Ultrathin Tropical Tropopause Clouds (UTTCs): II. Stabilization mechanisms

[摘要] Mechanisms by which subvisible cirrus clouds (SVCs) might contribute to dehydration closeto the tropical tropopause are not well understood.Recently Ultrathin Tropical TropopauseClouds (UTTCs) with optical depths around 10^-4 have been detected in the western Indianocean.These clouds cover thousands of square kilometers as 200-300 m thick distinct andhomogeneous layer just below the tropical tropopause.In their condensed phase UTTCs containonly 1-5% of the total water, and essentially no nitric acid.A new cloud stabilizationmechanism is required to explain this small fraction of the condensed water content in theclouds and their small vertical thickness. This work suggests a mechanism, which forces theparticles into a thin layer, based on upwelling of the air of some mm/s to balance the ice particles,supersaturation with respect to ice above and subsaturation below the UTTC.In situmeasurements suggest that these requirements are fulfilled.The basic physical properties ofthis mechanism are explored by means of a single particle model.Comprehensive 1-D cloudsimulations demonstrate this stabilization mechanism to be robust against rapid temperaturefluctuations of +/- 0.5 K.However, rapid warming (\Delta T > 2 K) leads to evaporation of theUTTC, while rapid cooling (\Delta T < -2 K) leads to destabilization of the particles with the potentialfor significant dehydration below the cloud

[学科分类] 大气科学