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Difference fields of zonal mean temperature (a) and zonal mean zonal wind (b) between two TIME-GCM simulations of the 2002 Southern Hemisphere stratospheric sudden warming. The two simulations are nearly identical, except that in the control case the eastward components of gravity wave source spectrum are reduced. As a result, there is less mesospheric cooling and lower thermospheric warming in the Southern Hemisphere. The eastward jet reversal is also reduced. |
Atmospheric Gravity Waves are important because they cause a redistribution of momentum and energy, trigger convection, and induce mixing, which changes the transport of chemical species such as ozone. The vast spatial and temporal extent of gravity waves has important implications for the atmosphere from the mesoscale to the global scale and poses a stiff challenge to improve weather and climate predictions at all ranges. Gravity waves play an important role in studying the coupling of lower and upper atmospheric regions and are therefore of tremendous inter-disciplinary interest.
Given this significance in atmospheric studies, the Institute for Integrative and Multidisciplinary Earth Studies (TIIMES) hosted a Gravity Waves Retreat between 19 June and 7 July 2006 which brought together leading experts in atmospheric gravity wave studies from the U.S. and international institutions to explore various aspects of gravity wave studies in the context of lower and upper atmosphere coupling. The retreat yielded a definitive report, Gravity Waves in Weather, Climate, and Atmospheric Chemistry: Issues and Challenges for the Community, containing several key recommendations. Among these were the formation of a working group under the Scientific Committee on Solar-Terrestrial Physics (SCOTEP), the Stratospheric Processes and their Role in Climate (SPARC) and NCAR to facilitate interactions between mesoscale modelers, gravity wave theorists and observationalists; the initiation of a collaboration between Fuqing Zhang (Texas A&M University) and the Whole Atmosphere Community Climate Model (WACCM) group (Rolando Garcia, Fabrizio Sassi, Jadwiga Richter) to develop a source spectrum parameterization for baroclinic jet-front systems for climate models. Stronger links with the convection community are envisioned (Mitch Moncrieff, Rit Carbone, M. Joan Alexander, and Richter), including a project to extend the Weather Research and Forecast (WRF) model with a radiative upper boundary condition.
An important part of the NCAR mission is to understand the coupling of lower and upper atmosphere through dynamical, chemical, and radiative processes. Sudden stratospheric warming (SSW) involves dynamical changes on vastly different scales from the troposphere to the lower thermosphere, and thus provides us an opportunity to understand the coupling process. Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics/Sounding of the Atmosphere using Broadband Emission Radiometry (TIMED/SABER) temperature measurements have shown that mesospheric cooling is both weaker and occurs at lower altitudes than previous studies had indicated (Siskind et al., 2005). Ground-based observations showed that the mesospheric cooling is generally weak in the subsequent major Southern SSW (Vincent, private communication). It was hypothesized that such discrepancies stem from uncertainties in gravity wave specification and parameterization in General Circulation Models (GCMs). For this reason, Hanli Liu and Raymond Roble performed Thermosphere-Ionosphere-Mesosphere-Electrodynamic General Circulation Model (TIME-GCM) simulations of the 2002 Southern SSW with varying wave sources. From these simulations, it was found that the mesospheric cooling and thermospheric warming and the mesosphere and lower thermosphere (MLT) wind change are reduced if the eastward components of the gravity wave sources are reduced during the southern winter season. Further wave source sensitivity studies and observations, will help to constrain such gravity wave sources. |
