Effect of Trade Wind Inversion on Precipitation in Hawaii

As a dominant control on local weather and climate in Hawaii , the trade wind inversion (TWI) is believed to have an important impact on Hawaiian ecosystem and hydrology, particularly through its effect on local rainfall. Several studies on two Hawaiian islands suggest that the TWI has influenced the island rainfall in Hawaii (e.g. Giambelluca 1991, Feng and Chen 2001). However, there are studies in contrast to above results, showing that the relationship between rainfall and inversion are not statistically significant at other times and locations (e.g. Larson 1978, Grindinger 1992). There is an urgent need to reexamine the relationship between rainfall and inversion from broad spatial and longer temporal scales for the further information. Based on our inversion climatology during 1979-2003 (Cao et al. in press), we compose the NCEP/NCAR daily and twice daily precipitation data. The following four inversion categories are used for composite analysis: no inversion, inversion, low (inversion base height in the lowest 25%) and high (inversion base height in the highest 25%) inversion. The means of the difference between above composite categories are calculated, and evaluated with t-test. The precipitation rate difference between non inversion and inversion is significant at 0.05 level, about 4*10^{-5} kg/m2/s, or 3.5 mm/day, with higher precipitation rate during days without inversion. For the times with inversion, high precipitation rate is associated with high inversion, and the difference between high and low inversion can reach 3.5 mm/day. The associated composite analysis on precipitable water, total cloud cover, and the vertical velocity show that both total cloud cover and precipitable water content are higher during times without inversion, or in the high inversion events; and strong upward motion is associated with times when inversion disappears. Therefore all the evidences support an association of higher precipitation with inversion absence, when there is no stable capping layer in the low troposphere to hinder the convection and vertical cloud development.