The shallow to deep convective transition is a key aspect of convection and has a significant influence on such aspects of tropical climate as the margins of convection zones, vertical thermodynamic structure and tropical climate variability spanning a wide range of spatio-temporal scales. In this study, deep convective onset characteristics over the tropics, as previously seen in satellite retrievals of precipitation, are probed using parcel stability calculations, coupled with a plume velocity equation. The deep convective onset boundary (defined using conditional instability and updraft velocity) in the temperature-water vapor thermodynamic plane, is found to be very sensitive to the entrainment assumptions used in the plume calculations. Only sufficiently entraining plumes reproduce the observed onset characteristics of deep convection, thus placing bounds on the range of entrainment values that should be used in convection schemes of climate models.

The deep convective transition characteristics, including the onset curve in the temperature-water vapor thermodynamic plane, can potentially serve as a useful constraint on climate models and a useful metric for their intercomparison. Analysis of model output from a recent high resolution version of the Community Climate System Model, whose cumulus scheme has a substantial entrainment, yields a reasonable match to satellite observations in several respects. Other models used in the fifth phase of the Coupled Model Intercomparison Project (CMIP5) exhibit a variety of results in this comparison.