Ice supersaturation in the tropopause region:

Ice supersaturation in clear air:

In contrast to water clouds where the saturated/supersaturated environment and the cloud are almost identical, for ice clouds there is a fundamental difference: For ice crystal formation high ice supersaturation (i.e. relative humidities over ice larger then 100%) are needed; this is confirmed by many measurements (e.g. Koop et al., 2000; DeMott et al., 2003). Thus, the existence of cloud-free air masses in the status of ice supersaturation (so-called ice supersaturated regions, ISSRs) is clear from a theoretical point of view and it was also proven by a variety of measurement techniques. For a better understanding of cirrus clouds and their radiative properties (e.g. their potential for a net warming to the Earth's radiation budget) we have to improve our knowledge on their potential formation regions, i.e. ISSRs.

Figure 1: Ice supersaturation on pressure levels 147/215 hPa as seen by the microwave limb sounder (MLS), figure adapted from Spichtinger et al. (2003b).

During the last years, many investigations on properties of ISSRs (thermodynamics, radiation, horizontal and vertical extensions, life cycle) were carried out (e.g. Gierens & Spichtinger, 2000; Spichtinger et al., 2003a,b; Spichtinger et al., 2005a,b; Fusina et al., 2007). However, our knowledge about the life cycle of ISSRs and their possible transformation into cirrus clouds is quite incomplete. Further research on this topic is still needed.

Ice supersaturation in cloudy air:
While ice supersaturation in clear seems to be quite natural, as long as water saturation is not surpassed, ice supersaturation inside cirrus clouds seems quite unusual: Ice crystals represent a strong sink for ice supersaturation, thus they should deplete water vapour until saturation is reached inside the cloud quickly. However, many measurements indicate that ice supersaturation inside cirrus clouds occurs quite often, constituting the ice supersaturation puzzle as termed in Peter et al. (2006).

Figure 2: Ice supersaturation inside cirrus clouds as measured in many campaigns, figure adapted from Krämer et al. (2009).

Currently, there are lots of efforts to explain this phenomenon. While most of the research focuses on microphysical or chemical properties, our research focus is more on the impact of dynamics. This work is carried out using box models and cloud resolving models including ice microphysics.