Upper tropospheric relative humidity from the GOES 6.7 μm channel: method and climatology for July 1987
Upper tropospheric relative humidity from the GOES 6.7 μm channel: method and climatology for July 1987
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DOI：
10.1029/93jd01283
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发表时间：
1993-09
影响因子：
--
通讯作者：
B. Soden;F. Bretherton
中科院分区：
文献类型：
--
作者：
B. Soden;F. Bretherton
本文对对流层上层相对湿度和云层进行了分析，这些湿度和云层是根据对地静止卫星对上升流红外辐射的观测而确定的。 6.7 μm 通道位于强水蒸气吸收带的中心附近，在晴空条件下主要对从 200 到 500 mbar 的大气深度上的平均相对湿度敏感。通过利用 11μm 辐射场的局部空间结构以及 11μm 和 6.7μm 之间的相关性，获得了 6.7μm 晴空辐射率的估计，以区分晴朗和阴天像素。这种方法被证明比仅基于 6.7-μm 通道的局部空间结构的云间隙更可靠，并且提供了纬度和经度为 2° × 2° 的区域的晴空 6.7-μm 亮度温度的估计，这些估计可以从间隔 30 分钟到大约 1 K 范围内的连续图像中重复。为了便于解释晴空亮度温度，提出了 6.7-μm 辐射传输的简化模型。该模型基于一组不规则间隔、强吸收、压力展宽的线，表明精确到实际相对湿度的大约 1 K 或 10% 范围内，6.7 μm 处的亮度温度与相对湿度的适当垂直平均值除以观看天顶角的余弦的自然对数成正比。 1987 年 7 月，我们提出了通过 GOES E 观测结果推断出的对流层上层水汽的估计值。地理分布反映了大尺度大气环流的许多众所周知的特征。还证明了对流层上层水蒸气的温室效应对大尺度动力学的明显依赖性。最后，提出了观测到的对流层上层相对湿度与对流层上层云盖发生之间的关系，并讨论了其对一般环流模型中云参数化的影响。
This paper presents an analysis of upper tropospheric relative humidity and clouds determined from geostationary satellite observations of the upwelling infrared radiation. The 6.7-μm channel is located near the center of a strong water vapor absorption band and under clear sky conditions is primarily sensitive to the relative humidity averaged over a depth of atmosphere extending from 200 to 500 mbar. Estimates of the clear sky radiance at 6.7-μm are obtained by utilizing the local spatial structure of the radiance field at 11-μm and the correlation between 11-μm and 6.7-μm, to discriminate between clear and cloudy pixels. This approach is demonstrated to be more reliable than cloud clearance based solely upon the local spatial structure of the 6.7-μm channel alone and provides estimates of the clear sky 6.7-μm brightness temperature for areas 2° × 2° of latitude and longitude which are repeatable from successive images 30 min apart to within approximately 1 K. To facilitate the interpretation of the clear sky brightness temperatures, a simplified model of the radiative transfer at 6.7-μm is presented. This model, based upon a set of irregularly spaced, strongly absorbing, pressure-broadened lines, demonstrates that accurate to within approximately 1 K or 10% of the actual relative humidity, the brightness temperature at 6.7-μm is proportional to the natural logarithm of the appropriate vertical average of the relative humidity divided by the cosine of the viewing zenith angle. Estimates of upper tropospheric water vapor inferred in this way from GOES E observations are presented for July 1987. The geographic distribution reflects many well-known features of the large-scale atmospheric circulation. A clear dependence of the greenhouse effect of upper tropospheric water vapor upon the large-scale dynamics is also demonstrated. Finally, the observed relationship between the upper tropospheric relative humidity and the occurrence of upper tropospheric cloud cover is presented and its implications for the parameterization of clouds in general circulation models are discussed.