Ground-Based Infrared Remote Sensing of Cloud Properties over the Antarctic Plateau. Part II: Cloud Optical Depths and Particle Sizes
Ground-Based Infrared Remote Sensing of Cloud Properties over the Antarctic Plateau. Part II: Cloud Optical Depths and Particle Sizes
复制标题
南极高原云特性的地基红外遥感。
DOI：
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发表时间：
2001
期刊：
影响因子：
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通讯作者：
S. Warren
中科院分区：
文献类型：
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作者：
A. Mahesh;V. Walden;S. Warren
一个完整的一年，每天两次长波大气发射光谱测量从表面在1厘米21分辨率被用来推断光学厚度和冰晶大小在南极高原对流层云。该方法利用云在10毫米和11毫米波长的发射率和云在9.6毫米波段平流层臭氧发射的透射率。需要了解云底温度以及臭氧和温度的垂直分布情况;南极站的无线电探空仪和臭氧探空仪可提供这些资料。10和11 mm之间的发射率差异对冰颗粒尺寸敏感，因为冰的吸收系数在这两个波长之间变化很大。光学深度的检索（表示为几何光学极限tg中的值）限于tg，5，有效粒子半径reff仅区分为reff，25 mm，但80%的云观察到的tg和reff在可检索的范围内。这些云在南极洲内部被发现是光学薄，通常与tg，1，与沿海云，通常有tg。20.大多数的reff在5 - 25 mm的范围内，模式为15 mm。夏季检索的reff大于冬季，与现场测量结果一致。从11月到4月，reff通常至少为10 mm，而在冬季（7月至9月）的3个月期间，没有检索到大于25 mm的reff值。从红外光谱检索的颗粒大小进行了比较的尺寸的冰晶下降到表面和测量的显微照片。有效的球面半径计算从照片中的三种方式：等面积，等体积，等体积面积比（V/A）。与来自辐射测量的reff的协议是最好的等V/A的领域。从发射光谱推断的光学厚度和底高与天气观测者的目视报告定性一致，因为光学上较厚的云通常被报告为雨层云，而具有最高检索底的云被报告为卷云或卷层云。层云往往被报道为低;高层云和高积云的高度居中。
One full year of twice-daily longwave atmospheric emission spectra measured from the surface at 1-cm 21 resolution are used to infer optical thicknesses and ice crystal sizes in tropospheric clouds over the Antarctic Plateau. The method makes use of the cloud’s emissivity at 10- and 11- mm wavelength and the cloud’s transmittance of stratospheric ozone emission in the 9.6-mm band. Knowledge of the cloud-base temperature and the vertical distributions of ozone and temperature is required; these are available at South Pole Station from radiosondes and ozonesondes. The difference in emissivity between 10 and 11 mm is sensitive to ice particle size because the absorption coefficient of ice varies greatly between these two wavelengths. The retrieval of optical depth (expressed as its value in the geometric-optics limit tg) is limited to tg , 5, and the effective particle radii reff are distinguished only for reff , 25 mm, but 80% of the clouds observed have tg and reff in the retrievable range. These clouds over the Antarctic interior are found to be optically thin, usually with tg , 1, in contrast to coastal clouds, which usually have tg . 20. Most have reff in the range of 5‐25 mm, with a mode at 15 mm. The retrieved reff is larger in summer than in winter, in agreement with in situ measurements. From November to April, reff was usually at least 10 mm, whereas, for a 3-month period in winter (July‐September), no reff values greater than 25 mm were retrieved. The particle sizes retrieved from the infrared spectra are compared with dimensions of ice crystals falling to the surface and measured on photomicrographs. Effective spherical radii are computed from the photographs in three ways: equal area, equal volume, and equal volumeto-area ratio (V/A). Agreement with the reff derived from radiation measurements is best for equal-V/A spheres. The optical thicknesses and base heights inferred from the emission spectra agree qualitatively with the visual reports of the weather observers, in that the optically thicker clouds are usually reported as nimbostratus and clouds with the highest retrieved bases are reported as cirrus or cirrostratus. Stratus clouds tend to be reported as low; altostratus and altocumulus are intermediate in height.