SCATTEROMETER OBSERVATIONS OF LOW-LEVEL WIND JETS OVER NEW-ZEALAND COASTAL WATERS

Wind data from the European Remote-Sensing Satellite (ERS-I) scatterom eter are used to depict wind fields in coastal waters around New Zeala nd. They include mesoscale detail not resolved by present operational models nor from general ships' data due to the low density of observat ions. These wind data reveal regions of local wind intensification (lo w-level jets) associated with the ends of orographic barriers. New Zea land has steep and high orography, which frequently induces orographic lee troughs. The jets are particularly evident streaming over coastal waters beyond the ends of these lee troughs. Using a year of scattero meter data, these jets are identified and characterized. Some necessar y conditions far their occurrence are established, namely, the general direction of the flow in relation to orographic features, the thresho ld stability of the airstream, and the degree of orographic blocking. The formation of these jets requires the Brunt-Vaisala frequency for t he lower atmosphere, up to a height commensurate with the height of th e orography, to be greater than 0.010-0.012 s(-1) depending on the loc ation and the Froude number to be lower than 0.7. The general flow con ditions determine the position and direction of the jet. Further, when blocking is strong, the Features are more intense. However, the autho rs are unable to predict the length and breadth from the synoptic situ ation. Given the limited sampling imposed by the coverage of the ERS-1 scatterometer, it has also not been possible to address conditions fo r sufficiency.