NASA-IPY: The Roles of Gravity Waves in the Polar Atmosphere

Flow over topography, latent heating associated with polar fronts, and jetstream fluctuations can generate gravity waves (GWs) that propagate into the stratosphere. While the coupling processes between near-surface disturbances and those in the middle atmosphere are still unclear, GWs are thought to play a key role in interacting with planetary-scale flows and distributing energy at different parts of the atmosphere. Damping of GW can significantly alter the stratospheric circulation, distorting the polar vortex and the underlying planetary-scale disturbances. Anomalous changes in the polar vortex have been suggested to bias near-surface climate conditions. Perturbed polar dynamics on mesoscale and planetary scales can have significant implications in the formation of polar stratospheric clouds (PSCs) which could facilitate stratospheric ozone destruction.

The goal of this study is to investigate the roles of GWs in coupling the polar troposphere to the stratospheric evolution. The study aims to better understand the dynamical connection between GWs and planetary waves, to access GW forcing on the polar vortex, and to examine the role of GWs in PSCs formation. As such, our objectives are (1) to characterize the sources/properties of GWs in the polar region and their propagation into the stratosphere, (2) to assess the interactions between GWs and planetary-scale disturbances in the polar region, and causes of wave intensification, and (3) to estimate impacts of GW perturbations on the formation of PSCs in the lower stratosphere.

Methods: To achieve these objectives, GW simulations will be performed using the ARPS weather model (extended up to the mesosphere) to explicitly resolve GWs of horizontal scales greater than 50 km. Focus will be placed on events in which synoptic disturbances significantly raise the tropopause. These cases are often associated with enhanced cloudiness (in the troposphere and the stratosphere) and strong GW generation. Model results will be compared with satellite observations like NASA AIRS radiance to elucidate wave properties and their sources. Gravity wave forcing of the polar stratosphere in relation to planetary waves will be analyzed using model results and reanalyses data from GEOS-4. In assessing the impact of gravity waves on PSC formations, model results are used to diagnose and interpret simultaneous satellite observations of temperature, water vapor, and PSCs, including those from Aura HIRDLS, ENVISAT SCIAMACHY, Aura MLS, and local soundings. The cloud occurrence frequency generated with/without the presence of gravity waves will be compared to quantify overall wave impacts in various meteorological conditions.

Significance: Improved understanding of GW can provide further insights on the formation of PSC (linked directly to ozone depletion) and the polar stratosphere (which are coupled to tropospheric climate). By shedding light on GW source/characteristics and influence on the polar stratosphere, this study can improve how GW effects are incorporated in global climate models and thereby increase the models' effective in assessing climate variability and changes. To this end, the proposed study addresses two scientific questions raised by NASA Earth Science Enterprise Strategy: "how will future changes in atmospheric composition affect ozone and climate?" and "how can predictions of climate variability and change be improved?" Importantly, the proposed investigation will complement and be enhanced by several IPY projects on modeling, observations, and data assimilation, in conjunction with ice clouds over the polar region. Like these projects, our objectives address the IPY themes of: (1) "Current state of the [polar] environment", (2) "Change in the polar regions", (3) "Polar-global linkages/teleconnections", and (4) "Exploring new frontiers [in polar science]".