Structure of Moderately Deep, Isolated Tropical Cumulonimbi

The structure of isolated cumulonimbus clouds, particularly those in the moderately deep (often incorrectly referred to as congestus) mode with heights of up to 7 km, as observed by the S-PolKa radar during DYNAMO is explored. Such elements are first identified following the algorithm of Powell et al (2016); smaller contiguous regions of echo are considered isolated convection, while large regions are classified as either convective, stratiform, or mixed. The regions of largest reflectivity within isolated echo objects are considered to be “isolated convective core”, and the surrounding weaker echoes in the same object are “fringe”. Observed composite vertical profiles of reflectivity and differential reflectivity (Z_DR) in isolated convective cores have similar distributions to those in deeper, wider, and more intense convection. Between the surface and 4 km, reflectivities of 30–40 (10–20) dBZ are most commonly observed in isolated convective core (fringe) echoes. Convective cores in echo objects too wide to be considered isolated have a Z_DR profile that peaks near the surface (with values of 0.5–1 dB common), and decays linearly to about 0.3 dB at and above an altitude of 6 km. Stratiform echoes have a minimum Z_DR below of ~0–0.5 dB below the bright band and a constant distribution centered on 0.5 dB above the bright band. The isolated convective core and fringe respectively possess composite vertical profiles of Z_DR that resemble convective and stratiform echoes.

The cloud population during the same period is then simulated using WRF. Simulated reflectivity and inbound/outbound velocities relative to convective cores of the modeled isolated convection is comparable to that which is observed. The model output thus allows for supplementing the observational dataset with information about vertical velocity profiles within isolated convection. It reveals that upward motion is typically present within the isolated core and fringe echoes. Fringe echoes contribute to only about 5% to total rainfall, but they represent an interesting “hybrid” type of convection: that which possesses a shallow layer of weak upward motion but also has a distribution of drop orientation similar to that of stratiform regions, in which more spherical, rather than horizontally oriented drops, are prevalent.