STORM-INDUCED WIND PATTERNS ON THE SEA FROM SPACEBORNE SYNTHETIC-APERTURE RADAR

The authors discuss the origin of a unique footprint on the sea induce d by storm winds and rainfall as seen by synthetic aperture radar (SAR ) from space. Two hypotheses are presented to explain the origin of an apparent wind shadow downwind of a storm cell. The first suggests tha t the cool air pool from the storm acts as an obstacle to divert the l ow-level easterly ambient winds and leaves a ''wind shadow'' on its do wnwind side. This theory is discarded because of the excessive storm l ifetime needed to cause the long downstream ''shadow.'' The second hyp othesis invokes the cool outflows from two preexisting storm cells suc h that their boundaries intersect obliquely leaving a triangular wedge of weaker winds and radar cross section (i.e., the shadow). A new pre cipitation cell is initiated at the point of intersection of the bound aries at the apex of the shadow, giving the illusion that this cell is the cause of the shadow. While the authors lack corroborative observa tions, this theory is consistent with prior evidence of the triggering of convective clouds and precipitation by intersecting cool air bound aries. The regular observation of such persistent cool air storm outfl ow boundaries both in satellite observations, and more recently in SAR imagery, suggests that such discontinuities are ubiquitous and serve to trigger new convection in the absence of large-scale forcing.