Condensation mass sink and intensification of tropical storms

Abstract

Intensification of tropical storms measured as the central pressure tendency represents a subtle imbalance, of the order of $10^{-3}$, between the inflow and outflow of air in the storm core. Factors driving this imbalance, especially in cases of rapid intensification, remain elusive. Here, using an analysis of intensification rates and precipitation in North Atlantic cyclones, it is shown that the storms on average deepen at a rate with which maximum local precipitation removes mass from the atmospheric column. Means for lifetime maximum intensification rate and maximum concurrent precipitation (multiplied by the acceleration of gravity) are, respectively, $23$ and $17$ hPa day$^{-1}$. This equivalence is not limited to average values: both intensification rates and precipitation have the same dependence on the inverse radius of maximum wind. It is further shown using a numerical model that with the mass sink switched off, storms driven by sensible and latent heat alone either do not develop at all or develop significantly more slowly reaching lower maximum intensities. It is discussed that the conclusions of previous studies about the relative insignificance of the mass sink arose from a long-standing misinterpretation of mass nonconservation assessments for assesments of the actual impact of the mass sink on storm dynamics. Condensation mass sink provides for a fundamental positive feedback between surface pressure and vertical velocity that was earlier shown to be instrumental in analytical descriptions of storm intensification. This feedback allows the storm circulation to get more compact during intensification in contrast to modeled heat-driven storms that increase their radius of maximum wind as they intensify. These findings indicate that the condensation mass sink is a dominant process governing the dynamics of tropical storms.