MODIS observations of ship tracks in summertime stratus off the west coast of the United States
MODIS observations of ship tracks in summertime stratus off the west coast of the United States
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DOI：
10.1175/2007jas2308.1
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发表时间：
2007-12-01
影响因子：
3.1
通讯作者：
Tahnk, William R.
中科院分区：
文献类型：
--
作者：
Segrin, Matthew S.;Coakley, James A., Jr.;Tahnk, William R.
分析了三年来对美国西海岸外夏季海洋层层的1公里中分辨率成像光谱仪(MODIS)上午(Terra)和下午(Aqua)的观测，以确定这些云对下面船只羽流中的气溶胶的反应。与附近含有未污染云的像素相比，被污染云的反射率和辐射率分别提高了2.1微米和3.7微米。开发了一个自动程序，以确定哪些像素包含被底层船舶污染的云层，然后在船舶航迹两侧尽可能接近污染云层的位置选择未受污染的控制像素，通常只有1个像素的间隔。虽然没有对选定像素内的云层覆盖率提出要求，但为了便于与早先基于1公里高级甚高分辨率辐射计(AVHRR)图像的研究进行比较，本文提供的结果仅限于船道沿线的20个像素段，其中所有被识别为污染的像素和被选为航道两侧对照的所有像素都被发现是阴天。在MODIS和AVHRR的观测中，污染导致液滴半径减少了约20%。在MODIS观测中，光学厚度增加了约15%，在置信度大于99.5%的情况下具有统计意义。这些变化比AVHRR观测到的2%的增长要大得多，在统计上可以忽略不计。与AVHRR观测一样，MODIS观测表明，大多数云(60%)在受到污染时失去了液态水，但与光学深度的不同结果一致，MODIS观测的损失大约是AVHRR观测发现的近20%损失的一半。在MODIS和AVHRR观测中，云层高度都不受污染的影响。对于上午和下午的云，对污染的反照率增加最大的云也显示出最大的云液态水增加。利用MODIS观测得到的液滴半径和液态水量的变化表明，与附近未受污染的云相比，污染云中的液滴生长受到了抑制，从云底到云顶的增长速度也有所降低。
Three years of I-km Moderate Resolution Imaging Spectroradiometer (MODIS) observations for morning (Terra) and afternoon (Aqua) summertime marine stratus off the west coast of the United States were analyzed to determine the response of the clouds to aerosols in the plumes of underlying ships. The polluted clouds were identified in pixels with elevated 2.1-mu m reflectivities and 3.7-mu m radiances when compared with nearby pixels that contained unpolluted clouds. An automated routine was developed to identify which of the pixels contained clouds polluted by underlying ships and then to select unpolluted control pixels on both sides of the ship track as near as was feasible to the polluted clouds, often with a separation of as little as 1 pixel. While no requirements were placed on the cloud cover fraction-within the selected pixels, to facilitate comparisons with an earlier study based on 1-km Advanced Very High Resolution Radiometer (AVHRR) imagery, the results presented here are restricted to 20-pixel segments along the ship tracks in which all of the pixels identified as polluted and all of the pixels selected as controls on both sides of the tracks were found to be overcast. The pollution caused about a 20% reduction in droplet radius in both the MODIS and the AVHRR observations. In the MODIS observations, the increase in optical depth was about 15% and statistically significant at a confidence level greater than 99.5%. These changes were much larger than the 2% and statistically negligible increases found for the AVHRR observations. Like the AVHRR observations, the MODIS observations indicated that the majority of clouds (60%) lost liquid water when polluted, but consistent with the different outcomes for the optical depths, the losses for the MODIS observations were about half the nearly 20% loss found in the AVHRR observations. Cloud altitudes were unaffected by the pollution for both the MODIS and the AVHRR observations. For both morning and afternoon clouds, those with the largest increases in albedo in response to the pollution also showed the largest increases in cloud liquid water amount. The changes in droplet radii and liquid water amounts retrieved using the MODIS observations suggested that droplet growth was suppressed in the polluted clouds and that the rate of growth from cloud base to cloud top was also reduced when compared with nearby unpolluted clouds.