ABSTRACT
PHAM Thi Thanh Nga
| In the present study, horizontal winds with high temporal and vertical resolution from the 400-MHz wind profiler radar (WPR) at Okinawa and the WPR network of the Japan Meteorological Agency (JMA) at three stations surrounding Okinawa are used to investigate the low level jet (LLJ) during the summer season, May-June-July of 2004-2006 over the southwest islands of Japan. The analysis focuses on the diurnal variation of the LLJ and its relation to precipitation associated with the Baiu front. 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, another peak also appears around a 3-km height. 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 the Okinawa Island, provide supporting evidences for the role of the atmospheric boundary layer in generating the diurnal variation of LLJ occurrence. To better understand the relationship between the LLJ and the Baiu rainfall system, the diurnal variation of precipitation associated with the Baiu front over this region is examined by using GSMaP (Global Satellite Mapping Precipitation) and Radar-AMeDAS (Automated Meteorological Data Acquisition System) rainfall data. The Baiu precipitation system over the studied region exhibits not only a diurnal variation with an intensity peak in the morning, but also a propagation from the northwest to the southeast while it is weakening through the day. The diurnal variation of LLJ occurrence is further investigated with the focus on rain cases relating to the Baiu system. In a close examination of the LLJ with precipitation in its propagating direction toward individual stations, it is revealed that different diurnal variations of LLJ occurrence are obtained in accordance with different variations of precipitation approaching these stations. The strongest enhancement of the LLJ in the morning is obtained at Okinawa when the precipitation peak is to the north of the site, while nighttime preferences of the LLJ and precipitation are found at Yonaguni. Naze and Yakushima show less clear diurnal variations in both the LLJ and precipitation. This study concludes that the LLJ over the Okinawa region is related to both atmospheric boundary layer (ABL) processes and precipitation activities, associated with synoptic-scale disturbances. The diurnal variation with nocturnal preferences in occurrence of LLJ frequency is largely controlled by ABL processes, while the morning enhancement of the LLJ is related to the intensity peak of precipitation. Although further investigations are needed to clarify the interaction between precipitation and ABL processes, this study emphasizes that both factors influence characteristics of the LLJ over the Okinawa region. |
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