Development and Decay Processes of Dual Inversion Layers in Winter over the Northwest Coast of the South China Sea

Inversion layers in the lower troposphere appear centered at two heights, 1.5 and 4 km, over the northwestern coast of the South China Sea in late boreal winter. The mechanisms of these dual inversion layers are investigated with thermal budget and composite analyses of the JRA-55 dataset. The thermal budget analysis classifies inversion layers between the levels of 700 and 600 hPa into two types. One type is related to high pressure moving southward along the eastern edge of the Tibetan Plateau; the arrival of the high coincides with subsidence that warms the 600-hPa level more strongly, and the stability increases between the 700- and 600-hPa levels. The other type is related to a synoptic-scale eastward-propagating wave in the upper troposphere. The upper-level high pressure in the wave is accompanied by a midtropospheric warm area whose intrusion at the 600-hPa level generates the inversion layers. Inversions between the levels of 925 and 850 hPa are related to heating and cooling by horizontal advection of potential temperature in the upper and lower levels, respectively. The vertical contrast of horizontal advection corresponds to a frontlike structure at the southern edge of a pressure surge. The pressure surge has a similar structure with the high pressure moving along the plateau edge as mentioned for the former type of inversion layers between the 700- and 600-hPa levels.