Disparate Controls of Convective Cell Deepening and Intensification Based on Cell Width

Numerous studies have shown that once deep convection initiates, its growth in size and intensity is sensitive to convective available potential energy (CAPE) and vertical wind shear. Others have shown sensitivity to mid-level relative humidity via entrainment-driven dilution. Central to the relative roles of these sensitivities is the updraft width. Much of the support for these findings depends on modeling in which specific sensitivities can be isolated in simplified, controlled conditions. Here, we use an extensive database of C-band scanning radar-tracked convective cells during the Cloud, Aerosol, and Complex Terrain Interactions (CACTI) field campaign to assess the relative roles of these and additional environmental variables on the deep convective cell-lifetime maximum depth, reflectivity, and precipitation rate. Tracked cell properties are derived from radar and satellite measurements, while environmental variables are obtained from 3-hourly soundings interpolated to the deep convective initiation (CI) time for each of the thousands of tracked cells. Sensitivities of cell growth to atmospheric conditions are found to significantly differ depending on cell width. Wide cell maximum depth and intensity increases as CAPE and equilibrium level (EL) increase. Narrow cell maximum depth and intensity lack any correlation with CAPE and EL but strongly correlate with mid-level relative humidity (RH). Relationships with vertical wind shear are less clear. Further sensitivities are found to pre-existing cells and reflectivity in the vicinity of CI locations in which narrow cells become more intense and reach greater depths when growing in a region of pre-existing cells and precipitation. The opposite is true for wide cells.

A km-scale simulation covering the CACTI campaign period reproduces the observed sensitivities and is used to further explore composited spatiotemporal evolution of conditions leading up to CI at and around CI locations for narrow and wide cells with varying depth and intensity outcomes. Narrow cell growth is strongly dictated by mid-level RH, which is modulated by mid-level mesoscale ascent. Much of this ascent is realized via groups of convective cells, leading to the positive correlation of narrow cell growth with the number and coverage of pre-existing cells. These correlations are not observed for wide cells. Narrow cell lifetime-maximum reflectivity and precipitation is particularly sensitive to pre-existing precipitation over the cell inflow, suggesting a potential role for precipitation recycling. This may be a particularly important process in orographic environments such as those studied here in which inflow commonly passes through pre-existing precipitation. On the other hand, wide cell depth and intensity negatively correlate with pre-existing condensate and precipitation in the opposite manner as narrow cell depth and intensity, responding instead to warmer and moister low levels over a mesoscale region that facilitate greater CAPE and EL. Cell width positively correlates with updraft width in the simulation, which suggests that the differing environmental sensitivities for narrow and wide cells may be at least partly controlled by the decreasing effects of entrainment-driven buoyancy dilution as updraft width increases, consistent with idealized modeling studies.