Abstract
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 %.
Highlights
As air rises into the stratosphere, it is freeze-dried by condensation as it passes through the coldest regions of the upper troposphere and lower stratosphere (UT/LS)
(1998), which suggests that the entropies of cubic and hexagonal ices are nearly identical, we assume the same is true of ice Isd and extrapolate the measured values of Shilling et al (2006) to temperatures higher than their measurement range of 181–191 K (Fig. 5, blue dashed curve)
The properties of metastable ice I are likely dependent on its method of preparation, and it is possible that ice grown through deposition from the vapor may have a different vapor pressure than ice prepared by annealing amorphous ice, as is done by Shilling et al (2006)
Summary
As air rises into the stratosphere, it is freeze-dried by condensation as it passes through the coldest regions of the upper troposphere and lower stratosphere (UT/LS). An apparent supersaturation of 20 % at 190 K over expected values (from the Murphy–Koop parametrization, MK; Murphy and Koop, 2005) corresponds to a difference of about 0.7 ppmv H2O. If uniformly distributed, this additional stratospheric water would increase global surface radiative forcing by about 0.2 W m−2 (Forster and Shine, 1999). Incomplete dehydration would change the radiative effect of the cirrus produced by freeze-drying ascending air, but the magnitude and even sign of this effect are not well known.
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