HR: 17:10h
AN: A54D-05 INVITED     [Abstracts]
TI: Recent Changes in Cloud Cover and Cloud Types over Land
AU: * Warren, S G
EM: sgw@atmos.washington.edu
AF: University of Washington, Department of Atmospheric Sciences, Box 351640, Seattle, WA 98195 United States
AU: Eastman, R M
EM: rmeast@atmos.washington.edu
AF: University of Washington, Department of Atmospheric Sciences, Box 351640, Seattle, WA 98195 United States
AU: Hahn, C J
EM: hahn@atmo.arizona.edu
AF: University of Arizona, Department of Atmospheric Sciences, Tucson, AZ 85721 United States
AB: From a worldwide dataset of weather observations from land stations, 5400 stations were selected as having long periods of record with reliable reports; they cover all continents and many islands. About 185 million synoptic reports were analyzed for total cloud cover and the amounts of nine different cloud types, for the 26-year period 1971-1996. Monthly and seasonal averages were formed for day and night separately. To select criteria for admitting stations to trend-computation, the average uncertainty in the slopes of trend lines was plotted as a function of the number of years required and the number of observations required per year-season. Based on these tests, a daytime trend will be computed for a season or month if we have a minimum of 75 observations in each of at least 15 years, spanning at least 20 years. For night we require 50 observations. Significant regional trends are found for many cloud types. The night trends are well-correlated with day trends for total cloud cover and for all cloud types except fog and cumulus. Where we lack sufficient observations to compute night-time trends, we therefore assume the daytime trend is representative. The regional changes are coherent across national boundaries. Cirrus trends are generally negative on all continents, but may reverse sign seasonally in the tropical monsoon regions. A previously-reported decline in total cloud cover over China and its neighbors is largely attributable to high and middle clouds. Time-series of cloud-cover anomalies for the continents show a large decrease for South America and a small decrease for Eurasia. Africa and North America show interannual variations of 1.8% and 1.4% for seasonal means, but no overall trend. The largest interannual variations (2.7%) are found for Australia, which is strongly influenced by ENSO variations. The zonal average trends of total cloud cover are mostly negative, but weakly positive at 10-30N and strongly positive in the Arctic, 60-80N. The global average trend in total cloud cover is small. It is -0.7% per decade for land, but offset by a positive trend over the ocean. Interannual variations in Europe, particularly of nimbostratus, are well correlated with the North Atlantic Oscillation; significant correlations are also found across northern Asia. Interannual variations in many parts of the tropics are well correlated with an ENSO index. Little correlation was found with an index of smoke aerosol, in seven regions of seasonal biomass burning. In the middle latitudes of both hemispheres, interannual anomalies of cloud cover are positively correlated with surface temperature in winter and negatively correlated in summer, as is to be expected if the direction of causality is from clouds to temperature. The slope of the correlation averages about 5% cloud cover per degree K in winter, and -12% per K in summer.
DE: 0321 Cloud/radiation interaction
DE: 1616 Climate variability (1635, 3305, 3309, 4215, 4513)
DE: 1655 Water cycles (1836)
DE: 3310 Clouds and cloud feedbacks
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005