Large-Eddy Simulation of a stratocumulus to cumulus transition as observed during the First Lagrangian of ASTEX Stephan de Roode and Johan van der Dussen.

Large-Eddy Simulation of a stratocumulus to

cumulus transition as observed during the First

Lagrangian of ASTEX

Stephan de Roode and Johan van der Dussen

Delft University of Technology, Netherlands

cumulus penetrating stratocumulus, Bretagne, France View on Delft, Johannes Vermeer, 1661

ASTEX First Lagrangian

12-14 June 1992

Contents

ASTEX First Lagrangian

- observations

- previous numerical studies

Large-Eddy Simulation results

- mean state

- turbulence

Observed stratocumulus to cumulus transition

Bretherton and Pincus, 1995

Bretherton et al, 1995

Duynkerke et al, 1995

De Roode and Duynkerke, 1997

Observed stratocumulus to cumulus transition

Bretherton and Pincus, 1995

Bretherton et al, 1995

Duynkerke et al, 1995

De Roode and Duynkerke, 1997

GCSS case,1995

EUCREM/GCSS,Duynkerke et al, 1999

like GCSS ATEX case,Stevens et al, 2001

Study of ASTEX First Lagrangian wtih SCM and 2D models by Bretherton et al, 1999:

"there are substantial quantitative differences in the cloud cover and liquid water path between models."

Satellite images Flights 1 and 5

Inversion jumps

-10

-8

-6

-4

-2

0

2

0 5 10 15 20

Δ2 = 0

Δθv = 0

FIRE I mean

ASTEX Flight 2

ASTEX Flight 3

ΔYCOMS II

Δ θl [ ]K

Buoyancy reversalcriterion

DYCOMS II

EUROCStransition to cumulus, increases

EUROCS FIRE case

ASTEX

Lock (2009)

Entrainment rates

0

0.5

1

1.5

2

2.5 DALES oldother LESDALES 3.1DALES 3.1 HRobservations

ASTEXA209

ASTEXRF06

FIRE I DYCOMS IIRF01

too large?

Dales paper to be submitted by Heus et al.

Set-up of simulation

Initialization

Take vertical profiles from GCSS ASTEX A209 intercomparison case

Time varying forcing

SST and subsidence rate from Bretherton et al. (1999)

Geostrophic winds from observations

Dutch Atmospheric Large-Eddy Simulation (Dales) model (Heus et al,

2009)

1283 grid points (6.4x6.4x2.5 km3) on 32 processors

Monotone advection ("kappa") scheme for passive scalars

Time step t = 0.5 s (Runge-Kutta)

Bulk microphysics (Khairoutdinov and Kogan 2000)

Longwave radiation: "GCSS WG1" parameterization

Shortwave radiation: "EUROCS FIRE" delta-Eddington, Independent Pixel

Approximation

Simulated mean cloud-top height

Simulated cloud boundaries

mean cloud top

mean cloud base

lowest cloud base

Entrainment rate

0

0.5

1

1.5

2

0 5 10 15 20 25 30 35 40

ASTEX Lagrangianold GCSS mean

time (hours)

Including microphysics (rain and cloud droplet sedimentation) decreases entrainment rate

Cloud liquid water path evolution

0

50

100

150

200

0 5 10 15 20 25 30 35 40

time (hours)

0 10

2000

cloud fraction0 1

0

2000

cloud fraction0 1

0

2000

cloud fraction

Mean state Hour 9 Mean state

Hour 19

2 4 6 8 10 120

500

1000

1500

2000

2500

3000

Total water content (g/kg)

285 290 295 300 305 3100

500

1000

1500

2000

2500

3000

Liquid water potential temperature [K]

290 295 300 305 3100

500

1000

1500

2000

2500

3000

Liquid water potential temperature [K]

0 2 4 6 8 10 120

500

1000

1500

2000

2500

3000

Total water content (g/kg)

Buoyancy flux

-0.004 0 0.004 0.008 0.012 0.0160

200

400

600

800

1000

w'θv [ / ]' Km s

( )b Flight 2Flight 3

-0.01 0 0.01 0.02 0.03 0.040

500

1000

1500

2000

w'θv [ / ]' Km s

( )c Flight 4Flight 5

Large buoyancy fluxesduring Flight 4

Vertical velocity variance

Flight 4Flight 5

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

0

500

1000

1500

2000

w2 [m

2/s

2]

0 0.05 0.1 0.15 0.2 0.25 0.30

200

400

600

800

1000

w2 [m

2/s

2]

Flight 2Flight 3

decoupledboundary layer

(double peak in w'2)

Drizzle and total humidity flux

-0.03 -0.015 0 0.015 0.030

200

400

600

800

1000

[m/s][g/kg]

Flight 2

-0.1 -0.05 0 0.05 0.1 0.15 0.20

200

400

600

800

1000

1200

Flight 4

[m/s][g/kg]

Summary

Strong points

+ aircraft observations of turbulence and entrainment rate

+ critical regime with regard to cloud fraction

+ passes through two previous intercomparison cases (revisiting Flights 2 and 3)

Weak points

- quite large uncertainty in total humidity fluxes

Status of the case

- refine forcing of free atmosphere (longwave radiative cooling, geostrophic forcing)

Research questions: Study and provide detailed 3D LES fields on a large horizontal domain

+ connection to radiation community (McICA)

+ length scale analysis (NWP horizontal resolution -> 1 km)