Exploring the Impact of the Diurnal Cycle on the Convective Intensity Contrast Between Land and Ocean

The intensity of deep convection on smaller scales is not always correlated with the amount of rainfall on larger scales. For example, geographic maps of lightning flash rates are correlated with maps of rainfall, but there are also substantial variations that can't be explained by mean precipitation alone. Lightning is used as a proxy for convective intensity in the real world, as higher flash rates are associated with greater updraft velocities. In particular, land has orders of magnitude more lightning than the oceans, despite generally having less rainfall. Motivated by these observations, we use a cloud resolving model to test a hypothesis for why convection over land might have stronger high-percentile updrafts than occur over the ocean. This hypothesis is that the diurnal cycle in surface heating leads to CAPE variations that explain this convective intensity contrast. We test this hypothesis by inserting an island with an oscillating sea surface temperature adjacent to an ocean with a fixed sea surface temperature. We find that convective intensity, and CAPE do not differ from the adjacent ocean, and hopefully explain why CAPE is not varying, despite the changes in surface temperature (from basic principles). We present evidence for a different hypothesis for explaining the land-ocean contrast in lightning, namely: increased saturation deficit, and potential for land to initiate convection at lower relative humidities than over the ocean.