Characteristics of Tropopause Polar Vortices Based on Observations Over The Greenland Ice Sheet
Tropopause Polar Vortices (TPVs) are long-lived, coherent vortices that are identified by closed material contours of potential temperature on the dynamic tropopause. They are characterized by PV anomalies that spend most of their lifetime in the Arctic, can impact their surrounding environment by introducing variability in sea ice, generate surface cyclones, and intensify midlatitude weather systems when influenced by midlatitude Rossby waves. While several studies have modeled the structure and climatology of TPVs, there is much to be discovered in terms of their evolution, intensification, and genesis. Case studies and composite studies from previous model simulations have shown that changes in TPV intensity can be attributed to diabatic forcings such as radiative cooling and latent heating. Specifically, models indicate that clear-sky longwave radiative cooling is an important factor in the maintenance of TPVs while clouds can contribute to large amplitude changes in response to cloud-top radiative cooling in the vortex core. This study uses cloud and atmospheric state observations from Summit Station, Greenland, in combination with single column experiments using the Rapid Radiative Transfer Model (RRTM) to investigate the effects of clear-sky, ice-only, and all-sky radiative cooling on TPV intensification.
As part of the Integrated Characterization of Energy, Clouds, Atmospheric State, and Precipitation at Summit (ICECAPS) project, observations of tropospheric and cloud properties at Summit Station, Greenland have been collected since 2010. This ground-based observing system combined with temperature and humidity profiles from the European Centre for Medium-Range Weather Forecasts’ fifth Reanalysis dataset (ERA5), which assimilates the twice-daily soundings launched at Summit, provides novel details of local characteristics of TPVs. Longwave radiative contributions to TPV diabatic intensity changes from both clouds and clear-sky water vapor effects are analyzed with these resources. A case study is broken down to consider cloud properties and the details of the radiative effects, followed by an in-depth composite study used to compare observed results to previously simulated results. Cloud and atmospheric state properties associated with TPVs are also analyzed in this study via a breakdown of strengthening and weakening cases. Results from the case study show that clouds can sometimes be influential in the intensification of a TPV. Composite results share promising similarities to previous studies in terms of atmospheric state structure and radiative structure. Vertical gradients of clear-sky positive radiative heating rate (RHR) anomalies show positive (negative) signatures for strengthening (weakening) TPVs, contributing positively (negatively) to EPV tendency, as expected. Cloud property results show little indication that various TPVs have similar cloud features, however the presence of cloud water may be more common in smaller and/or weaker TPVs.