Aerosols effects on turbulence in mixed-phase deep convective clouds investigated with a 2D cloud model

with spectral bin microphysicsThe 26th Annual Meeting of the Israeli Association of Aerosol

Research

Nir Benmoshe, Alexander Khain

Atmospheric science departmentThe Hebrew University in Jerusalem

HUCM

• A 2D cloud model with 43 bins spectral bins

• 7 different hydrometeors type • Aerosols• Diffusional growth, collision, freezing,

melting, advection• Model resolution of 50 m x 50 m was

used.

Droplet fall and collisions in non-turbulent air

Formation of eddies

Absolute velocitiesRelative velocities

Physical mechanisms of effects of turbulence on collisions

Formation of relative velocitybetween particles and

environment

Formation of concentration inhomogeneity

(droplet clustering)

21 2 1 2 1 2 ker( , ) ( ) ( , )turb grav clustK r r r r E r r K P P 1 2V V

Swept volume Collision efficiency Fluctuations of concentration

is the collision kernel

References:

Saffman and Turner (1956);

Khain and Pinsky, 1995;

Pinsky and Khain, 1996,1997a,b;

Pinsky et el, 2000, 2001;

Zhou et al, 1998;

Wang et al, 1998, 2000;

Elperin and Dodin, 2012

References: Maxey, 1987,Wang and Maxey, 1993;Pinsky et al. 1997; 1999, Pinsky and Khain 2001, 2003.Shaw et al, 1998; Shaw and Kostinsky 2003; Elperin et. al 1996; 1998, 2002Falkovich et al, 2001, 2002;

References:

Pinsky et al, 1999, 2000; 2001; 2004; 2008

Khain et al, 2000;

Pigeonneau and Feuillebois, 2002

Wang et al, 2004; Ayala et al 2010

,,,,,,,,

0

2/

0

mdtmfmmKtmfdmtmmfmmmKtmft

tmfcc

m

ccc

),()(),( 212

2121 rrErrrrK 21 VV

How does turbulence influence droplet collisions?

Benmoshe et al. (2012)combined effect of all factors

Mean normalized collision kernel in turbulent flow for three cases: stratiform clouds (left panel), cumulus clouds (middle) and cumulonimbus (right panel). Pressure is equal to 1000mb. (After Pinsky et al, 2008)

Stratocumulus

Cumulus

Cumulonimbus

Collision kernel enhancement factors for different dissipation rates and Reynolds numbers

Novel approach for calculation of collisions:

a) Calculation of dissipation rate in each grid point at each time step

b) Calculation of Reynolds number in each grid point at each time step;

c) Calculation of collision enhancement factor in each grid point at each time step

This method makes it possible to investigate effects of turbulence on precipitation formation.

Turbulence kinetic energy equation

εαKNxW

zU

2zW

2xU

2KzE

WxE

UtE 2

222

0.5klECK

0Nif

NE

,0.76ΔxΔzmin

0NifΔxΔz

l2

2

ΔxΔz

l1.9C0.931.9CC k

k

0.2Ck

zθ

θg

N2

Calculation of dissipation rate

Benmoshe et al. (2012)

lCE

ε3/2

Dissipation rate

cm^2/sec^3

Calculating λRe

νλu'

Reλ

TKE32

u'

2/3tot )(TKE L

ε15ν

u'λ Taylor microscale

Characteristic velocity fluctuation

Reynolds lambda

L is the external turbulent scale

Benmoshe et al. (2012)

• Blue ocean - CCN concentration 200 cm-3

• Green ocean – CCN concentration 700-900 cm

-3

• Smoky clouds - CCN concentration 5000-10000 cm

-3

CASE STUDIES: LBA-SMOC FIELD EXPERIMENT

Andreae et al, 2004

Turbulent structure of deep cumulus clouds

Turbulence properties

Benmoshe et al. (2012)

spatial vs. averaged values

0 1000 2000 3000 4000 5000 6000 7000 80000

1

2

3

4

5

6

TU

RB

CO

EF

FIC

IEN

T,m

2 /s

averaged values

Time, s

S-exp CCN-wat-tur

Figure 8. Vertical profiles of time averaged maximum values of the dissipation rates in Sgr, Stur, BOgr and BOtur (averaging over the period 3600 s to 4800 s)

Aerosols effect on cloud turbulence

Accumulated rain: effects of turbulence and aerosols

Effect of turbulence on

collisions in mixed-phase clouds

The turbulence effect on ice particles collision is larger than on water droplets

Effects of turbulence on ice collisions should be larger because of lower sedimentation velocity at the same

mass (inertia)

von Blohn et.al. 2005Nowell 2010

Pinsky, M.B., A.P. Khain, D. Rosenfeld and A. Pokrovsky, 1998

Increase in the collision kernel

EFFECTS WITH ENHANCED RIMING

graupel graupel

CWC CWC

CONTROL

EFFECTS WITH ENHANCED RIMING

Graupel, grav Graupel, turb

Snow, grav

Snow, turb

CONTROL

Accumulated rain

• High resolution of the model gives us real fractal cloud structures.

• This is the first time that time and spatial depended turbulence characteristics were calculated for cumulus clouds

• Turbulence in clouds is highly inhomogeneous: mean values do not reflect effects of turbulence on collisions

Conclusions – turbulence structure

• Turbulent intensity in clouds increase in the presence of higher aerosols concentration

• Increase in the collision rate between droplets reduces the total amount of precipitation since it eventually weakens cold precipitation processes

• Turbulence substantially accelerates formation of warm rain, especially in polluted clouds.

• Turbulence in mixed phase clouds increases the rate of riming, mass and size of graupel and accelerates formation of cold rain

Questions ?

