Exploring the Tropical Land-Ocean Convective Intensity Difference through Surface Bowen Ratio and Island Size Variations

Friday, 19 December 2014
Zachary Hansen, University of Wisconsin Madison, Madison, WI, United States and Larissa E Back, UW-Madison, Madison, WI, United States
A better understanding of the mean distribution and variability in convective patterns is important for improving our understanding of the climate system. General circulation models (GCMs) still have trouble simulating climate variability in the tropics, where deep convection plays an important role in determining the mean climate and its variability. 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 hope to elucidate the physical mechanisms that influence updraft intensity using a cloud resolving model (CRM), which, unlike a GCM, is able to explicitly simulate individual convective updrafts.

There are numerous hypotheses regarding what causes the discrepancy between land and ocean convection in the tropics. The hypotheses that we explore are variations in the Surface Bowen Ratio (SBR), as well as the existence of horizontal heterogeneities interacting with an ocean. We test these hypotheses in two different contexts: initial condition simulations, and radiative convective equilibrium (RCE) simulations. We modified the SBR of a region in RCE which by extension changes its boundary layer depth. The expectation in this case was that higher SBRs would lead to more enhanced convection. However, we find that higher SBRs have no effect or a slightly negative effect on updraft statistics in RCE simulations.

We also looked at how the inclusion of differently sized islands affected convection in initial condition simulations. Previously, there had been discussion of a resonant island size that may produce more intense convection (Robinson, Sherwood, and Li 2008; Robinson et al. 2011,). We were able to reproduce predicted variations in convective intensity using differently sized islands. It remains to be seen whether these variations in convective intensity are due to resonant island size or some other mechanism.