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Revision of convection and vertical diffusion schemes in the NCEP Global Forecast System
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  • Description:
    A new physics package containing revised convection and planetary boundary layer (PBL) schemes in the National Centers for Environmental Prediction's Global Forecast System is described. The shallow convection (SC) scheme in the revision employs a mass flux parameterization replacing the operational turbulent diffusion based approach. For the deep convection scheme, the random cloud top selection in the current operational scheme is replaced by an entrainment rate parameterization with the rate dependent upon environmental moisture. The effects of the convection-induced pressure gradient force on cumulus momentum transport and convective overshooting are parameterized in both deep and shallow convection schemes and a modification of the trigger function has been developed. In addition, the PBL model is revised to enhance turbulence diffusion in stratocumulus regions. A remarkable difference between the new and operational SC schemes is seen in their heating behavior in lower atmospheric layers above the PBL. While the operational SC scheme using the diffusion approach produces a pair of layers with cooling above and heating below in lower atmospheric layers, the new SC scheme using the mass-flux approach produces heating throughout the convection layers due to the dominant environmental subsidence warming. In particular, the new SC scheme helps to form stratocumulus clouds in the regions off the West coasts of South America and Africa, whereas the old scheme destroys them. Significant improvements in the forecasts of the global 500 hPa height, vector wind, and precipitation over the continental US are found with the revised model. In particular, unrealistic forecasts of excessive heavy precipitation have been significantly reduced due to the revision of the convection
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