(Long version of an article that appears in GEWEX News February 2004)

    Klaus Gierens, Ulrich Schumann

    DLR, Institute of Atmospheric Physics, Oberpfaffenhofen, Germany

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.

rhi_comparison-inca-ecmwfFigure 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


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List of Abbreviations that are not explained in the text

AASE        Airborne Arctic Stratospheric Expedition
AAOE        Airborne Antarctic  Ozone Experiment
INCA        Interhemispheric differences in cirrus properties from anthropogenic emissions
FIRE        First ISCCP Regional Experiment
GVaP        GEWEX Water Vapour Project
GCSS        GEWEX Cloud System Study
MOZAIC    Measurement of ozone from Airbus in-service aircraft
POLINAT    Pollution from Aircraft Emissions in the North Atlantic Flight Corridor
SONEX    SASS ozone and NOx experiment
SUCCESS    Subsonic aircraft contrail and cloud effects special study

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