Precipitation convective-stratiform partition in relationship to the thermodynamic environment: dependence on moisture convergence for CMIP6 model diagnostics

Abstract:

Precipitation-buoyancy relationships estimated using column water vapor (CWV) capture leading-order effects of conditional instability on deep convection. Here we examine how these relationships are altered in presence of strong moisture convergence or of convective organization. When conditioned on CWV, precipitation sharply increases as CWV exceeds a critical threshold, and this relation is insensitive to mean moisture convergence from subdaily to monthly timescales (using TRMM precipitation retrievals and ERA5 reanalysis). Moisture convergence, especially monthly mean, determines the probability distribution of CWV, thus the frequency and accumulation of precipitation. Examining these relationships in CMIP6-class models, some of the models can simulate the observed precipitation-CWV relationship. However, among these models, both convective and large-scale precipitation exhibit a dependence on moisture convergence, at odds with observations. We suspect that some behavior is being simulated at the grid scale that should be occurring at smaller scales. For perspective from observations for precipitation organized at different scales, we further examine the onset statistics for precipitation within clusters meeting a criterion for mesoscale organized systems (MCS) versus other convection. Precipitation within MCS occurs for a noticeably smaller critical CWV threshold, and greatly alters the partition between convective and stratiform rain. MCS precipitation exhibits ledges for below-critical CWV values, due primarily to increased stratiform contribution in MCS. Moisture convergence also shows a greater influence on precipitation-buoyancy relations in or near organized systems, with strong moisture convergence (divergence) associated with higher rain rate at a given CWV leading (lagging) MCS events. Differences in how the environmental temperature/moisture adjusts inside vs outside MCS are also noted. The extent to which global model grid size occurring within the mesoscale might affect such statistics will be discussed.