Temperature inversions in the lower troposphere over the Sichuan Basin, China: Seasonal feature and relation with regional atmospheric circulations

Frequently generated lower troposphere inversions (LTIs) above the planetary boundary layer (PBL) are a unique feature of the Sichuan Basin (SB), adjacent to the Tibetan Plateau (TP). However, the knowledge of LTI genesis and development mechanisms remains limited. In this study, radiosonde measurements and the fifth generation European Centre for Medium-Range Weather Forecasts (ECMWF) global atmospheric reanalysis (ERA5) from June 2013 to May 2019 were used to investigate LTI seasonality and the influences of the seasonal contrasts and diurnal variations of regional circulations on LTI. LTIs are most frequent in winter, with inversion frequency of 87.8%. They often exist between ~2000 and 3800 m above the ground. Due to TP-SB topographical forcing, the airflows over the SB feature a highly stratified structure and exhibit seasonal variations. LTI is an orographically induced, diurnally driven, and elevated inversion. Its genesis and evolution result from the synergistic effects of the orographically induced stratified flows at different heights and the topographical sheltering by the leeward slope of the TP. In the absence of special weather conditions, the key factors for LTI genesis are 600-hPa westerly warm advection, originating from the elevated heat source of the southeastern TP, and 750–700-hPa southwesterly warm advection in winter, due to topographically dynamic forcing and surface heating. Strong descending motion at 550–700 hPa improves the warm air mass downward extension and maintenance. Easterly flow at 850–775 hPa and PBL Mountain-Plains Solenoid circulation prevent the lower atmosphere from warming, making LTI non-surface based. These factors dominate the diurnal evolution of the LTI, which forms in the afternoon, grows strongest at dusk, and weakens at night. The present work lays a foundation to understand the roles of LTI in the regional air pollution, weather, and climate in the downstream regions of TP.