An attempt to describe the SAS convection scheme

1. Parcel starting level : Chosen from the lowest 30% of the atmosphere the level with the largest moist static energy

2. Downdraft originating level : Chosen to be the level above the minimum moist static energy.

3. Trigger : The level of free convection must exist and must be within 150 hPa of the parcel starting level. Cloud depth must be greater than 150 hPa.

4. Parcel property : parcel moist static energy and moisture are modified below the level of free convection by the environment but is not modified for the rest of the updraft.

5. Cloud base and top: Cloud base is defined as the level of free convection and cloud top is the level where the parcel has less moist static energy than the environment. Cloud mass flux is assumed to detrain entirely at the cloud top.

5.1  A random cloud top is taken between the level of minimum moist-static energy and the tallest cloud top. Entrainment is computed to ensure that the cloud is neutrally buoyant at the new cloud top.

6. Cloud updraft: Cloud model used in the Arakawa-Schubert_Lord scheme is employed to modify the moisture property as the parcel rises and precipitation results (for a unit parcel mass flux).

7. Cloud workfunction is also computed as the vertical integral of the buoyancy term (with virtual temperature corrections).

8. Downdraft : saturated downdraft is used to calculate the evaporation for a unit downdraft mass flux. Downdraft mass flux is allowed to detrain below the cloud base.

9. Downdraft cloud workfunction : Modification of the cloud workfunction due to downdraft is similar to the updraft.

10. The column temperature and moisture (and momentum) change due to a unit of updraft and the corresponding downdraft (ratio determined by the evaporation efficiency, precipitation efficiency and a function of the vertical wind shear) are estimated using the 'compensating subsidence' concept of Arakawa and Schubert. Pressure-gradient effect for momentum is crudely parameterized in the form of entrainment.

11. A small-step forecast is done to modify the air column for unit mass flux.

12. Cloud workfunction for the modified column is evaluated. This is the cloud workfunction change due to a unit parcel of updraft.

13. Closure is determined using quasi-equilibrium assumption to determine the cloud mass flux. The climate cloud workfunction and the time-scale of adjustment are functions of the environmental vertical motion at the level of free convection.

14. Column temperature and moisture changes are scaled from the resulting mass flux.

15. Precipitation is scaled from the mass flux.

16. Re-evaporation of precipitation is scaled following the Kessler type microphysics.