Dropsonde Atmospheric Surface Flux Estimates in Tropical Cyclones

Flux measurements in tropical cyclones (TCs) are extremely challenging to obtain. Previous field campaigns, such as CBLAST, have been successful in collecting a limited number of flux measurements. However, in order to understand surface fluxes in the extreme environment of TCs, more flux observations are needed. A core assumption for the near surface boundary layer is that the mean vertical wind, temperature, and humidity profiles define a single Monin-Obukhov similarity (MOS) solution that is scaled by the surface flux values. Importantly, the MOS solution defines all three vertical profiles. Fluxes can be estimated by fitting an MOS solution to near-surface mean measurements of wind, temperature and humidity. Dropsondes measure high resolution vertical profiles of wind vectors, potential temperature, and specific humidity. Because the TC boundary layer is strongly turbulent, dropsonde profiles are single members of a (virtual) ensemble rather than mean profiles. Foster and Patoux (2013) found that least-squares optimal MOS solutions could be fit to the near-surface dropsonde profiles, which in turn provides an estimate of the surface fluxes consistent with the profile. In effect, we seek the best overall MOS solution to the multiple near-surface observations in the dropsonde profile. The primary assumption behind MOS is that the typical “large” turbulent eddy size in the surface layer is proportional to the height above the surface. Analysis of the deviates from the MOS solution show that temperature rarely deviates from MOS and that the deviations of wind and humidity are anti-correlated. This is consistent with overturning eddies in which parcels with lower momentum air and near-saturated humidity are ejected from the surface and/or higher momentum and lower humidity are swept downward. Some of the very lowest measurements might be inconsistent with standard MOS parameterizations due to gust fronts, sea spray, or ocean surface waves. We evaluate several MOS parameterizations, including a new parameterization that incorporates the contribution of sea spray to the stress. Applying this new technique for estimating surface fluxes to NOAA’s extensive dropsonde dataset will greatly increase the quantity of surface flux observations in TCs and lead to a better understanding of surface fluxes in TCs.