Geoscience Reference
In-Depth Information
Equations for Updraught and Downdraught
The equations describing the evolution with height of the convective updraught and
downdraught mass fluxes,
M
u
and
M
d
, respectively, are
@M
u
@
D
.
u
ı
u
/M
u
(11.28)
z
@M
d
@
D
.
d
ı
d
/M
d
(11.29)
z
where
respectively denote the entrainment and detrainment rates. A
second set of equations is used to describe the evolution with height of any other
characteristic,
and
ı
, of the updraught or downdraught, namely
@
u
@
D
u
.
u
/
(11.30)
z
@
d
@
D
d
.
d
/
(11.31)
z
where
in the large-scale environment.
In practice, (
11.28
)and(
11.29
) are solved in terms of
is the value of
base
u
D
M=M
,where
base
M
u
is the mass flux at cloud base (determined from the closure assumption as
described further down).
Triggering of Moist Convection
The determination of the occurrence of moist convection in the model is based on
whether a positively buoyant test parcel starting at each model level (iteratively from
the surface and upwards) can rise high enough to produce a convective cloud and
possibly precipitation. For an updraught starting from the lowest model level, its
initial temperature and moisture departures with respect to the environment and its
initial vertical velocity depend on surface sensible and latent heat fluxes, following
Jakob and Siebesma
(
2003
). When starting from higher model levels, the ascent is
initially set to 1 m s
1
and its initial thermodynamic characteristics are assumed to
be representative of a few hundred metre deep mixed-layer, with typical excesses
of 0.2 K for temperature and 110
4
kg kg
1
for moisture. A 200 hPa threshold for
cloud depth is prescribed to distinguish between shallow and deep convection. Mid-
level convection is treated as deep convection.
Search WWH ::
Custom Search