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中国区域云特性分析及在FY-2云检测中的应用
Cloud Properties Analysis and Its Application in FY-2 Cloud Detection
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摘要: 云检测中所使用的云检测阈值正确与否是关系到云检测精度的重要因素。该文利用1983年7月-2007年6月ISCCP数据对覆盖我国及周边区域不同云类云顶温度的年、日变化特征进行分析, 得到云顶温度的分布特征; 对晴空下垫面与最暖云云顶温度差随纬度分布特征、不同区域晴空下垫面及云顶温度与晴空地表温度差日变化特征进行分析, 这些特征及每3 h的晴空下垫面24年平均亮温作为云检测算法的背景场, 用以判识实时动态提取云检测阈值的合理性。个例分析表明:利用多年平均晴空下垫面温度及最暖云云顶温度与晴空下垫面温度之差, 可有效识别云检测阈值的合理性, 合理的阈值有助于提高连续多日被云覆盖及冰雪下垫面条件下的云检测精度。Abstract: One pixel is cloud or not dependent on cloud detection scheme. Cloud detection approach includes histogram analysis, threshold detection, deviation analysis and so on. Cloud threshold is an important factor for cloud detection scheme. Now, cloud threshold usually can be gotten by one time and multi channel data in operational cloud detection method. But if surface is covered by snow or ice, or cloud covers one area for a long time, dynamic cloud threshold method always fails. So it should depend on clear temperature background. On the other hand, climate mean surface temperature can help to distinguish the dynamic cloud threshold' s validity. ISCCP data are proved by many researches that it is one of the best satellite cloud climate data in the world now. ISCCP data are used to analyze properties of cloud and clear temperature over China and neighboured areas. Cloud can be divided into three kinds, such as low, middle and high cloud. Different kind of cloud has different distribution property of cloud top temperature on a seasonly or daily basis along the latitude. Different kinds of cloud top temperature basically show strip distribution along longitude. The lower latitude, the higher cloud top temperature is. Along the same latitude, different area has different cloud properties during one day or one year.The analysis shows that the low cloud top temperature increases progressively in the fall and winter seasons from north to south, and has obvious linear increase tendency with reduced latitude. The middle level cloud top temperature distribution has distinct properties. Except July, August and September, the distribution of middle level cloud top temperature presents the meridional strip basically.The middle level cloud top temperature gradient is large in fall and winter, which is small in spring. The high cloud top temperature presents the meridional strip distribution in fall, winter and spring. The high cloud top temperature is lower than 240 Kand it' s latitudinal gradient is small to the north of 35°N. The high cloud top temperature meridional gradient increases and it' s diurnal variation is small to the south of 35°N. The temperature difference between clear surface and the warmest cloud top has stabile daily and seasonly change in the southeast part of China. At the same time, in the north part of China, the temperature difference between clear surface and the warmest cloud top has distinct daily and seasonly change. Above analyzed properties and mean clear temperature can be used as background information to distinguish the dynamic cloud detection threshold value' s validation and offer cloud threshold for areas that are covered by low cloud for a long time. An example is showed to testify mean clear temperature and temperature difference between clear and the warmest cloud can offer great information for cloud detection scheme.
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Key words:
- ISCCP;
- clear temperature;
- cloud top temperature;
- cloud detection
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图 1 多年平均1, 4, 7, 10月06:00晴空下垫面与最暖云云顶亮温差
Fig. 1 Multi-annual mean temperature difference between clear surface and the warmest cloud top at 06:00 in January, April, July and October
图 2 多年平均1月晴空下垫面与最暖云云顶亮温差每3h间隔沿83.75°E, 106.25°E
Fig. 2 Distribution pattern of multi-annual temperature difference between clear surface and cloud to p along 83.75°E, 106.25°E and 126. 25°E for every 3 hours in January
图 3 多年平均4月晴空下垫面与最暖云云顶亮温差每3h间隔沿83.75°E, 106.25°E和126.25°E的分布
Fig. 3 Distribution pattern of multi-annual temperature difference between clear surface and cloud to p along 83.75°E, 106.25°E and 126.25°E for every 3 hours in April
图 4 多年平均7月晴空下垫面与最暖云云顶亮温差每3h间隔沿83.75°E, 106.25°E
Fig. 4 Distribution pattern of multi-annual temperature difference between clear surface and cloud to p along 83.75°E, 106.25°E and 126.25°E for every 3 hours in July
图 5 多年平均10月晴空下垫面与最暖云云顶亮温差沿83.75°E, 106.25°E和126.25°E的分布
Fig. 5 Distribution pattern of multi-annual temperature difference between clear surface and cloud to p along 83. 75° E, 106. 25° E and 126.25°E for every 3 hours in October
图 6 被雾覆盖区扫描线上观测像元的FY-2C红外亮温、提取的动态阈值及ISCCP晴空多年平均下垫面亮温
Fig. 6 Pixel' s FY-2C infra red brightness temperature, the dynamic threshold value and the ISCCP multi-annual mean clear sky surface temperature in selected fog area
图 7 云检测阈值合理性检验前后的判识对比图(白色为云像元, 黑色为晴空像元; 方框为所选目标区域)
(a) FY-2C红外窗区通道亮温, (b) 用云检测阈值直接判识结果图, (c) 利用气候背景场订正云检测阈值后的云检测判识图
Fig. 7 Cloud detection image s before and after validation for cloud detection threshold value (white area is cloud pixel an d black area is clear pixel; square frame denotes the target region)
(a) brightness temperature at infared channel of FY-2C, (b) cloud detection image using cloud threshold directly, (c) cloud detection image after adjusted cloud threshold by climate background data
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