Next time you are in an air pocket think about its good side….

Questions?

IMPORTANCE OF THE STUDY

• increasing the collision rate in highly turbulent clouds by order of the magnitude.

• cloud turbulence determines processes of entrainment of dry air into the cloud and affects the cloud height.

• The knowledge of the cloud turbulence intensity is important for purposes of flights safety.

• why the shape of DSD is wider than it is supposed to be according to the equation for the diffusion droplet growth (e.g., Brenguier and Chaumat, 2001)

• and why warm rain formation, as shown by Jonas (1996), occurs significantly faster than it is supposed to in accordance with the classical theory of gravitational coagulation.

• Pinsky et al 2008 tell how turbulence kernel effect a DSD• Falkovich et al (2002); Pinsky et al (1997a,b; 2008); Xue et al (2008);

Wang and Grabowsky (2009), the authors presented solutions of the stochastic collision equation in which turbulent effects on the evolution of the initially given DSD were simulated.

So, what are we talkingסרטון של ענן מצולםabout

Previous work

• The mean kinetic energy dissipation rate in stratocumulus clouds (Sc) is estimated as (Siebert et al. 2006) and in small cumuli as (MacPherson and Isaac, 1977; Mazin et al 1989; Pinsky and Khain 2003).

• According to Panchev (1971) and Weil et al (1993), the values of measured in deep cumulus clouds range from several hundreds to .

• The recent measurements of the turbulent structure of the boundary layer using a helicopter (Siebert et al 2006) indicated dramatic spatial inhomogeneity of: while the typical mean values of are , in some zones of Sc clouds (possibly in zones of imbedded convection) the values of can increase up to .

• the typical values of were estimated by Pinsky et al (2007, 2008) as ranging from ~ in stratiform clouds to ~in strong deep convective clouds (Cb).

• According to Siebert et al (2006), turbulent intensity varies dramatically within stratocumulus clouds. One can expect a high variability of and in cumulus and Cb clouds as well.

• To our knowledge, there have been no regular measurements of the fine spatial distribution of and in deep cumulus clouds.

• Turbulence determines small scale spatial fluctuations of the liquid water content (e.g., Spyksma and Bartello, 2008).

• turbulence affects droplet size distributions (DSD) thus having an impact on diffusion growth/evaporation of drops (e.g., Jensen and Baker, 1989; Khvorostyanov and Curry 1999a,b).

Where are the first drops forms?

X, km

Hei

ght,

km

eps,1500sec,M2

/S3

2.5 5 7.5 10 12.5

10.35

7.85

5.35

2.85

0.35

0.00

0.00

0.01

0.02

0.03

0.05

0.08

0.13

0.22

X, km

Hei

ght,

km

RAIN DROP mass,1800sec,g/m3

2.5 5 7.5 10 12.5

10.35

7.85

5.35

2.85

0.350

0.1

0.2

0.3

0.4

0.5

0.6

0.7

X, km

Hei

ght,

km

RAIN DROP mass,1500sec,g/m3

2.5 5 7.5 10 12.5

10.35

7.85

5.35

2.85

0.350

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Rain water content, gm-3 ,t=1500s Rain water content, gm-3 ,t=1800s

Dissipation rate, m2s-3 ,t=1500s

X, km

Hei

ght,

km

eps,1800sec,M2

/S3

2.5 5 7.5 10 12.5

10.35

7.85

5.35

2.85

0.35 0.00

0.00

0.00

0.01

0.02

0.03

0.05

0.08

0.13

0.22

Dissipation rate, m2s-3 ,t=1800s

X, km

Hei

ght,

km

eps,1500sec,M2

/S3

2.5 5 7.5 10 12.5

10.35

7.85

5.35

2.85

0.35

0.00

0.00

0.01

0.02

0.03

0.05

0.08

0.13

0.22

X, km

Hei

ght,

km

RAIN DROP mass,1500sec,g/m3

2.5 5 7.5 10 12.5

10.35

7.85

5.35

2.85

0.350

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Rain water content, gm-3 ,t=1500s Rain water content, gm-3 ,t=1800s

Dissipation rate, m2s-3 ,t=1500s

X, km

Hei

ght,

km

eps,1800sec,M2

/S3

2.5 5 7.5 10 12.5

10.35

7.85

5.35

2.85

0.35 0.00

0.00

0.00

0.01

0.02

0.03

0.05

0.08

0.13

0.22

Dissipation rate, m2s-3 ,t=1800s

E200T E2000T

Data Size Distributions

How is the first precipitation influenced by turbulence?

GO-turb

GO-grav

S-turb

S-grav

Strange notations

Reff

1 2 3 4 5 61

2

3

4

5

6

7

8

9

Stur

, time=4140

Adiabatic LWC, g/m3

Hei

gh

t, m

0

1

2

3

4

5

6

7

8

0 1 2 30

1

2

3

4

5

6

7

8

0 1 2 30

1

2

3

4

5

6

7

8

0 1 2 3

0

1

2

3

4

5

6

7

8

0 1 2 3 4 50

1

2

3

4

5

6

7

8

0 1 2 3 4 50

1

2

3

4

5

6

7

8

0 1 2 3 4 5

Hei

ght

)km

(

August 24 August 25August 23

June 16 June 22 June 21

Liquid water content )gm-3(

Hei

ght

)km

(

LWC LWCad

0.5 1 1.5 2 2.5 3 3.5 4 4.50

100

200

300

400

500

600

X, km

DR

OPL

ETS

num

,cm

- 39502,4800sec, Height1.1km

1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5150

200

250

300

350

400

X, km

DR

OPL

ETS

num

,cm

- 3

9502,4800sec, Height1.1km