Ice supersaturations and cirrus cloud crystal numbers

Abstract

Upper tropospheric observations outside and inside of cirrus clouds indicate water vapour mixing ratios sometimes exceeding water saturation. Relative humidities over ice (RHice) of up to and more than 200% have been reported from aircraft and balloon measurements in recent years. From these observations a lively discussion continues on whether there is a lack of understanding of ice cloud microphysics or whether the water measurements are tainted with large uncertainties or flaws. Here, RHice in clear air and in ice clouds is investigated. Strict quality-checked aircraft in situ observations of RHice were performed during 28 flights in tropical, mid-latitude and Arctic field experiments in the temperature range 183–240 K. In our field measurements, no supersaturations above water saturation are found. Nevertheless, super- or subsaturations inside of cirrus are frequently observed at low temperatures (ice deviating from saturation, we analysed the number densities of ice crystals recorded during 20 flights. From the combined analysis – using conventional microphysics – of supersaturations and ice crystal numbers, we show that the high, persistent supersaturations observed inside of cirrus can possibly be explained by unexpected, frequent very low ice crystal numbers that could scarcely be caused by homogeneous ice nucleation. Heterogeneous ice formation or the suppression of freezing might better explain the observed ice crystal numbers. Thus, our lack of understanding of the high supersaturations, with implications for the microphysical and radiative properties of cirrus, the vertical redistribution of water and climate, is traced back to the understanding of the freezing process at low temperatures.

Full text

Page 1

ACPD
8, 21089–21128, 2008
Ice supersaturations
M. Kr¨ amer et al.
Title Page
Abstract Introduction
ConclusionsReferences
Tables Figures
??
??
BackClose
Full Screen / Esc
Printer-friendly Version
Interactive Discussion
Atmos. Chem. Phys. Discuss., 8, 21089–21128, 2008
www.atmos-chem-phys-discuss.net/8/21089/2008/
© Author(s) 2008. This work is distributed under
the Creative Commons Attribution 3.0 License.
Atmospheric
Chemistry
and Physics
Discussions
This discussion paper is/has been under review for the journal Atmospheric Chemistry
and Physics (ACP). Please refer to the corresponding final paper in ACP if available.
Ice supersaturations and cirrus cloud
crystal numbers
M. Kr¨ amer1, C. Schiller1, A. Afchine1, R. Bauer1, I. Gensch1, A. Mangold1,*,
S. Schlicht1,**, N. Spelten1, N. Sitnikov2, S. Borrmann3, M. de Reus3, and
P. Spichtinger4
1FZ J¨ ulich, ICG-I, Germany
2CAO, Region Moscow, Russia
3Univ. Mainz, Inst. Phys. Atmos., Germany
4ETH Z¨ urich, Inst. Atmos. andClimate, Switzerland
*now at: Royal Met. Inst. of Belgium, Brussels, Belgium
**now at: M¨ uhlenbachstraße 5, 52134 Herzogenrath, Germany
Received: 20 October 2008 – Accepted: 20 October 2008 – Published: 17 December 2008
Correspondence to: M. Kr¨ amer (m.kraemer@fz-juelich.de)
Published by Copernicus Publications on behalf of the European Geosciences Union.
21089

