Seasonal Changes in a Vertical Thermal Structure Producing Stable Lower-Troposphere Layers over the Inland Region of the Indochina Peninsula

The authors performed a thermal budget analysis to understand the nature of seasonal changes in stable lower-troposphere layers over the inland region of the Indochina Peninsula, using atmospheric reanalysis data. The analysis focuses on subseasonal stable layers. Stability increase in the generation of stable layers is classified into three dominant thermal factors: vertical differences in horizontal potential temperature advection, vertical potential temperature advection, and their residual component Q1. The largest contributor to the stability increase is defined as the dominant thermal factor. Climatological typical heights where stable layers most frequently appear are the 850–700-, 700–600-, and 600–500-hPa levels in November–January, February–March, and April, respectively, according to a previous study. From November to January, most of the stable layers in the typical height are generated by vertical differences in horizontal potential temperature advection. Their generation (dissipation) is characterized by strong (weak) cooling due to horizontal advection below the stable layers. The strong cooling is related to cold surges in the winter monsoon. Generation of the stable layers in the typical height from February to April is characterized by vertical differences in Q1. Here, Q1 cooling below the stable layers is demonstrated in February and March. The authors propose a mixed boundary layer process in explaining the Q1 cooling. In April, the analyses demonstrate Q1 heating above the stable layers, coincident with a peak in the apparent moisture sink. The results indicate that the thermal processes of stable-layer generation change the height of the stable layer along the seasonal advance.