First attempt to identify sources of
water vapor variability during the 29
May 2002 boundary layer
heterogeneity mission
Sophie Bastin
Cyrille Flamant, Philippe Drobinski
Institut Pierre Simon Laplace
Service d’Aéronomie
The problem and the tools…
The problem
Deconvolution of water vapor sources : surface forcing versus
advection
PBL heterogeneity: time/spatial variability
The tools
IHOP observations (surface composite, radiosoundings,
LEANDRE-2)
MM5 model
Study of the 29 May 2002 BLH mission
The 29 May
2002 BLH
mission
East-west water vapour gradient
observed by LEANDRE-2
Large E/W moisture gradient (5 g kg-1)
PBL higher than 3 km MSL to the west
LEANDRE-2-derived water vapor mixing
ratio values in PBL consitent with aircraft
measurements to the west and Vici to the
east
The 29 May 2002 BLH mission
Persistent west to east
soil moisture gradient
Intense rainfall associated
with frontal passage.
1
4
7
From Ken
Davis et al.
(IHOP
workshop
2003)
Station7(E)
Station4(C)
Station1(W)
Station 1 = west.
Station 4 = central.
Station 7 = east.
Rainfall spatial structure on 28 May
Sparse convective precipitating systems on 28 May 2002
Northeast/southwest oriented
accumulated precipitation
pattern on the eastern side of
the investigated target area
Rainfall impact on ECMWF soil
moisture analysis
29 May 2002
28 May 2002
Good agreement between the
observed precipitation pattern
and the ECMWF soil moisture
(Drusch et al., 2004)
MM5 simulations
Simulation forced by ECMWF analyses
Starting time: 29 May 2002 – 0000 UTC
Domain 1: 15 km x 15 km; Domain 2: 5 kmx 5 km
Comparison observations/MM5
Comparison with surface data
Sparse data
2-m humidity
2-m
temperature
10-m wind
Good agreement but uncertainty on the large gradient
region where the measurement density is low
Comparison observations/MM5
Comparison with radiosoundings
Radiosounding VICI (-99.2°E;36.07°N) on 29 May 2002 at 1730 UTC
MM5
Obs.
Good agreement but water vapor mixing ratio overestimation by MM5
Evaluation of water vapor sources:
western side
West of -101.5°E: no vegetation, no latent heat
flux, no soil moiture, continental advection
No atmospheric water
vapor
Evaluation of water vapor sources:
eastern side
N.B.: The values of the data are rescaled
(arbitrary units) for a clearer visualization
Correlation
Small
scales
Large
scales
2-m humidity
vs. latent
heat flux
0.72
0.97
2-m humidity
vs.
vegetation
-0.09
0.97
2-m humidity
vs. soil
moisture
-0.50
0.72
Significant when
>0.15 or <-0.15
Large correlation up
to about 1000 m AGL
Evaluation of water vapor sources:
eastern side
correlation
no correlation
anti-correlation
N.B.: The values of the data are rescaled
(arbitrary units) for a clearer visualization
Evaluation of water vapor sources:
eastern side
No correlation
between 2-m
humidity and
surface forcing
Dominant advective
regime?
N.B.: The values of the data are rescaled
(arbitrary units) for a clearer visualization
Evaluation of water vapor sources:
preliminary summary
No forcing
source
Dominant
surface
forcing
Probable
advection
Impact on PBL structure
The Bowen ratio is smaller over
moist surfaces where most of
the energy goes into evaporation
The Bowen ratio is larger over
dry surfaces where most of the
energy goes into sensible heating
Typical Bowen ratio values:
5 over semi arid regions
0.5 over grasslands/forests
0.2 over grass
0.1 over the sea
<0 over oases
Sensible heat fluxes are larger on the
western side (dry surfaces) of the
domain so the PBL is deeper. The PBL
depth variability is also correlated
with the surface forcing variability
No apparent correlation between the PBL
depth variability with surface forcing
(sensible heat flux) in the southern part
of the domain: cold air advection?
Impact on PBL structure
PBL height: Craig
and Davis
Impact of
Canadian valley
underestimated
by MM5?
The MM5 pbl depth
deepens west of
-100°E vs. -101.4°E
for LEANDRE-2
Local effect?
The MM5 pbl depth
deepens west of
-100°E vs. -101.1°E
for LEANDRE-2
Consistent
with aircraft
measurements
Upper moist
layer
MM5 overestimates the PBL depth compared to LEANDRE-2 but the zonal
evolution is well predicted especially north of 37°N
Small scale PBL variability underestimated by MM5
Conclusion and perspectives
First evaluation of the possible origin of the
atmospheric water variariability with respect to
surface forcing and large-scale advection
In the near future: re-run MM5 on a longer period (2730 May 2002) in order to simulate the precipitating
event using:
ECMWF analyses (including soil moisture)
combined ECMWF analyses AND « IHOP soil moisture
reanalysis »
increased grid resolution (small scale variability, wave
activity,…)