Rafael Eigenmann [email protected] University of Bayreuth – Department of...

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




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




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




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




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




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)




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




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




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




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)




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 (?)




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)




Thank you for your

attention !




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




Definition of free convection

Stull, R.B., 2000. Meteorology for Scientists and Engineers. 2nd edition.BROOKS/COLE, Pacific Grove, 502 pp.




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]




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


june 17:36 0:55 7

july 17:23 1:35 5

august 17:56 1:10 10

whole 17:42 1:10 22


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)




Example for the calculation of the persistence

)(

)()()(

22

tv

tVtUtP

h

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