CONVECT 4.3    

                          Preparation Essentials

The following steps should be taken to set up CONVECT for use in atmospheric models:

1. Set the parameter NA in the initial PARAMETER call statement to a value
equal to at least ND +1, where ND is the vertical dimension of the arrays
of pressure, temperature, etc. that will be input to the subroutine.

2. Set the values of the switches IPBL and MINORIG. IPBL should be set to 0
if the calling model has a boundary layer scheme; otherwise it should be
set to any other integer. If IPBL is not 0, then CONVECT performs dry
adiabatic adjustment and returns altered values of the arrays T,Q,QS,U,V
and TRA; otherwise, these arrays are unaltered. The dry adiabatic
adjustment is useful in simple models, such as single-column models, with
no boundary layer schemes of their own. MINORIG is the minimum level from
which moist convection is allowed to originate.  Ordinarily, it should be
set to 1, the lowest model level at which temperature is defined. But it
should not be set to 1 in models with very high resolution near the bottom
of the boundary layer.  In this case, MINORIG should be set to the lowest
model level above the surface layer. The layer thickness in the layer
corresponding to MINORIG should be at least 75 meters or so for a time step
of 10 minutes, larger for larger time steps. If the layer thickness is too
small, CONVECT will return IFLAG=4.

3. Examine the values assigned to thermodynamic constants CPD, CPV, etc. in
BOTH CONVECT and in SUBROUTINE TLIFT. Change them, if necessary, to be
consistent with values used by the calling model.(But note that CL is artifically 
small; please do not change this value.)

4. Decide whether you want CONVECT to act on passive tracers or not. If
not, define a dummy array TRA of dimension (ND,1) filled with 0's, to pass
to CONVECT, and set NTRA to 0.

5. Define a storage array CBMFIJ of dimensions I,J, where I and J are the
horizontal indices of the calling model variables. (For two-dimensional
models, CBMFI should be a one-dimensional array.) CBMFIJ should be
initialized to zero at the beginning of the time integration. At each time
step at each grid column, set the scalar CBMF to CBMFIJ for that column
before passing CBMF to CONVECT. After the call to CONVECT, set CBMFIJ equal
to the new value of CBMF returned by CONVECT. (CBMFIJ contains the cloud
base mass fluxes at each grid column; these must be "remembered" between
calls to CONVECT.  The scalar CBMF is therefore essential both as input to
and output from CONVECT.)

6. At each grid column at each call to convect, assign one-dimensional
arrays of temperature (T, in Kelvin), specific humidity (Q, in gm/gm),
saturation specific humidity (QS, in gm/gm), first horizontal velocity
component (e.g. zonal wind) (U, in m/s) second horizontal velocity
component (e.g. meridional wind) (V, in m/s), pressure (P, in mb) and
pressure at layer interfaces (PH, in mb). Also assign values to the
two-dimensional array TRA, whose second index corresponds to the tracer
number. (Thus, if the model carries ozone and methane, for example,
TRA(ND,1) might correspond to ozone while TRA(ND,2) corresponds to
methane. The tracer content is defined as a mass mixing ratio.) MAKE SURE
THAT THE FIRST VALUE OF EACH ARRAY CORRESPONDS TO THE LOWEST MODEL LEVEL.

7. The values of T,Q,QS,U,V,TRA and P should all be defined at the same
levels. But the array PH must be defined at the model half-levels, such
that the first value of PH is LARGER than the first value of P. (For
example, P(1) might be 1000 mb while PH(1) might be 1020 mb.) It is not
necessary that P and PH remain exactly constant in successive calls to
CONVECT (as will be the case, for example, in models whose vertical
coordinate is other than pressure.)

PERFORMANCE OPTIMIZATION

The only internal parameter that might need to be adjusted is ALPHA, which
controls the rate of approach to statistical equilibrium. A value of ALPHA
should be sought such that changing it by a factor of 2 in either direction
produces little change in the performance of the scheme. In general, when
ALPHA is too large, detailed time series of precipitation at many grid
points will be noisy. If ALPHA is too small, convection may be
under-active. Some experiments should be performed to insure ALPHA lies in
a range for which the model is relatively insensitive to its value.