Clouds as Seen by Satellite Sounders (3I) and Imagers (ISCCP). Part III: Spatial Heterogeneity and Radiative Effects

Abstract

Their relatively good spectral resolution makes infrared sounders very useful for the determination of cloud properties (day and night), and their coarse spatial resolution has less effect on clouds with large spatial extents like cirrus clouds. The Improved Initialization Inversion (3I) algorithms convert TIROS-N Operational Vertical Sounder observations from the NOAA Polar Orbiting Environmental Satellites into atmospheric temperature and humidity profiles and into cloud and surface properties. On the other hand, the relatively high spatial resolution of the imagers from the geostationary and polar orbiting satellites used in the International Satellite Cloud Climatology Project (ISCCP) is important for the determination of properties of clouds with smaller spatial extents like boundary layer clouds. By combining these quite different datasets some insight into the behavior of retrieved cloud properties with spatial heterogeneity is gained. The effective cloud amount as determined by 3I and ISCCP agrees very well for homogeneous cloud types at all heights, but heterogeneous cloud scenes lead to a smaller 3I effective cloud amount than the one retrieved by ISCCP. In the case of thin cirrus overlying low clouds, 3I will determine the effective cloud amount of the cirrus, whereas ISCCP?s information from the visible channel includes the lower cloud. The correlation between infrared cloud emissivity (3I) and visible cloud optical thickness (ISCCP) agrees quite well for high clouds with the expected exponential behavior. Mesoscale heterogeneities in midlevel and low-level cloud fields, however, cause a flatter behavior between 3I retrieved cloud emissivity and ISCCP retrieved cloud optical thickness, which can be simulated in a GCM with a subgrid-scale cloud overlapping scheme. Cloud radiative effects are studied in combination with Earth Radiation Budget Experiment fluxes. The warming effect of clouds depends on cloud height and effective cloud amount, but also on atmospheric conditions like near-surface temperature and humidity. The variability ranges from nearly no effect for partially covered low clouds up to 150 W m?2 for overcast high opaque clouds in the Tropics. The reprocessing of ISCCP produced a better distinction between the radiative effects of high opaque and cirrus clouds, in better agreement with the 3I results. Still, 3I high opaque clouds produce about 10 W m?2 larger warming. The cooling effect of clouds during the daytime depends very much on solar inclination as well as cloud optical thickness and cover.