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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 Assembly Montréal, Canada, July 24 th , 2009.

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slide1

The potential impact of mineral dust on cirrus (and other) cloud formation:a trajectory modeling perspectiveAldona Wiącek* and Thomas Peter ETH, Zürich, Switzerland* Now at Dalhousie University, Halifax, CanadaMOCA-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

outline
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
why mineral dust
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
how does mineral dust affect cirrus and mixed phase clouds
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
location of preferential dust sources
Location of preferential dust sources

Gobi

W. African

Taklamakan

Bodélé

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].

case studies from regions of high omi ai
Case studies from regions of high OMI AI

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

  • 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

pressure

[hPa]

Why so different?

unusual asian topography encourages lifting but only in the taklimakan not in the gobi
Unusual Asian topography encourages lifting(but only in the Taklimakan, not in the Gobi)
  • Topography [m]
slide9

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

statistical trajectory study setup
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 ECMWFfields (T799 – 25 km)
  • Traced p, T, Q
  • Calculated RHwater, RHiceusing Q (t=0)

6 km

4 km

2 km

0

10 km

5 km

0

same procedure for other dust sources
Same procedure for other dust sources

42 starting points roughly span each region at 1° resolution

Bodélé depression

(Africa, active all year)

West Africa

(peak activity JJA)

Gobi desert divided into East and West but results very similar

(peak activity MAM)

slide12

Cloud formation processes

200

150

100

50

0

2000

1000

0

“Classical cirrus”

Mixed-phase clouds

Classical cirrus’

MPC

Liquid water cloud

MPC’

RHice [%]

“Warm thin cirrus”

“Cold thin cirrus”

dry

avg

wet

Distribution of trajectories from Taklamakan

# saturating trajectories [K-1]

210

220

230

240

250

260

270

280

290

Temperature [K]

Wiacek & Peter [2009, GRL, in press]

results by region of 1 8 million points originating from each region
Results by region (% of ~ 1.8 million points originating from each region)

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

breakdown of cloudy points only
Breakdown of cloudy points only

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

Potentially big effect here

invisible

Dust gets into “classical cirrus” only via MPC

% of all trajectory points (1.8 Mio in each region)

Unlikely to play a big role

Do “warm thin cirrus” exist at all?

No effect on “cold thin cirrus”

slide15

Details of Temperature

for selected cloud types

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

some lab measurements of ice nucleation old and new
Some lab measurements of ice nucleation, old and new

AIDA chamber experiments: Arizona test dust [Möhler et al., 2006].

AIDA chamber experiments: Saharan and Taklimakan dust [Möhler et al., 2006].

AIDA warm measurements: Saharan and Asian dust [Field et al., 2006].

CFDC measurements: kaolinite (white squares), montmorillonite (white diamonds) [Salam et al., 2006].

Thermal diffusion chamber: kaolinite (upper blue curve), Denver local soil (center blue curve), silver iodide (lower blue curve) [Schaller & Fukuta, 1979].

Microscope cold stage: Montmorillonite (upper horizontal white line: unprocessed; lower horizontal white line: preactivated [Roberts & Hallett, 1968].

SEM cold stage: illite and kaolinite [Zimmermann et al., 2008]

summary
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