11

The potential impact of mineral dust on cirrus (and other) cloud formation:a trajectory modeling perspective

Aldona Wiącek* and Thomas Peter ETH, Zürich, Switzerland

* Now at Dalhousie University, Halifax, Canada

MOCA-09 Joint AssemblyMontréal, Canada, July 24th, 2009

Funded by a Marie Curie Incoming International Fellowship under FP6 and the Canadian Foundation for Climate and Atmospheric Science

22

Outline

Why mineral dust? Where/when is it found?

How does mineral dust affect clouds?

African and Asian dust emission trajectory case studies

Statistical studies of trajectories from Africa and Asia

Discussion and Summary

33

Why mineral dust?

Highest burden and emitted mass of all aerosols: ~17 Tg and ~1500 Tg/year [Satheesh & Moorthy, 2005]

Anthropogenic contribution estimated at 0-20% [IPCC, AR4]

Efficient ice nucleus (IN) insoluble and contains mineral lattice defects crystallographic and chemical bond similarities to ice particle size > 0.1 μm

Modulates the ice phase of clouds and precipitation (much smaller effect on water clouds due to poor CCN ability)

Indirect radiative effects highly uncertain

44

How does mineral dust affect cirrus and mixed-phase clouds?

Cirrus cloud radiative properties are very sensitive to ice crystal number concentration and size

These are influenced by the ice formation mechanism homogeneous nucleation on, e.g. H2SO4 solution droplets

heterogeneous nucleation on, e.g. on dust IN

Similarly, mineral dust will alter the radiative properties and lifetime of mixed-phase clouds

Vast majority of atmospheric dust mass remains confined to altitudes < 7 km, but a little dust can go a long way to modify cirrus and mixed-phase clouds

55

Where does mineral dust come from?

66

Percentage of model grid box that is a preferential dust source, calculated from the extent of potential lake areas, excluding areas of actual lakes [Tegen et al., 2003, Quat. Sci. Rev].

Location of preferential dust sources

W. African

Bodélé

Taklamakan

Gobi

77

Case studies from regions of high OMI AI

Calculated ~ 600 7-day forward trajectories from each the following regions

(1) West Sahara,

15 July 2007

4.4 % of trajectories

ascended from

700 to 450 hPa

(2) Taklimakan,

20 May 2007

96 % of trajectories

ascended from

700 to 450 hPa

Why so different?

pressure

[hPa]

88

Unusual Asian topography encourages lifting(but only in the Taklimakan, not in the Gobi)

Topography [m]

99

Why so different?

(1) Potential temperature (θsurf) as high in Asia as in Africa(due to higher dust source elevation in Asia)

(2) Altitude corresponding to θsurf higher in Asia

θsurf 320 K

1010

Statistical trajectory study setup

7-day forward trajectories:

4 times per day (00, 06, 12, 18)

365 days in 2007

42 points covering the Tarim basin at 1°

61,320 trajectories (1,778,280 saved points)

High-resolution ECMWF fields (T799 – 25 km)

Traced p, T, Q

Calculated RHwater, RHice using Q (t=0) 10 km

5 km

0

6 km

4 km

2 km

0

11

Same procedure for other dust sources

Gobi desert divided into East and West but results very similar(peak activity MAM)

Bodélé depression (Africa, active all year)

West Africa (peak activity JJA)

42 starting points roughly span each region at 1° resolution

12

210 220 230 240 250 260 270 280 2900

20

40

60

80

100

120

140

160

180

200

avgdry wet

RH

ice [

%] Liquid

water cloud

MPC

MPC’

“Warm thin

cirrus”

“Cold thin

cirrus”

“Classical cirrus” Mixed-phase

cloudsClassical

cirrus’

Cloud formation processes

Temperature [K]220 230 240 250 260 270 280 290

# s

atu

rati

ng

tr

aje

cto

rie

s [

K-1]

200

150

100

50

0

2000

1000

0210

Distribution of trajectories from Taklamakan

Wiacek & Peter [2009, GRL, in press]

13

Results by region (% of ~ 1.8 million points

originating from each region)

Taklimakan Gobi West Africa Bodélé

Crashed 13.0 8.9 0.4 0.3

Clear sky 57.3 57.6 84.5 86. 5

Oscillatory Clear

Sky 7.7 11.0 5.7 2.7

Cloudy 13.4 15.2 6.9 7.6

Oscillatory Cloudy 8.5 7.4 2.6 3.0

Wiacek et al., [to be submitted to ACPD]

14Tak

Gobi

W A

frBod

TakG

obi

W A

frBod

TakG

obi

W A

frBod

TakG

obi

W A

frBod

TakG

obi

W A

frBod

0

2

4

6

8

10

12

14

16

18

Classical cirrus

Cold thin cirrus

Liquid wa-ter clouds

Mixed-phase clouds

Warm thin cirrus

Breakdown of cloudy points only%

of

all t

raje

cto

ry p

oin

ts (

1.8

Mio

in e

ach

reg

ion

)

No effect on “cold thin

cirrus”

Do “warm thin cirrus” exist at all?

Dust gets into

“classical cirrus” only

via MPC

Potentially big effect

here

Unlikely to play a big

role

Wiacek et al., [to be submitted to

ACPD]

invisible

15

Wiacek et al., [to be submitted to ACPD]

Details of Temperaturefor selected cloud types

16

Some lab measurements of ice nucleation,

old and new

(1) AIDA chamber experiments: Arizona test dust [Möhler et al., 2006]. (2) AIDA chamber experiments: Saharan and Taklimakan dust [Möhler et al., 2006]. (3) AIDA warm measurements: Saharan and Asian dust [Field et al., 2006]. (4) CFDC measurements: kaolinite (white squares), montmorillonite (white diamonds)

[Salam et al., 2006]. (5) Thermal diffusion chamber: kaolinite (upper blue curve), Denver local soil (center

blue curve), silver iodide (lower blue curve) [Schaller & Fukuta, 1979]. (6) Microscope cold stage: Montmorillonite (upper horizontal white line: unprocessed;

lower horizontal white line: preactivated [Roberts & Hallett, 1968].(7) SEM cold stage: illite and kaolinite [Zimmermann et al., 2008]

1717

Summary

The availability of bare ice nuclei for ‘classical’ cold cirrus is negligible from both African and Asian source regions

Mineral dust unlikely competitor to homogeneous nucleation

The availability of bare ice nuclei for ‘warm thin cirrus’ could be significant as a dehydration pathway, and is higher from Asian deserts, however, their existence remains speculative given the lack of field measurements

The greatest influence of mineral dust is found to be on mixed-phase clouds (0°C < T < -40°C), especially from Asian deserts