generation of free convection in a valley due to changes of the local circulation system
DESCRIPTION
Generation of free convection in a valley due to changes of the local circulation system. Rafael Eigenmann, Thomas Foken Dept. Micrometeorology, University of Bayreuth www.bayceer.uni-bayreuth.de/mm. 7 th COPS Workshop 27 th -29 th October 2008, Strasbourg. Content. Data basis of the study - PowerPoint PPT PresentationTRANSCRIPT
Rafael [email protected]
University of Bayreuth – Department of Micrometeorology www.bayceer.uni-bayreuth.de/mm
Bayreuth Center of Ecology and Environmental Research
Generation of free convection in a valley due to changes of the local
circulation system
Rafael Eigenmann, Thomas Foken
Dept. Micrometeorology, University of Bayreuth www.bayceer.uni-bayreuth.de/mm
7th COPS Workshop 27th-29th October 2008, Strasbourg
Rafael [email protected]
University of Bayreuth – Department of Micrometeorology www.bayceer.uni-bayreuth.de/mm
Bayreuth Center of Ecology and Environmental Research
Content
• Data basis of the study
• Local circulation system in the Kinzig valley
• Near-ground generation of free convection
• Parameters indicating free convection
• Characterisation of free convection event days
• Conclusion
• Future plans
Rafael [email protected]
University of Bayreuth – Department of Micrometeorology www.bayceer.uni-bayreuth.de/mm
Bayreuth Center of Ecology and Environmental Research
Flux calculation with the software package TK2 Footprint analysis Check for possible internal boundary layers
Data basis – COPS energy balance network
• Station near Fussbach (UBT1ETG) in the Kinzig valley
(Eigenmann, 2008; diploma thesis)
Turbulence system Sodar/RASSRadiation and soil
Rafael [email protected]
University of Bayreuth – Department of Micrometeorology www.bayceer.uni-bayreuth.de/mm
Bayreuth Center of Ecology and Environmental Research
Kinzig valley – local circulation system
• Sodar measurements at COPS IOP8b: – Down-valley winds (S) at night, up-valley winds (N) during the day
Wind direction [°] Wind speed [ms-1]
Strong collapse of the horizontal wind speed in the morning hours:
6:30 – 8:50 UTC; vh < 1.5 ms-1
Rafael [email protected]
University of Bayreuth – Department of Micrometeorology www.bayceer.uni-bayreuth.de/mm
Bayreuth Center of Ecology and Environmental Research
Near-ground generation of free convection
• Detection of free convection events (FCEs) by the surface
eddy-covariance system:
• Free convection for:
– small friction velocities u*
– high buoyancy fluxes
1
3*
0''
u
wgz
L
z
v
v
Induced by the wind
speed collapse during
the wind direction change
'' vw
Rafael [email protected]
University of Bayreuth – Department of Micrometeorology www.bayceer.uni-bayreuth.de/mm
Bayreuth Center of Ecology and Environmental Research
Further parameters indicating free convection (I)
• Ratio of the Deardorff velocity w* to the friction velocity u*:
31
0* ''
v
v
i wzg
w
zi: visual inspection of a secondary maximum in the reflectivity profile
Reflectivity [dB]
(Beyrich, 1997)
Rafael [email protected]
University of Bayreuth – Department of Micrometeorology www.bayceer.uni-bayreuth.de/mm
Bayreuth Center of Ecology and Environmental Research
Further parameters indicating free convection (II)
• Buoyancy (B) and shear term (S) of the TKE equation:
0'' vv
wg
B
dz
duwuS ''
Wind shear :profile mast
dzdu
Rafael [email protected]
University of Bayreuth – Department of Micrometeorology www.bayceer.uni-bayreuth.de/mm
Bayreuth Center of Ecology and Environmental Research
Circulation system regarding all COPS days
• Visualisation with the persistence P of wind direction:
)(
)()()(
22
tv
tVtUtP
h
(Lugauer and Winkler, 2005)
0
0,25
0,5
0,75
1
00:00 06:00 12:00 18:00 00:00
time [UTC]
per
sist
ence
P [
-] all days (n=92)
event days (n=23)
intermittent days (n=19)
non-event days (n=37)
Values between 0 and 1
Rafael [email protected]
University of Bayreuth – Department of Micrometeorology www.bayceer.uni-bayreuth.de/mm
Bayreuth Center of Ecology and Environmental Research
Free convection events regarding all COPS days
2007-06-01
2007-06-24
2007-07-17
2007-08-09
2007-09-01
0:00 6:00 12:00 18:00 0:00
time [UTC]
onset
cessation
z/L event(s)
sunrise
sunset
23 free convection event days (25%) Mean duration: 1h 24min Adjustment to the annual cycle of sunrise
Rafael [email protected]
University of Bayreuth – Department of Micrometeorology www.bayceer.uni-bayreuth.de/mm
Bayreuth Center of Ecology and Environmental Research
Mean diurnal courses of the classified days
Above average values on ‘event days’ (QE: ~100 Wm-2; QH: ~40 Wm-2)
Influence on temperatureand moisture profiles of the ABL
Density effects
Support of cloud formation(fair-weather cumuli)
0
50
100
150
200
250
300
350
400
0:00 6:00 12:00 18:00 0:00
time [UTC]
late
nt
he
at
QE [
W m
-2] latent heat QE [Wm-2]
-40
-20
0
20
40
60
80
100
120
0:00 6:00 12:00 18:00 0:00
time [UTC]
mea
n d
iurn
al Q
H [
W m
-2] sensible heat QH [Wm-2]
all days (n=92)event days (n=23)intermittent days (n=19)non-event days (n=37)
Rafael [email protected]
University of Bayreuth – Department of Micrometeorology www.bayceer.uni-bayreuth.de/mm
Bayreuth Center of Ecology and Environmental Research
Conclusions
• The local circulation system in the Kinzig valley is a powerful trigger mechanism for the generation of free convection events (FCEs)
• Eddy-covariance systems are able to detect FCEs with the stability parameter ζ for ζ < -1
• Further parameters indicate FCEs: w*/u* and B/S• FCEs may have a not negligible
impact on ABL thermodynamics and its structure
• Contribution to the pre-convective environment of the cell at IOP8b (?)
