In this first paper of a three-part series on cloudiness we intercompare the simultaneous cloudiness data obtained from Meteor satellites, Nimbus 7, and the International Satellite Cloud Climatology Project (ISCCP) for the one-year period, July 1983 to June 1984. Four versions of ISCCP cloudiness are obtained from analyses of the ISCCP-C1 data. These versions differ in their requirements for temporal and spectral sampling. ISCCPs 1 and 2 require for each 2.5¿¿2.5¿ latitude-longitude cell that there be observations at least (Nd=)20 of the 28--31 possible days per month and at least (Nh=)5 of the 8 possible 3 hourly times each such day; ISCCPs 3 and 4 require only Nd=1 and Nh=1. ISCCPs 1 and 3 use IR information only, while ISCCPs 2 and 4 use both IR and visible (VIS) information. The ISCCP-C2 data produced by the ISCCP Global Processing Center is also included in this intercomparison. The ISCCP 1--4 intercomparison shows that (1) the cloudiness differences due to the above temporal sampling are smaller than those due to the above spectral sampling; (2) both spectral and temporal sampling effects are larger for the northern hemisphere than for the southern hemisphere; and (3) the difference between zonal mean cloudiness with and without visible information generally increases with latitude from polar night to about 60¿ latitude in the summer hemisphere.

The comparison between ISCCP-C2 and ISCCP 4 cloudiness shows that (1) ISCCP-C2 cloudiness is larger for both hemispheres and the globe; (2) ISCCP-C2 zonal mean cloudiness is larger from 55¿ latitude in the winter hemisphere to 65¿ latitude in the summer hemisphere; and (3) ISCCP-C2 cloudiness is larger than the ISCCP 4 cloudiness over most of the Earth, with largest differences generally located over water and the adjacent coastal regions. The comparison among Meteor, Nimbus 7, and ISCCP-C2 cloudiness shows many features, including (1) hemispheric mean and global mean cloudiness for Nimbus 7 are smaller than for Meteor and ISCCP-C2; (2) for Meteor, Nimbus 7, and ISCCP-C2, mean southern hemisphere cloudiness exceeds mean northern hemisphere cloudiness in July 1983 and in January 1984; (3) for Meteor, Nimbus 7, and ISCCP-C2, hemispheric mean cloudiness increases from winter to summer in both hemispheres, with a larger increase in the northern hemisphere; (4) for Meteor, Nimbus 7, and ISCCP-C2, zonal mean cloudiness maxima are located in the intertropical convergence zone (ITCZ) in the summer hemisphere, in the winter hemisphere storm-track region and same latitudinal belt in the summer hemisphere, and generally increase with decreasing latitude from about 60 ¿N to 60 ¿S; (5) over much of the Earth, particularly in the tropical-subtropical minima, Nimbus 7 shows the smallest zonal mean cloudiness, with values as much as 0.4 less than those for Meteor and ISCCP-C2; (6) the spatial cloudiness patterns and amplitudes of ISCCP-C2 and Meteor are most alike, with correlation and regression coefficients of 0.81--0.85 and 0.81--0.90, respectively;

(7) the temporal cloudiness patterns and amplitudes of ISCCP-C2 and Nimbus 7 are most alike, with average correlation and regression coefficients of 0.70 and 0.83 for the northern hemisphere and 0.64 and 0.74 for the southern hemisphere; (8) Meteor, Nimbus 7, and ISCCP-C2 cloudiness values show considerable disagreement in the polar regions, particularly in winter, with Nimbus 7 cloudiness exceeding both Meteor and ISCCP-C2 cloudiness in the Antarctic during winter, Meteor cloudiness being less than both ISCCP-C2 and Nimbus 7 cloudiness in the Arctic during winter, and ISCCP-C2 cloudiness being less than both Meteor and Nimbus 7 cloudiness in summer in both hemispheres; and (9) both Nimbus 7 and ISCCP-C2 show an increase in cloudiness from summer to winter over the Arctic and Antarctic, while Meteor shows a decrease. A special observational program in both the Arctic and the Antarctic is proposed to resolve the discrepancies among the satellite and ground-based cloudiness observations in polar latitudes. ¿ American Geophysical Union 1992