Geoscience Reference
In-Depth Information
11.5.1.6
Large-Scale Condensation and Precipitation
The original version of the simplified diagnostic large-scale clouds and precipitation
scheme currently used in the minimization of 4D-Var is described in
Tompkins and
Janiskova
(
2004
). Only a summary of its main features is given here.
The physical tendencies of temperature and specific humidity produced by moist
processes on the large-scale can be written as
@q
@t
D
C
ce
C
E
prec
C
D
conv
(11.32)
@T
@t
D
L.C
ce
E
prec
D
conv
/
C
L
f
.F
rain
M
snow
/
(11.33)
where
C
ce
denotes large-scale condensation/evaporation,
E
prec
is the moistening due
to the evaporation of precipitation and
D
conv
is the detrainment of cloud water from
convective clouds.
F
rain
and
M
snow
correspond to the freezing of rain and melting
of snow, respectively.
L
and
L
f
are the latent heats of vaporisation/sublimation and
fusion, respectively.
Condensation
The subgrid-scale variability of humidity is assumed to be represented by a uniform
distribution. This allows the estimation of the stratiform cloud fraction,
C
strat
,and
strat
c
cloud condensate amount,
q
, from the grid-box relative humidity,
RH
,as
s
1
RH
1
RH
crit
.RH
RH
crit
/
C
strat
D
1
(11.34)
D
q
sat
C
strat
˚
.1
RH/
C
.1
/.1
RH
crit
/
strat
c
q
(11.35)
where
q
sat
is the saturation specific humidity. The critical relative humidity thresh-
old,
are specified as in
Tompkins and Janiskova
(
2004
).
A simple diagnostic partitioning based on temperature is used to separate cloud
condensate into liquid and ice.
The impact of convective activity on large-scale clouds, which is particularly
important in the tropics and mid-latitude summers, is accounted for through the
detrainment rate produced by the convection scheme. This detrainment term is
used to compute the additional cloud cover and cloud condensate resulting from
convection (i.e. convection called before condensation).
RH
crit
, and the coefficient
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