The four cumulus cloud modes and their progression during rainfall events: a C-band polarimetric radar perspective

Description

Cumulus clouds have historically been thought of as primarily consisting of two modes: shallow cumulus, with cloud-top heights (CTH) near the trade inversion layer, 1–2 km above the surface and deep cumulonimbus clouds, with CTH near the tropopause layer (~ 15 km). Recent recognition of a third cumulus cloud mode, the mid-level cumulus congestus cloud, has attracted significant interest to this field since identification of the congestus mode and importantly their role in the pre-conditioning of tropical deep convection is still unclear. In this study, a three-wet season data set of the Darwin C-band polarimetric radar is analysed to investigate the properties of convective cells as a function of its CTH with the motivation of identifying clear differences between the cumulus modes. The convective cells are identified using the Steiner algorithm at 2.5 km height. Then for each cell the CTH is taken as the maximum height reached by the 0-dBz radar echo above the cell, provided there is a continuous radar echo from the near ground to the CTH. We also make use of reflectivity data, drop-size distributions (DSD) parameters, and rain rate retrievals from the 2.5-km height bounded by the convective cells, as well as the vertical gradient in reflectivity (lapse rate) in the convective cells. Several interesting features in the near-ground convective cell properties and the reflectivity lapse rates associated with the shallow (CTH < 3 km), congestus (3–7 km), deep cloud (7–15 km), and over-shooting tops (> 15 km) are found and will be discussed during the conference. The temporal evolution of these convective cell characteristics and the four cumulus modes around heavy rain-rate events are also investigated.