P2.1 Redistribution of Angular Momentum in a Global Forecast Model Due to Change in Drag Parameterizations

Contemporary numerical models of the global atmosphere include parameterizations of various drag mechanisms, e.g., orographic and convective gravity wave drag (GWD), mountain drag, surface friction drag, and artificial model top drag. These mechanisms, except for friction drag, directly affect the magnitude of the polar night jet, whose variation significantly affects the tropospheric circulation (e.g., Boville 1984; Kodera et al. 1990; Kuroda 2002). Moreover, these mechanisms in the model induce indirect secondary circulations that significantly affect the troposphere through the “downward control (Haynes et al. 1991)”. Validation of these drag mechanisms in a global model can be done by comparing simulations or forecasts with observations or analyses of such first-order variables as the zonal wind and temperature, sea-level pressure and/or such derived quantities as the EliassenPalm (E-P) flux (e.g., Andrews et al. 1987) and the atmospheric angular momentum (AAM). In particular, the AAM is a globally conserved quantity and its global imbalance in an atmospheric model is considered to produce systematic errors in short-term forecasts as well as climate drift in long-term simulations (Huang et al. 1999). It is regarded as a useful diagnostic quantity for evaluating global numerical weather prediction models (Bell et al. 1991; Salstein et al. 1993). The budget of the AAM calculated from analyses can be indirectly compared with the model-simulated torques due to various drag parameterizations as an attempt to validate the parameterizations and also to understand the redistribution of the AAM due to the parameterizations (e.g., Swinbank 1985; Boer and Lazare 1988; Boer 1990; Lejenäs et al. 1997).