A Sample Computation of Kinematic Properties from Cloud Motion Vectors

Abstract

Distributions of relative vorticity and balanced height are computed from the cloud velocities associated with the cloud structure of an extratropical cyclone over the continental United States during a three-day period in March 1970. Using the SRI/NASA Electronic Display System, cloud motions are obtained from the abundance of cloud elements shown in series of ATS III photographs. Because of the absence of data on the specific height of cloud elements measured, cloud motions are assigned to a ?mid-level? (when subjectively classified as middle clouds), or to a ?high-level? (when subjectively classified as cirrus clouds). Derived vorticity and balanced height are compared with concurrent NMC analyses and also with similar kinematic quantities obtained from rawins at three constant-pressure levels. The computations of relative vorticity using ?mid-level? cloud motion vectors show encouraging results. Patterns of computed cyclonic vorticity are related to the development, location and movement of the surface cyclone. Computed absolute values of relative vorticity show moderate correlation with 500-mb NMC values but show good positive correlation with vorticity computed from concurrent rawinsonde data for the 700-mb level. The analyses suggest that the ?mid-level? corresponds best to the 700-mb level. The vorticity analysis from the ?high-level? motion vectors presented difficulties resulting from the wide latitudinal range in the height of cirrus. It is concluded that, with the addition of concurrent radiometric data, kinematic computations from cloud motions will improve beyond the results found in this investigation.