Abstract
Abstract. An extremely fast model to estimate the degree of stratospheric ozone depletion during polar winters is described. It is based on a set of coupled differential equations that simulate the seasonal evolution of vortex-averaged hydrogen chloride (HCl), nitric acid (HNO3), chlorine nitrate (ClONO2), active forms of chlorine (ClOx = Cl + ClO + 2 ClOOCl) and ozone (O3) on isentropic levels within the polar vortices. Terms in these equations account for the chemical and physical processes driving the time rate of change of these species. Eight empirical fit coefficients associated with these terms are derived by iteratively fitting the equations to vortex-averaged satellite-based measurements of HCl, HNO3 and ClONO2 and observationally derived ozone loss rates. The system of differential equations is not stiff and can be solved with a time step of one day, allowing many years to be processed per second on a standard PC. The inputs required are the daily fractions of the vortex area covered by polar stratospheric clouds and the fractions of the vortex area exposed to sunlight. The resultant model, SWIFT (Semi-empirical Weighted Iterative Fit Technique), provides a fast yet accurate method to simulate ozone loss rates in polar regions. SWIFT's capabilities are demonstrated by comparing measured and modeled total ozone loss outside of the training period.
Highlights
The importance of stratospheric ozone as a climate-active gas has long been recognized (e.g., Forster and Shine, 1997; Gauss et al, 2006; Forster et al, 2007)
Accounting for the interactions between climate change and ozone in climate models is usually accomplished by interactively coupling a stratospheric chemistry module to a global climate model (GCM, defined as a model consisting of a dynamical core and parameterizations for physical processes, but without a chemistry module): dynamical fields from the GCM provide input to the stratospheric chemistry module at a time step compatible with the GCM
A semi-empirical approach to modeling stratospheric ozone loss in both the Arctic and Antarctic has been developed with the goal of simulating as faithfully as possible the chemical mechanisms that drive polar ozone loss with a simple and fast model of vortex-average quantities
Summary
The importance of stratospheric ozone as a climate-active gas has long been recognized (e.g., Forster and Shine, 1997; Gauss et al, 2006; Forster et al, 2007). The radiative forcing induces changes in atmospheric temperatures, which in turn influence dynamics, the distribution of trace gases and temperature-dependent chemistry Such models are generally referred to as chemistry–climate models (CCMs), in contrast to GCMs without a chemistry scheme (Austin, 2002; Eyring et al, 2006, 2007). Prescribed ozone fields are unlikely to be aligned with the internal dynamics of the model; i.e., values typical of the polar vortex may be specified in regions outside of the vortex as a result of vortex excursions within the model, or lower stratospheric air may be prescribed in the upper troposphere if the model has an anomalously high tropopause on that day In such a model configuration atmospheric dynamics cannot interact with polar ozone chemistry.
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