DIURNAL-VARIATIONS OF CLOUD COVER AND THEIR RELATIONSHIP TO CLIMATOLOGICAL CONDITIONS

The ISCCP-C2 cloud climatology is used to describe the three-dimension al structure of cloud diurnal variations and to investigate their rela tionship to local climatological conditions. The latter follows from t he regression of diurnal components onto climatological state variable s. Four important diurnal cloud categories are identified. The diurnal variation of maritime high-cloud fraction C-hi' maximizes at 1700 loc al solar time (LST) and is strongest over maritime convective location s where the mean high-cloud fraction is (C) over bar (hi)) 0.1. The di urnal variation of maritime low-cloud fraction maximizes at 0400 LST a nd is strong over maritime nonconvective locations where <(C)over bar (hi)> < 0.1. Diurnal variations of high-cloud fraction (persistent dur ing the night, minimum at 1100 LST) and low-cloud fraction (1300 LST m aximum) are strong over all continental locations in the latitude band 40 degrees S-40 degrees N. In each cloud category, most of the diurna l amplitude and phase at individual locations is explained by the regr ession of diurnal amplitude onto only three climatological state varia bles. For most categories, the diurnal amplitude has its strongest rel ationship with mean cloud fraction. The relationship between relative diurnal amplitude (amplitude divided by the mean) and other climatolog ical properties is then particularly meaningful. The relative amplitud e of maritime high-cloud fraction is related to the mean total-cloud f raction and the noontime solar zenith angle, which measures the solar diurnal amplitude. The diurnal amplitude of maritime low-cloud fractio n does not have its strongest relationship with the mean low-cloud fra ction, but has strong relationships to the upper-level cloud fraction, cloud-top height, and the solar diurnal amplitude. The relative ampli tude of continental high-cloud fraction is related most strongly to th e time-mean surface temperature, the diurnal amplitude of surface temp erature, and the solar diurnal amplitude. The relative amplitude of co ntinental 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 relati onship with any climatological variable. Instead, it is uniform within individual categories, which makes cloud diurnal variations independe nt of geographical location and, therefore, highly spatially coherent. The spatial coherence of cloud diurnal variations makes them an impor tant ingredient of climate, one that affords some predictability in te rms of local climatological conditions.