Storm-Induced Wind Patterns on the Sea from Spaceborne Synthetic Aperture Radar

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The authors discuss the origin of a unique footprint on the sea induced 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 that the cool air pool from the storm acts as an obstacle to divert the low-level easterly ambient winds and leaves a “wind shadow” on its downwind side. This theory is discarded because of the excessive storm lifetime needed to cause the long downstream “shadow.” The second hypothesis invokes the cool outflows from two preexisting storm cells such that their boundaries intersect obliquely leaving a triangular wedge of weaker winds and radar cross section (i.e., the shadow). A new precipitation cell is initiated at the point of intersection of the boundaries at the apex of the shadow, giving the illusion that this cell is the cause of the shadow. While the authors lack corroborative observations, this theory is consistent with prior evidence of the triggering of convective clouds and precipitation by intersecting cool air boundaries. The regular observation of such persistent cool air storm outflow 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.

*Distinguished Visiting Scientist, Laboratory for Atmospheres, NASA Goddard Space Flight Center, Greenbelt, Maryland.

+Communications Research Laboratory, Ministry of Posts and Telecommunications, Tokyo, Japan.

#Science Systems and Applications Inc., Lanham, Maryland.

Corresponding author address: David Atlas, Code 910.2, Goddard Space Flight Center, Greenbelt, MD 20771.

The authors discuss the origin of a unique footprint on the sea induced 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 that the cool air pool from the storm acts as an obstacle to divert the low-level easterly ambient winds and leaves a “wind shadow” on its downwind side. This theory is discarded because of the excessive storm lifetime needed to cause the long downstream “shadow.” The second hypothesis invokes the cool outflows from two preexisting storm cells such that their boundaries intersect obliquely leaving a triangular wedge of weaker winds and radar cross section (i.e., the shadow). A new precipitation cell is initiated at the point of intersection of the boundaries at the apex of the shadow, giving the illusion that this cell is the cause of the shadow. While the authors lack corroborative observations, this theory is consistent with prior evidence of the triggering of convective clouds and precipitation by intersecting cool air boundaries. The regular observation of such persistent cool air storm outflow 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.

*Distinguished Visiting Scientist, Laboratory for Atmospheres, NASA Goddard Space Flight Center, Greenbelt, Maryland.

+Communications Research Laboratory, Ministry of Posts and Telecommunications, Tokyo, Japan.

#Science Systems and Applications Inc., Lanham, Maryland.

Corresponding author address: David Atlas, Code 910.2, Goddard Space Flight Center, Greenbelt, MD 20771.
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