CAPE Threshold for Lightning Over the Tropical Ocean

Abstract:

We investigate the relationship between convective available potential energy (CAPE), precipitation, storm size, and lightning flash rate (F) in the tropics. A particular focus is on the difference between marine and land convective systems in their associated CAPE-F relationship. While F increases almost linearly with CAPE^1/2 over land, F is nearly muted until CAPE reaches a sufficiently large value over the ocean. In the high-CAPE regime, on the contrary, oceanic storms produce as much lightning flashes as land storms. To understand why lightning flashes are suppressed over the ocean in the low-CAPE regime, a simplified entraining plume model is employed to derive an analytical relationship between maximum vertical velocity and plume size. The analytical model presents an L-shape curve for the relationship between CAPE and a minimum plume size required for a storm to yield lightning flashes, and is able to qualitatively capture the observed relationship between CAPE, storm size and F. We applied a CAPE threshold is applied to the CAPE-based lightning parameterization of Romps (2014) and found that it improves the performance of the lightning parameterization scheme, in particular, at representing the land-sea contrast in F. Our results suggest that CAPE-based lightning parameterizations need to consider the CAPE threshold to properly represent the land-sea lightning frequency contrast.