Climate Characteristics of the CWB Global Forecast System:Hydrological Processes and Atmospheric Circulation

Abstract

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 documents the winter (December-February) and summer (June-August) climate characteristics 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 compared to observations and analyses of the atmosphere. As for the climate characteristics of hydrological processes, the major features of observed precipitation, such as the Intertropical Convergence Zone (ITCZ), the Asian monsoon 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 subsidence zones and polar areas, are also well defined by the GFS.

The simulated precipitation pattern, however, exhibits some obvious discrepancies from the observed in the tropics. Excessive precipitation is simulated by the GFS over some tropical regions where there are complex topographic variations among oceans and lands. Otherwise, the GFS precipitation in the remaining tropical regions is generally underestimated. In particular, the underestimate of model precipitation over the tropical eastern Pacific results in a local ITCZ that is less organized in spatial structure than the observed. This model precipitation deficiency is linked to underestimates of 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 mean climatologies of temperature and zonal wind are adequately simulated by the GFS when compared to analyses. The major difference between the simulated and analyzed zonal mean temperatures is a systematic cold bias in the model troposphere. This cold bias is generally within 4°K of the analyses for most of the tropospheric domain bounded by 40°S and 40°N. The model cold bias becomes significant at the polar tropopause, where the simulated zonal mean temperature can be from 8°K to 18°K colder than the analyzed. Also noteworthy is the spatial relationship between the zonal mean temperature bias and zonal mean zonal wind bias. This is found to be consistent with the spatial relationship between the real temperature and the real zonal wind fields known as the thermal wind relationship. This finding suggests that interactions between the thermal and dynamic fields in the GFS simulation must be to a great extent consistent with analyses with regard to the thermal wind relationship.

Regarding the climate characteristics of atmospheric circulation, the primary circulation features associated with the summer monsoon system and winter teleconnection pattern are well represented in the GFS simulation when compared with analyses. Nevertheless, in winter, major differences between the analyzed and simulated circulation fields include the underestimate of the East Asia subtropical jet features and the overestimate of the North America subtropical jet features in the GFS simulation. In the summer simulation, the major circulation bias is that the zonal wavenumber-2 component of the Northern-Hemisphere 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 America/Caribbean Sea region.

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