Climate characteristics of the CWB Global Forecast System: Hydrological processes and atmospheric circulation

The purpose of this paper is to document the climate characteristics of the Global Forecast System (GFS), which is an atmospheric general circulation model developed at the Central Weather Bureau (CWB), Taiwan. This paper doc uments the winter (December-February) and summer (June-August) climate char acteristics of simulated hydrological processes and atmospheric circulation from a 2-year control simulation conducted with the GFS using an annually repeating prescribed sea surface temperature climatology. In most regards, the climate characteristics of hydrological processes and atmospheric circulation are reproduced reasonably well by the GFS when comp ared to observations and analyses of the atmosphere. As for the climate cha racteristics of hydrological processes, the major features of observed prec ipitation, such as the Intertropical Convergence Zone (ITCZ), the Asian mon soon regimes, and the extratropical storm tracks, are well captured in the GFS simulation. Similarly, other climate features of observed precipitation , namely the regions of low precipitation rates over the subtropical subsid ence zones and polar areas, are also well defined by the GFS. The simulated precipitation pattern, however, exhibits some obvious discrep ancies from the observed in the tropics. Excessive precipitation is simulat ed by the GFS over some tropical regions where there are complex topographi c variations among oceans and lands. Otherwise, the GFS precipitation in th e remaining tropical regions is generally underestimated. In particular, th e underestimate of model precipitation over the tropical eastern Pacific re sults in a local ITCZ that is less organized in spatial structure than the observed. This model precipitation deficiency is linked to underestimates o f precipitable water content and water vapor convergence over the tropical eastern Pacific in the GFS simulation. Regarding the climate characteristics of the zonal mean state, the zonal me an climatologies of temperature and zonal wind are adequately simulated by the GFS when compared to analyses. The major difference between the simulat ed and analyzed zonal mean temperatures is a systematic cold bias in the mo del troposphere. This cold bias is generally within 4 degrees K of the anal yses for most of the tropospheric domain bounded by 40 degrees S and 40 deg rees N. The model cold bias becomes significant at the polar tropopause, wh ere the simulated zonal mean temperature can be from 8 degrees K to 18 degr ees K colder than the analyzed. Also noteworthy is the spatial relationship between the zonal mean temperature bias and zonal mean zonal wind bias. Th is is found to be consistent with the spatial relationship between the real temperature and the real zonal wind fields known as the thermal wind relat ionship. This finding suggests that interactions between the thermal and dy namic fields in the GFS simulation must be to a great extent consistent wit h analyses with regard to the thermal wind relationship. Regarding the clim ate characteristics of atmospheric circulation, the primary circulation fea tures associated with the summer monsoon system and winter teleconnection p attern are well represented in the GFS simulation when compared with analys es. Nevertheless, in winter, major differences between the analyzed and sim ulated circulation fields include the underestimate of the East Asia subtro pical jet features and the overestimate of the North America subtropical je t features in the GFS simulation, in the summer simulation, the major circu lation bias is that the zonal wavenumber-2 component of the Northern-Hemisp here stationary eddy is simulated with larger amplitude than analyses. This circulation bias is accompanied by excessive precipitation biases over the subtropical central North Pacific west of the date line and the Central Am erica/Caribbean Sea region.