Observations of cloud liquid water path over oceans: Optical and microwave remote sensing methods
Observations of cloud liquid water path over oceans: Optical and microwave remote sensing methods
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DOI：
10.1029/94jd01831
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发表时间：
1994-10
影响因子：
--
通讯作者：
B. Lin;W. Rossow
中科院分区：
文献类型：
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作者：
B. Lin;W. Rossow
发表的估计云液态水路径（LWP）从卫星测量的微波辐射显示很少的协议，甚至在热带和中纬度地区的LWP的相对大小。为了了解这些差异，并获得更可靠的估计，光学和微波LWP反演方法进行了比较，使用国际卫星云气候学计划（ISCCP）和特殊传感器微波/成像仪（SSM/I）的数据。微波LWP反演与地面，大气和云特性的不确定性的错误进行了评估。如果在反演中使用精确的同期测量，海表温度可能不会产生很大的LWP误差。估计的近地面风速的不确定性高达2米/秒，LWP的不确定性约为5毫克/平方厘米。云液态水温度只有一个小的影响LWP检索（均方根误差小于2毫克/平方厘米），如果温度误差小于5 ℃，但是，这样的错误可以产生虚假的LWP随纬度和季节的变化。大气柱水汽（CWV）的错误强烈耦合在LWP（对于某些反演方法）的错误，造成高达30毫克/平方厘米的误差。由于微波辐射对LWP较小（小于7 mg/cm 2）的云的敏感性远远低于可见波长辐射，因此微波结果对用于区分晴朗和多云条件的过程非常敏感。不同的微波反演方法对云的探测灵敏度不同，对LWP的估计值有偏差。比较ISCCP和SSM/I LWP，我们发现，这两个估计值是一致的，在全球，纬向和区域的手段温暖，非降水云，平均LWP值约为5毫克/平方厘米，发生更频繁地比降水云。冰水路径（IWP）可以粗略地从ISCCP总的水路径和SSM/I LWP之间的差异冷，非降水云。冬季半球的IWP约为LWP的3倍，而夏季半球的IWP仅为LWP的一半。对流云对由微波确定的月纬向平均LWP值有显著贡献，特别是在热带辐合区（ITCZ），因为它们平均具有几乎10倍于非降水云的液态水（云加降水）。与降水云的处理有关的微波LWP估计值之间存在显着差异。
Published estimates of cloud liquid water path (LWP) from satellite-measured microwave radiation show little agreement, even about the relative magnitudes of LWP in the tropics and midlatitudes. To understand these differences and to obtain more reliable estimate, optical and microwave LWP retrieval methods are compared using the International Satellite Cloud Climatology Project (ISCCP) and special sensor microwave/imager (SSM/I) data. Errors in microwave LWP retrieval associated with uncertainties in surface, atmosphere, and cloud properties are assessed. Sea surface temperature may not produce great LWP errors, if accurate contemporaneous measurements are used in the retrieval. An uncertainty of estimated near-surface wind speed as high as 2 m/s produces uncertainty in LWP of about 5 mg/sq cm. Cloud liquid water temperature has only a small effect on LWP retrievals (rms errors less than 2 mg/sq cm), if errors in the temperature are less than 5 C; however, such errors can produce spurious variations of LWP with latitude and season. Errors in atmospheric column water vapor (CWV) are strongly coupled with errors in LWP (for some retrieval methods) causing errors as large as 30 mg/sq cm. Because microwave radiation is much less sensitive to clouds with small LWP (less than 7 mg/sq cm) than visible wavelength radiation, the microwave results are very sensitive to the process used to separate clear and cloudy conditions. Different cloud detection sensitivities in different microwave retrieval methods bias estimated LWP values. Comparing ISCCP and SSM/I LWPs, we find that the two estimated values are consistent in global, zonal, and regional means for warm, nonprecipitating clouds, which have average LWP values of about 5 mg/sq cm and occur much more frequently than precipitating clouds. Ice water path (IWP) can be roughly estimated from the differences between ISCCP total water path and SSM/I LWP for cold, nonprecipitating clouds. IWP in the winter hemisphere is about 3 times the LWP but only half the LWP in the summer hemisphere. Precipitating clouds contribute significantly to monthly, zonal mean LWP values determined from microwave, especially in the intertropical convergence zone (ITCZ), because they have almost 10 times the liquid water (cloud plus precipitation) of nonprecipitating clouds on average. There are significant differences among microwave LWP estimates associated with the treatment of precipitating clouds.