UC San Diego

Low-level marine clouds are abundant over the global ocean and are an important element of Earth’s energy budget. They are sensitive to numerous meteorological variables, including downward longwave radiation, a parameter that has received little attention in observational studies. Using satellite observations and reanalysis data, we show that enhanced downward longwave radiation reduces low-level marine cloud fraction, liquid water path, and cloud-top height. The relationships between low-level cloud properties and downward longwave radiation are isolated from the effects of other cloud-controlling factors through both a compositing and multiple linear regression analysis. These relationships are quantified for clouds over the eastern subtropical oceans and used to calculate the radiative adjustment due to a perturbation in downward longwave radiation under the radiative forcing from 4xCO2. Both methods achieve comparable results. The study area is then expanded to the global ocean from 60° N to 60° S, and another multiple linear regression analysis is performed. Owing to reductions in cloud fraction, liquid water path, and cloud-top height, both the shortwave and longwave cloud radiative effects are weakened globally by 5.5 % and 6.5 %, respectively. This reduces the net cloud radiative effect of low-level clouds by approximately 0.5 Wm-2 or 5.3 %. The relative importance of downward longwave radiation as a cloud-controlling factor is also examined. Low-level cloud fraction and liquid water path are influenced more by a typical 16-day anomaly in downward longwave radiation than any other cloud-controlling factor. Downward longwave radiation also exhibits confounding effects on estimated inversion strength and the relative humidity above the cloud top by weakening the influence of these cloud-controlling factors on low-level cloud properties. These results provide observational evidence that downward longwave radiation substantially impacts low-level cloud properties.