Cloud cover characteristics in Sichuan-Chongqing region based on geostationary satellite data
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摘要: 利用气象卫星风云四号A星和葵花8号观测资料，根据川渝地区独特的地形、地势将其分为东部低海拔地区和西部高海拔地区，建立高分辨率的网格化无云背景场，作为无云与有云辐射差异对比，使用阈值法进行晴空、水云、冰云、低层云雾、霾和积雪的检测，进而提高云识别信度，统计分析2016—2021年川渝地区云覆盖时、空分布特征。研究表明：川渝地区云的区域分布特征明显，东、西部存在明显差异，总体呈东多西少，云覆盖频率常年存在高值中心，云覆盖面积有明显月变化。东部低海拔地区云覆盖频率常年在70%—80%，而西部高海拔地区云覆盖频率在50%—65%。云覆盖频率的高、低值过渡区对应青藏高原与四川盆地之间的陡峭地形区，这与地形地势、水汽条件和大气环流特征有关。各月的云覆盖面积在东部低海拔地区相对稳定（占比在60%—80%），而西部高海拔地区差异较大。其中东部的冰云和西部的总云面积具有显著月变化（单峰）特征，峰值出现在7月，分别为37%和76%。6 a中各类型云覆盖面积占比年际波动较小，东部的云覆盖面积占比常年超过70%，其中水云占比最大（35%—40%），冰云次之（20%左右），低层云雾最小（约13%）；西部的云覆盖面积占比常年约60%（水云24%—26%，冰云22%—25%，低层云雾约10%）。Abstract: This study is based on observations of meteorological satellites Fengyun-4A and Himawari-8 and combined with the unique terrain over the Sichuan-Chongqing region, which is divided into the eastern low-altitude and the western high-altitude regions. A high-resolution gridded cloudless background field is established for comparison of radiation between cloudless and cloudy environments. The detection of clear sky, water cloud, ice cloud, low cloud/fog, haze, and snow using threshold method is realized to improve the confidence of cloud recognition. Temporal and spatial distributions of all the above types are analyzed from 2016 to 2021. Results show that clouds in the Sichuan-Chongqing region have significant regional characteristics and generally show a notable difference between the east and the west. The overall pattern of cloud cover frequency is larger in the east and smaller in the west. The cloud cover frequency has a high-value center throughout the year, and the cloud cover area has obvious monthly variations. The cloud cover frequency in the eastern low-altitude region is 70%—80% all the year round, while in the western high-altitude region it is 50%—65%. The high- and low-value transition area of cloud cover frequency corresponds to the steep terrain area between the Qingzang plateau and Sichuan basin, and is related to terrain, water vapor condition, and atmospheric circulation characteristics. The cloud cover area in the eastern low-altitude region is relatively stable in each month, accounting for 60%—80%, while the difference is significant in the western high-altitude region. The ice cloud area in the eastern region and the total cloud area in the western region both exhibit significant monthly variations (single peaks) with respective peaks of 37% and 76% appearing in July. In these six years, the interannual fluctuation of all the types is relatively small, and the cloud cover area in the eastern region exceeds 70% throughout the year with water clouds accounting for the largest proportion (35%—40%), ice cloud accounting for the second largest (about 20%), and low cloud/fog accounting for the smallest (about 13%). The cloud cover area in the westernregion accounts for about 60% throughout the year (water clouds 24%—26%, ice clouds 22%—25%, and low cloud/fog about 10%).
