Statistical analysis of satellite-observed trade wind cloud clusters in the western North Pacific

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Abstract
Composite upper-air soundings have been constructed relative to 1 257 individual satellite-observed mesoscale (3–6° latitude) trade wind cloud clusters in the western tropical North Pacific. Clusters have been stratified into five categories: pre-storm clusters (166 cases treated), developing clusters (211), conservative clusters (537), non-conservative clusters (135), and dying clusters (208). A sixth category has been specified for clear areas (553). Rawinsonde observations from 14 island stations for the two-year period of October 1966 to October 1968 provide composited data for 16 pressure levels from the surface to 30 mb. Approximately 12 000 observations make up the data sample. Computerized composited summaries for each group of clusters have been made from Northern Hemisphere Data Tabulations (NHDT) tapes for wind, vorticity, divergence, kinetic energy, temperature, moisture, stability, and isobaric heights. Mass and thermal balances are very well satisfied by the composited data. Significant differences in the low-level horizontal wind shears exist among the six categories. The pre-storm and developing clusters exhibit large low-level cyclonic shears, whereas the remaining cloud categories show weaker cyclonic shears. The clear areas show a marked anticyclonic low-level wind shear. The relative vorticities are mostly determined by shears of the zonal wind. East-west shears of the meridional wind are of secondary importance. These cloud clusters may be viewed as typical of the usual easterly trade wave only if the latter is primarily interpreted as a shearing phenomena of the zonal trade wind. These observations generally support the contention of the importance of low-level frictionally forced convergence in the zonal shearing environment north of the equatorial trough as the mechanism for producing and maintaining these clusters (some of which later develop into typhoons). DOI: 10.1111/j.2153-3490.1973.tb00617.x