Shallow cumulus : cloud-subcloud
layer interaction and the role of
water vapour
Fleur Couvreux, Phil Austin
BOMEX
HUM3B
ARM
E1120
September 2006
Cumulus characteristics
Inferred from observations :
Stommel 1947 : significant mixing with environment
 Squires 1958: evidence of downdrafts
 Austin et al. 1985: strong heterogeneity in the cloud

radar
Kollias et al. (2001)
Updraft=5 m/s, downdraft=-3 m/s
spectral width: turbulence
Cumulus characteristics
Inferred from observations :
Stommel 1947 : significant mixing with environment
 Squires 1958: evidence of downdrafts
 Austin et al. 1985: strong heterogeneity in the cloud

No observation of initiation
(small spatial and temporal scales)
radar
No observation to study the
interaction between the cloud layer
and the subcloud layer
Kollias et al. (2001)
Updraft=5 m/s, downdraft=-3 m/s
spectral width: turbulence
Cumulus characteristics
Inferred from LES:
- Constraint for the entrainment rate
(Siebesma and Cuijpers 1995) ~10-3
- Understanding cloud characteristics
(Neggers et al. 2002)
- Description of transport inside cumulus
(Zhao and Austin 2005) : pulsating,
indirect transport, life cycle (dissipation)
-
Neggers et al. 2002
Cumulus characteristics
Inferred from LES:
- Constraint for the entrainment rate
(Siebesma and Cuijpers 1995) ~10-3
- Understanding cloud characteristics
(Neggers et al. 2002)
- Description of transport inside cumulus
(Zhao and Austin 2005) : pulsating,
indirect transport, life cycle (dissipation)
A look at individual clouds :
to study the life cycle of the cloud
A closer look at the relation between
the subcloud layer and the cloud
layer
-
Neggers et al. 2002
Importance of humidity variability
Boundary layer humidity :
Mahrt 1976: LCL sensitivity to small changes in humidity
Crook 1996: sensitivity of cloud initiation to qsfce
Weckwerth et al. 2000: CIN threshold if wv variability taken into account
Importance of humidity variability
Boundary layer humidity :
Mahrt 1976: LCL sensitivity to small changes in humidity
Crook 1996: sensitivity of cloud initiation to qsfce
Weckwerth et al. 2000: CIN threshold if wv variability taken into account
Tropospheric humidity:
Derbyshire et al. 2004: sensitivity to
rh of mass flux profiles -> deep
Loijen, de Roode 2005: shallow
Loijen & de Roode 2005
Derbyshire et al. 2004
90%
70%
50%
25%
water vapour role evident but need to be better understood
std
-0.2
-0.4
+0.4
+0.7
Our focus
Questions :
● What controls cloud initiation?
•What is the relevance of the water vapour variability in the BL?
● What are the interactions between clouds and the subcloud layer?
● What controls cloud organisation (not looking at wind as for rolls) ?
● How does the cloud environment impact the cloud ? -> humidity
Methodology :
LES simulations : individual clouds and then total domain
Overview :
● Cloud base characteristics through the cloud life cycle
● Subcloud layer characteristics at initiation
● Modification of subcloud layer during the cloud life cycle
● Role of water vapour
Methodology : LES
Advantages :
- to address the lack of observations (cloud-subcloud layer interaction)
- cloud realism : fractal dimension, cloud size distribution
(Siebesma and Jonker 2000, Neggers et al. 2003)
- simultaneous 3D fields of different variables at high resolution
cloud+rv(0.4km)
cloud+w(0.4km)
Available simulations :
- six isolated clouds in BOMEX simulation (Zhao and Austin, 2005)
- ~6 cases : LES BOMEX, ARM and IHOP cases (modification of initial profiles,
stability and humidity, in order to get different cloud cover)
The six clouds (Zhao and Austin 2005)
3 small clouds and 3 large clouds : same cld base, diff. cld top
spatial and temporal resolution x=25m, t=1.5s
Cld top
wmax
Cld base
small clouds:
height ~ 500 m
hal L< 200 m
1 updraft
life time~1000s
large clouds:
height ~ 1000 m
hal L~ 400 m
2-3 pulse-like updrafts
life time ~ 1500s
wmin
A
Zhao and Austin (2005)
B
C
D
E
F
Cloud base caracteristics at initiation
Narrow distribution at initiation except for w': l'~min,qt'~max, v'~0
qt'~max
v'~0
(%)
l'~min
(%)
(%)
(g/kg)
(K)
w~1m/s
(%)
(m/s)
(K)
cloud A
cloud B small clouds
cloud C
cloud D large clouds
cloud E
cloud F
Standard deviation
Cloud base characteristics after initiation
Variation of the distribution through the life cycle of the clouds: ex: cloud A
qt'~max
v'~0
l'~min
(%)
(%)
(%)
(g/kg)
(K)
w~1m/s
(%)
(m/s)
same results for other clouds
(K)
t=0s
t=180s
t=360s
t=450s
t=540s
t=720s
t=870s
Cloud base characteristics in LES
Wider distribution but similar characteristics : l~min,qt~max, v'~0
qt~max
thl~min
thv'~0
w~2m/s
=0.8 g/kg
=0.1 K
=0.05 K
=0.3 m/s
=1 g/kg
=0.2 K
=0.06 K
=1 m/s
l'<0
v'~0
w'>0
rv'>0
Cloud base characteristics at initiation
Cloud D,E,F -> larger clouds have wider perturbations
but same intensity at initiation
perturbation size evolution at cloud base
large clouds
small clouds
Determination of the height of cloud from initiation?
