Cloud and Radiation Variations Associated with Northern Midlatitude Low and High Sea Level Pressure Regimes

Abstract

A global meteorological dataset, a global satellite dataset, and a radiative transfer model are combined to map the cloud types in low, near-normal, and high sea level pressure regimes in the northern midlatitudes, and to calculate the radiative balance in those regimes. The prominent cloud feature is a background cloud field that is present most of the time and is modulated by changes in dynamic regime. It consists of a low cloud deck, which becomes optically thicker in the warm seasons over ocean and in the cold seasons over land, and a population of optically thin middle-to-high-top clouds that is mostly middle-top in the cold and mostly high-top in the warm seasons. This background cloud field is modulated by the emergence of a population of optically thick high-top clouds in the low pressure regime and by an increase in the optical thickness of the low clouds in the high pressure regime. The top-of-the-atmosphere (TOA) shortwave flux differences between dynamic regimes show that more sunlight is reflected in the low than in the high pressure regime. In January TOA shortwave flux differences between regimes range between 5 and 20 W m?2 and in July between 20 and 50 W m?2, and those differences are manifested as a net excess cooling at the earth?s surface. The TOA longwave budget shows more heat trapped in the troposphere in the low pressure than in the high pressure regime. The differences in the TOA outgoing longwave fluxes between the two extreme regimes range in all seasons between 5 and 35 W m?2 and are manifested mostly as an additional warming in the atmospheric column. The TOA total flux differences between the low and high pressure regimes change both sign and magnitude with season;in the winter an excess warming of 5?15 W m?2 is found in the low pressure regime while in all other seasons an excess cooling, which ranges between 10 and 40 W m?2, is found. Preliminary investigations with the Goddard Institute for Space Studies GCM show that changes in midlatitude dynamics with climate can produce significant radiation feedbacks.