Doppler radar investigation of Hawaiian rain¹

1 Present affiliation: Department of Meteorology, McGill University, Montreal, Canada.

Abstract

Doppler radar measurements in a number of Hawaiian rain echoes indicated a variable amount of convection, with vertical air velocities ranging from unmeasurably small values in some cases to values of 7 m/sec in the more convective cases. The echoes exhibiting weak vertical motions usually had a stratiform appearance when plotted on coordinates of height versus time: contours of signal intensity and Doppler velocity tended to be approximately horizontal. The intensity decreased with altitude in these cases, and vertical gradients as strong as 20 db/km were not uncommon.

In the echoes exhibiting strong vertical air motions, signal intensities, echo depth, and surface rainfall rate tended to be higher. Radar reflectivity factors as high as 105 mm6/m3 were observed. Although height-time patterns revealed less order than in the echoes with weak updraft velocities, there was a tendency for the leading (upslope) portion of a convective echo to be more active than the trailing portion. As echoes progressed further upslope they tended to become more stratiform in character.

The fact that moderate rain could at times fall from shallow, stratiform clouds, taken in conjunction with the strong vertical reflectivity gradients observed in these cases, indicates a high efficiency of the precipitation process in Hawaiian orographic clouds. The observations in convective echoes clearly demonstrate that convective motions enhance the process.