ENTHALPY FUELS THE STORM

Surface fluxes under tropical storm conditions remain dificult to measure due to the extreme environmental conditions and limitations in a laboratory setting. As a means of simply quantifying the dominant processes within tropical cyclones, Emanuel (1986) proposed an expression for the potential intensity of a storm that goes as the ratio of the enthalpy flux to the momentum drag, Ck/Cd. More simply this is the ratio between the energy fueling the storm from the ocean to the drag holding it back. With a lack of obersvations beyond moderate wind speeds, Emanuel (1995) postulated that the ratio of Ck/Cd could not be less than roughly 0.75 for a tropical storm to exist. The goal of my research is to model hurricane enthalpy and momentum fluxes to further quantify and validate the Ck/Cd ratio and thresholds.

 

HURRICANE ENTHALPY FLUXES

Over the past two decades a set of field studies were conducted to further quantify the role of air-sea heat and moisture fluxes (collectively enthalpy fluxes) on atmospheric processes. The results from the Humidity Exchange over the Sea (HEXOS) experiment (DeCosmo et al., 1996) and initial results from the Coupled Boundary Layers Air-Sea Transfer (CBLAST) experiment (Zhang et al., 2008) provide the first large data set of fluxes for wind speeds up to 40 m/s. More recently, higher resolution data from CBLAST has become available and includes wind speed measuremets (U10) in excess of 50 m/s during Hurricanes Isabel and Fabian. This data was analyzed by Bell et al. (2012) and represents the only observations of enthalpy fluxes at tropcial cyclone wind speeds. In light of these recent measurements, my work is to model the enthalpy fluxes under windspeed conditions ranging between 1 and 100 m/s and test the results against the results from DeCosmo et al. (1996) and Zhang et al. (2008) for sub-tropical cyclone winds, and Bell et al. (2012) for tropical cyclone winds, in addition to some recent laboratory estimates by Jeong et al. (2012).

The molecular sensible heat and specific moisture fluxes are modeled using the "wavy wall law" approach from Mueller and Veron, 2009, while accounting for air-flow separation for molecular fluxes. Within this separation region, turbulent processes dominate and the viscous sublayer is not present (Veron et al., 2007). On the atmospheric side of the air-sea interface, the scalar diffusive sublayers are entirely within the viscous sublayer. Therefore as the viscous sublayer is erroded in favor of a turbulent regime within the separation regions, as would the diffusive sublayers.

Figure (a) shows how the diffusive flux scale for heat (theta_*mol) varies with fetch and windspeed against the smooth flow limit flux scale (i.e. the non wave model run, theta_*s). The solid line is for the smooth flow limit flux scale at 100km fetch, while the diffusive flux scale for heat for 100 km (dashed-dotted) and 10 km (dashed) are also shown. The results show a significant decrease in molecular heat fluxes at high wind speeds. The specific humidity results are similar but not shown.

As seen in the Figure (d), the displacement parameter for specific humidity, alpha, (and similarly sensible heat) is reduced in the presense of increase windspeed due to an increase in separation regions. Air flow separation (A) as a function of wind speed and fetch is shown in Figure (c). Figure b shows the fraction of surface heat flux due to molecular (mol) vs. non-molecular processes (non-mol) as a function of both wind speed and fetch. The transition point between molecular and non-molecular dominated surface scalar fluxes occurs roughly 5 m/s earlier than previous theoretcial work, representing an improvement in model accuracy. From the surface scalar fluxes, the surface specific enthalpy flux can be determined. In addition, sea spary plays an important role and is an on-going area of research.

 

   
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