Evaluating Simulated Convective Hydration Processes in the Midlatitude Stratosphere Against Airborne Observations

Recent analyses of data collected during the Dynamics and Chemistry of the Summer Stratosphere (DCOTSS) field campaign have revealed the routine diversity of processes by which tropopause-overshooting convection hydrates the midlatitude lower stratosphere. Such hydration is of potential significance to the radiative budget and climate, since the radiative forcing of water vapor is most sensitive to changes in its lower stratosphere concentration and increases driven by convection exacerbate observed warming. While many previous studies have revealed the existence of key constraining processes for stratospheric hydration by convection, such as air transport & mixing, ice sublimation, and thermodynamic constraints, DCOTSS revealed the frequency at which each operates in observed events.

This study leverages the Weather Research and Forecasting (WRF) model and a custom passive tracer suite to determine whether a numerical model can reproduce the observed diversity of constraining stratospheric hydration processes in overshooting convection. DCOTSS cases are simulated at convection-allowing resolution (3-km horizontal grid spacing, 250-m vertical grid spacing), with sensitivities to WRF microphysics and boundary layer parameterizations explored. Output from the model is compared to DCOTSS observations of ozone, water vapor, and meteorological quantities collected by the NASA ER-2. It is found that the commonly-used WRF design for studies of overshooting convection is able to reproduce the diversity of constraining processes, but substantially under-represents the relative significance of ice sublimation as a pathway for stratospheric hydration. Implications for future modeling efforts including ideal design and required development efforts will be emphasized.