Exploring the Tropical Land-Ocean Convective Intensity Difference through Surface Bowen Ratio Variations and High Percentiles of the CAPE Distribution

It has been observed that lightning flash rates over land in the tropics are orders of magnitude higher than those over the ocean. Because higher lightning flash rates tend to be associated with higher updraft velocities, deep convective updrafts over land are expected to be stronger than those over the tropical oceans. We attempt to elucidate the physical mechanisms that influence updraft intensity using a cloud resolving model (CRM) as well as using reanalysis data.

We used a CRM (the System for Atmospheric Modeling at 200m resolution, with Morrison microphysics, 50km domain) to test the hypothesis that higher surface Bowen ratios (SBR) result in stronger high intensity updraft velocities. This hypothesis was tested in two different contexts of CRM simulation: initial condition simulations and radiative-convective equilibrium (RCE) simulations. In RCE we find that higher SBRs have very little effect on updraft statistics, cloud sizes, and the distribution of entrainment rates. In initial condition simulations, strong updrafts are weaker for high surface Bowen ratio simulations. Thus, the hypothesis is not supported. We speculate on other hypotheses these type of simulations could support for land-ocean variations.

We also use reanalysis data to examine the hypothesis that convective available potential energy (CAPE) variations between land and ocean can explain land-ocean variations in lightning and by extension potentially updraft velocities. We calculate 9 years of 4 times daily CAPE from ERA-interim data. It It is found that regions with higher 99 percentile CAPE values do not correlate with regions of increased lightning activity.