The Link between Summertime Cloud Radiative Forcing and Extratropical Cyclones in the North Pacific

Abstract

This paper examines the role of extratropical cyclones in determining the cloud radiative forcing over the North Pacific during summer. Specifically, this study uses daily and monthly ERBE cloud radiative forcing, monthly ISCCP cloud-type distributions and optical depth, daily ECMWF meteorological analyses, and a climatology of cloud-type distributions based on surface observations. The geographic correspondence between monthly mean fields of cloud radiative forcing, cloud type and optical depth, and quantities such as baroclinicity and transient eddy flux suggests that large-scale, stratiform cloud systems associated with traveling cyclones are largely responsible for the band of strongly negative shortwave cloud forcing (Cs) over the Pacific between 40° and 60°N. Analysis of daily ERBE cloud forcing for July 1985, in conjunction with daily ECMWF geopotential, demonstrates the evolution of highly reflective cloud systems associated with several traveling, closed lows. The southwest to northeast extension of the broad maximum in monthly mean Cs, magnitude is in agreement with the observed pattern of traveling cyclones whose genesis is near Japan and Korea and whose trajectory continues to the Aleutian Islands and the Gulf of Alaska. The scale of each individual cloud system during July 1995 is a significant fraction of the entire Pacific basin north of 40°N. In addition, the mean shortwave cloud forcing of each system is approximately ?150 W m?2. Over the month, most of the basin north of 40°N is covered by clouds associated with one or more such systems.