The ISCCP-C2 cloud climatology is used to describe the three-dimensional structure of cloud diurnal variations and to investigate their relationship to local climatological conditions. The latter follows from the regression of diurnal components onto climatological state variables.

Four important diurnal cloud categories are identified. The diurnal variation of maritime high-cloud fraction Chi maximizes at 1700 local solar time (LST) and is strongest over maritime convective locations where the mean high-cloud fraction is Chi > 0.1. The diurnal variation of maritime low-cloud fraction maximizes at 0400 LST and is strong over maritime nonconvective locations where Chi < 0.1. Diurnal variations of high-cloud fraction (persistent during the night, minimum at 1100 LST) and low-cloud fraction (1300 LST maximum) are strong over all continental locations in the latitude band 40°S–40°N.

In each cloud category, most of the diurnal amplitude and phase at individual locations is explained by the regression of diurnal amplitude onto only three climatological state variables. For most categories, the diurnal amplitude has its strongest relationship with mean cloud fraction. The relationship between relative diurnal amplitude (amplitude divided by the mean) and other climatological properties is then particularly meaningful. The relative amplitude of maritime high-cloud fraction is related to the mean total-cloud fraction and the noon-time solar zenith angle, which measures the solar diurnal amplitude. The diurnal amplitude of maritime low-cloud fraction does not have its strongest relationship with the mean low-cloud fraction, but has strong relationships to the upper-level cloud fraction, cloud-top height, and the solar diurnal amplitude. The relative amplitude of continental high-cloud fraction is related most strongly to the time-mean surface temperature, the diurnal amplitude of surface temperature, and the solar diurnal amplitude. The relative amplitude of continental low-cloud fraction has strong relationships with atmospheric moisture content and the diurnal amplitude of surface temperature.

In contrast to amplitude, diurnal phase does not exhibit a strong relationship with any climatological variable. Instead, it is uniform within individual categories, which makes cloud diurnal variations independent of geographical location and, therefore, highly spatially coherent. The spatial coherence of cloud diurnal variations makes them an important ingredient of climate, one that affords some predictability in terms of local climatological conditions.

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