ICE-SUPERSATURATION IN THE UPPER
(Long version of an article that appears in GEWEX News February 2004)
Klaus Gierens, Ulrich Schumann
DLR, Institute of Atmospheric Physics,
In recent years it has become evident that the relative humidity with
respect to the ice phase of water (RHi) in the upper troposphere (UT) is
often above saturation, i.e. RHi > 100 %. Regions containing air
masses with RHi > 100 % have been termed "ice-supersaturated regions"
(ISSRs, Detwiler and Pratt, 1984; Gierens et al., 1999). As a
phenomenon in the water vapor field, essential for weather and climate,
and because of their importance as cirrus formation regions, ISSRs
should be a topic both in the GVaP and GCSS parts of GEWEX.
Although the first detection of ice-supersaturation in the UT dates
back at least to the 1940es (Glückauf, 1945; Weickmann, 1945;
Brewer, 1946), it was generally believed that ice-supersaturation occurs
only exceptionally and that clouds of ice particles form soon after the
humidity exceeds saturation. Even today, humidity measurements
indicating ice-supersaturation are often classified as being unreliable
and interpreted as implying the presence of cirrus clouds (Wang et al.,
2003). Also most weather and climate models still assume that cirrus
clouds form immediately when the humidity reaches ice saturation.
Examples are the operational weather prediction model of the European
Centre for Medium-Range Weather Forecasts (ECMWF) and climate models
derived from it, see Fig. 1.
a) 1 min averages of relative humidity obtained on research flights
during the INCA campaigns with a frostpoint hygrometer vs. temperature.
Ice supersaturation occurs often and at all temperatures in the plot.
b) Relative humidities from the ECMWF analyses for the locations and
times of the INCA flights. The model shows practically no ice
supersaturation, because it assumes cirrus to be present as soon as
saturation is reached.
The existence of ice-supersaturated air masses in the UT has been
confirmed by airborne measurements with various types of hygrometers
during several campaigns. In the 1980es the investigation of the
feasibility of clear-air seeding (artificial cloud production) gave
first insights about location and extent of ISSRs (Detwiler and Pratt,
1984). Later campaigns where ice supersaturation was detected include:
AASE/AAOE (Murphy et al., 1990), FIRE (Heymsfield and Miloshevich,
1995), POLINAT (Ovarlez et al., 2000), and MOZAIC (Marenco et al., 1998;
Gierens et al., 1999). Radiosonde measurements in the past mostly
underestimate relative humidity in the UT; however carefully calibrated
and corrected RS80A radiosondes (Nagel et al., 2001) can now be used to
detect ice supersaturation (Spichtinger et al., 2003a). More than 40 %
of all data collected during POLINAT over the North Atlantic and more
than 15% of all MOZAIC data were taken in ISSRs (Schumann et al., 2000;
Gierens et al., 1999).
Ice-supersaturation occurs both inside and outside cirrus clouds as
simultaneous measurements of humidity and particles have shown during
SUCCESS (Jensen et al., 1998), SONEX (Vay et al., 2000), and INCA
(Ovarlez et al., 2002).
Ice particles in cirrus clouds form by homogeneous or heterogeneous
freezing, depending on the availability of ice nuclei and speed of
vertical motions (Gierens, 2003; Kärcher and Ström, 2003).
When a stratiform cold (T<-40°C) cirrus forms by homogeneous
freezing of aqueous solution droplets, the cloud must form in highly
supersaturated air, because such solutions freeze only at ice
supersaturation exceeding 40% (and the supersaturation necessary for
freezing increases with decreasing temperature, see Koop et al., 2000).
Heterogeneous nucleation probably needs less (but finite)
supersaturation, but the existence of cloud-free supersaturated air
masses indicates that there is often a lack of suitable ice nuclei.
Evidence for ice supersaturation occurring in the UT is provided by
cirrus fallstreaks that grow while falling through supersaturated air
layers (Ludlam, 1980) and by contrails (Brewer, 1946). Contrails can
decorate the sky when no cirrus clouds are around. Since contrail
persistence requires ice saturation, a sky full of contrails but without
cirrus shows that there must be ice-supersaturated air above (Schumann,
1996). The potential coverage of contrails (Sausen et al., 1998;
Mannstein et al., 1999) in the northern midlatitude agrees well with
the fractional coverage of ISSRs determined from MOZAIC data (namely
about 15-20%). The average horizontal extension of ISSRs is of the
order 150 km (Gierens and Spichtinger, 2000).
