Ice nucleation and dehydration in the Tropical Tropopause Layer
Ice nucleation and dehydration in the Tropical Tropopause Layer
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DOI：
10.1073/pnas.1217104110
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发表时间：
2013-02-05
影响因子：
11.1
通讯作者：
Gao, Rushan
中科院分区：
文献类型：
--
作者：
Jensen, Eric J.;Diskin, Glenn;Gao, Rushan
靠近热带对流层顶的光学薄卷云调节进入平流层的空气湿度，这反过来又对地球的辐射收支和气候产生强烈影响。最近的高空无人驾驶飞机测量为热带对流层顶区域形成的两种不同类型的卷云提供了证据：(i)垂直扩展的卷云，冰数浓度低，灭绝率低，与冰相关的过饱和度大（高达70%）；（ii）垂直薄的卷云层，具有更高的冰浓度，可以有效地消耗超过饱和的蒸汽。前一类卷云的持续过饱和与在低冰浓度和寒冷的热带对流层顶温度下，超过饱和的水汽猝灭的长时间尺度（几个小时或更长）是一致的。低浓度的云很可能是在浓度低于100 L-1（通常低于20 L-1）的不溶性颗粒的背景群上形成的，而高浓度的冰层（浓度高达10,000 L-1）只能通过大量的水性气溶胶的均匀冻结来产生。这些测量，连同过去的高空飞机测量，表明低浓度的卷云在热带对流层顶区域频繁发生，而高浓度的卷云很少发生。低浓度云的优势意味着热带对流层顶附近的卷云通常允许空气进入平流层，其冰饱和混合比高达1.7倍。
Optically thin cirrus near the tropical tropopause regulate the humidity of air entering the stratosphere, which in turn has a strong influence on the Earth's radiation budget and climate. Recent high-altitude, unmanned aircraft measurements provide evidence for two distinct classes of cirrus formed in the tropical tropopause region: (i) vertically extensive cirrus with low ice number concentrations, low extinctions, and large supersaturations (up to similar to 70%) with respect to ice; and (ii) vertically thin cirrus layers with much higher ice concentrations that effectively deplete the vapor in excess of saturation. The persistent supersaturation in the former class of cirrus is consistent with the long time-scales (several hours or longer) for quenching of vapor in excess of saturation given the low ice concentrations and cold tropical tropopause temperatures. The low-concentration clouds are likely formed on a background population of insoluble particles with concentrations less than 100 L-1 (often less than 20 L-1), whereas the high ice concentration layers (with concentrations up to 10,000 L-1) can only be produced by homogeneous freezing of an abundant population of aqueous aerosols. These measurements, along with past high-altitude aircraft measurements, indicate that the low-concentration cirrus occur frequently in the tropical tropopause region, whereas the high-concentration cirrus occur infrequently. The predominance of the low-concentration clouds means cirrus near the tropical tropopause may typically allow entry of air into the stratosphere with as much as similar to 1.7 times the ice saturation mixing ratio.