fraction, and thus the shortwave (SW) cloud radiative effect (CRE). Here we explore a set of parameters that characterize the degree to which a precipitating cloud cools the surface or heats the atmosphere, respectively, over multiple spatial scales. The surface radiative cooling efficiency, r_c, represents the ratio of the SW CRE to LH release from precipitation. The atmospheric heating efficiency, r_h, describes the cloud’s ability to heat the atmosphere per unit LH. At the native resolution of satellite datasets, the LH term is larger, leading to smaller radiative efficiencies, but as scales approach the sizes of global climate models (GCM) resolutions, cloud fraction effects can dominate. Estimates of precipitation and radiative fluxes from CloudSat/CALIPSO observations are used to examine the scale dependence of light precipitation impacts on cloud fraction, r_c, and r_h in low clouds. Results show that cloud fractions in areas of predominantly stratocumulus are less sensitive to the presence of precipitation, while cloud fraction in areas dominated by shallow cumulus actually increases when precipitation is present. The area of transition from stratocumulus to shallow cumulus is especially sensitive to the presence of precipitation, with large reductions in surrounding cloud fraction when precipitation is present. The impact of these changes on the radiative efficiencies, especially r_c, will be quantified from pixel-level to GCM grid scales, and the sensitivity of the link between precipitation and radiative effects to cloud characteristics will be explored. Understanding this scale dependence should provide insights into the time and space variability of precipitation impacts on CREs and could potentially be used to assess global models’ ability to capture sub-grid variability associated with cloud and precipitation processes in low cloud regimes.