Toulouse IHOP meeting 15 June 2004 Water vapour variability
within the growing convective boundary layer of 14 June 2002 with
large eddy simulations and observations OUTLINE Methodology and
Objectives Observational data: several scales of variability LES
simulations: comparisons obs/model Conclusions & Perspectives
Fleur Couvreux Franoise Guichard, Jean-Luc Redelsperger, Cyrille
Flamant, Jean-Philippe Lafore, Valry Masson r v (g/kg) W (m/s) ENE
WSW 6 4 2 0 -2 -4 11 10 8 9 7 12 Time (UTC) 12131415161718 Slide 2
Toulouse IHOP meeting 15 June 2004 Methodology & Objectives 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 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? Slide 3
Toulouse IHOP meeting 15 June 2004 Time 12h 13h 14h 15h 16h 17h 18h
19h (K) 306 294 1 0 Zi (km) Time 12h 13h 14h 15h 16h 17h 18h 19h 2
morning Early afternoon Boundary layer mean value of & r v Time
in UTC=local time+5h 2 1 0 300 r v (g/kg) 7 1210 A classic
convective BL growth but with large fluctuations of r v Zi (km)
Slide 4 Toulouse IHOP meeting 15 June 2004 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 The 14 june 2002 case Data : Soundings (35)
Lidars (DLR-DIAL, LEANDRE et SRL) In-situ aircraft data (NRL-P3 et
UWKA) Surface flux measurements (ISSF) Slide 5 Toulouse IHOP
meeting 15 June 2004 Different scales of variability: evidence in
soundings 2 soundings separated from less than 10 km 1g.kg -1 1830
UTC Soundings : Different variability at different scales rvrv
Soundings in a 200 km wide domain 1130 UTC 1700 UTC 3 g/kg 5 g/kg
rvrv rvrv Slide 6 Toulouse IHOP meeting 15 June 2004 EW Different
scales of variability : aircraft data and lidars + rvrv 3g.kg -1
rvrv 1.5 g.kg -1 Aircraft r v measurement 1710Time (UTC) 1710 12 8
7 8 +0.7 -0.7 Time (UTC) 1745 rvrv 1710 1745 r v measured by the
DLR lidar 1500 1000 500 Height (m) 1500 1000 500 Height (m) WSW ENE
1500 1000 500 Height (m) Slide 7 Toulouse IHOP meeting 15 June 2004
Modelling: LES with Mso-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 Initial
profile observations rvrv Slide 8 Toulouse IHOP meeting 15 June
2004 Mean profiles Temporal evolution of mean profiles rvrv
Comparison obs/LES at 1800 UTC rvrv Slide 9 Toulouse IHOP meeting
15 June 2004 Time variations of boundary layer characteristics
Sensitivity to surface fluxes : +Bo -> +z i Sensitivity to
large- scale advection : +ADV -> +z i +Bo -> + m +Bo -> +
qm Cf Bo ie SSH et SLH +ADV -> + m +ADV -> + qm Several
factors : Ws -> z i -> , q Adv -> -> zi Adv q -> q
Validated reference simulation, quantification of sensibility to
different forcings rvrv zi Slide 10 Toulouse IHOP meeting 15 June
2004 Horizontal cross sections Z/zi=0.3 Z/zi=0.8 Z/zi=1. Z/zi=0.8
Z/zi=1. 11 10 9 8 7 6 5 11 10 9 8 7 6 5 11 10 9 8 7 6 5 305 304.5
304. 303.5 5 3 1 -3 5 3 1 -3 5 3 1 -3 W (m/s) v (K) 305 304.5 304.
303.5 306 304.5 304. 303.5 305.5 305 r v (g/kg) 10 km Slide 11
Toulouse IHOP meeting 15 June 2004 Characteristic length scale Los
C( )= Reference simulation at 17h r v length scale is larger than
length scale of v, , w from Lohou et al. (2002) (m) z/zi Rv v w
Slide 12 Toulouse IHOP meeting 15 June 2004 LEANDRE and LES
horizontal cross-sections Measurements from LEANDRE LES Simulations
At 1600 UTC r v -r v rv ~10 km 1.2 3.5 1.2 3.5 Slide 13 Toulouse
IHOP meeting 15 June 2004 Vertical cross sections Evidence of dry
downdrafts Several thermals in one humid zone LES r v & w
DLR-DIAL r v 6.5 5.5 6 7 7.5 8 8.5 9.5 10 9 10.5 6.5 5.5 6 7 7.5 8
8.5 9.5 10 9 10.5 Slide 14 Toulouse IHOP meeting 15 June 2004
Evaluation of histograms of , r v, w P3 aircraft KA aircraft Z=0.4z
i model max --- min w rvrv Dry downdraft Overshooting updrafts zi
Equivalent gaussian Slide 15 Toulouse IHOP meeting 15 June 2004
Boundary layer precipitable water Slide 16 Toulouse IHOP meeting 15
June 2004 Second order moments __ simulation __ lidar DIAL o P3 o
KA Strong relation between The inversion strength and the variance
Max( rv ) (g.kg -1 ) q z i (g.kg -1 ) 1.2 0.72 0.81 0.41 0.56 5.7
MNH 4.4 MNH 4.3 DIAL 3.9 DIAL 4.1 DIAL 3 lidar 10km-long
Cross-section at 1730 UTC LES 1700 & 1800 UTC rvrv Slide 17
Toulouse IHOP meeting 15 June 2004 Joint probability distribution
At z/zi=0.3 + z/z i + larger spectrum of w + and q - At z/zi=0.8
Slide 18 Toulouse IHOP meeting 15 June 2004 Conclusions: 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)
Observations from 14 juin 2002 during IHOP_2002 : Several scales of
variability ( 10 km) Slide 19 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 Understand the impact of such a variability on
cloud formation and maintenance Quantify time scales concerned by
mechanisms involved in the water vapour variability: dry intrusion
life time, transient state Slide 20 Toulouse IHOP meeting 15 June
2004 F I N Slide 21 Toulouse IHOP meeting 15 June 2004 Development
of the CBL ( courtesy of Bart Geerts) aspect ratio: 1:1 1330 UTC
1415 UTC 1530 UTC 1630 UTC 1730 UTC Slide 22 Toulouse IHOP meeting
15 June 2004 Surface fluxes Sensible heat flux Latent heat flux
Bo~1. Bo~0.5 Slide 23 Toulouse IHOP meeting 15 June 2004 Large
scale forcings (advection) Large-scale forcings Deduced from MM5
Horizontal advection of Horizontal advection of r v Subsiding w
