ISCCP Cloud Properties Associated with Standard Cloud Types Identified in Individual Surface Observations

Abstract

Individual surface weather observations from land stations and ships are compared with individual cloud retrievals of the International Satellite Cloud Climatology Project (ISCCP), stage C1, for an 8-yr period (1983?91) to relate cloud optical thicknesses and cloud-top pressures obtained from satellite data to the standard cloud types reported in visual observations from the surface. Each surface report is matched to the corresponding ISCCP-C1 report for the time of observation for the 280 km ? 280 km grid box containing that observation. Classes of the surface reports are identified in which a particular cloud type was reported present, either alone or in combination with other clouds. For each class, cloud amounts from both surface and C1 data, base heights from surface data, and the frequency distributions of cloud-top pressure (pc) and optical thickness (τ) from C1 data are averaged over 15° latitude zones, for land and ocean separately, for 3-month seasons. The frequency distribution of pc and τ is plotted for each of the surface-defined cloud types occurring both alone and with other clouds. The average cloud-top pressures within a grid box do not always correspond well with values expected for a reported cloud type, particularly for the higher clouds Ci, Ac, and Cb. In many cases this is because the satellites also detect clouds within the grid box that are outside the field of view of the surface observer. The highest average cloud tops are found for the most extensive cloud type, Ns, averaging 7 km globally and reaching 9 km in the ITCZ. Nimbostratus also has the greatest average retrieved optical thickness, τ ≈ 20. Cumulonimbus clouds may actually attain far greater heights and depths, but they do not fill the grid box. The τ?pc distributions show features that distinguish the high, middle, and low clouds reported by the surface observers. However, the distribution patterns for the individual low cloud types (Cu, Sc, St) occurring alone overlap to such an extent that it is not possible to distinguish these cloud types from each other on the basis of τ?pc values alone. Other cloud types whose τ?pc distributions are indistinguishable are Cb, Ns, and thick As. However, the τ?pc distribution patterns for the different low cloud types are nevertheless distinguishable when all occurrences of a low cloud type are included, indicating that the different low types differ in their probabilities of co-occurrence with middle and high clouds.