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February 12, 2004

Recent observational
studies on ice nuclei and ice
formation in clouds

Paul J. DeMott

Colorado State University

February 12, 2004


Focus primarily on some examples of studies ice
and mixed
phase clouds in last 10 years in
which IN and ice concentrations were measured.

Some inferences about our understanding of
upper tropospheric ice formation from studies of
natural IN and clouds at low temperatures.

A general realization/validation: mineral dust is
an important source of atmospheric IN.

A few thoughts on future needs.

February 12, 2004

Wave cloud studies (1990

Heymsfield and Milosevich 1993
1995 papers suggest that there are few
heterogeneous IN in upper troposphere and that ice formation by
homogeneous freezing dominates in cold wave clouds

Some during WISP studies (1993
1994): IN collected from around clouds
and processed in CFDC and controlled expansion cloud chamber. IN
concentrations reasonably consistent with ice in clouds, but method not
sufficient to explain variability in time and space. Tests for evaporation IN in
controlled expansion cloud chamber find no more than 2
3 enhancement.

Wave clouds below
40C during SUCCESS (1996) show total ice
concentrations consistent with homogeneous freezing, but also evidence
consistent with presence of IN up to 100 per liter at low temperatures. Such
high IN not always there and sometimes few deposition nuclei present.

U.K. studies (Field, Cotton, et al.) using SID show evidence of strong ice
formation mechanism in evaporation portion of modestly supercooled wave
clouds. Does not always occur.

WAVEICE studies (2000): Little apparent evidence for enhanced ice
formation in downstream portion of modestly supercooled wave clouds. IN
upstream of wave cloud are reasonably consistent with ice formed in cloud.
Some evidence for springtime dust impacts on cloud ice formation.

Some early optimism that IN measurements
are meaningful: Winter Icing in Storms Project

WISP 1994

NCAR Electra,
Wyoming KingAir

Six wave clouds


Ice concentration
from PMS 2DC and

Upwind aerosol bag
samples, analyzed at
CSU lab with CFD &
Dynamic Cloud

February 12, 2004

WAVEICE 2000: March 17, 2000

Wind 12
25 m s

February 12, 2004

WAVEICE 2000 Ice Concentrations in 35
Cloud Passes

March 17

200X probe

Parcel model uses IN and CCN measurements:

Data from Wyo. KA; March 17, 2000

Inference that, in absence of secondary
processes, and at T >
C: [IN] = [Ice]

February 12, 2004

A number of observations have been made of
enhanced ice formation in evaporation region of waves

Cooper (1995,
AMS Cloud
Conf.). See
also Cotton
and Field
(2002, QJRMS)

February 12, 2004

Preliminary Inferences from AIRS
2 Studies on the
Role of IN in the Evolution of Mixed Phase Clouds

Extremely inhomogeneous spatial distributions
of IN may exist in the atmosphere prior to winter

This heterogeneity is reflected by the IN
detected from cloud particle residuals (sampled
by CVI) but the interpretation of these data may
not be straightforward.

February 12, 2004

November 14: Lower clouds with and without ice, deeper clouds
and cirrus in some areas. Were in process of transitioning CFDC
conditions to equal those in lower clouds at this time

February 12, 2004

November 14: Distinct layers with high IN aloft, sometimes
reaching down to lower clouds. Some clouds had ice and some
not. Likewise, some had IN, some not.

[IN] in deep precipitating cloud system on November
19, 2003 and relation to cloud residual aerosol

CFDC processing T


= 102%

February 12, 2004

Other Mixed Phase Cloud Studies
Including IN Measurements

: IN relate to cloud ice in lake

: Generally lower IN in Arctic,
possible sources from open ocean leads,
silicate/sulfur chemistry of IN (Rogers et al.

