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Excerpted summary documentation of the representations of precipitation and planetary boundary layer in the AMIP version of the NCAR CCM2 model. Note usage of hyperlinks (indicated by underlined text) to cross-reference related summaries of convection, diffusion, and surface fluxes in the model, as well as bibliographic information on cited references (indicated by bracketed numerical footnotes).
Precipitation
Subgrid-scale precipitation is generated in unstable conditions by the moist convective scheme (see Convection). Grid-scale precipitation forms as a result of supersaturation under stable conditions. In this case, the moisture is adjusted so that the layer is just saturated, with the excess condensing as precipitation; the layer temperature is adjusted according to the associated latent heat release. (Moisture and temperature are mutually adjusted in two iterations.) Subsequent evaporation of falling precipitation is not simulated. Cf. Hack et al. (1993)[3] for details. Planetary Boundary Layer
The PBL height is determined by iteration at each 20-minute time step following the formulation of Troen and Mahrt (1986)
[26]; the height is a function of the critical bulk Richardson number for the PBL, u-v winds and virtual temperature at the PBL top, and the 10-meter virtual temperature, which is calculated from the temperature and moisture of the surface and of the lowest atmospheric level (at sigma = 0.992) following Geleyn (1988)[27]. Within the PBL, there is nonlocal diffusion of heat and moisture after Holtslag and Boville (1993)[9]; otherwise (and under all conditions for momentum), properties are mixed by the stability-dependent local diffusion that applies in the model's free atmosphere. See also Diffusion and Surface Fluxes.
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Last update September 8, 1995. For further information, contact Tom Phillips (phillips@tworks.llnl.gov)
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