Conditional instability and the buoyancy of convective plumes drive moist convection in the tropics but have a variety of representations in model convective schemes. Generally, instability—generated on the large-scale—is removed by local convection and precipitation. This process is reflected in statistical relationships between precipitation and bulk column measures of relevant variables, such as humidity and CAPE. A first-order relationship that can be leveraged for climate model diagnostics is the precipitation ‘pickup’ in which precipitation conditionally averaged on humidity undergoes a sharp, sudden increase. The strength and location of the pickup—referred to as the critical point—is used to assess model sensitivity to lower-free tropospheric moisture. Observational data from the Atmospheric Radiation Measurement (ARM) permanent field observational sites are augmented with satellite observations of precipitation and temperature as an observational baseline. Coupled Model Intercomparison Project Phase 6 (CMIP6) models are assessed. Models perform relatively well in regards to the critical point of the pickup when model temperature biases are accounted for. The conditional average precipitation is decomposed into the product of the probability of raining and mean precipitation during raining times (conditional intensity), showing that models which do perform well do so through compensating biases—model conditional intensity that is too low at a given humidity is compensated in part by excessive probability of precipitation.

Biases uncovered are further explored with the use of an entraining plume model. The estimation of a bulk plume entrainment rate which we term the “pseudo-entrainment,” captures the trade-off between lager temperature lapse rates and smaller subsaturation across the models. This pseudo-entrainment diagnostic is also a reasonable indicator of the critical value of integrated buoyancy for precipitation onset.

Using a combination of reanalysis and observational products (ARMBE, COSMIC2, and ERA5), the precipitation onset is expressed as a weighting between CAPE-like and subsaturation-like factors, here including the condensate loading and direct water vapor effect on buoyancy. The evolution of convection and its feedback to the environment are shown to have a considerable impact in making this relationship more precise. Virtual and condensate effects have a significant effect on plume buoyancy; however, the weighting between instability and subsaturation proves robust when these effects on plume buoyancy are introduced.