Lower tropospheric states revealed in high vertical-resolution radiosonde data in Korea and synoptic patterns for instability based on a self-organizing map

Abstract

Seasonal and spatial characterizations of wind and stability in the lower troposphere below 1500 m are investigated using 1 s high-resolution operational radiosonde data observed at five stations (Baengnyeongdo, Heuksando, National Typhoon Center (NTC), Bukgangneung, and Pohang) in South Korea for 4 years (July 2016–June 2020), concentrating on four variables: horizontal wind speed (HWS), vertical wind shear (VWS), squared Brunt–Väisälä frequency (N2), and Richardson number (Ri). Significant seasonal variations are identified in these variables due to the heterogeneity of the geographical locations as well as seasonal variations in the synoptic patterns. VWS is high in spring and summer at two island stations on the west of the Korean Peninsula, Baengnyeongdo, and Heuksando, whereas it is high in winter at NTC, Bukgangneung, and Pohang, leading to frequent Kelvin–Helmholtz (KH) instability (KHI; 0 ≤ Ri < 1/4). Below z = 1000 m, static stability is lower in winter than in summer at all stations, resulting in a higher convective instability (Ri < 0) in winter, which is attributed to the inflow of the marine boundary layer. The frequent convective instability observed in spring at Bukgangneung suggests a potential relationship with mountain wave breaking and downslope windstorms. The microstructures of the lower tropospheric instabilities revealed in the current radiosonde observations are not properly represented in the high-resolution local numerical weather prediction and analysis system, namely local Data Assimilation and Prediction System (LDAPS) of Korea Meteorological Administration. In particular, the observed convective and KH instabilities, which are approximately 45% for the whole period, are rarely (approximately 15%) represented in LDAPS. The synoptic patterns favorable for the convective and KH instabilities cases are identified based on a self-organizing map clustering algorithm: (1) west-high and east-low patterns with extended Siberian high in winter, (2) strong low-pressure systems susceptible to KHI from spring to fall, (3) weak high-pressure systems with potential convective instability, accompanying northerly or north-westerly flows with cold advection at 925 hPa.