A Climatology of Nocturnal Low-level Jest for the Topographically Complex Site at Boseong

We examine nocturnal low-level jets for the topographically complex site at Boseong, South Korea. The study site is located in southern coast of Korean peninsula. Although the site is surrounded by flat rice paddy field within a few kilometers, to the southeast are sea and relatively high mountains (576 m) and valley are located to the north and to the west, respectively. We define LLJs as local wind maximum in the vertical profile which is at least 2 m s^-1 greater than the speeds both above and below it within the lowest 300 m of the atmosphere. LLJ characteristics are derived from a 3-yr half-hourly database of wind speed profile (11 levels), obtained from the 300-m mast. LLJ days with at least one LLJ during nocturnal period occupy more than half of total days but duration of LLJ is about 3 hours so LLJs occur in about 20 % of the nighttime based on half hourly data. High hourly frequency of LLJ is shown between 0000 and 0600 LST. LLJs are typically situated at 40 m above the ground and have a minor peak at 100 m. Jet speed is not proportional to the jet height. Half of LLJs have a speed of 3-5 m s^-1 and jet direction is predominantly west-northwesterly regardless of time, height, and season. Seasonal distribution of LLJ occurrence shows maxima during March and October and minimum in July. LLJ occurs in clear sky and weak wind conditions. Mechanisms of LLJs are discussed in terms of local wind developed in the valley.