|
B. Geerts and E. Linacre |
3/’02 |
Low-level clouds are most affected by the diurnal cycle of surface heating and cooling. The diurnal cycle of cloudiness over the oceans is very weak, and this cycle relates more to the radiation balance near the cloud top than to the variation of sea surface temperature, which normally is less than 2K. For instance, at night a layer of stratus cloud cools at its top, due to longwave radiation loss, and this induces static instability (negative buoyancy), causing the stratus to break up into stratocumulus.
Over land cumuliform clouds are most common in the early afternoon, when the planetary boundary layer is best mixed, and convective thermals reach highest. In most land areas cumulonimbus clouds are most common in the afternoon, however in some areas thunderstorms are more common in the evening or even around midnight, depending on the regional topography. Shallow cumulus clouds often dissipate to become stratocumulus, therefore Sc is more common in the evening. Fog and shallow stratus clouds are most common around dawn.
Deep, precipitating clouds appear to have become more frequent over land during the 20th century (1). The observed anomalies of precipitation, cloudiness, and diurnal temperature range support the notion that recent global warming has enhanced the global hydrological cycle and resulted in deeper, precipitating clouds, which are likely responsible for the decreasing trend in diurnal temperature range over land (the nighttime minima have increased more than the daytime maxima, see Section 15.4).
(1) Dai, A. 1996. Global Precipitation Variability and Its Relationship with Other Climate Changes. Ph.D. thesis. Columbia University. New York, New York.