Air-sea enthalpy fluxes and evidence of spray effects within tropical cyclones

Monday, 15 December 2014: 9:45 AM
David H Richter, University of Notre Dame, Notre Dame, IN, United States and Daniel P Stern, National Center for Atmospheric Research, Boulder, CO, United States
Within tropical storm systems, reliable measurements are often prohibitively difficult to obtain, which leads to a continued uncertainty about near-surface turbulent processes in the high-wind, tropical cyclone boundary layer. One such source of uncertainty is the feedback effect of sea spray, whose suspension above the water surface has been postulated to be a significant source of additional latent and sensible heat flux, beyond what can be achieved by turbulent exchange alone. This spray-mediated thermodynamic exchange has significant implications for tropical cyclone development and prediction. To estimate surface enthalpy fluxes from within tropical cyclones, we analyze data from over 2000 dropsonde profiles taken from 37 different storms. Mean thermodynamic profiles are constructed and binned according to wind speed and sea surface temperature, and Monin-Obukhov similarity theory is invoked to estimate surface fluxes based on average profiles. In agreement with the few existing measurements at high winds, we find no statistical dependence of the bulk enthalpy coefficient with 10-meter wind speed. More importantly, however, by computing the power-law scaling of enthalpy flux versus of wind speed, it is found that in extreme conditions (wind speeds from 20 - 50 m/s), a significantly stronger scaling dependence is found when compared to cases where no spray effects would be expected. This stronger scaling agrees with spray-mediated flux models and suggests an additional route of thermodynamic exchange within the high-wind boundary layer.