Diurnal cycle of clouds and how they affect polar orbiting satellite data

Many of data on global cloud cover and climate records of cloudiness come from polar orbiting satellites. The ISCCP, for example, has used both polar and geostationary orbiting satellites. But the geostationary time series records have shown problems related to the increase in satellites over time and the changes in the regions they covered. The polar orbiting NOAA satellites have produced more consistent records. But most polar orbiting meteorological satellites are in sun synchronous orbits which sample in fixed locations with respect to diurnal cycles. Polar sun synchronous orbits also are the preference of experimental satellite, specifically Tera and Aqua from the EOS program and the future A-Train series. The sensors on these experimental satellites will be a large source of information for global cloud cover and precipitation for present and near future studies. While diurnal cycles present a problem, they are mostly in geographical areas and seasons where the radiative forcing mechanisms appear to be large and are minimal in areas and seasons where radiative forcing is small and synoptic dynamics are large. For example, large diurnal cycles in cloud cover are typically found in mountain ranges and tropical coasts in summer where cumulus convection develops from solar heating. Over oceans diurnal cycles have a different phase than land from infrared cooling with cloudiness maxima in the morning rather than the later afternoon. This occurs in the marine stratus clouds on the eastern sides of the Atlantic and Pacific oceans. Deep convective clouds in the western Pacific also exhibit a similar diurnal cycle for the same reason. On the other hand, the diurnal cycles in cloudiness tend to be small in the winter season and in higher latitudes where solar forcing is small. What is known about diurnal cycles in cloudiness from satellite observations will be discussed.