The Influence of Vertical Air Velocity on the Remote Microwave Measurement of Rain

Abstract

Atlas and Ulbrich showed a close theoretical relation between gage-measured rain rate and 1 cm microwave absorption; and other remote techniques for potentially accurate rain estimation have been developed. More recently, Ulbrich cast doubt on the absorption relation, suggesting an important influence by vertical air velocity. This paper uses a mass-continuity argument to show that over flat terrain vertical air velocity has no influence on the relation between gage-measured rain rate and rain rate remotely sensed aloft, although it introduces a discrepancy between the area of the rain sensed aloft and the area of surface rainfall. Thus point rainfall may be correctly estimated, but areal rainfall will be erroneous where rain falls systematically in significant convective updrafts or downdrafts. This conclusion affects all remote techniques for rain estimation, whether ground or satellite based, although techniques incorporating continuous raingage calibration may be excepted. Evidence of agreement with gage measurements cannot be taken as evidence that any technique will estimate rainfall correctly, unless (averaged) vertical air velocity effects can be accounted for.