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ABSTRACT
A worldwide climatology of the diurnal cycles of low clouds is obtained from surface observations made eight or four times daily at 3- or 6-h intervals from weather stations and ships. Harmonic fits to the daily cycle are made for 5388 weather stations with long periods of record, and for gridded data on a 5° × 5° or 10° × 10° latitude-longitude grid over land and ocean areas separately.
For all cloud types, the diurnal cycle has larger amplitude over land than over ocean, on average by a factor of 2. Diurnal cycles of cloud amount appear to be proprietary to each low cloud type, showing the same cycle regardless of whether that type dominates the diurnal cycle of cloud cover. Stratiform cloud amounts tend to peak near sunrise, while cumuliform amounts peak in the afternoon; however, cumulonimbus amounts peak in the early morning over the ocean. Small latitudinal and seasonal variation is apparent in the phase and amplitude of the diurnal cycles of each type. Land areas show more seasonality compared to ocean areas with respect to which type dominates the diurnal cycle.
Multidecadal trends in low cloud cover are small and agree between day and night regardless of the local climate.
Diurnal cycles of base height are much larger over land than over the ocean. For most cloud types, the bases are highest in the midafternoon or early evening.
1. Introduction
Clouds have two competing effects on surface temperature. They emit longwave (infrared) radiation throughout the day and night, causing a warming at the surface relative to a cloudless sky. When the sun is high during the day their effect on albedo exceeds their reduction of longwave emission to space, causing cooling. Globally averaged, the negative shortwave cloud radiative effect (CRE; 47Wm22) exceeds the positive longwave CRE (26Wm22), so that clouds have a net cooling effect of 21Wm22 (Loeb et al. 2009). The radiative effect of clouds is strongly negative during the daytime and strongly positive at night, so to assess the effects of cloud cover changes on climate it is important to determine the diurnal cycles of the clouds and to determine whether regional cloud amount changes are occurring during daytime or at night, or both. Even in a steady...