The Role of the Free-Tropospheric Moisture for Convective Onset from Radio Occultation

Previous work has examined the precipitation-buoyancy relationship using radiosonde data and suggested the importance of lower free-tropospheric moisture to convective onset. While soundings provide vertical moisture distribution needed for estimating buoyancy (i.e., conditional instability), they can only be acquired at designated sites, leaving much of the globe uncovered, especially ocean regions. Utilizing the Global Navigation Satellite System (GNSS) signals, satellite radio occultation (RO) can provide the desired moisture profile with much higher frequency and near-global coverage. In this study, the precipitation-buoyancy relationship is compiled from the COSMIC RO data to assess the relative importance of moisture in different atmospheric layers to convective onset. The resulting average precipitation rate conditioned on plume buoyancy estimated with deep-inflow mixing shows a sharp pickup as buoyancy increases, and exhibits little quantitative variation across climate regimes. The contributions of free-tropospheric water vapor to buoyancy variations in the key regime of strong precipitation are estimated, as is the sensitivity to various assumptions relevant to model parameterizations. The extent to which removal of condensate by precipitation below the freezing level reduces the sensitivity to freezing processes in the upper troposphere is quantified. In addition, analytic approximations to the plume buoyancy that are useful for understanding dominant physical processes and to reduce computational cost are tested. These results from the COSMIC RO data provide more precise constraints on convective onset for convective parameterizations and model diagnostics.