Radiative Impacts of Free-Tropospheric Clouds on the Properties of Marine Stratocumulus

Abstract

bservations from multiple satellites and large-eddy simulations (LESs) from the Regional Atmospheric Modeling System (RAMS) are used to determine the extent to which free-tropospheric clouds (FTCs) affect the properties of stratocumulus. Overlying FTCs decrease the cloud-top radiative cooling in stratocumulus by an amount that depends on the upper-cloud base altitude, cloud optical thickness, and abundance of moisture between the cloud layers. On average, FTCs increase the downward longwave radiative flux above stratocumulus clouds (at 3.5 km) by approximately 30 W m?2. As a consequence, this forcing translates to a relative decrease in stratocumulus cooling rates by about 20%. Overall, the reduced cloud-top radiative cooling decreases the turbulent mixing, vertical development, and precipitation rate in stratocumulus clouds at night. During the day these effects are greatly reduced because the overlying clouds shade the stratocumulus from strong solar radiation, thus reducing the net radiative effect by the upper cloud. Differences in liquid water path are also observed in stratocumulus; however, the response is tied to the diurnal cycle and the time scale of interaction between the FTCs and the stratocumulus. Radiative effects by FTCs tend to be largest in the midlatitudes where the clouds overlying stratocumulus tend to be more frequent, lower, and thicker on average. In conclusion, changes in net radiation and moisture brought about by FTCs can significantly affect the dynamics of marine stratocumulus and these processes should be considered when evaluating cloud feedbacks in the climate system.