CGD's Dr. Fabrizio Sassi

Abstract

Chemical ozone loss in the polar lower stratosphere is derived from an ensemble of simulations from the Whole Atmosphere Community Climate Model (WACCM3) for the period 1950-2003, using the tracer-tracer correlation technique. We describe a detailed evaluation of diagnostics in the polar region: vortex temperature, sharpness of the vortex edge, and the potential of activated chlorine (PACl). PACl is a measure that includes meteorological and chemical information about the polar vortex (temperature, vortex size, and activation time, and level of Equivalent Effective Stratospheric Chlorine). Discrepancies of the relationship between chemical ozone loss and PACl between model and observations are discussed. Simulated PACl for Antarctica is in good agreement with observations, owing to slightly lower simulated temperatures and a larger vortex volume than observed. Observed chemical ozone loss of 140 ±; 30 DU in the Antarctic vortex core are reproduced by the WACCM3 simulations. However, WACCM3 with the horizontal resolution used here (4° latitude × 5° longitude) is not able to simulate the observed sharp transport barrier at the polar vortex edge. Therefore, the model does not produce an homogeneous cold polar vortex. Warmer temperatures in the outer region of the vortex result in less chemical ozone loss over the entire polar vortex than observed. For the Arctic, WACCM3 temperatures are biased high (by 2-3 degrees in the annual average) and the vortex volume and chlorine activation period is significantly smaller than observed. WACCM3 Arctic chemical ozone loss only reaches 20 DU for cold winters, where observations would suggest ~80-120 DU.

Figure caption: Distribution of different species, O₃ (first row), N₂O (second row), HN)₃ (third row) and H₂) (fourth row) in the Antarctic polar vortex on the 475 K isentropic surface for different days of one WACCM3 realization. White lines indicate the poleward edge, the edge and the equatorward edge of the polar vortex using the definition derived by Nash, et al, [1996].

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Modeling the whole atmosphere response to solar cycle changes in radiative and geomagnetic forcing.

Marsh, D.R., R.R. Garcia, D.E. Kinnison, B.A. Boville, F. Sassi, and S.C. Solomon. Modeling the whole atmosphere response to solar cycle changes in radiative and geomagnetic forcing. J. of Geophys. Res. In press, 2007.

Abstract

The NCAR Whole Atmosphere Community Climate Model (WACCM), version 3, is used to study the atmospheric response from the surface to the lower thermosphere to changes in solar and geomagnetic forcing over the 11-year solar cycle. WACCM3 is a general circulation model that incorporates interactive chemistry that solves for both neutral and ion species. Energy inputs include solar radiation and energetic particles, which vary significantly over the solar cycle. This paper presents a comparison of simulations for solar cycle maximum and solar cycle minimum conditions. Changes in composition and dynamical variables are clearly seen in the middle and upper atmosphere, and these in turn affect the terms in the energy budget. Generally good agreement is found between the model response and that derived from satellite observations. A small but statistically significant response is predicted in tropospheric winds and temperatures which is consistent with signals observed in reanalysis datasets.

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Simulation of Secular Trends in the Middle Atmosphere, 1950-2003

Garcia, R.R., D.R. Marsh, D.E. Kinnison, B.A. Boville, F. Sassi. Simulation of Secular Trends in the Middle Atmosphere, 1950-2003. J. of Geophys. Res. In press, 2007.

Abstract

We have used the Whole Atmosphere Community Climate Model to produce a small (three-member) ensemble of simulations of the period 1950-2003. Comparison of model results against available observations shows that, for the most part, the model is able to reproduce well the observed trends in zonal-mean temperature and ozone, both as regards their magnitude and their distribution in latitude and altitude. Calculated trends in water vapor, on the other hand, are not at all consistent with observations from either the HALOE satellite instrument or the Boulder, Colorado, hygrosonde dataset. We show that such lack of agreement is actually to be expected because water vapor has various sources of low-frequency variability (heating due to volcanic eruptions, the quasi-biennial oscillation and ENSO) that can confound the determination of secular trends. The simulations also reveal the presence of other interesting behavior, such as the lack of any significant temperature trend near the mesopause, a decrease in the stratospheric age of air, and the rare occurrence of an extremely disturbed southern hemisphere winter.

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The SPARC DynVar Project: A SPARC Project on the Dynamics and Variability of the Coupled Stratosphere-Troposphere System

Kushner, P.J., J. Austin, M.P. Baldwin, N. Butchart, M.A. Giorgetta, P.H. Haynes, E. Manzini, N.A. McFarlane, A. O'Neill, J. Perlwitz, L.M. Polvani, W. Robinson, F. Sassi, J.F. Scinocca, T.G. Shepherd. SPARC Newsletter, 29, 9-14, 2007.

In light of the growing need to understand the global climate system and its future evolution, stratospheric science requires a renewed and sustained research focus. Although we have known for some time that the tropospheric circulation influences the stratosphere, we have more recently learned that the stratosphere can in turn influence the tropospheric circulation all the way to the surface. This two-way stratosphere-troposphere coupling implies that the stratosphere can significantly influence the global climate system and the pattern and magnitude of global climate change. The problem of stratospheric ozone depletion has already demonstrated how human activity can affect a critical component of the global climate system, how a systematic international research effort is required to understand and solve a global environmental problem, how this research needs to be communicated to society, and how ongoing scientific assessment is essential to evaluate the effectiveness of solutions to the problem. All this makes clear that the stratosphere is an integral part of the climate change problem and will continue to be a crucial component of research on climate change science, impacts, and mitigation