object-oriented identification of coherent structures: a multi-case … · 2019. 10. 23. ·...

Object-oriented identification of coherent structures: A multi-case analysis of boundary-layer Large-Eddy Simulations

4 October 2019

Florent BrientFleur Couvreux, Catherine Rio, Rachel Honnert

CNRM/Météo-France/CNRS

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@�

@z� @

@z!0�0 + ↵(c� e) +Qrad

resolved parameterized

Turbulent and convective vertical transport

!0�0 = �K@�

@z

!0�0 = �!c(�c � �e)

turbulence

convection

Closure assumptions

Witek et. al 11

Errors in representing boundary-layer clouds significantly influence cloud feedbacks, the hydrological sensitivity, regional climate projections…

—> Investigating coherent structures in Large Eddy Simulations for better understanding of PBL motions (MESO-NH model)

Representing the boundary layer in climate models

Object-oriented sampling• Definition of coherent structures : 1. Ensemble of grid boxes satisfying the conditional sampling CS = {s’(x,y,z)>m*𝜎s(z)} based

on Couvreux et. al (10) (with s’(x,y,z) anomalies of tracer concentrations) 2. Object = Contiguous cells of positive CS (sharing face, edge, corner) 3. Selected object = Object with volume larger than Vmin

https://gitlab.com/tropics/objectsCollaborative project:

StCu by FIRE

https://gitlab.com/tropics/objects

Object-oriented sampling

• Advantages: • 3D geometrical coherence • Individual object characterisation • No a priori assumptions

of flow characteristics (𝜔, ql)

• Main results for StCu: • Objects cover 20% of the volume, but

contribute to ~80% of moisture and temperature resolved fluxes

• Coherent downdrafts matter !

StCu by FIRE

Brient et. al, 19 (GRL)

• Definition of coherent structures : 1. Ensemble of grid boxes satisfying the conditional sampling CS = {s’(x,y,z)>m*𝜎s(z)} based

on Couvreux et. al (10) (with s’(x,y,z) anomalies of tracer concentrations) 2. Object = Contiguous cells of positive CS (sharing face, edge, corner) 3. Selected object = Object with volume larger than Vmin

https://gitlab.com/tropics/objectsCollaborative project:

https://gitlab.com/tropics/objects

A diversity of boundary-layer situations

Clear-sky Stratus/Fog Stratocumulus St-to-Cu Cumulus

Continental

Marine

Are coherent structures consistent across boundary layers?


Continental

Marine

IHOP

FIRE ASTEX RICO

BOMEXCONSTRAIN

ARMCu

CGILS (s12)DYCOMS

AYOTTE

WarmCold

Microphysics



Continental

Marine



Continental

Marine

IHOP

FIRE ASTEX RICO

BOMEXCONSTRAIN

ARMCu

CGILS (s12)DYCOMS

AYOTTE

WarmCold

Microphysics


• A new tracer emitted above the domain-average cloud base —> Tracking structures entrained in the mixed layer

• Updrafts are separated by positive/negative vertical motions —> Returning shells

• Definition of “PBL-top” tracers for clear skies

First layer where:

Update since Brient et. al (19):


Continental

Marine


Characteristics of boundary layers All Structures

Entrained downdraftUpdraft

Returning shells

Mean Precipitable Water (mm/day)

Convective Clear Sky (IHOP)

!0✓0l !0q0t

(K m/s) (g/kg m/s)

Domain-mean resolved fluxes

Alti

tude

(km

)

X (m)

Y (m)



!0✓0l !0q0t

(K m/s) (g/kg m/s)

Alti

tude

(km

)

Domain-mean resolved fluxesMean Precipitable Water (mm/day)


Returning shells

X (m)

Y (m)

Characteristics of boundary layers

11% 1% 13%Domain-mean volume

All Structures


!0✓0l !0q0t

(K m/s) (g/kg m/s)


Alti

tude

(km

)


Returning shells

X (m)

Y (m)


11% 1% 13%

• Objects contribute to 74/86% of total heat/moisture fluxes (for 25% of volume)

All Structures


!0✓0l !0q0t

(K m/s) (g/kg m/s)

Boundary-layer top


Alti

tude

(km

)


Domain-mean volume

Returning shells

X (m)

Y (m)


Liquid Water Content (g/m2)

All Structures


Sub-cloud downdraftStratocumulus (FIRE)

Cloud top

Cloud base


!0✓0l !0q0t

(K m/s) (g/kg m/s)

Alti

tude

(km

)

Returning shells

X (m)

Y (m)



1%

7%

All Structures


Sub-cloud downdraft

Cloud top

Cloud base


!0✓0l !0q0t

Alti

tude

(km

)

Domain-mean volume:

Stratocumulus (FIRE)

(K m/s) (g/kg m/s)

Returning shells

X (m)

Y (m)



10%

All Structures


Sub-cloud downdraft

1%

7%

Cloud top

Cloud base


!0✓0l !0q0t

(K m/s) (g/kg m/s)

Alti

tude

(km

)

Domain-mean volume:


Returning shells

X (m)

Y (m)



10%

All Structures


Sub-cloud downdraft

1%

7%

Cloud top

Cloud base


!0✓0l !0q0t

(K m/s) (g/kg m/s)

