Scatterometer Observations of Low-Level Wind Jets over New Zealand Coastal Waters

Abstract

Wind data from the European Remote-Sensing Satellite (ERS-1) scatterometer are used to depict wind fields in coastal waters around New Zealand. They include mesoscale detail not resolved by present operational models nor from general ships' data due to the low density of observations. These wind data reveal regions of local wind intensification (low-level jet) associated with the ends of orographic barriers. New Zealand 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 scatterometer data, these jets are identified and characterized. Some necessary conditions for their occurrence are established, namely, the general direction of the flow in relation to orographic features, the threshold stability of the airstream, and the degree of orographic blocking. The formation of these jets requires the Brunt-Väisälä frequency for the lower atmosphere, up to a height commensurate with the height of the orography, to be greater than 0.010–0.012 s⁻¹ depending on the location and the Froude number to be lower than 0.7. The general flow conditions determine the position and direction of the jet. Further. when blocking is strong, the features are more intense. However, the authors are unable to predict the length and breadth from the synoptic situation. Given the limited sampling imposed by the coverage of the ERS-1 scatterometer, it has also not been possible to address conditions for sufficiency.