North Dakota Tracer Experiment
: Bag
samples from cumulus cloud base levels and
surface sampling suggests agreement between
IN and young updraft ice (Stith et al. 1994;
DeMott et al. 1995)

February 12, 2004

Free tropospheric sampling of concentration
and composition of nuclei for cirrus formation

Storm Peak Laboratory (3220 m
MSL; Steamboat Springs, CO, USA)

Aerosol processing methodology (Spring 2004)

Cziczo et al. 2003, AS&T

INSPECT: Nov. 2001

February 12, 2004

DeMott et al. 2003,
December, PNAS


ice nucleation

Cirrus ice formation conditions and ice
concentrations nucleated on ambient
tropospheric aerosol particles

February 12, 2004

Inferences based on atmospheric observations of
T conditions required for cirrus formation

freezing of pure
sulfates from
Chen et al. (2000)
or Koop et al.


inside/outside cirrus, |w|<|1m/s
(Jensen et al., JGR, 2001)

Ice saturation

February 12, 2004

Heterogeneous nucleation at low temperatures on ambient
tropospheric aerosol particles suggest the range of cirrus
types impacted

Gierens (2003): “critical” concentration of
heterogeneous IN triggering a switch of
predominant mechanism from homogeneous
freezing to heterogeneous nucleation, as a
function of T and updraft speed

Synoptic lifting and Subvisual

Smaller scale
wave forcing
and anvil cirrus


DeMott et al. 2003, PNAS

IN formulations for numerical modeling: need for better
parameterizations and more fundamental understanding

Meyers et al



February 12, 2004

Do [IN] relate to aerosol particle
concentrations in a certain size range?

T =
42 to

= 90

February 12, 2004

What is the composition of heterogeneous
ice nuclei active under cirrus conditions?

Statistics of PALMS cluster analyses of particle types


80% (1/4 with any
detectable S)

Note: Untold story
about organic
aerosol components
and ice nucleation

February 12, 2004

Many recent results concerning
cirrus from INCA

Atmos. Chem. Phys., 3, 1791
1806, 2003

Freezing thresholds and cirrus cloud formation
mechanisms inferred from in situ measurements of relative humidity

W. Haag, B. Kärcher, J. Ström, A. Minikin, U. Lohmann, J. Ovarlez, and A. Stohl

The analysis of field data taken at northern and southern midlatitudes in fall 2000 reveals distinct differences in cirrus cl

Homogeneous freezing is found to be the most likely mechanism by which cirrus form at southern hemisphere
midlatitudes. The results provide evidence for the existence of heterogeneous freezing in cirrus in parts of the polluted
northern hemisphere, but do not suggest that cirrus clouds in this region form exclusively on heterogeneous ice nuclei.

Atmos. Chem. Phys., 3, 1807
1816, 2003

Cirrus cloud occurrence as function of ambient relative humidity: a
comparison of observations obtained during the INCA experiment

J. Ström, M. Seifert, B. Kärcher, J. Ovarlez, A. Minikin, J.
F. Gayet, R. Krejci, A. Petzold, F. Auriol, W. Haag, R. Busen, U. S
chumann, and H. C.

Discusses the cloud presence fraction (CPF) defined as the ratio between the number of data points determined to represent cl

at a
given ambient relative humidity over ice (RHI) divided by the total number of data points at that value of RHI. The CPFs take
n a
t Southern
Hemisphere (SH) and Northern Hemisphere (NH) midlatitudes differ from each other.
Above ice saturation, clouds occurred more
frequently during the NH campaign. Clouds during the SH campaign formed preferentially at RHIs between 140 and 155%,
whereas clouds in the NH campaign formed at RHIs somewhat below 130%.

Observed distributions of cloud water content differ only
slightly between the NH and SH campaigns and seem to be only weakly, if at all, affected by the freezing aerosols.

Atmos. Chem. Phys., 3, 1037
1049, 2003

situ observations of aerosol particles remaining from evaporated cirrus
crystals: Comparing clean and polluted air masses

M. Seifert, J. Ström, R. Krejci, A. Minikin, A. Petzold, J.
F. Gayet, U. Schumann, and J. Ovarlez

situ observations of aerosol particles contained in cirrus crystals are presented and compared to interstitial aerosol size d
activated particles in between the cirrus crystals).
Size distribution measurements of crystal residuals show that small aerosol
particles (Dp< 0.1 um) dominate the number density of residuals. On average the residual size distributions were shifted
towards larger sizes and the calculated particle volume was three times larger in the Southern Hemisphere. The form of the
residual size distribution did not depend on temperature as one might have expected considering different modes of

