When water vapor changes phase to liquid cloud water or cloud ice in the atmosphere, the gaseous atmospheric mass is reduced. If the resulting hydrometeors are subsequently removed to the surface via precipitation, there is a net hydrostatic pressure decrease due to the reduction of total mass in the overlying column. Atmospheric mass sinks (sources) involving precipitation (evaporation) are currently neglected in most meteorological applications. In tropical cyclones, where precipitation rates may exceed 200 mm d^-1, the pressure reduction due to the precipitation mass sink is significant (³ 20 hPa d^-1). The resulting inward directed pressure gradient force leads to air and moisture convergence, as well as vorticity generation in the highly rotational tropical cyclone environment.

Hurricane Lili (2002) is used as a case study to quantify the effects of the precipitation mass sink. Using MM5 model output, the mass loss needed to explain the area-averaged surface pressure decrease during the rapid intensification phase of Lili is compared with the total precipitation mass for an area of 100-km radius centered on the storm. The mass loss due to precipitation (7.25 hPa 5h^-1) exceeds that needed to explain the area-averaged pressure decrease (2.29 hPa 5h^-1), but the precipitation mass sink pressure reduction is not fully realized because of compensating convergence, which contributes to vorticity generation. These results indicate that the significance of the precipitation mass sink in tropical cyclones and perhaps other heavily precipitating systems should not be dismissed.

Continuing research involves performing sensitivity experiments with and without the precipitation mass sink term in the prognostic pressure tendency equations of Rotunno and Emanuel’s idealized nonhydrostatic axisymmetric numerical model and the workstation version of the NCEP Eta model. Preliminary results from both models reveal that the differences in central pressure, maximum wind speed, and precipitation rate between the CTRL and MSNK runs are non-negligible. More research is needed to quantify the various feedbacks relating to the precipitation mass sink in tropical cyclones and other heavily precipitating weather systems.