The moisture and MSE budget of developing tropical convection: Vertical modes and free-tropospheric moisture convergence

It is well known that vigorous tropical convection is fed largely by moisture convergence, producing heavy rainfall overwhelming the local moisture supply through surface evaporation. During such convective events, a substantial portion of moisture convergence often occurs above the lifting condensation level (LCL) and hence the condensation from any cumulus clouds rooted in the sub-cloud layer would seemingly account only for a minor fraction of the total precipitation even at the intensifying stage of convective organization. In this paper, DYNAMO/CINDY2011/AMIE sounding array data are analyzed to study the moisture and thermal budget to address this question. To quantify the behavior of large-scale dynamics, vertical mode decomposition is applied to the soundings. As expected, the variation of vertical motion is primarily governed by the first baroclinic mode. The second and higher modes, although minor in amplitude, also exhibit familiar features from time to time: a buildup of the "congestus mode" as convective intensifies, which gives way to the "stratiform mode" as convection dissipates. The MSE import by the congestus mode, while frequently observed prior to convection, does not always lead to a subsequent burst of convection. A key element leading to the development of vigorous convection appears to be the presence of a layer of moisture convergence thickening to far beyond LCL, which is assisted by a buildup of the deep (first) mode. The environmental factors facilitating the condensation of free-tropospheric vapor will be also discussed.