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Current issues in GFS moist physics

This article discusses current issues and challenges related to moisture physics in the GFS model, including convective cloud fraction, moist turbulence, cloud fraction scheme, extending SAS to meso-scale models, hurricane intensity, and fast and slow physics.

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Current issues in GFS moist physics

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  1. Current issues in GFS moist physics Hua-Lu Pan, Stephen Lord, and Bill Lapenta

  2. Convective cloud fraction for radiation • Convection scheme leaves only the detrained condensate to the condensate budget. Convective cloud is assumed to disappear at the end of each step. • The mass flux scheme can be used to derive an ‘implied cloud’ that existed during the step. We can use this for radiation only. It does add to the total cloud fraction (mainly low and middle clouds in the convective region)

  3. Moist turbulence • Current GFS turbulence mixes temperature and moisture separately. This is not good for stratus and stratocumulus regions. Moist conserving variables would be a better way to go. • In partial cloudy region, there is a computational problem separating the cloudy and clear region after mixing. • A cloud fraction scheme is needed. We are looking into using a PDF assumption to solve the problem.

  4. Cloud fraction • We need cloud fraction for radiation, turbulence, and microphysics. Most of the advanced microphysics scheme assumes the grid to be saturated. For global models, we must deal with partial cloudiness. • We are working on using simple PDF assumptions to provide a consistent cloud fraction scheme for all physics.

  5. Extending SAS to meso-scale models • The basic assumption in the SAS (and in the original AS) is that the updraft area is small. Compensating subsidence is the primary physics that does the warming and drying. • Work is ongoing to remove this assumption is a way that will allow the SAS to work for model grids down to 1 km.

  6. Hurricane intensity issue • We are working with GFDL and HWRF to examine the impact of physics to the intensity problem. • Cumulus momentum exchange may need to be studied more with LES and CRM.

  7. ‘Fast’ and ‘Slow’ physics • Weather models need to deal with short-range forecasts of weather. So we have to emphasize fast physics : turbulence, onset of convections, diurnal signals of precipitation and evaporation. • Climate models need to deal with longer-term response of the physics, e.g. cloud-radiation feedback, impact and interactions with aerosols, etc. • Mixed problem also exist : land water budget. Sea ice budget, response to convective heating, restoration of CISK.

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