Characteristics of Low Level Jets over Okinawa in the Baiu and post-Baiu Seasons Revealed by Wind Profiler Observations

Abstract

Hourly data from a 400-MHz Wind Profiler Radar (WPR) during the warm season months of three years are used to develop a climatology of the low level jet (LLJ) over Okinawa, a Japan subtropical island. Long-term observations of the wind with high temporal and vertical resolution allowed an investigation of the LLJ statistics. Characteristics of the LLJ are investigated in two subperiods: during the Baiu season over Okinawa Island and the post-Baiu season when the Baiu front has moved further north of Okinawa. The investigation result shows that stronger LLJs occur more frequently during the Baiu season, when heavy precipitation is mostly observed compared to the post-Baiu season. The appearance frequency of the maximum speed height of LLJs has a double peak structure in the vertical; lower and higher peaks appear around heights of 600 m and 1.3 km, respectively. In the rain cases with daily rainfall equal and more than 1 mm, another peak also appears around a height of 3 km.
The frequency of LLJ occurrence exhibits a diurnal variation with the maximum in the nighttime and early morning, especially in the non-rain cases. The examination with upper-air sounding observations suggests that the nocturnal preference of LLJ occurrence is brought from the reduction of vertical mixing in the atmospheric boundary layer after sunset. Additional investigations of LLJs, using data from the WPR network of the Japan Meteorological Agency for three other stations surrounding Okinawa Island, provide more evidence for the role of the atmospheric boundary layer in generating the diurnal variation of LLJ occurrence.