Atmospheric Intermediate Physics (SPEEDY) 

The atmospheric physics of intermediate complexity developed by Franco Molteni (called SPEEDY, because they are fast) are:

Surface fluxes of momentum and energy
Fluxes are defined by bulk aerodynamic formulas with different exchange coefficients between land and sea. Coefficients for (sensible and latent) heat fluxes also depend on the vertical gradient of potential temperature between the surface and the lowest model level.

Convection
A simplified mass-flux scheme is activated where conditional instability is present (namely, where saturation moist static energy decreases with height between the lowest layer (PBL) and the two upper-tropospheric layers), and where relative humidity in the PBL exceeds a fixed threshold. The cloud-base mass flux (at the top of the PBL) is such that the PBL relative humidity is relaxed towards the threshold value on a time-scale of 6 hours. Detrainment occurs only at the cloud-top level (determined by the conditional instability criterion), while entrainment occurs in the lower troposphere if the cloud top is at the highest tropospheric level. The air in the updrafts is assumed to be saturated.

Large-scale condensation
When relative humidity exceeds a fixed threshold, specific humidity is relaxed towards the corresponding threshold value on a time-scale of 6/8 hours, and the latent heat content removed from the atmosphere is converted into dry static energy.

Cloud cover
Cloud cover is defined diagnostically from the maximum relative humidity in an air column including all tropospheric layers except the PBL.

Short-wave radiation
SW radiation is reflected by clouds at the top of the troposphere and at the surface; the cloud albedo is proportional to the total cloud cover. SW transmissivity is a function of layer mass, specific humidity and cloud cover.

Long-wave radiation
Only one LW band is used at present (a two-band scheme is planned). LW transmissivity is a function of layer mass, specific humidity and cloud cover. The downward grey-body emission from each layer is a function of the temperature at the full level in the centre and of the (interpolated) temperature at the half-level below. For the upward emission, the temperature at the full level and at the half-level above is used.

Vertical diffusion (shallow convection)
Vertical diffusion only acts between the two lowest model layers. Dry static energy and specific humidity are diffused when a conditional instability criterion is satisfied. Otherwise, only humidity is diffused, at a slower rate.