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VII Waves

9.2 The ECMWF ozone parameterization

The parameterization used in the ECMWF model is an updated version of Cariolle and Déqué (1986), which has been used in the ARPEGE climate model at Météo-France. This parameterization assumes that chemical changes in ozone can be described by a linear relaxation towards a photochemical equilibrium. It is mainly a stratospheric parameterization. The relaxation rates and the equilibrium values have been determined from a photochemical model, including a representation of the heterogeneous ozone hole chemistry. The updated version of the parameterization (with coefficients provided by Pascal Simon, Météo-France) is

(9.2)

where

(9.3)

Here are the relaxation rates and , , and are photochemical equilibrium values, all functions of latitude, pressure, and month. is the equivalent chlorine content of the stratosphere for the actual year, and is the only parameter that varies from year to year (see Fig. 9.1 ). For the ECMWF model it was necessary to replace the photochemical equilibrium values for ozone with an ozone climatology (Fortuin and Langematz, 1995) derived from observations. The heterogeneous part is only turned on below a threshold temperature of 195 K.

Figure 9.1 Equivalent chlorine content of the stratosphere in ppt for the heterogeneous chemistry part of the ozone source/sink parameterization (provided by Pascal Simon, Météo-France).