Chaff studies during the Hawaiian Rainband Project

The 1990 Hawaiian Rainband Project (HaRP) was designed to investigate the structure and dynamics of the rainbands that form on the upwind side of the island of Hawaii. A major part of the investigation is to understand the interaction between the island barrier and the trade winds. As part of this effort, we conducted a number of experiments in which we released radar reflective chaff from a locally rented light plane into the clear air upwind of the island, and observed the subsequent evolution of the chaff cloud using project's two C-band Doppler. The close proximity of the two radars limited the main dual-Doppler analysis lobe, where good wind measurements can be recovered from the Doppler data, to a relatively small area. This area, essentially circular, is roughly 30 km in diameter. In this area, dual-Doppler analysis of the chaff experiments could be expected to return useful wind vectors. The effect of the island on the trade winds, however, extends much offshore. The offshore winds, however, can be monitored by analyzing the advection of the chaff cloud anywhere within the scan volume. At the suggestion of David Atlas, we developed a specific flight plan for studying the larger scale flow structure, in which we flew in a straight line, alternately turning the chaff cutter on and off. Easch on or off period was maintained for between one and two minutes, resulting in an easily identifiable set of discrete chaff elements. The resultant echo patterns can be readily analyzed to estimate the larger scale divergence of the trade winds as they approach the island (Lee and Johnson, 1991). In this earlier study, Lee and Johnson relied on manual analysis of the chaff positions to estimate the upwind air motions. In the current extension of the that work, we are employing an automated pattern tracking and analysis program. TREC. TREC is particularly well suited for this application, since the chaff itself is a passive tracer and is not subject to rapide developments and changes in storm structure that can be a problem for natural radar echoes from convective storms.