THE NATURE OF INTERMEDIATE-SCALE CLOUD SPIRALS.
NATIONAL ENVIRONMENTAL SATELLITE CENTER WASHINGTON D C
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The purpose of this investigation was to learn the nature of the so-called secondary cloud spiral and associated cloud enhancement that is viewed by satellites. Eight case sequences of two or three days each were compared with their mid-tropospheric vorticity fields and each of the various forcing function fields contributing to vorticity production at sea level. In the situations studied, it was found that the cloud spirals could be related to mid-tropospheric vorticity maxima associated with short-wave systems moving through the larger scale patterns. Thermal advection namely, the Laplacian of thickness advection in the 1000-mb. to 500-mb. layer was at least as significant as the mid-tropospheric 500-mb. vorticity advection in areas of cloud enhancement. It is important to note that the contribution of the thermal term may be either positive or negative, so that sometimes it serves to diminish and at other times to augment the effects of vorticity advection. Generally speaking, as the cloud system develops or becomes better organized, its effects become more positive. The other forcing functions examined, including that one involving the sensible heat exchange with the ocean surface, appeared to have had negligible or minor effects. Author
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