Satellite-Tracked Cumulus Velocities

Velocities of tracer clouds have been computed by NOAA, NASA, Stanford Research Institute, University of Wisconsin, University of Chicago, and others. Despite the fact that their methods and inherent computation speeds are different from each other, the present state-of-the-art permits the computations with 1 m s−1 speed and 4° direction in standard deviation. Such an accuracy in the cloud velocity is satisfactory for most practical purposes. The research presented in this paper warns that we have to exercise extreme caution in converting cloud velocities into winds. The motion of fair-weather cumuli obtained by tracking their shadows over Springfield, Mo., revealed that the standard deviation in the individual cloud motion is several times the tracking error. The motion of over-ocean cumuli near Barbados clearly indicated the complicated nature of cumulus velocities. Analysis of whole-sky images obtained near Tampa, Fla., failed to show significant continuity and stability of cumulus plumes, less than 0.3 mi in diameter. Cumulus turrets 0.3 to 2 mi in size appear to be the best target to infer the mean wind within the sub cloud layers. Cumulus or stratocumulus cells consisting of multi-turrets do not always move with wind. The addition and deletion of turrets belonging to a specific cell appear to be the cause of the erratic motion of a tracer cell. It may be concluded that the accuracy of wind estimates is unlikely to be better than 2 m s−1 unless the physical and dynamical characteristics of cumulus motion is further investigated.