The observations of ambient aerosol particles were consistent with the expected higher pollution level in the Northern
Hemisphere. The fraction of residual particles only contributes to approximately a percent or less of the total number of par

February 12, 2004

Dust and IN: transports affect different parts of the world at
different times (E.g., Asian dust in N. America)

Fine (PM2.5) soil concentration at
the Mt. Zirkel IMPROVE site

VanCuren and Cahill

[JGR, 2002].
Continental transect of inferred fine
Asian dust frequency (top) and
concentrations (bottom) in ng m

February 12, 2004

Cirrus forming within Asian Dust layer
Sassen (2002)

Polarization lidar data in
Salt Lake City, UT on
April 29, 2001

Considerable warmer and
lower than climatological
means for cirrus

February 12, 2004

Possible dust impacts (Sassen 2002 GRL introduced
PDL evidence)

Also evidence in WAVEICE (2000)

Early morning 3/25 ruby lidar relative
backscattered power and linear depolarization
ratio at Salt Lake City (FARS
Ken Sassen)

March 25: Hazy day aloft

Mt Zirkel IMPROVE network sampler
indicates dust intrusion

c] =16 to
206 l
; T=

February 12, 2004

WAVEICE 2000 Ice Concentrations in 40
Cloud Passes

March 25

February 12, 2004

Recent Lab Studies Corroborate Ice Formation by Dust
Particles (resuspended Asian dust

Cassie Archuleta thesis)

200 nm

200 nm

Ca, Si, S, Mg

Si, Al, Fe

freezing points
of sulfuric acid

nucleation by

February 12, 2004

Saharan dust aerosol sampled in

(DeMott et al., GRL, 2003 and Sassen et al., GRL, 2003)

Processing at T =

= 86%,

= 123% assured
heterogeneous ice nucleation only

MODIS aerosol optical depth, July
27; July 29, 2003 back trajectory

S. Florida PDL lidar data on 7/29

July 29: CFDC operating mostly in expected
homogeneous freezing regime at low temperature
during anvil ascent profile.

High IWC contents

No Citation FSSP
data during period

correspond with
[2D] within factor 2

CPI concentrations
also correspond
well with [IN]

[IN] up to 600/liter

CPI data: C. Scmitt, A. Bansemer, A.

February 12, 2004

TEM analyses of IN from July 29, 2002

February 12, 2004

PALMS analysis of particles from high tropical cirrus
during CRYSTAL
FACE (Source D.J. Cziczo, NOAA)

Histogram of the area of the sodium peak in each positive polarity mass spectrum.

This area can be used as a rough indicator of particle type.

Most ice residue, particles outside cloud, and interstitial aerosols are
sulfate /

: this is consistent with our understanding of
homogeneous freezing

Sulfates plus

Organics (~70%)

Mineral Dust, Fly Ash, Meteoritic, Etc. (~25%)

Sea Salt (<5%)

February 12, 2004

PALMS analyses of anvil cirrus particles (July 29, 2002)

Ice residue from July 28
29 2002 have a much higher sodium signal than out of
cloud particles or interstitial aerosol.

~20% are consistent with frozen sea salt. <10% Sulfates and organics.

Most of the remainder (70%) are consistent with mineral dust or fly ash

heterogeneous freezing…

Sulfates plus



Mineral Dust (~70%)

Sea Salt (20%)

February 12, 2004

Thoughts on future studies

Still believe that wave clouds have much to offer in
understanding ice formation mechanisms.

Do not yet have IN measurements at appropriate
conditions in and around cirrus (Tropical cirrus
missions and high altitude capabilities coming).

Need IN versus ice concentration in convective clouds.
Likely missing an important ice formation mechanism.

Validate the impacts of mineral dusts on clouds in
programs such as AMMA?

Continued need for laboratory studies of aerosol
effects on ice nucleation and more fundamental work.

Definition of ice versus water is still a critical issue for
studies relating IN to ice formation.

Need to take real
time combined IN and residual
composition measurements aloft.

February 12, 2004

Dust versus no dust adiabatic simulation
of cumuli parcels (12 m s

updraft; 15
cloud base, maritime CCN)