Turbulent Fluxes and Surface Meteorology during the Landfall of Four Typhoons in the South China Sea

Abstract

Abstract We examine landfall of four typhoons in the South China Sea to better understand their air–land and air–sea energy exchanges at both an inland site and a nearby oversea observational site, comparing the exchanges to those during the monsoon period. We found the following: 1) Counter-gradient sensible heat fluxes could occur for all wind directions. For the latent heat, counter-gradient flux occurred at the inland site mainly with onshore winds. At the oversea site, such flux regularly occurred during the typhoons as well as during rainy periods in the monsoon. 2) Monin–Obukhov similarity theory (MOST) fits the inland and oversea cases well for only the vertical and horizontal wind speeds. Also, the nondimensional vertical gradient of wind speed had larger MOST parameter values over sea than inland. 3) Inland, when the wind speed increased, the downward heat flux from air to ground increased. 4) When the typhoon center was distant, the oversea latent heat flux was larger than that during the monsoon period. However, when the typhoon center was nearby, the latent heat flux was smaller. We suggest that the observed heat flux behavior over sea may arise from boundary layer rolls and sea spray during a typhoon with rainfall, but significant gaps exist in the latent heat flux data from the two stronger typhoons. 5) Detailed measurements of Typhoons Vicente and Kai-tak suggest two mechanisms for their SST decrease: Vicente had a wind-driven upwelling and mixing of the sea’s upper layer, whereas Kai-tak had a much stronger wind-driven vertical turbulent mixing, resulting in whole-layer mixing and less cooling than Vicente. Significance Statement The energy exchanges of air–land and air–sea during a typhoon landfall remain unclear. To investigate these exchanges, we studied four typhoons using two coastal observational sites (inland and over sea). When the site has no rainfall and is far away from the typhoon center, the latent heat flux is much larger than that in the monsoon period. However, when a typhoon is closer and has high winds and rainfall, a greater variation in heat fluxes occurs with larger negative and positive values. The variation results in a small average sensible heat flux and more uncertainty in the latent heat flux. Such complex heat flux transfer over sea may be due to boundary layer rolls and sea spray during a typhoon with rainfall.