Structure of tropical variability from a vertical mode perspective

A composite mesoscale precipitation event and a convectively coupled Kelvin wave produced by a diabatically accelerated cloud resolving model are compared. Special emphasis is placed on the vertical structure of moisture perturbations and the interaction of these perturbations with the composited dynamical fields.

Both composites share the same general features, a gradual deepening and strengthening of convection followed by deep convection and a stratiform region, quite similar in character to observations. Composited frozen moist static energy (FMSE) perturbations are several times larger than virtual temperature perturbations and are found to control convective parcel buoyancy.

A two vertical mode decomposition of the dynamical and moisture fields is found to reproduce both composites quite well. The vertical modes are computed empirically and provide a self-consistent way to link both FMSE and virtual temperature variability with well defined modes of vertical velocity variability. Deep convection is associated with nearly uniform moistening in the lower and middle troposphere, while shallow convection is associated with a moist lower troposphere and dry middle and upper troposphere.

The FMSE budget leads to an interpretation of the convective life-cycle as a recharge-discharge mechanism in column integrated FMSE. The budget analysis places diabatic forcing -- surface and radiative fluxes -- into the moist energetic framework. In particular, they are seen to prolong active convection, but play a passive role in its initiation. The modally decomposed FMSE budget highlights the dynamical importance of the second baroclinic mode in moistening the lower and middle troposphere before convective onset (recharging), and then discharging stored FMSE in the stratiform region.