The precipitation mass sink in tropical cyclones

When water vapor is converted to cloud and precipitation and subsequently removed to the surface via precipitation, there is a corresponding hydrostatic pressure decrease due to the reduction of mass in the overlying column. Pressure changes resulting from the addition or removal of water vapor are currently neglected in most applications, because the continuity equation, which is integrated vertically to obtain the pressure-tendency equation, is not typically written to include mass source/sink terms. However in heavily precipitating systems such as tropical cyclones, the exclusion of vapor sources and sinks is questionable. An earlier study of Hurricane Lili (2002) demonstrated that the mass sink effect was not negligible in model simulations of that case using the NCEP Eta model. Although the results confirmed that the mass sink effect contributed up to 5 hPa additional sea-level pressure deepening and a 10-15 kt increase in wind speed, the Eta simulations of the Lili case failed to generate a storm that approached the intensity of the observed system. Here, simulations of Hurricane Ivan (2004) are presented in which the Eta was able to deepen the cyclone more strongly, allowing a better assessment of the magnitude of the mass sink process. Comparison of simulations from (i) an unmodified version of the Workstation Eta model, (ii) a version with a modified surface-flux parameterization scheme, and (iii) a version of the model modified to include the mass-sink effect are used to quantify the importance of this process for more realistic simulations of a strong hurricane.