WRRCTR No.90 An Analysis of Four Showers with Rainfall Rates > 250 mm/hr

A computer monitored network of rapid-response Raymond-Wilson rainfall intensity gauges is operating at the Cloud Physics Observatory (CPO), on the windward coast of the island of Hawaii, recording the passage of both warm and cold cloud rain showers. During a one-year (1 July 1973 to 30 June 1974) monitoring period, 57 rainfall events were recorded with intensities greater than or equal to 95mm/hr. Four of these events exhibited rainfall rates greater than 250 mm/hr. Each of these events is analyzed as a case study documenting the network rainfall intensity records, the associated local variations in potential gradient, surface wind velocity and atmospheric pressure, rawinsonde soundings of temperature, dew point and winds, satellite and 250-mb wind data, and an isohyetal presentation of the accumulated rainfall over the island. Of the four cases studied, three were clearly of cold cloud origin, two being synoptic-scale events associated with low pressure centers west of Hawaii and the third a highly localized thunderstorm related to an upper level trough. The most interesting case was a 268 mm/hr rainfall intensity event almost certainly of warm cloud origin. This case (22 June 1974) may represent a record high rainfall rate from clouds known to lie entirely beneath the freezing level. Finally, a comparison is made of the winds aloft with a calculated disturbance velocity indicating, in most cases, that the surface wind gust occurring just before the onset of rainfall is approximately equal to the wind vector at cloud level. Two anomalous cases are discussed briefly.U.S. Department of the Interior, Water Resources Research Act Grant/Contract No. 14-31-0001-5011 Project No. B-032-H