Woods Sarah, Lawson R Paul, Jensen Eric, Bui T P, Thornberry Troy, Rollins Andrew, Pfister Leonhard, Avery Melody
SPEC, Inc., Boulder, CO, USA.
NASA Ames Research Center, Moffet Field, CA, USA.
J Geophys Res Atmos. 2018 May 4;123(11):6053-6069. doi: 10.1029/2017JD028068.
Pervasive cirrus clouds in the tropical tropopause layer (TTL) play an important role in determining the composition of stratospheric air through dehydration of tropospheric air entering the stratosphere. This dehydration affects Earth's energy budget and climate, yet uncertainties remain regarding the microphysical processes that govern TTL cirrus. TTL cirrus were sampled with the NASA Global Hawk UAV for over 30 hr in the Western Pacific in 2014 during the Airborne Tropical TRopopause EXperiment. In situ measurements by a Fast Cloud Droplet Probe and Hawkeye probe (combination Fast Cloud Droplet Probe, Two-Dimensional Stereo optical array probe, and Cloud Particle Imager) provided particle concentrations and sizing between 1- and 1,280-μm diameter and high resolution images for habit identification. We present the variability in ice concentrations, size distributions, and habits as functions of temperature, altitude, and time since convective influence. Observed ice particles were predominantly small and quasi-spheroidal in shape, with the percentage of quasi-spheroids increasing with decreasing temperature. In comparison to the large fraction of the population consisting of quasi-spheroids, faceted habits (columns, plates, rosettes, and budding rosettes) constituted a smaller percentage of the overall population and exhibited the opposite correlation with temperature. The trend of higher percentages of faceted crystals occurring at warmer temperatures may be due to diffusional growth or aggregation as particles descend through cloud, and/or the more rapid diffusional growth rate at warmer temperatures. Sampling was typically well away from deep convection, however, and very few aggregates were observed, so the trend of higher percentages of faceted habits is likely attributable to diffusional growth.
热带对流层顶区域（TTL）中普遍存在的卷云在通过使进入平流层的对流层空气脱水来决定平流层空气成分方面发挥着重要作用。这种脱水作用影响着地球的能量平衡和气候，然而，关于控制TTL卷云的微物理过程仍存在不确定性。在2014年的机载热带对流层顶实验期间，美国国家航空航天局（NASA）的全球鹰无人机在西太平洋对TTL卷云进行了超过30小时的采样。使用快速云滴探针和鹰眼探针（快速云滴探针、二维立体光学阵列探针和云粒子成像仪的组合）进行的原位测量提供了直径在1至1280微米之间的粒子浓度和尺寸信息，以及用于习性识别的高分辨率图像。我们展示了冰粒子浓度、尺寸分布和习性随温度、高度以及自对流影响以来的时间的变化情况。观测到的冰粒子主要较小且形状为准球形，准球形粒子的百分比随温度降低而增加。与由准球形组成的大部分群体相比，多面习性（柱状、片状、玫瑰花状和芽状玫瑰花状）在总体群体中所占百分比更小，并且与温度呈现相反的相关性。在较温暖温度下出现更高百分比多面晶体的趋势可能是由于粒子在云层中下降时的扩散生长或聚集，和/或在较温暖温度下更快的扩散生长速率。然而，采样通常远离深对流区域并且观测到的聚集体非常少，所以多面习性更高百分比的趋势可能归因于扩散生长。
