Tropical precipitation relationship to the thermodynamic environment in CMIP6 models at ARM sites

Abstract:

A set of diagnostics based on simple, statistical relationships between deep convection and the thermodynamic environment in observations is implemented to assess Coupled Model Intercomparison Project Phase 6 (CMIP6) model behavior with respect to convective precipitation-related processes. Observational data from the Atmospheric Radiation Measurement field campaign (ARM) sites are augmented with satellite observations of precipitation and temperature as an observational baseline. A robust relationship across observational datasets between column water vapor (CWV) and precipitation in which conditionally-averaged precipitation exhibits a sharp pickup at some critical CWV value provides a useful convective onset diagnostic for climate model comparison. While a few models reproduce an appropriate precipitation pickup, most models begin their pickup at too low CWV and the increase in precipitation with CWV is too weak.. The discrepancies in model behavior are addressed and examined in the context of temperature, precipitation intensity, and event duration and accumulation biases. The onset of the pickup exhibits strong temperature dependence through column-averaged saturation specific humidity and most model pickups display some degree of temperature dependence; however, statistics compiled in column relative humidity (CRH) do very little in correcting the model pickups. Differences in the slopes of model pickups are conjectured to relate to event durations: slower pickups are a consequence of longer event durations. All models show a bias toward longer events and their distributions of precipitation intensities do not extend to the more intense rates recorded in the observations.