Clouds in Polar Regions

• Poor representation of clouds a likely major source of error in forecasting models

• Clouds are mixed phase often dominated by super-cooled water

• Models of all scales fail to reproduce these clouds well. Tend to over predict ice content

• Break-up of the clouds poorly predicted

• Moist bias. Models predict coupled boundary layer structure often observed to be decoupled

Clouds in Polar regions

• Key Questions • What is the droplet number concentration. What are the CCN ? • How many at what is the nature of IN ? • Are secondary ice particle processes important. Is it just H-M or are

there others ? • What is the role of entrainment at cloud top and how does this

change? • What is the contribution of ‘warm rain’ ie supercooled drizzle in

controlling the water budget ? • How does this compare with the role of snow ? • How does precipitation affect the boundary layer structure and CCN

/IN number

Structure of talk

• Microphysics of arctic stratus cloud from ACCACIA-Primary ice secondary ice

• Observations of the evolution of arctic clouds from ASCOS

• Microphysics of deep frontal clouds

• Evolution of clouds in arctic air outbreaks, transition from stratus to cumulus

Example Microphysics – Spring Case

2500200015001000500

0Alt

itu

de

[m]

13:05

13:05

13:10

13:10

13:15

13:15

13:20

13:20

13:25

13:25

13:30

13:30

Time [UTC]

-20-16

-12

-8

Tem

pera

ture [ºC

]

0.4 0.4

0.3 0.3

0.2 0.2

0.1 0.1

0.0 0.0

LW

C [

g m

-3]

120120

8080

4040

00

Co

nc

[cm

-3]

10 10

8 8

6 6

4 4

2 2

0 0

Ice

Co

nc

[L-1

]

The Vertical Structure – Probe Imagery

The Vertical Structure – Probe Imagery

Example Microphysics – Summer Case

3600

3200

2800

2400

Alt

itu

de

[m]

10:10

10:10

10:15

10:15

10:20

10:20

10:25

10:25

10:30

10:30

10:35

10:35

Time [UTC]

-12

-8

-4

0

Tem

p [°C

]

0.30 0.300.25 0.250.20 0.200.15 0.150.10 0.100.05 0.050.00 0.00

LW

C [

g m

-3]

200 200

150 150

100 100

50 50

0 0

Co

nc

[cm

-3]

40 40

30 30

20 20

10 10

0 0

Ice

Co

nc

[L-1

]

The Vertical Structure – Probe Imagery

The Vertical Structure – Probe Imagery

Timeseries of LWC and IWC

Simulation period

(From Sotiropoulou et al, 2014 ACP)

Liquid water content profiles B814

Top Right PSD

Bottom Left PSD

Bottom Right PSD

12 December 2011 – A Warm Front Case Study

12 December 2011 – A Warm Front Case Study

The Vertical Structure - Number

1.28mm

(a)

(c)

(b)

-35

-25

-10

-5

Ice Concentration [L-1]

The Vertical Structure - Size

1.28mm

(a)

(c)

(b)

Ice Size[µm]

Alt

itu

de

[m]

The Vertical Structure - Mass

1.28mm

(a)

(c)

(b)

Ice Size[g m-3]

Alt

itu

de

[m]

The Role of Big Drops?

Cold Front

Warm Front

Occluded Front

Clouds in Polar regions

• What is the droplet number concentration. What are the CCN ? Droplet

number concentration declines as cloud breaks up (washout of CCN similar to VOCALS)

• How many at what is the nature of IN ? This is very variable • Are secondary ice particle processes important. Is it just H-M or are there

others ? Certainly H-M but others probably have a role (drop freezing splinters and crystal break-up, surface frost)

• What is the role of entrainment at cloud top and how does this change? • What is the contribution of ‘warm rain’ ie supercooled drizzle in

controlling the water budget ? Sometimes key when few IN and little secondary ice

• How does this compare with the role of snow ? • How does precipitation affect the boundary layer structure and CCN /IN

number evaporative cooling causes decoupling