Parameterization of the effects of Moist Convection in GCMs • Mass flux schemes – Basic concepts and quantities – Quasi-steady Entraining/detraining plumes (Arakawa&Schubert and similar approaches) – Buoyancy sorting • Raymond-Blythe, Emanuel • Kain-Fritsch – Closure Conditions, Triggering • Adjustment Schemes – Manabe – Betts-Miller
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Parameterization of the effects of Moist Convection in GCMs Mass flux schemes –Basic concepts and quantities –Quasi-steady Entraining/detraining plumes.
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Parameterization of the effects of Moist Convection in GCMs
• Mass flux schemes– Basic concepts and quantities– Quasi-steady Entraining/detraining plumes (Arakawa&Schubert
Convective-scale average (for a cumulus up/downdraft) :
cAcc dA
A 1
Environment average (single convective element):
AecL
e dAAA
1
Where 1 Lc AA
ec )1( ˆ)1(*
Vertical velocity: ec www )1(
)1(/, Oec
ec www ,
Vertical flux
eeeccc wwwwwww )]ˆ)(ˆ)[(1()])([( **
But since 1 ; ec ww
)()ˆˆ)(1()()( 2** Owwww eccc
For simplicity in the following we ignore the last term (to focus on predominantlyconvective processes) . Also ignore sub-scale horizontal fluxes on the boundaries of the large-scale area (but account for exchanges between convective elements and the environment via entrainment/detrainment processes).
Time average over cloud life cycle:
2/
2/
)(1
)(L
L
t
tL
tdtt
Cumulus effects on the larger-scales
Qz
wV
t H
)(
)()(
Start with a general conservation equation for
Plus the assumption:
(i) Average over the large-scale area (assuming fixed boundaries):
(similar to using anelastic assumption for convective-scale motions)
;
In practice (e.g. in a GCM) the prognostic variables are also implicitly time averages over convective cloud life-cycles
(ii) Apply cumulus scale sub-average to the general conservation equation, accounting for temporally and spatially varying boundaries:
cccc
bnc
c Qz
wwdlv
At)(
]))([()( **
Mass continuity gives:
0)(
z
awdlv
Atc
nc
; the outward directed normal flow velocity (relative to the cloud boundary)
nv
Entrainment (inflow)/detrainment (outflow):
dlvHvA
E nnc
)](1[
dlvHv
AD nn
c
)(
{ 0;10;0
)(
ff
fH
Define: dlvHvEA nb
c
nc
E )](1[ dlvHvDA nb
c
nc
D )(
Top hat: eE cD ; ;
Summary for a generic scalar, (top hat in cloud drafts):
otherQDz
Mz
wV
t
therefore
Qz
wMED
t
z
MED
t
otherQz
wM
z
wV
t
eDc
cccc
Dc
c
ccc
)1(
:
)(
0
])([
**
**
When both updrafts and downdrafts are present, both entraining environmental air:
dduucdduucc
dudududuc
DDDMMM
MMDDDEEEMMM
;
0;0;;;
Large-scale equations for dry static energy and water vapour
)(
/
..
qlqDz
qM
Dt
qD
dryingmoistening
HDsLlsDz
sM
Dt
sD
heating
uucc
uucc
Note that
..)(
..
..
HDz
hhMHDhEDh
z
hM
HDhhDz
hM
Dt
qDL
Dt
sD
uuu
u
uu
At the conv. layer top: ;0uM At c.l. base: hhu
[ Effects on horizontal momentum and associated dynamical heating: talk by Tiffany Shaw ]
Basic cumulus updraft equations (top-hat)
0
)(
)(
)(
)(
0
z
hMhEhD
t
h
Pcz
lMlD
t
l
cz
qMqEqD
t
q
Lcz
sMsEsD
t
sz
MED
t
uuuu
u
uuuu
uuu
uuu
uuuu
uuu
uuuu
uuu
{Dry static energy: s=CpT+gz; Moist static energy : h=s+Lq; }
v
vcvcuueu
u gz
PP
z
wMEwDw
t
M
])[()/)(()()(
0
mass conservation
dry SE
moist SE
condensate
vapour
vertical velocity
uu wM
Quasi-steady assumption: effects of averaging over a cumulus life-cycle can be represented in terms of steady-state convective elements .Transient (cloud life-cycle) formulations: Kuo (1964, 1974); Fraedrich(1974), Betts(1975), Cho(1977), von Salzen&McFarlane (2002).
