Assessing Relative Contributions of Instability and Lower-Free Tropospheric Moisture to Precipitation Onset in Observations and Reanalysis

In the bulk plume framework of convection, the convective updraft is initiated by warm and moist air rising out from the boundary layer into the free troposphere. The convective entity mixes with the ambient air in its ascent, diluting its buoyancy and bringing qualities of the plume, such as its humidity and temperature, closer to those of the environment. Framing plume buoyancy as a combination of these contributions poses buoyancy as a weighting between undilute instability (a CAPE-like quantity, CL) and the state of the ambient environmental air (a subsaturation-like quantity, SL). The weighting between these quantities is determined by the mixing rate---more mixing necessitates more humid ambient air or a stronger initial instability for the updraft to remain buoyant. Previous studies seek a value of this weighting that gives a strong relationship of buoyancy to precipitation, finding that nearly-equal contributions from CL and SL most optimally describe increases in precipitation or the precipitating state. However, the evolution of convection and its feedback to the environment complicates the estimations of this weighting. Using a combination of reanalysis and observational products (ARMBE, COSMIC-2 radio occultations, and ERA5) we distinguish between aspects of the convective time evolution, showing that buoyancy with CL and SL contributions evolves into a state with subsaturation components later. Virtual and condensate effects have a significant effect on plume buoyancy; however, the near-equal weighting between CL and SL proves robust when these effects on plume buoyancy are introduced.