Thermodynamic structure of monsoon boundary layer over the west coast of India

Abstract

The study investigates thermodynamic structure of the atmospheric boundary layer (ABL) over the west coast of India during the southwest monsoon season. Radiosonde profiles at 00 UTC (0530 IST) are utilised for three stations, namely Thiruvananthapuram (TVM), Mangalore (MNG) and Mumbai (MUM) to represent south, central and north parts of the west coast, respectively. The high vertical resolution radiosonde data were collected from India Meteorological Department (IMD) for the period 1st May to 31st October 2018. Variations in the thermodynamic parameters associated with the monsoon and active and weak surges are examined. The active and weak monsoon conditions are identified using the values of OLR (outgoing longwave radiation) and rainfall in the respective stations. The study delineates interesting dynamic and thermodynamic features over the region. The thermodynamic parameters possess remarkable variations associated with the approach of the monsoon surges. TVM is characterised by absolutely stable or conditionally neutral (conditionally unstable) layer above a shallow conditionally unstable layer over the ground during the active (weak) monsoon situations; however, over MNG and MUM, ABL is conditionally unstable during both active and weak situations. Stable atmosphere is expected to cause stratiform clouds under favourable dynamic factors, whereas unstable atmosphere can aid vertical development of clouds and this can be attributed to the vertical shear of zonal wind. Zonal wind has a sharp increase from the ground to above ABL (~ 1200 m) in TVM; however, it is in the lower levels of ABL over MNG and MUM. In MUM, strong zonal wind is observed during the active (>20 m s−1) and weak (>12 m s−1) situations that causes intense mixing and maximum ABL height. The enhanced monsoon activity in MUM (MNG) is mainly associated with the active phase of the monsoon intraseasonal oscillation (active phase of the monsoon intraseasonal oscillation and strengthening of low level zonal wind), whereas in TVM it is mainly associated with the strengthening of low level zonal wind. The ABL heights are elevated during weak monsoon conditions due to convective turbulence added to the mechanical turbulence associated with the strong wind during the season.