Ice supersaturations exceeding 100% at the cold tropical tropopause: implications for cirrus formation and dehydration

[摘要] Recent in situ measurements at tropical tropopause temperatures as low as187 K indicate supersaturations with respect to ice exceeding 100% withlittle or no ice present. In contrast, models used to simulate cloudformation near the tropopause assume a supersaturation threshold for icenucleation of about 65% based on laboratory measurements of aqueous aerosolfreezing. The high supersaturations reported here, along with cloudsimulations assuming a plausible range of temperature histories in thesampled air mass, indicate that the vast majority of aerosols in the airsampled on this flight must have had supersaturation thresholds for icenucleation exceeding 100% (i.e. near liquid water saturation at thesetemperatures). Possible explanations for this high threshold are that (1) theexpressions used for calculating vapor pressure over supercooled water at lowtemperatures give values are at least 20% too low, (2) organic films on theaerosol surfaces reduce their accommodation coefficient for uptake of water,resulting in aerosols with more concentrated solutions when moderate-rapidcooling occurs and correspondingly inhibited homogeneous freezing, and (3) ifsurface freezing dominates, organic coatings may increase the surface energyof the ice embryo/vapor interface resulting in suppressed ice nucleation.Simulations of in situ cloud formation in the tropical tropopause layer (TTL)throughout the tropics indicate that if decreased accommodation coefficientsand resulting high thresholds for ice nucleation prevailed throughout thetropics, then the calculated occurrence frequency and areal coverage of TTLcirrus would be significantly suppressed. However, the simulations also showthat even if in situ TTL cirrus form only over a very small fraction of thetropics in the western Pacific, enough air passes through them due to rapidhorizontal transport such that they can still effectively freeze-dry airentering the stratosphere. The TTL cirrus simulations show that even if verylarge supersaturations are required for ice nucleation, these largesupersaturations should occur very rarely.

[学科分类] 大气科学