Investigating the Model Representation of Cloud Tops: Toward a Better Understanding of Overshoot Dynamics

Overshooting convection can significantly impact the chemical and radiative properties of the upper troposphere and lower stratosphere (UTLS), and thus affect climate change, through the transport of various chemical species. Convective updrafts provide a mechanism for transporting boundary layer air and its chemical constituents into the UTLS on time scales and in extratropical locations that are not possible via large-scale circulations. Additionally, overshooting updrafts are often correlated with increased reports of strong turbulence by commercial aircraft and with more significant severe storm impacts at the surface. For each of these reasons, accurately predicting the occurrence of overshooting tops and understanding their dynamics is of high importance.

We will present an analysis of the performance of the High-Resolution Rapid Refresh (HRRR) operational model in representing overshoots. Climatologies of echo top heights, derived from approximately 35,000 individual updrafts, were produced and compared to distributions developed from radar observations. These distributions showed a noticeable low bias in echo top heights forecast by the HRRR, both in ground- and tropopause-relative coordinates. Inconsistencies in the diurnal cycle of overshooting from the HRRR were also found and will be discussed. These errors persisted at shorter forecast lead times and show that work is needed to improve model simulations of overshooting convection.

To investigate the potential effects of grid resolution on poor prediction of overshoots in operational models, we will also introduce preliminary findings from idealized, high-resolution (dx, dz ~100 m) Cloud Model 1 (CM1) simulations of overshooting convection in environments sampled during the NASA Dynamics and Chemistry of the Summer Stratosphere (DCOTSS) field campaign. With these simulations the processes that control the depth of overshooting, deviations from parcel-theory-based predictions of overshoot height, and the initial transport of mass from updrafts into the surrounding stratosphere will be explored.