Deep Convection Lids: Origins and Implications for Monsoon Onset

Abstract

Past literature has identified several factors that limit deep convection, including sea surface temperature (SST) and the humidity within and around an atmospheric column. These limiting factors often have a nonlinear relationship to deep convection, imposing thresholds below which convection is rare. Empirical values for these thresholds are well known, but there is not a well-known theoretical basis for the values themselves. Why is deep convection rare below SSTs of 26°C rather than 24°C? Why is it rare for water vapor content below 35 kg/m2? This paper uses reanalysis data to compute moist static energy (MSE), a metric which accounts for temperature and humidity simultaneously. We test the hypothesis that deep convection begins when local MSE surpasses the rainy tropical maximum MSE (i.e. “lid” MSE) and find that in many regions, a local MSE within 5 kJ/kg of the lid MSE is enough for deep convection onset to occur. However, it is hard to find universal support for the hypothesis, as behavior varies widely between regions. Factors including latitude, topography, land-sea ratio, distance to lid origin location, and dry air entrainment may play a role in this variability.