Subcloud layer characteristics
Characteristics at
cloud initiation
w (m/s)
l (K)
* every column
*cloudy column
<q>BL (kg/kg)
e (K)
d<w'q'>/dz (kg/kg/s)
<q>BL (kg/kg)
Cloud C
Criteria:
moist and ascending
positive div(wqt)
<w'q'>
(kg/kg.m/s)
<q>BL
(kg/kg)
Subcloud layer characteristics
Evolution with time: criteria of initiation only during the first 3-4 minutes
w
w
(m/s)
(m/s)
<q>BL (kg/kg)
e
(K)
(K)
d<w'q'>/dz (kg/kg/s)
<q>BL (kg/kg)
w
w
(m/s)
(m/s)
<q>BL (kg/kg)
e
cloud C
d<w'q'>/dz (kg/kg/s)
e
e
(K)
(K)
d<w'q'>/dz (kg/kg/s)
<q>BL (kg/kg)
cloud C
w
w
e
(m/s)
e
(m/s)
(K)
(K)
d<w'q'>/dz (kg/kg/s)
Cloud C
d<w'q'>/dz (kg/kg/s)
d<w'q'>/dz (kg/kg/s)
Dry thermals/”cloudy” thermals
Thermodynamical properties of the dry thermals versus cloudy thermal
(conditional sampling on w' and cld col)
z
(m)
z
(m)
mean
std deviation
q
l (K)
(g/kg)
z
(m)
z
(m)
v
(K)
w (m/s)
A(cld thermals)~0.1
A(dry thermals)~0.3
“Cloudy” thermals moister and faster in the upper half of the boundary layer
Dry thermals/cldy thermals: ind. clds
qt
qt
cloud B
(m)
cloud D
Same conclusions:
moister and faster in the
upper boundary layer
+ evidence of vertical
transport of moisture
(m)
(g/kg)
dry thermals
cld thermals
0
2'
4'
6'
8'
10'
w
(g/kg)
cloud B
w
cloud D
(m)
(m)
(m/s)
(m/s)
Moisture flux enhancement under clouds
Enhancement of water vapour flux in the 1/3 upper BL
wqt
wl
wv
BOMEX: cloudD
mean profile
contribution according
to cloud fraction
under cld contribution
assymetry between
moisture and temperature
(g/kg.m/s)
(K.m/s)
(K.m/s)
Moisture flux enhancement under clouds
Enhancement of water vapour flux in the 1/3 upper BL
wqt
wv
wl
BOMEX: cloudD
mean profile
contribution according
to cloud fraction
under cld contribution
assymetry between
moisture and temperature
(g/kg.m/s)
(K.m/s)
wqt
(g/kg.m/s)
(K.m/s)
wl
(K.m/s)
wv
(K.m/s)
LES: ARM 6h
mean profile
contribution according
to cloud fraction
under cld contribution
cld contribution
~no impact on temperature
flux (wl and wv)
Evolution of the moisture flux below ind. clds
Mean flux under clouds / Flux contribution from under clouds
cloudcloud
B : mean
B : flux
fluxcontribution
cloud B : mean flux
30s
150
270
390
510
630
750
950
30s
150
270
390
510
630
750
950
evolution of the area
that feeds the cloud
at initiation, maximum of moisture transport but small area covered by updrafts
afterwards this area increases but less and less vertical transport
Role of humidity
CAPE (colors) and rv (isolines) at surface
Evidence of a strong impact of water vapour
variability on stability indexes for
CAPE and CIN
(m)
CIN (colors) and rv (isolines) at surface
(m)
ARM – 16 UTC
(m)
(m)
Role of water vapour
ARM
evolution of the total fraction covered by perturbations
relationship between
clouds and the “intensity”
but not the size of
the moisture perturbation
evolution of the mean size of perturbation
Transitory aspect of the water vapour in
the dry boundary layer – IHOP simulation
Couvreux (2005)
at 0.5zi
At the end of the day large scale of variability is still present
but smaller intensity
Water vapour variability in dry BL
at 12 local time
Lidar observations
LES simulation
5.
