Transcript Water vapour variability within the growing convective boundary layer of 14 June 2002 with large eddy simulations and observations (Couvreux)

Water vapour variability within the growing
convective boundary layer of 14 June 2002 with
large eddy simulations and observations
Fleur Couvreux
Françoise Guichard, Jean-Luc Redelsperger, Cyrille Flamant, Jean-Philippe Lafore, Valéry Masson
6
OUTLINE
4
Methodology and Objectives
2
Observational data:
0
several scales of variability
-2
LES simulations:
-4
12
comparisons obs/model
Conclusions & Perspectives
W (m/s)
WSW
11
10
rv (g/kg)
9
ENE
8
7
12
Toulouse IHOP meeting 15 June 2004
13
14
15
16
17
18
Time (UTC)
Methodology & Objectives
IHOP observations:
What are the fluctuations of water vapour mixing ratio observed within the
growing convective boundary layer?
14 june 2002: BLE case
What are the scales involved ?
Is this day typical?
LES :
Is such a high resolution model able to represent the observed water vapour
variability ?
first goal : from observations and simulations, to explain the mecanisms
responsible for the water vapour variability
second goal: to understand the role of such variability on cloud formation and
maintenance
Toulouse IHOP meeting 15 June 2004
A classic convective BL growth …
but with large fluctuations of rv
2
Early afternoon
Zi
Time
12h
13h
14h
15h
16h
17h
18h
19h
(km)
1
morning
0
294
300  (K)
306
Boundary layer mean value of
 & rv
2
Time
12h
13h
14h
15h
16h
17h
18h
19h
Zi
(km)
1
0
7 rv (g/kg)
Time in UTC=local time+5h
Toulouse IHOP meeting 15 June 2004
10
12
The 14 june 2002 case
Main characteristics :
•« Relatively » simple case of a growing boundary layer: a well mixed boundary
layer reaching 1.5 km
•High Pressure system, homogeneous temperature field
•Weak subsidence constant whole day
•Low shear and weak wind (< 5m/s) from N to NE
•Existence of thermals (cf Cloud radar)
•Small cumuli developed after 1500 UTC
•NE/SW moisture gradient
•Heavy precipitation the day before, coherent
distribution with moisture gradient
Data :
•Soundings (35)
•Lidars (DLR-DIAL, LEANDRE et SRL)
•In-situ aircraft data (NRL-P3 et UWKA)
•Surface flux measurements (ISSF)
Toulouse IHOP meeting 15 June 2004
Different scales of variability: evidence in soundings
rv
1830 UTC
2 soundings
separated
from less
than 10 km

1g.kg-1
Soundings in a 200 km wide domain
1130 UTC
1700 UTC
rv
rv
3 g/kg
5 g/kg
Soundings : Different variability at different scales
Toulouse IHOP meeting 15 June 2004
Different scales of variability : aircraft data and lidars
rv measured by the DLR lidar
12
Aircraft rv
measurement
1000
E
12
Height (m)
W
7
1500
500
rv
ENE
WSW
1500
8
1710
12
Time (UTC)
rv
Height (m)
1000
1745
500
+
+0.7
1500
3g.kg-1
8
1710
Time (UTC)
Height (m)
1000
1.5 g.kg-1
-0.7
Toulouse IHOP meeting 15 June 2004
1710
500
rv’
Time (UTC)
1745
Modelling: LES with Méso-NH (Lafore et al. 1998)
Simulation :
• x=y=100m, z streched (< 50m in BL)
• 3D turbulence scheme (Cuxart et al. 2000)
•early morning to early afternoon (duration 7h)
• a ‘realistic’ simulation:
• ISFF2 surface fluxes (prescribed) homogeneous
• initial sounding = composite of soundings at 1130
UTC
• large scale advection estimated from MM5
simulations and soundings
Toulouse IHOP meeting 15 June 2004

Initial profile
observations
rv
Mean profiles
Temporal evolution of mean profiles

rv
Toulouse IHOP meeting 15 June 2004
Comparison obs/LES at 1800 UTC

rv
Time variations of boundary layer characteristics
zi
Sensitivity to
Sensitivity to largescale advection :
surface fluxes :

