2A.3 EXPLORING THE TROPICAL LAND-OCEAN CONVECTIVE INTENSITY DIFFERENCE THROUGH SURFACE BOWEN RATIO VARIATIONS AND HIGH PERCENTILES OF THE CAPE DISTRIBUTION

Introduction: There are large differences in the intensity of deep convection between land and ocean in the tropics (Zipser 2003). This is clearly seen in order of magnitude differences in lightning flash rate between land and ocean, with land have much greater flash rates (Cecil et al.2014). Lightning flash rate is generally thought to be a good proxy for updraft velocity (Boccipio 2002). The few in situ observations of deep convective tropical updrafts seem to confirm this idea (Lucas et al. 1994). It is our goal to focus on two specific hypotheses that may explain these differences. The first is that convective available potential energy (CAPE) is simply greater over land than over ocean at high percentiles. We test this using 9 years of 4 times daily ERA-interim data to calculate CAPE between 45N and 45S. The second hypothesis is that differences in the surface Bowen ratio result (SBR) in deeper boundary layers over land, resulting in wider updrafts at cloud base, resulting in less entrainment, and thus more intense convection over land. We explore this hypothesis using cloud resolving model simulations. CAPE differences over land and ocean would be a parsimonious explanation as to why there are large observed lightning differences. However, previous studies have suggested that there are not very large differences in CAPE over land and ocean (Williams and Renno 1993, Riemann-Campe et al. 2009). A previous study from reanalysis data only captured the mean CAPE of the tropics (Riemann-Campe et al. 2009) and suggested that while mean CAPE was slightly higher over land, the differences didn’t seem to be enough to explain the regional differences in lightning activity. It also seems more reasonable to us that a thunderstorm is likely using a higher CAPE than the climatological mean to produce deep convection, and so we wished to test whether high percentiles of CAPE over land and ocean are different. Horizontal entrainment of environmental air into a convective plume weakens the buoyancy of the plume (Williams and Stanfill 2002). Varying the surface Bowen ratio affects the depth of the boundary layer. Deeper boundary layers wider clouds at cloud base that could limit environmental entrainment’s impingement into the updraft core (Zipser 2003, Lucas et al. 1994, Williams and Stanfill 2002, Lucas et al. 1996, Williams et al. 2005). Conceptually, this can be represented by a formulation for entrainment where the entrainment is inversely proportional to boundary layer depth. Using radiative convective equilibrium (RCE) and initial condition simulations in a cloud resolving model, we test the impact of differing SBR on highpercentile updraft strength. We used a parcel model to test whether entrainment that is inversely proportional to boundary layer depth is representative for our simulations. CAPE is explored using ERA-interim reanalysis data from 2000-2008 and separated by percentile as well. Below, we separate into the two main ideas that were tested. First we explore the CAPE distribution and its relationship to lightning flash rate. Following that, we explore the surface Bowen ratio.