Abstract. High-altitude cirrus clouds are climatically important: their formation
freeze-dries air ascending to the stratosphere to its final value, and their
radiative impact is disproportionately large. However, their formation and
growth are not fully understood, and multiple in situ aircraft campaigns have
observed frequent and persistent apparent water vapor supersaturations of
5 %–25 % in ultracold cirrus (T<205 K), even in the presence of ice
particles. A variety of explanations for these observations have been put
forth, including that ultracold cirrus are dominated by metastable ice whose
vapor pressure exceeds that of hexagonal ice. The 2013 IsoCloud campaign at
the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) cloud and
aerosol chamber allowed explicit testing of cirrus formation dynamics at
these low temperatures. A series of 28 experiments allows robust estimation
of the saturation vapor pressure over ice for temperatures between 189 and
235 K, with a variety of ice nucleating particles. Experiments are rapid
enough (∼10 min) to allow detection of any metastable ice that may
form, as the timescale for annealing to hexagonal ice is hours or longer over
the whole experimental temperature range. We show that in all experiments,
saturation vapor pressures are fully consistent with expected values for
hexagonal ice and inconsistent with the highest values postulated for
metastable ice, with no temperature-dependent deviations from expected
saturation vapor pressure. If metastable ice forms in ultracold cirrus
clouds, it appears to have a vapor pressure indistinguishable from that of
hexagonal ice to within about 4.5 %.
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