+ADV -> +zi
+Bo
+Bo
-> +zi
-> +m
+Bo
-> + qm
+ADV -> + m
Cf Bo ie SSH
+ADV -> + qm
et SLH
Several factors :
rv
Ws
-> zi ->, q
Adv  ->  -> zi
Adv q -> q …
Validated reference simulation, quantification of sensibility to different forcings
Toulouse IHOP meeting 15 June 2004
Horizontal cross sections
v (K)
rv (g/kg)
11
10
10 km
W (m/s)
306
Z/zi=1.
9
8
5
305.5
Z/zi=1.
305
304.5
7
6
5
11
-1
303.5
-3
305
5
304.5
Z/zi=0.8
Z/zi=0.8
7
6
304.
5
303.5
3
1
-1
305
11
10
9
8
1
304.
10
9
8
3
304.5
Z/zi=0.3
Z/zi=0.3
304.
7
6
5
Toulouse IHOP meeting 15 June 2004
-3
5
3
1
-1
303.5
-3
C()=

x ' (  ) x ' (   )
x
2
Characteristic length scale

Reference simulation at 17h
Los
from Lohou et al. (2002)
z/zi
Rv

v
w
(m)
rv length scale is larger than length scale of v, , w
Toulouse IHOP meeting 15 June 2004
LEANDRE and LES horizontal cross-sections
rv-rv
rv
Measurements from LEANDRE
1.2
3.5 1.2
3.5 1.2
3.5 1.2
3.5
~10 km
~10 km
At 1600 UTC
Toulouse IHOP meeting 15 June 2004
LES Simulations
Vertical cross sections
LES rv & w
5.5
6
6.5
7
7.5
8
8.5
9
9.5
10
10.5
DLR-DIAL rv
5.5
6
6.5
Evidence of dry downdrafts
Several thermals in one humid zone
7
7.5
8
8.5
9
9.5
10
Toulouse IHOP meeting 15 June 2004
10.5
Evaluation of histograms of , rv, w
Z=0.4zi
P3 aircraft
KA aircraft
… max
--- min
Overshooting
updrafts
model
w’
Dry downdraft
’
Equivalent gaussian
rv’
Toulouse IHOP meeting 15 June 2004
zi
Boundary layer precipitable water
Toulouse IHOP meeting 15 June 2004
Second order moments
__ simulation
Strong relation between
The inversion strength and
the variance
Max(rv)
(g.kg-1)
q à zi
(g.kg-1)
1.2
5.7 MNH
0.72
4.4 MNH
0.81
4.3 DIAL
0.41
3.9 DIAL
0.56
4.1 DIAL
Toulouse IHOP meeting 15 June 2004
__ lidar DIAL
o P3
o KA
rv
3 lidar 10km-long
Cross-section at 1730 UTC
LES 1700 & 1800 UTC
Joint probability distribution
At z/zi=0.8
At z/zi=0.3
+ z/zi
Toulouse IHOP meeting 15 June 2004
+ larger spectrum of w+ and q-
Conclusions:
 Observations from 14 juin 2002 during IHOP_2002 :
Several scales of variability ( < 10 km et > 10 km)
 Evaluation of the LES
Able to represent the variability observed at scales lower than 10 km
(comparisons to soundings, lidars (DIAL et SRL), aircraft time-series)
Quantification the impact of scales > 10 km on variability at scales < 10km
 At first order, the boundary layer dynamics explain the observed
variability at scales lower than 10 km even without surface heterogeneities
and variability in the initial atmospheric state
 Dry narrow downdrafts as a signature of the BL growth (via dynamics at
the top) [Crum et al. (1987) and Weckwerth et al. (1996)]
impact on length scale, skewness, vertical transport..
Negative skewness is common (cf other IHOP days)
Toulouse IHOP meeting 15 June 2004
Perspectives:
 Systematic analysis of IHOP data :
- objective : to identify pertinent parameters controlling the water vapour
variability in the boundary layer (such as strength of inversion (,q),
fluxes…) from more idealised simulations -> 1D parameterizations
 Quantify time scales concerned by mechanisms involved in the water
vapour variability: dry intrusion life time, transient state
 Understand the impact of such a variability on cloud formation and
maintenance
Toulouse IHOP meeting 15 June 2004
FIN
Toulouse IHOP meeting 15 June 2004
Development of the CBL (courtesy of Bart Geerts)
1330 UTC
1415 UTC
1530 UTC
1630 UTC
1730 UTC
aspect ratio: 1:1
Toulouse IHOP meeting 15 June 2004
Surface fluxes
Sensible heat flux
Bo~1.
Bo~0.5
Toulouse IHOP meeting 15 June 2004
Latent heat flux
Large scale forcings (advection)
Horizontal advection of 
Horizontal advection of rv
Large-scale forcings
Deduced from MM5
Subsiding w
Toulouse IHOP meeting 15 June 2004