ESTIMATION OF SMALL-SCALE WIND FIELDS FROM SYNTHETIC APERTURE RADAR DATA David R. Lyzenga General Dynamics Advanced Information Systems Ann Arbor, MI 48113-4008 USA Email: David.Lyzenga@gd-ais.com ABSTRACT This paper explores the possibility of estimating small- scale vector wind fields from SAR data by applying certain dynamical constraints to the wind field. The method is illustrated for the case of a diverging surface flow field that is intended to represent a precipitation- induced downdraft. A simulated radar cross section map generated from this flow field is used to reconstruct the wind field by choosing the wind direction at each point so as to minimize the vorticity. The method works well for this case because the input wind field was chosen to be irrotational. For cases in which the vorticity is not zero, it may be possible to generalize the method by applying other dynamical constraints. 1. INTRODUCTION The surface wind vector over the ocean can be estimated from radar cross section measurements taken from two or more look directions. If the radar cross section is measured from a single look direction, as is typical for synthetic aperture radar images, there is an inherent ambiguity in the estimated winds because of the fact that the radar cross section is determined by both the wind speed and direction. Previous investigators have solved this problem by using independent estimates of the wind direction from models [1] or from spatial patterns in the imagery [2-4]. However, these methods are not applicable for small- scale wind fields such as those due to convection cells or downdrafts. Some examples of such features are shown in Fig. 1. This figure shows a Radarsat-1 image collected over the Gulf of Alaska on May 10, 1998, with a wind front extending diagonally across the image and a number of circular features to the left of the front. The front was moving eastward at 30 km/hr, as inferred from a sequence of SSM/I images collected on May 9-10, 1998. Although the wind front itself was clearly visible on the SSM/I imagery, the smaller features behind it are not resolved by this sensor. The passage of the front is also evident in the wind measurements made by NOAA Buoy 46001 which is located about 250 km west of the image. The circular features appear to be precipitation- induced downdraft cells similar to those reported in [5]. This interpretation is supported by the NOAA buoy measurements, which show variations in the wind speed and direction behind the front as would be expected due to the passage of downdraft cells over the buoy. The mean wind speed indicated by the buoy in this region was 7-8 m/s from 230 o T. The winds within a downdraft cell as envisioned by Atlas [5] are shown in Fig. 2. Fig. 1. Gulf of Alaska Radarsat image, ext ending from approximately 55 o to 60 o N and 136 o to 144 o W. Fig. 2. Diagram depicting winds in a precipitation- induced downdraft (from [5]). N look direction © RADARSAT International, 1998