Quantifying the Effects of Downward Longwave Radiation on Low Clouds

Abstract:

Stratocumulus clouds cover a large area of the world’s oceans in regions of subsidence and are vital to understanding Earth’s energy budget. These are low clouds that are maintained through radiative cooling at their cloud tops. A change in radiative cooling should affect cloud properties.

While many studies have been conducted measuring the response of low clouds to increases in temperature due to global warming, few observational studies have been conducted measuring their response to increases in greenhouse gases and longwave (LW) radiation.

Using NASA’s CERES, CALIPSO, CloudSat, and MODIS data product, low cloud properties were measured in the subtropical Pacific Ocean from 2007-2010. This region is home to persistent low cloud decks. Linear regressions were performed between downward LW radiation and the maximum cloud radiative cooling rate, cloud top height, and cloud fraction, while keeping meteorological constraints constant.

We found that with increased downward LW radiation, the maximum cloud radiative cooling rate decreased in low clouds. A decrease in cloud top height and cloud fraction was also observed with increased downward LW radiation. This was especially true when estimated inversion strength and atmospheric temperature were constrained.

Increased downward LW radiation reduces the amount of radiative cooling at low cloud tops. This reduces convection and lowers cloud tops and reduces cloud fraction. Therefore, with increased greenhouse gases, as downward LW radiation increases low clouds are reduced. This in turn allows more incoming shortwave radiation to reach Earth’s surface and increases warming.