Closed cellular cloud patterns are characterized by approximately polygonal cloud-covered areas bounded by clear or less cloudy walls. Atmospheric conditions necessary for the formation of closed cells are weak convective mixing in the lower levels, with a cap to this mixing aloft. The convective mixing is the result of surface heating of the air or radiational cooling of the cloud tops. This type of convection is not as intense as that associated with open cells. The cap to the instability associated with closed cellular cloud patterns is in the form of a subsidence inversion in both polar and subtropical situations. Closed cellular cloud patterns are made up of stratocumulus elements in both the polar and subtropical air masses. In addition to the stratocumulus elements, trade-wind cumulus may also be present with the subtropical highs. When associated with the subtropical highs, closed cellular clouds are located in the eastern sections of the high-pressure area. Closed cells are associated with limited low-level instability below the subsidence inversion. Extensive

vertical convective activity is not likely. Figure 4-10 shows closed cells in the southeastern portion of a polar high near A.

Figure 4-11 shows closed cells in the eastern portion of a subtropical high in the South Pacific. West of A, the closed cells are composed of stratocumulus with some clear walls, and east of the walls, the cells are composed of thinner clouds.

The practical training publications, Satellite Imagery Interpretation in Synoptic and Mesoscale Meteorology, NAVEDTRA 40950, and Tropical Clouds and Cloud Systems Observed in Satellite Imagery, Volume 1, NAVEDTRA 40970, offer further information on the subject of satellite interpretation.

VERTICAL MOTION AND WEATHER

Upward vertical motion (convection) is associated with increasing cloudiness and precipitation, and downward vertical motion (subsidence) with improving weather.

Vertical motion analyses and prognostic charts are currently transmitted by the NWS and FNMOC. The values are computed and are on the charts. charts. Plus values represent upward vertical motion (convection), and minus values represent downward vertical motion (subsidence).

Vorticity was discussed in chapter 1 of this manual, as well as the AG2 TRAMAN, volume 1. We have seen that relative vorticity is due to the effects of both curvature and shear. Studies have led to the following rules:

Cloudiness and precipitation should prevail in regions where the relative vorticity decreases downstream.

Fair weather should prevail where relative vorticity increases downstream.

The fact that both shear and curvature must be considered when relative vorticity changes are investigated results in a large number of possible combinations on upper air charts. When both terms are in agreement, we can confidently predict precipitation or fair weather. When the two are in conflict, a closer examination is required.

Figure 4-10.-Closed cells in polar high.

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