Sensitivity of Medium-Range Weather Forecasts to the Use of Reference Atmosphere

In this paper, the authors develop the earlier work of Chen Jiabin et al. (1986). In order to reduce spectral truncation errors, the reference atmosphere has been introduced in ECMWF model, and the spectrally-represented variables, temperature, geopotential height and orography, are replaced by their deviations from the reference atmosphere. Two modified semi- implicit schemes have been proposed to alleviate the computational instability due to the introduction of reference atmosphere. Concerning the deviation of surface geopotential height from reference atmosphere, an exact computational formulation has been used instead of the approximate one in the earlier work. To re duce aliasing errors in the computations of the deviation of the surface geopotential height, a spectral fit has been used slightly to modify the original Gaussian grid-point values of orography.A series of experiments has been performed in order to assess the impact of the reference atmosphere on ECMWF medium- range forecasts at the resolution T21, T42 and T63. The results we have obtained reveal that the reference atmosphere introduced in ECMWF spectral model is generally beneficial to the mean statistical scores of 1000-200 hPa height 10-day forecasts over the globe. In the Southern Hemisphere, it is a clear improvement for T21, T42 and T63 throughout the 10-day forecast period. In the Northern Hemisphere, the impact of the reference atmos phere on anomaly correlation is positive for resolution T21, a very slightly damaging at T42 and almost neutral at T63 in the range of day 1 to day 4. Beyond the day 4 there is a clear improvement at all resolutions.

Seasonally Varying Reference Atmospheres for East Asia

Vertical profiles of seasonally varying pressure, temperature, water vapor, and trace gases （O3, N2O, CO, CH4）, representing atmospheric conditions up to a height of 100 km over the East Asia region （30°-50°N, 110°-150°E） were constructed by using various observation data, model outputs of atmospheric thermodynamic parameters, and gaseous concentrations. Optical characteristics of the obtained East Asia reference atmospheres were compared with those from typical midlatitude summer and winter atmospheres. It was noted that, in the water vapor field, there are major differences between the two model atmospheres during the summer. The resultant impact during the summer of water vapor difference on incoming solar fluxes at the surface and emitted terrestrial fluxes at the top of the atmosphere are 14.3 W m^-2 and 6.5 W m^-2, respectively. On the other hand, the winter difference between East Asian and midlatitude atmospheres appears to be insignificant. Reference atmospheres for the spring and fall are also available. Utilizing the constructed atmospheric profiles as inputs to the radiative transfer model, it is expected that the constructed seasonally varying reference atmospheres can facilitate better descriptions of optical properties in East Asia.

Development of 1km-Scale Operational Model in South China

To support short-range weather forecasts,a high-resolution model(1km)is developed and technically upgraded in the South China Regional Center,including the improvement of the 3D reference scheme and the predictor-corrector method for semi-implicit semi-Lagrangian(SISL)in model dynamical core,as well as the improvement of physical parameterization.Furthermore,the multi-process parallel I/O and parallel nudging techniques are developed and have facilitated rapid updating in the assimilation prediction system and fast-output post processing process.The experimental results show that the improved 3D reference scheme and upgraded physic schemes can effectively improve the prediction accuracy and stability with a longer integration time step.The batch test shows that the precipitation forecast performance of 1-km model is significantly better than that of 3-km model.The 1-km model is in operation with a rapidly updating cycle at 12-minute intervals,which can be applied to short-range forecasts and nowcasting.