Alti

tude

(km

)

3%

Domain-mean volume:


Returning shells

X (m)

Y (m)



3%

All Structures


Sub-cloud downdraft

Domain-mean volume:

10%

1%

7%


Cloud top

Cloud base


!0✓0l !0q0t

(K m/s) (g/kg m/s)

Alti

tude

(km

)


Returning shells

X (m)

Y (m)



Sub-cloud downdraft


Cumulus (BOMEX)

Cloud top

Cloud base


!0✓0l !0q0t

(K m/s) (g/kg m/s)

Alti

tude

(km

)

Returning shells

X (m)

Y (m)



Sub-cloud downdraft

2%

6%

Liquid Water Content (g/m2)Domain-mean

volume:

Cumulus (BOMEX)


!0✓0l !0q0t

(g/kg m/s)

Alti

tude

(km

)

(K m/s)Cloud top

Cloud base

Returning shells

X (m)

Y (m)



Sub-cloud downdraft

2%


volume:

2%

6%

Cumulus (BOMEX)


!0✓0l !0q0t

(g/kg m/s)

Alti

tude

(km

)

(K m/s)Cloud top

Cloud base

Returning shells

X (m)

Y (m)



Sub-cloud downdraft

2%

3%


volume:

2%

6%


Cumulus (BOMEX)


!0✓0l !0q0t

(g/kg m/s)

Alti

tude

(km

)

(K m/s)Cloud top

Cloud base

Returning shells

X (m)

Y (m)

Time evolution of boundary layersRelative Humidity (%)

Average contribution to fluxes


Sub-cloud downdraftReturning shells

65%

25%10%

Clear Sky





65%

25%10%

Clear Sky

Cloud Fraction (%)

50%35%4%4%

40%40%2%1%

Day Night

StCuAverage contribution

to fluxes





65%

25%10%

Clear Sky

Cloud Fraction (%)

50%35%4%4%

40%40%2%1%

Day Night


to fluxes

St-to-Cu

Cloud Fraction (%)

t+8h t+26h

60%5%4%5%

95%2%1%

17%

Contribution to fluxes





65%

25%10%

Clear Sky

Cloud Fraction (%)

50%35%4%4%

40%40%2%1%

Day Night


to fluxes

St-to-Cu

Cloud Fraction (%)

t+8h t+26h

60%5%4%5%

95%2%1%

17%

Contribution to fluxes

Cu

85%2%10%10%

Cloud Fraction (%)


60%1%20%5%

sub-cloud only

Coherent structures in observationsAre LES realistic?

Can we observe boundary-layer coherent structures?

Image Satellite Suomi07/09/2019

Coherent structures in observationsAre LES realistic?

Can we observe boundary-layer coherent structures?

100 km


Coherent structures in observations

Updrafts

Returning shells?

Are LES realistic?Can we observe boundary-layer coherent structures?

100 km


Coherent structures in observations

Updrafts

Returning shells?

Entrained downdraftSaturated

environment

Are LES realistic?Can we observe boundary-layer coherent structures?

100 km


Conclusions

Future work about boundary-layer coherent structures:

Coherent structures in LES using the MESO-NH model

Updrafts contribute the most to heat and moisture transportDowndrafts contribute significantly to transport only in StCu simulations (FIRE)Downward coherent structures exist below the cloud base and ressemble coherent downdrafts of convective clear-sky situations (IHOP)

Returning shells are close to updrafts but contribute only to ~10% of fluxes

Conclusions

Future work about boundary-layer coherent structures:

• Building better unified parameterizations for low-cloudsHow should be represented downward motions? Is the top-hat approximation relevant for downdrafts?

• Process-oriented analysis of the meso-scale low-cloud organisationWhat are the drivers of boundary-layer characteristics? Is the distance between updrafts important?

• Low-cloud feedback mechanismsChanges in coherent structures drive changes in cloudiness. How do coherent structures will change with global warming?

Coherent structures in LES using the MESO-NH model

Updrafts contribute the most to heat and moisture transportDowndrafts contribute significantly to transport only in StCu simulations (FIRE)Downward coherent structures exist below the cloud base and ressemble coherent downdrafts of convective clear-sky situations (IHOP)

Returning shells are close to updrafts but contribute only to ~10% of fluxes

Physical understanding of downdrafts

Zhou and Bretherton (19)

StCu (FIRE)



Downdraft objects Updraft objects

StCu (FIRE)




StCu (FIRE)

rH · V (s-1)Divergence




• Divergence at the top of updrafts

• Convergence where downdrafts start (20-30m below the updraft’s divergence)

• Updrafts trigger downdrafts? Then, amplify by buoyancy reversal?

StCu (FIRE)

rH · V (s-1)Divergence

! (m s-1)Vertical velocity


Witek et. al (11)

Downdraft objects

Updraft objects

• Divergence at the top of updrafts

• Convergence where downdrafts start (20-30m below the updraft’s divergence)

• Updrafts trigger downdrafts? Then, amplify by buoyancy reversal, or not?

Clear-sky (IHOP)

Divergence(s-1)rH · V

Vertical velocity! (m s-1)

object-oriented identification of coherent structures: a multi-case … · 2019. 10. 23. ·...

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