The effects of islands on trade wind flow can be determined by observing trade wind cloud bands. In this study, changes in size, speed, visible brightness, and infrared cloud top temperature of bands approaching the Northern Hawaiian Islands of O’ahu and Kauai will be quantified. Trade wind bands are an important source of precipitation for this populous region. They can also produce flash floods. This investigation of trade wind bands will provide a better understanding of trade wind flow in the vicinity of islands and specific information concerning bands as they approach the Northern Hawaiian Islands.
A trade wind regime was defined by the presence of an established high pressure center to the northeast of the Hawaiian Islands, causing winds in the islands to exceed 5.1 ms^-1 (10 knots) from the north to east directions. Trade wind cloud bands were identified during the daytime hours of each "trade wind day." These bands consisted of a group of clouds organized in a line, moving in the direction of the trade winds.
GOES-9 visible imagery of the Hawaiian Islands (18.49N to 22.52N latitude, -154.13W to -159.87W longitude) was acquired at 1 kilometer resolution in 15 minute intervals on days exhibiting a trade wind regime from May-September 1997. Both the average and the maximum length, width, and brightness were recorded from the visible imagery, as were the positions of bands and their direction of movement. A common band was roughly 10.7 kilometers long by 4.6 kilometers wide. Preliminary results reveal that an average band was tracked for an hour and forty five minutes over a distance of 39 kilometers from first to final viewing (corresponding to an average velocity of 6 ms^-1). GOES-9 infrared imagery was also acquired for these days at
4 kilometer resolution in 15 minute intervals. The infrared imagery was used to determine the average and minimum cloud top temperature of the bands. Images with obstructive cirrus were excluded from the data set.
The height of the inversion and the lifting condensation level of each "trade wind day" were determined from sounding data collected at Lihue, Kauai at 0000Z and 12000Z daily. In addition, rainfall totals were collected in 15 minute intervals using a telemetered rain gage network that includes 20 gages in windward regions of the islands. The brightness and cloud top temperature of the 4 square kilometer area above each rain gage were recorded.
The collected data establish the behavior of trade wind bands as they approach the Northern Hawaiian Islands. Conditions favorable for changes in size, speed, visible brightness, and infrared cloud top temperature upon approach to the islands are being investigated. In addition, the rain gage data are being used to determine any relationships between satellite derived cloud data over the gages and rainfall rates. These relationships may facilitate the establishment of rainfall estimates for trade wind bands over the open ocean.