Rafael [email protected]
University of Bayreuth – Department of Micrometeorology www.bayceer.uni-bayreuth.de/mm
Bayreuth Center of Ecology and Environmental Research
Future plans
• Follow-on project: Turbulent Fluxes and thermal convection in a valley (SALVE)
• Analysis of the development of free convection situations in the whole Kinzig valley atmosphere with LES modelling
• Supported by the flux measurements as initial field and boundary conditions
• Dependence on valley width and land use ?
• Virtual towers: attempt to close the energy balance by landscape-scale flux averaging
(Prof. Th. Foken, Prof. V. Wirth, Dr. N. Kalthoff)
Rafael [email protected]
University of Bayreuth – Department of Micrometeorology www.bayceer.uni-bayreuth.de/mm
Bayreuth Center of Ecology and Environmental Research
Thank you for your
attention !
Rafael [email protected]
University of Bayreuth – Department of Micrometeorology www.bayceer.uni-bayreuth.de/mm
Bayreuth Center of Ecology and Environmental Research
Further flux measurements at IOP8b
• Fußbach (UBT1ETG)
Latent heat flux QE Bowen ratio Bo Available energy at the surface: net radiation QS
* minus ground heat flux QG
Rafael [email protected]
University of Bayreuth – Department of Micrometeorology www.bayceer.uni-bayreuth.de/mm
Bayreuth Center of Ecology and Environmental Research
Definition of free convection
Stull, R.B., 2000. Meteorology for Scientists and Engineers. 2nd edition.BROOKS/COLE, Pacific Grove, 502 pp.
Rafael [email protected]
University of Bayreuth – Department of Micrometeorology www.bayceer.uni-bayreuth.de/mm
Bayreuth Center of Ecology and Environmental Research
Mean diurnal courses of the classified days
circulation systemtriggered by u* minimum
FCEs on both ‘intermittent’ and ‘event days’
Above-average values ofQH and global radiation
on ‘event days‘
-2
-1
0
1
2
3
0:00 6:00 12:00 18:00 0:00
time [UTC]
stab
ilit
y p
aram
eter
ζ [
-] all days (n=92)'event days' (n=23)'intermittent days' (n=19)'non-event days' (n=37)
0
0,05
0,1
0,15
0,2
0,25
0,3
0,35
0:00 6:00 12:00 18:00 0:00time [UTC]
mea
n d
iurn
al u
* [m
s-1]
-40
-20
0
20
40
60
80
100
120
0:00 6:00 12:00 18:00 0:00
time [UTC]
mea
n d
iurn
al Q
H [
W m
-2]
0
150
300
450
600
750
900
0:00 6:00 12:00 18:00 0:00
time [UTC]
stability parameter
global radiation [Wm-2]
sensible heat QH [Wm-2]
friction velocity u* [ms-1]
Rafael [email protected]
University of Bayreuth – Department of Micrometeorology www.bayceer.uni-bayreuth.de/mm
Bayreuth Center of Ecology and Environmental Research
Mean, standard deviation and number
period mean stdev number
june 8:01 1:14 8
july 8:03 0:37 5
august 9:06 0:46 10
whole 8:29 1:02 23
period mean stdev number
june 17:36 0:55 7
july 17:23 1:35 5
august 17:56 1:10 10
whole 17:42 1:10 22
period mean stdev number
june 7:33 1:08 8
july 8:14 0:51 5
august 8:48 0:42 10
whole 8:14 1:01 23
onset of up-valley winds
cessation of up-valley winds
z/L event(s)
Rafael [email protected]
University of Bayreuth – Department of Micrometeorology www.bayceer.uni-bayreuth.de/mm
Bayreuth Center of Ecology and Environmental Research
Example for the calculation of the persistence
)(
)()()(
22
tv
tVtUtP
h