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图 1 川渝地区地形地势 （红色实线是近1500 m等高线）
Figure 1. Topography of the Sichuan-Chongqing region （the solid red line indicates the elevation contour close to 1500 m）
图 3 检测结果对比 （a. AHI，b. AGRI，c. RGB真彩图，d. CALIOP； AHI和AGRI数据时间是2020年1月1日05时00分、04时53分，相应的CALIOP时间是2020年1月1日05时37分 （世界时，下同）；紫色实线为CALIOP的轨迹）
Figure 3. Comparison of detection results （a. AHI，b. AGRI，c. RGB true color，d. CALIOP；the AHI and AGRI observations are at 05:00 and 04:53 UTC 1 January 2020，respectively，and simultaneous CALIOP observation is at 05:37 UTC 1 January 2020；the solid purple line represents the CALIOP track）
图 7 2019年AHI检测结果 （晴空、水云和冰云） 和CALIOP产品验证对比
Figure 7. Comparison of AHI results （clear，water，and ice） and CALIOP products in 2019
图 8 川渝地区2021年AGRI和AHI的检测结果对比 （a. 东部低海拔，b. 西部高海拔）
Figure 8. Comparison of AGRI and AHI results in the Sichuan-Chongqing region during 2021 （a. Low altitude，b. high altitude）
图 9 2016—2021年月平均云覆盖率分布
Figure 9. Distributions of monthly mean cloud cover frequency in each month over 2016—2021
图 10 2016—2021年云覆盖率空间分布
Figure 10. Spatial distributions of cloud cover frequency over 2016—2021
图 11 2016—2021年东部低海拔 （Low） 和西部高海拔 （High） 地区云覆盖率 （a） 和晴空率 （b） 分布
Figure 11. Distributions of cloud cover （a） and clear （b） frequency in the east low-altitude region and the west high-altitude region over 2016—2021
图 12 2016—2021年东部低海拔地区 （a） 和西部高海拔地区 （b） 各类型面积占比月际变化
Figure 12. Monthly variation characteristics of area proportions for all types in the east low-altitude region （a） and the west high-altitude regions （b） over 2016—2021
图 13 2016—2021年东部低海拔地区 （a） 和西部高海拔地区 （b） 各类型面积占比逐年变化情况
Figure 13. Annual variation characteristics of area proportions for all types in the east low-altitude region （a） and the west high-altitude region （b） over 2016—2021
表 1 所用AGRI和AHI通道
Table 1. AGRI and AHI channels used for the present study
通道中心波长 （ μm ） AGRI AHI 中心波长
光谱带宽（ μm ）分辨率 （ km ） 中心波长
光谱带宽（ μm ）分辨率 （ km ） 0.47 0.47 （0.45—0.49） 1.0 0.47 （0.45—0.49） 1.0 0.65 0.65 （0.55—0.75） 0.5—1.0 0.64 （0.63—0.66） 0.5 1.61 1.61 （1.58—1.64） 2.0 1.61（1.60—1.62） 2.0 3.8 3.75 （3.50—4.00） 2.0 3.85 （3.74—3.96） 2.0 8.5 8.50 （8.00—9.00） 4.0 8.60 （8.44—8.76） 2.0 11.0 10.80 （10.30—11.30） 4.0 11.20 （11.10—11.30） 2.0 注：当两个仪器通道的中心波长存在差异时，为表述方便，选择一个数值代表与两者最接近的通道。 
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表 2 各类型检测方法及阈值
Table 2. Methods and thresholds for the detection of each type
检测类型 通道及阈值 参考文献 云雾 R（0.47）−BG（0.47）＞0.10
且R（0.47）＞0.12
或BTD（11.0−3.8）＜−14.0 KFrey，et al，2008 低层云雾 AGRI：ΔR（0.65−1.61）＜
−0.025
AHI: ΔR（0.65−1.61）＜0张培等，2019；
Ryu，et al，2020；
Yang，et al，2021冰云 AGRI：BT（8.5）＜283.0 K且
BTD（8.5−11.0）＞−1.5 K
AHI：BT（8.5）＜285.0 K且
BTD（8.5−11.0）＞−1.0 KBaum，et al，2012；
朝鲁门等，2019积雪 NDSI＞0.36且 DEM≥1500 m Xiao，et al，2001；
Shang，et al，2017霾 R（0.47）＞0.11 注：R表示反射率，例如R（0.65）表示0.65 μm通道反射率，ΔR（0.65−1.61）表示两通道反射率差，BT表示亮度温度，例如BT（8.5）表示8.5 μm通道亮度温度，BTD（8.5−11）表示两通道亮温差。
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