Page 2

ACPD
8, 21089–21128, 2008
Ice supersaturations
M. Kr¨ amer et al.
Title Page
AbstractIntroduction
ConclusionsReferences
Tables Figures
??
??
BackClose
Full Screen / Esc
Printer-friendly Version
Interactive Discussion
Abstract
Upper tropospheric observations outside and inside of cirrus clouds of water vapour
mixing ratios sometimes exceeding water saturation, yielding up to more than 200%
relative humidities over ice (RHice) have been reported from aircraft and balloon mea-
surements in recent years. From these observations a lively continuous discussion
arose on whether there is a lack of understanding of ice cloud microphysics or if the
water measurements are tainted with large uncertainties or flaws.
Here, RHicein clear air and in ice clouds is investigated: strictly quality checked
aircraft in-situ observations of RHicewere performed during 28 flights in tropical, mid-
latitude and Arctic field experiments in the temperature range 183–250K. In our field
measurements, no supersaturations above water saturation are found. Nevertheless,
super- or subsaturations inside of cirrus are frequently observed at low temperatures
(<205K) in our field data set. To explain persistent RHicedeviating from saturation, we
analysed the number densities of ice crystals recorded during 20 flights. From the com-
bined analysis – using conventional microphysics – of supersaturations and ice crystal
numbers, we show that the high, persistent supersaturations observed inside of cirrus
are caused by unexpected, frequent very low ice crystal numbers that could hardly be
explained by homogeneous ice nucleation. Heterogeneous ice formation or the sup-
pression of freezing might better explain the observed ice crystal numbers. Thus, our
lack of understanding of the high supersaturations with implications to the microphysi-
cal and radiative properties of cirrus, the vertical redistribution of water and climate, is
traced back to the understanding of the freezing process at low temperatures.
5
10
15
20
1Introduction
The relative humidity over ice RHicecontrols the formation of cirrus clouds in the up-
per troposphere. Prior to ice formation, when an air parcel cools while rising, RHice
increases up to the freezing threshold necessary to nucleate ice in the ambient aerosol
25
21090

Page 3

ACPD
8, 21089–21128, 2008
Ice supersaturations
M. Kr¨ amer et al.
Title Page
Abstract Introduction
ConclusionsReferences
TablesFigures
??
??
BackClose
Full Screen / Esc
Printer-friendly Version
Interactive Discussion
particles. Alfred Wegener was probably the first who recognized that atmospheric air
can be supersaturated with respect to ice without forming ice crystals. During his sec-
ond expedition to Greenland in 1911/1912 he recognized that moist breathing of his
horses produced small ice crystals (Wall, 1942) growing in the ice supersaturated air.
Gierens et al. (2000) and Spichtinger et al. (2003) recalled the work of Gl¨ uckauf (1945)
and Weickmann (1945), who already mentioned that cirrus clouds form not as soon
as ice saturation is reached and that ice-forming regions in the upper troposphere are
regions of high ice supersaturation that should occur frequently.
Today’s state of knowledge is that the freezing thresholds depend on the compounds
of the available ice forming aerosol particles. In case these particles are pure liquid so-
lutions (of arbitrary composition), the – homogeneous – freezing thresholds range for
140...180% for T=240...180K and are well described by the theory derived by Koop
et al. (2000). In the presence of aerosol particles containing an insoluble impurity
(so called ice nuclei, IN, such as soot, mineral dust or biological particles), the – het-
erogeneous – freezing thresholds are determined by the composition of the particles.
Therefore, up to now no simple parametrisation scheme exists for the heterogeneous
freezing thresholds. In most cases they are lower than the homogeneous freezing
thresholds and can be significantly different. Thus, injection of aerosol particles with
lower freezing threshold would directly impact to the cirrus cloud cover and thus the
radiation balance of the atmosphere (Gettelman and Kinnison, 2007).
Once the ice cloud has formed, the gas phase water and thus RHiceis depleted by
the growing ice crystals in dependence on their number and size. The ice cloud micro-
physics interacts with in-cloud RHice(see e.g. Gensch et al., 2008) because it affects
the water vapour condensation rate and fall speed of the ice crystals (Khvorostyanov
et al., 2006) that in turn influences the vertical redistribution of water in the upper tro-
posphere.
Forced by the recent insight in the importance of both, the clear sky and in-cloud
RHice, for the Earth’s climate, many airborne and remote sensing experiments as well
as model studies were recently performed to investigate the distributions of RHicein the
5
10
15
20
25
21091

