LIDAR OBSERVATIONS CONSTRAINT

FOR CIRRUS MODELISATION

IN Large Eddy Simulations

O. Thouron, V. Giraud (LOA - Lille)

H. Chepfer, V. Noël(LMD - Palaiseau) / J. Pelon (SA - Paris) / J-L Redelsperger (CNRM - Toulouse)

The reason that cirrus clouds are not well understood is that many atmospheric processes affect their development, structure and evolution:

• on the locale scale : radiation, aerosol properties, gravity waves, shear instability, latent heating, microphysical properties, ….

• On a larger scale : interaction with jet streams, interaction with planetary-scale waves, passing pressure systems, large scale lifting or descent ...

Successful parameterization of cirrus clouds needs to be based on an understanding of all the processes and their interactions.

Introduction

How active remote sensing bring the signatures of processes and their interactions at local scales?

How active remote sensing may be convenient to constrain physical parameterizations in Cloud Resolving Models?

Introduction

- strategy

- model used to make LES simulations

- cirrus cloud generation

- sensitivity study: microphysical processes

- conclusion

- perspective

Plan

Aircraft + ECMWF+radio sonde data

Synthetic Observations

fields 2/3D

Active Observations

Passive Observations

Radiatif transfer calculation

Microphysical Scheme

MESO-NH

Observations

Modelisation

Comparison

Idealized case

Sensibility Study of the lidar

to the microphysical processes

Strategy

Use the French atmospheric simulated system meso-NH

Run in 1, 2 or 3 dimensions

Non hydrostatic meso scale model

Bulk microphysical scheme

Designed to study convective cloud or precipitating cloud

It was necessary: - to adapt the microphysical scheme to simulate cirrus

- to prognostic ice number concentration

to be abble to calculate synthetic observations

The model

- spherical particles

- size distribution : gamma modified

First type SP :

Second type NSP :

- non spherical particles

(columns or plates)

- size distribution : gamma modified

Microphysical scheme

Sedimentation

AggregationTransformation

Water Vapor

SP

NSP

Sublimation

Sedimentation

Nucleation

Deposition

Resolution domain

0

2000

4000

6000

8000

10000

12000

14000

16000

0 1 2 3 4 5 6 7 8 9 10 11 12

Height (m)

Ho

rizo

nta

l E

xten

t (k

m)

50 m

100 m

Sponge zone

Limit Conditions: Cyclic2D simulations

Resolution domain

Cirrus clouds are generated in a similar way to the GEWEX Cloud Systems Study (GCSS) cirrus cloud intercomparison (Starr et al. 2000)

Cloud Forcing: - cooling equivalent to ascent at 3 cm/s- between 7 and 10 km

Turbulent structure: - initialized by artificial heat perturbation (+/- 0.01K) between 8 and 9 km

Cirrus cloud generation

Duration : 5 hours

Radiation turned off

Base run:

Nu=1000 l-3

Ri*=20mg.m-3

Adjustment on 100%

Velocity: Starr (1985)

nucleation:

Deposition

Sedimentation

Meyer: - Supersaturation ratio with respect to ice

- Ice nuclei number: Nu

Transformation

Depend on the primary ice water content threshold

Depend on the sursaturation in the cirrus

Depend on velocity-mass relation parameters c and d:dcDDV )(

The sensitivity study

Base run results after 4 hours: Direct output

Base run results after 4 hours: Deducted output

Sedimentation

AggregationTransformat

Water Vapor

SP

NSP

Sublimation

Sedimentation

Nucleation

Deposition

Sensitivity study: Ice nuclei numberIce primary backscattering(km-1) Ice cristal backscattering(km-1)Total backscattering(km-1) Depolarisation

1 h

2 h

4 h

500 l-1

1000 l-1

1500 l-1

Sedimentation

AggregationTransformat

Water Vapor

SP

NSP

Sublimation

Sedimentation

Nucleation

Deposition

Sensitivity study: the primary ice water content threshold

Mean backscatterring Depolarisation

1 h

2 h

4 h

10 mg.m-3

20 mg.m-3

30mg.m-3

Water Vapor

Sedimentation

AggregationTransformat

SP

NSP

Sublimation

Sedimentation

Nucleation

Deposition

Sensitivity study: Fall speed velocity

Fall speed velocity

0

10

20

30

40

50

60

0 100 200 300 400 500

diameter en micro m

Sp

ee

d (

cm

/s)

Primary ice

Ice cristal: Star(1985)

Small ice cristal:Heymsfield (2001)

Larger ice cristal:Heymsfiel (2001)

DepolarisationMean backscatterring

1h

2h

4h

0

10

20

30

40

50

60

0 100 200 300 400 500

Primary ice

Ice cristal: Star(1985)

Small ice cristal:Heymsfield (2001)

Larger ice cristal:Heymsfiel (2001)

mg.m-3

Sensitivity study: Deposition

Water Vapor

Sedimentation

AggregationTransformat

SP

NSP

Sublimation

Sedimentation

Nucleation

Deposition

Mean backscatterring Depolarisation

1h

2h

4h

50%

100%

80%

DepolarisationLidar

BackscaterringLidar

BackscaterringDepolarisation

Nucleation Transformation Fall speed

velocityDéposition

Sublimation

v

v v

v

v

v

v

Conclusion

v

Perspectives•Structure analysis: FFT

•Radar data

•Used of real case in order to constrain the model