HR: 16:50h
AN: A54D-04 INVITED     [Abstracts]
TI: Implications of Long-Term Variations of Cloudiness
AU: * Rossow, W B
EM: wrossow@giss.nasa.gov
AF: NASA Goddard Institute for Space Studies, 2880 Broadway, New York, NY 10025 United States
AB: Cloud and climate changes are often discussed in terms of global, as well as monthly, averages of cloud properties, usually determined separately. Yet global mean changes in the radiation budget, for example, are not proportional to global mean changes in clouds. So a proper determination of the global mean effects of clouds on the radiation budget requires integrating over the variations of the non-linear relations of cloud properties and radiative fluxes. To examine this integral effect, we can now use several global, long-term cloud data records with sufficient detail to characterize the regional-to-global variations of clouds on time scales from weather-scale up to about decadal scale. Of the satellite-based cloud datasets, the most detailed and complete is from the International Satellite Cloud Climatology Project (ISCCP), which now covers a period of 21.5 years. This dataset reports three cloud properties (fractional cover, optical thickness and top pressure/temperature) at 30 km, 3 hr intervals. There is also a somewhat longer record of cloud cover and morphological type based on surface weather observations; although it is not globally complete, it is the only other cloud dataset providing information about diurnal cloud variations. The ISCCP time record of the monthy (deseasonalized) anomalies of global mean total cloud cover fraction shows an initial increase of about 0.02 from mid-1983 until about 1987 followed by a steady decrease of about 0.05 from 1987 until about 2000. Recently completed data extends this record showing an increase of about 0.02 from 2004 until the end of 2004. During this same period there is notable variability but little systematic change in either global mean cloud top temperature or cloud optical thickness, except for a small increase of the latter since about 2000. Corresponding changes in top-of-atmosphere radiative fluxes, calculated based on the ISCCP cloud properties, show a small overall change from the 1980s to the 1990s, in excellent quantitative agreement with direct measurements by the Earth Radiation Budget Experiment (ERBE), Clouds and Earth Radiant Energy Study (CERES) and the Baseline Surface Radiation Network (BSRN). However, the global mean flux changes exhibit a different pattern of variation than the global mean cloud properties. For example, the decrease in reflected shortwave flux begins later at about 1990 and ends earlier at about 1998 than the decrease of cloud cover. Much of the variation of total cloud cover is associated with changes of the optically thinner, low-level clouds (called cumulus and stratocumulus in the ISCCP dataset) with a slight increase in high-level cirrus hinted at. In this presentation, we compare the variability exhibited in the several cloud datasets to look for evidence for and against the hypothesis that there have been "significant" variations of clouds in the past couple of decades. Given the differences in detection sensitivity for different cloud types among these data, we examine whether the variations of the different cloud types agree or not. The differences of the variations by cloud type, along with accompanying changes in radiation budget and precipitation, suggest some implications for climate change that will be highlighted.
UR: http://isccp.giss.nasa.gov
DE: 0321 Cloud/radiation interaction
DE: 3305 Climate change and variability (1616, 1635, 3309, 4215, 4513)
DE: 3309 Climatology (1616, 1620, 3305, 4215, 8408)
DE: 3310 Clouds and cloud feedbacks
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005