Page 4

ACPD
8, 21089–21128, 2008
Ice supersaturations
M. Kr¨ amer et al.
Title Page
AbstractIntroduction
ConclusionsReferences
TablesFigures
??
??
BackClose
Full Screen / Esc
Printer-friendly Version
Interactive Discussion
upper troposphere (Kelly et al., 1993; Heymsfield and Milosevitch, 1995; Heymsfield
et al., 1998; Gierens et al., 1999; Gierens et al., 2000; Jensen et al., 2001; Ovar-
lez et al., 2002; Haag et al., 2003; Spichtinger et al., 2003; Spichtinger et al., 2004;
Gayet et al., 2004; Comstock and Ackerman, 2004; Lee et al., 2004; Gao et al., 2004;
Jensen et al., 2005a; Jensen et al., 2005b; Gayet et al., 2006; Gettelman et al., 2006;
MacKenzie et al., 2006; Popp et al., 2007; V¨ omel and David, 2007; Immler et al., 2008).
The major results of these studies are sorted into two temperature ranges (T<200K
and T=200−240K) and are listed in Table 1. The warmer temperature range corre-
sponds to cirrus at altitudes between about 6 and 15km in Arctic, mid-latitude and
tropical regions, while cirrus in the colder temperature range are found in the tropics
between about 15 and 20km. Since most aircraft can reach only the lower altitudes,
the warmer cirrus clouds and their environment are more extensively investigated and
thus already a quite consistent picture exists.
At higher temperatures (T>200K), supersaturations up to the homogenous freez-
ing threshold occur frequently under clear sky conditions as well as inside of
cirrus clouds. Occasionally higher supersaturations were observed.
temperatures(T<200K), where the H2O concentrations are much lower so that the
measurements become challenging for the water instruments, the observations be-
come less frequent. In many of the aircraft and balloon studies RHiceup to or even
more than water saturation were reported outside and inside of the cold cirrus clouds.
As outlined earlier, we can understand supersaturations up to the freezing thresholds
in both, clear air as well as inside cirrus. But, supersaturations up to water saturation
or even above raise the question if these are caused by instrument artefacts or if “the
basic principles underpinning the current understanding of ice cloud formation and alter
the assessment of water distribution in the upper troposphere are called into question”,
as Peter et al. (2006) summarised.
Another crucial point in the frame of this discussion is the existence of persistently
high supersaturations inside of cirrus clouds. It is believed that the in-cloud initial high
supersaturation is reduced to saturation very quickly – in the timescale of minutes –
5
10
15
At lower
20
25
21092

Page 5

ACPD
8, 21089–21128, 2008
Ice supersaturations
M. Kr¨ amer et al.
Title Page
Abstract Introduction
ConclusionsReferences
Tables Figures
??
??
Back Close
Full Screen / Esc
Printer-friendly Version
Interactive Discussion
by consumption of gas phase water by the numerous growing ice crystals formed by
homogeneous freezing, which is believed to be the major process forming ice in the
upper troposphere (Hoyle et al., 2005). However, Korolev and Mazin (2003) showed
that one important parameter controlling the water vapour relaxation time and RHiceis
the product of the mean number and size of the ice crystals, Nice· Rice, the so called
integral ice crystal radius, which is inversely linked to RHice. Thus, in case of low
Nice· Rice, RHicecould also become persistent.
Here, we present an extensive data set of strongly quality checked in-situ clear sky
and in-cloud aircraft observations of RHiceand Nice, Ricein the temperature range
183–250K. The measurements are performed during 28 flights in the frame of ten field
campaigns in the Arctic, at mid-latitudes and in the Tropics. Based on the compre-
hensive field data set, we examine the possible atmospheric range of supersaturations
and relaxation times resulting from the observed cirrus microphysical parameters for
the complete ice cloud temperature range. We further derive frequencies of occur-
rence of RHicein 1K temperature bins and discuss the pattern of RHicefound in clear
air and inside of cirrus as well as those of Nice, Rice. Finally, we investigate the freezing
mechanism consistent with the observed ice crystal numbers for warmer and colder
cirrus. We show that there is strong indication that cold ice clouds (<205K) contain
a lower ice crystal number than expected.
5
10
15
2 Experimentals
20
Water vapour and ice crystal measurements from several instruments operated on
three different research aircraft, i.e. the high-altitude Russian M55 Geophysika and
the German research aircraft enviscope-Learjet and DLR Falcon, are analyzed in the
present study. Only a brief description of each instrument is given here as greater detail
is available in the referenced literature. The instruments and the parameters derived
from their measurements are listed in Table 2, the campaigns and flights are listed in
Table 3.
21093
25