Verification of precipitation calculation method based on parameterization of distribution function evolution. I.V.Akimov Hydrometeorological Centre of Russia 11-13 Bolshoy Predtechensky per., 123242, Moscow-242, Russia. E-mail: [email protected] The new precipitation calculation method presented in [1] was evaluated. The method is based on different approach, than used in Kessler-type parameterizations. In Kessler approach [2] the precipitation beginning from cloudy layer is permitted after cloud water content reaches some value, named as autoconversion threshold. But the value of this threshold could not be obtained from the measurements and the hypothesis about such threshold contradicts with the observed facts that precipitation starts from clouds when cloud water content rises up to quite different values [3,4]. Therefore the new method is based on consideration of precipitation beginning process as formation of part of cloud spectra consisting of large drops which falls out as a precipitation. This is realized by parametric description distribution function evolution during precipitation formation process. The main processes that influence on distribution function evolution are gravitational and turbulent coalescence in liquid clouds and sublimation of water vapour on ice crystals in mixed phase clouds. The proposed method was included as precipitation parameterization module in global spectral model of the Hydrometcentre of Russia T85L31. The precipitation calculation module that was used in old version of model is based on Kessler parameterization. Therefore the comparison of precipitation fields obtained from new version of model with results obtained from old version allows to evaluate the distinction between two different parameterization approaches. Two scores were chosen for verification analysis. The accuracy of precipitation spatial distribution forecast was characterized by Pearcy-Obychov criterion T (also known as Hanssen and Kuipers discriminant). The value of this score vary from T=1 for ideal forecast to T=-1 for absolutely incorrect forecast. The accuracy of precipitation quantity calculation was characterized by root-mean-square error σQ. The evaluation of precipitation forecasts obtained from two versions of model was conducted during time period March-December 2002. The values of T and σQ scores are presented on the figure. The estimation was performed using precipitation observation data from four regions in northern hemisphere. Figure shows that T criterion values resulted from new model version precipitation forecast is greater during all estimations period than values obtained from old model version integration. So for European part of Russia in period April-July criterion T value rises from level 0.2-0.3 to level 0.3-0.5. The usage of new precipitation calculation method also reduces forecast error σQ in all regions except North America. The values σQ is became smaller on 10-20% in comparison with errors resulted from old version of model. In North America at time period June-August the new version of model gives σQ values that overcame error level of old model version. The reason of this fact that new precipitation calculation method uses empirical relations and values of parameters, which were obtained from aircraft measurements data performed in middle latitudes region. But the weather conditions in south coast of North America in summer season is close to tropical weather. Therefore the usage of such parameters in this season is not quite correct. The possible way to eliminate this is to select different values of parameters included in new method for different climatic regions. In common figure shows that incorporation of new method in global spectral atmospheric model resulted in improvement of precipitation forecast in midlatitude region. a) b) c) d) Pearcy-Obychov criterion T and forecast root-mean square error σQ for 24 hours precipitation forecast, calculated for verification period March - December 2002. Versions of model: 1- old version, 2- version including new method of precipitation calculation. Regions of estimations: European part of Russia (a), Central Europe (b), West Europe (c), North America (d). References: 1. Akimov I.V., 2003. Method of precipitation intensity calculation based on microphysical processes parameterization in liquid and mixed phase clouds. Izvestia. Atmospheric and oceanic physics. v. 39. 2. Kessler E., 1969. On the distribution and continuity of water substance in atmospheric circulations. Meteor. Monogr., vol. 10. 3. Matveev L.T., 2000. Physics of atmosphere. S.Pb. Hydrometeoizdat. 4. Mason B.J., 1971. The physics of clouds. Clarendon Press, Oxford. ¢¡¤£¦¥¨§ © ¥ ¦¥ ! 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