Cases with very large ice supersaturation are rare. Inside mature
cirrus clouds the humidity is close to saturation (Ovarlez et al.,
2002). Once ice particles form, the humidity relaxes back to saturation
at time-scales of the order of minutes to hours, which can make a
considerable fraction of the life time of an individual cirrus cloud.
The time scale increases with decreasing temperature and hence ISSRs
occur mainly in the UT.
The probability density function of the occurrence of ice
supersaturation derived from measurements decreases about exponentially,
i.e. p(Si) is proportional to exp(-b*Si), where b is a constant and Si
= RHi- 1 is the ice supersaturation (Gierens et al., 1999; Spichtinger
et al., 2002). The threshold for homogeneous nucleation (Si exceeding
40%) is reached rarely, which could imply that cirrus forms mostly
heterogeneously. Evidence for homogeneous ice nucleation was derived
recently from the INCA campaign data (Haag et al., 2003) in regions
with strong upward motion and low aerosol loading (Minikin et al.,
2003; Kärcher and Ström, 2003; Gayet et al., 2004).
Supersaturation is also observed in the stratosphere in the polar
winter and also but rarely at mid-latitudes in the lowermost
stratosphere up to a few hundred meters just above the tropopause.
Global distribution maps of ISSRs on the nominal pressure levels 147 hPa
and 215 hPa have been produced from MLS RHi data (Spichtinger et al.,
2003b). Annual and seasonal distributions have been derived.
Geographical regions where ISSRs occur most frequently are the tropics
on both pressure levels, the midlatitude storm belts on 215 hPa in the
respective hemispheric summer and fall seasons, and Antarctica in
southern winter and spring. There is a remarkable similarity between the
features of the global distribution of ISSRs and the global
distribution of high clouds (Wylie et al., 1999, their Fig. 3), which
points to the role of ISSRs as cirrus formation sites.
Date from 15 months of radiosonde soundings at Lindenberg, Germany,
have been used to locate ice-supersaturation layers relative to the
local tropopause and to measure their vertical extensions (Spichtinger
et al., 2003a). Ice supersaturation over Lindenberg occurs mostly within
a broad layer between 450 and 200 hPa, with seasonal shifting, and
mainly below the tropopause (directly from the tropopause and 200hPa
down). The observed altitude distributions are similar but more narrow
than those of sub-visible cirrus derived from satellite observations
(SAGE II data, Wang et al., 1996), and similar to cirrus and
sub-visible cirrus distributions at the Observatoire de Haute Provence
(Goldfarb et al., 2001). Vertical extensions of such layers rarely
exceed 1 km, the mean is about 500m.
Contrasts in temperatures and specific humidity values between ISSRs
and their subsaturated surroundings have been studied using MLS and
MOZAIC data. In the tropics, ISSRs are mainly distinguished by a
moisture contrast from their environment, whereas the temperature
contrast is very small (about 0.1 K). In the extratropics and at the
tropical tropopause the temperature contrast reaches 2-4 K. The specific
humidity inside UT ISSRs is generally a factor of about 3 higher than
outside, and the moisture contrast is higher in the troposphere than in
the tropopause region. These contrasts indicate that ISSRs result from
a combination of various processes.
Additional information on the subject of ice supersaturation can be
found in the quoted papers and on http://www.pa.op.dlr.de/issr.
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List of Abbreviations that are not
explained in the text
AASE Airborne Arctic Stratospheric
AAOE Airborne Antarctic
INCA Interhemispheric differences
in cirrus properties from anthropogenic emissions
FIRE First ISCCP Regional
GVaP GEWEX Water Vapour Project
GCSS GEWEX Cloud System Study
MOZAIC Measurement of ozone from Airbus in-service
POLINAT Pollution from Aircraft Emissions in the
North Atlantic Flight Corridor
SONEX SASS ozone and NOx experiment
SUCCESS Subsonic aircraft contrail and cloud effects