Tuesday, 13 January 2004
Redistribution of angular momentum in a global forecast model due to change in drag parameterizations
Various drag mechanisms are currently represented by parameterizations in numerical models of the atmosphere. For global models that include the middle atmosphere, these mechanisms profoundly affect weather forecast as well as climate simulation. A global spectral weather forecast model is used to investigate the response of the model to various drag mechanisms, such as orographic and convective gravity wave drag, mountain drag, surface friction drag, and artificial model top drag. A series of ensemble simulations corresponding to January and July 2000 have been performed with the forecast model of NOGAPS (Navy Operational Global Atmospheric Prediction System). From the analyses of atmospheric angular momentum budget, it is found that the various drag mechanisms of the model - when modified - interact with one another by redistributing their drag while conserving the total amount. An overestimation of one drag mechanism, for example, can result in an under-estimation of others thereby breaking an optimal balance among the mechanisms. This – although apparently obvious - has an important implications for numerical modeling of the atmosphere: when a drag parameterization or equivalent mechanism is added into (or removed from) a model, not only the amount of the new drag, but also its balance with existing mechanisms must be checked, which may require adjustment of the existing mechanisms.