5.0
5.5
0
6.0
5.
6.5
5
7.0
6.
7.5
0
8.0
6.
8.5
5
9.0
7.
9.5
0
10.
7.
5
8.
0
5.
5.0
5.5
0
6.0
5.
6.5
5
7.0
6.
7.5
0
8.0
6.
8.5
5
9.0
7.
9.5
0
10.
7.
5
8.
Explained variance (%)
P3 aircraft
KA aircraft
qv’
v’²
w’²
Couvreux et al. 2005
rv’²
dry tongues
largely responsible
for the water vapour
in the dry boundary
layer
Can the dry tongues play a role in organisation ?
ARM – 16 UTC
need to investigate
possible role of dry
tongues in the
organization of
clouds
clouds
qt(BL) & cloud
CIN & cloud
BOMEX
Cloud organisation
variations of size, distance to neighbour, height: links with subcloud variables ?
ARM
HUM3h
BOMEX HUM3b
E1120
E1133
more smaller clouds
enough resolution
cldbase~blh
relationship with dry tongues need to be more investigated ...
Preliminary conclusions
Summary :
- at cloud base: narrow distribution at initiation
wider distribution during the cloud life cycle
width of cloud base at initiation discriminates small/large clouds
- in subcloud layer : w' max, qt' max , e' max, dwqt/dz max at initiation
- enhancement of moisture flux under cloud
- importance of the qt distribution (more “intensity” than size of perturbation)
Future work:
- pursue the study of the link between the qt distribution in BL and clouds
- look more in details at the exchange at cloud base through the cloud life cycle
- application to parameterization
Thanks for
your attention
Dry thermals/wet thermals
Thermodynamical properties of the dry thermals versus cloudy thermal
(conditional sampling on w' and cld col)
A(wet thermals)~0.2
A(dry thermals)~0.2
Wet thermals moister and faster in the upper half of the boundary layer
Role of humidity
CAPE (colors) and rv (isolines) in BL
Evidence of a strong impact of water vapour
variability on stability indexes for
CAPE and CIN
(m)
CIN (colors) and rv (isolines) in BL
(m)
ARM – 20 UTC
(m)
15
10
5
CIN
0
r²=0.45
-1.5
-1.
-0.5
0 rv (g/kg) 0.5
1.
(m)
Subcloud layer characteristics
Evolution with time: criteria of initiation only during the first 3-4 minutes
w
w
(m/s)
(m/s)
<q>BL (kg/kg)
e
(K)
(K)
d<w'q'>/dz (kg/kg/s)
<q>BL (kg/kg)
w
w
(m/s)
(m/s)
<q>BL (kg/kg)
e
cloud C
d<w'q'>/dz (kg/kg/s)
e
e
(K)
(K)
d<w'q'>/dz (kg/kg/s)
<q>BL (kg/kg)
cloud C
w
w
e
(m/s)
e
(m/s)
(K)
(K)
d<w'q'>/dz (kg/kg/s)
Cloud C
d<w'q'>/dz (kg/kg/s)
d<w'q'>/dz (kg/kg/s)
Cloud base characteristics after initiation
Variation of the distribution through the life cycle of the clouds: ex: cloud A
qt'~max
v'~0
l'~min
(%)
(%)
(%)
(g/kg)
(K)
w~1m/s
(%)
(m/s)
same results for other clouds
(K)
t=0s
t=180s
t=360s
t=450s
t=540s
t=720s
t=870s