Traditional organized (e.g.plume) entrainment assumption:
cc
c
nn PwdlvHv )](1[
cc
ccc
c wR
MwA
PE 2
c
c dlP (local draft perimeter)]
Entrainment/Detrainment
Arakawa & Schubert (1974) (and descendants, e.g. RAS, Z-M): - is a constant for each updraft [saturated homogeneous (top-hat) entraining plumes]- detrainment is confined to a narrow region near the top of the updraft, which is located at the level of zero buoyancy (determines )
[ where
Kain & Fritsch (1990) (and descendants, e.g. Bretherton et al, ):- Rc is specified (constant) for a given cumulus (not consistent with varying ) - entrainment/detrainment controlled by bouyancy sorting (i.e. the effective
value of is constrained by buoyancy sorting)
Episodic Entrainment and non-homogeneous mixing (Raymond&Blythe, Emanuel, Emanuel&Zivkovic-Rothman):-Not based on organized entrainment/detrainment - entrainment at a given level gives rise to an ensemble of mixtures of undiluted and environmental air which ascend/descend to levels of neutral buoyancy and detrain
zb
zt
iii hhz
h
)()( bbi zhzh ))((),)(( *
itiiti zhzh
zdzzzhzzzhzh it
z
z
iibitibit
it
b
)])((exp[)()])((exp[)())(()(
*
Determining fractional entrainment rates (e.g. when at the top of an updraft)
2*
**
)()1)(()),(( TTOT
q
c
LTTpTqq
c
LTT
c
hhi
pii
pi
p
i
Note that since updrafts are saturated with respect to water vapour above the LCL:
This determines the updraft temperature and w.v. mixing ratio given its mse.
ec TT
Fractional entrainment rates for updraft ensembles
0;)( uutu DMzE tb zzz
(a) Single ensemble member detraining at z=zt
))((exp btbu zzzMM
Detrainment over a finite depth ttut zzMzD /)()(:tz
(b) Discrete ensemble based on a range of tops
))(();,( itii
iu zzMM ii
iuu hMhM
ii
iu ME ti i
iu z
MD
Buoyancy Sorting
Entrainment produces mixtures of a fraction, f, of environmental air and (1-f) ofcloudy (saturated cumulus updraft) air. Some of the mixtures may be positively buoyant with respect to the environment, some negegatively buoyant, some saturated with respect to water, some unsaturated
cv
v
f0 1
ev cf *f
saturated (cloudy)
positivelybuoyant
Kain-Fritsch (1990) (see also Bretherton et al, 2003):
Suppose that entrainment into a cumulus updraft in a layer of thickness z leads to mixing of Mcdz of environmental air with an equal amount of cloudy air. K-F assumed that all of the negatively buoyant mixtures (f>fc) will be rejected from theupdraft immediately while positively buoyant mixtures will be incorporated into the updraft. Let P(f) be the pdf of mixing fractions. Then:
cf
uo dfffPME0
)(2 dffPfMDcf
u )()1(21
0
This assumes that negatively buoyant air detrains back to the environment without requiring it to descend to a level of nuetral bouyancy first).
Emanuel:
Mixtures are all combinations of environement air and undiluted cloud-base air. Each mixture ascends(positively buoyant)/descends (negatively buoyant), typically without further mixing to a level of nuetral buoyancy where it detrains.
Shallow convection:
Including decent to a nuetral buoyancy level (with evaporation of cloud water) before final detrainment requires gives rise to cooling associated with evaporatively driven downdrafts in the upper levels of cumulus cloud systems – noted as a diagnostic requirement by Cho(1977)
Closure and Triggering
• Triggering:– It is frequently observed that moist convection does
not occur even when there is a positive amount of CAPE. Processes which overcome convective inhibition must also occur.
• Closure:– The simple cloud models used in mass flux schemes
do not fully determine the mass flux. Typically an additional constraint is needed to close the formulation.
– The closure problem is currently still poorly constrained by theory.
Closure Schemes In Use
• Moisture convergence (Kuo, 1974- for deep precipitating convection)
• Quasi-equilibrium [Arakawa and Schubert, 1974 and descendants (RAS, Z-M, Zhang&Mu, 2005)]