An attempt to describe the SAS
1. Parcel starting level : Chosen from the lowest 30% of
atmosphere the level with the largest moist static energy
2. Downdraft originating level : Chosen to be the level
minimum moist static energy.
3. Trigger : The level of free convection must exist and
within 150 hPa of the parcel starting level. Cloud depth must be
greater than 150 hPa.
4. Parcel property : parcel moist static energy and
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
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
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
12. Cloud workfunction for the modified column is
is the cloud workfunction change due to a unit parcel of updraft.
13. Closure is determined using quasi-equilibrium
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
resulting mass flux.
15. Precipitation is scaled from the mass flux.
16. Re-evaporation of precipitation is scaled following the