Characteristics of surface boundary layer during active and weak phases of southwest monsoon over Kochi: A tropical station

This study explores the features of atmospheric surface layer over Kochi during active and weak phases of the southwest monsoon season. The classification of active and weak phases of monsoon is made on the basis of monsoon organized convection over the region. When the monsoon organized convection is over (away from) Kochi, considered it as an active (weak) phase. The study primarily utilizes sonic anemometer data. The diurnal variation of surface fluxes and turbulent kinetic energy (TKE) during active and weak conditions are examined in addition to surface wind and temperatures. It is found that monsoon clouds spread extensively over large area stabilizes the surface layer and a drastic decrease in sensible heat flux is observed during the active monsoon conditions. The average value of momentum flux and TKE decreases, surface layer becomes stable, and Atmospheric Boundary Layer (ABL) height lowers during active monsoon phase. A significant oscillation of 3.5 h periodicity is found to be embedded in the momentum flux and in the wind speed which is attributed to the penetration of the low level monsoon circulation. Weak monsoon phase is characterized by higher value of fluxes and TKE with higher amplitude of diurnal variation due to local heating and sea breeze circulation. Unstable condition between 00:00 IST and 01:00 IST and stable conditions in the early morning is observed during both active and weak phases. It is observed that there are two different relations for the growth and dissipation of fluxes and TKE. The dissipation is faster than growth in the case of surface fluxes with temperature. The sensible heat flux, momentum flux and TKE are logarithmically related with wind speed. Sensible heat flux has logarithmic relation with temperature, whereas momentum flux and TKE have exponential relation.