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Evaluation of ASCAT Coastal Wind Product Using Nearshore Buoy Data
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摘要: 利用美国西海岸7个近岸浮标2012年全年和中国近岸8个气象浮标2012年1—6月的风场观测数据，检验了卫星散射计ASCAT近岸风场产品中的风速和风向在近岸海域的精度。检验结果表明：在美国西海岸近岸海域，ASCAT近岸风场产品中的风速与浮标的风速一致性高，但ASCAT近岸风场产品中风向的精度受离岸距离、风速和风向等因素的影响，在离岸近的海域ASCAT近岸风场产品与浮标观测风场的一致性较差。统计发现，将低风速 (不超过3 m·s-1) 剔除可明显提高ASCAT近岸风场产品在近岸海域的精度。另外，ASCAT近岸风场产品的风向精度在不同风向上存在差异，表现为从陆地吹向海洋风向精度较小，而从海洋吹向陆地风向精度较高。在中国近岸海域，受地形影响，渤海海域ASCAT近岸风场产品与气象浮标观测的风向差异大，在其他近岸海域的ASCAT近岸风场产品和气象浮标的观测风场的对比结果与美国西海岸风场的对比结果特征相似。Abstract:
The new scatterometer advanced scatterometer (ASCAT) on board MetOp-A satellite provides surface wind speed and direction over global ocean. Providing accurate nearshore wind data from satellites is challenging because satellite data are unavailable very close to shore due to the contaminating effect of the land. Besides, land-sea breezes and shore topography produce small space scale and time-scale wind variations that can be smoothed by the satellite's space averaging and aliased by the satellite's twice-a-day sampling. The complexity of nearshore winds is one of the prime causes that the regions are so important. For example, over one-third of the total marine fish catch occurs within nearshore zone.The accuracy of ASCAT coastal wind product is determined through various comparisons with buoys. The nearshore buoys used in the comparisons locate in US West Coast and China Coast. As the time interval of US West Coast buoy wind is 10-minute interval and the spatial resolution of ASCAT wind product is 12.5 km, a scatterometer wind and a buoy wind measurement are considered to be collocated if the distance between the wind vector cell center and the buoy location is less than 12.5 km and if the acquisition time difference is less than 5 minutes in US West Coast. As the time interval of China Coast buoy wind is 1 hour, the acquisition time difference is less than 30 minutes in China Coast. The buoy winds at a given anemometer height are converted to 10 m neutral winds in order to enable a good comparison with the 10 m scatteromter winds. The time ranges of wind data used for comparison from US West Coast buoys and China Coast buoys are the whole year of 2012 and the first half year of 2012 individually.It shows that the accuracy of the wind speed of ASCAT product is high and the accuracy of the wind direction of ASCAT product is influenced by several factors, such as the distance from coast, wind speed and wind direction. The overall wind speed correlation coefficient between buoy data and ASCAT product is 0.94, and wind speed correlation coefficients between each buoy and ASCAT product are all above 0.9. The overall wind direction correlation coefficient between buoy and ASCAT product is 0.71, and the wind direction correlation coefficient between the nearest buoy and ASCAT product is only 0.55. Processing the satellite data by discarding observations recorded in light winds (below 3 m·s-1) can improve the accuracy of the ASCAT wind products by reducing the mean bias of wind direction from 21.89°to 11.83°, and the wind direction correlation coefficient increased from 0.71 to 0.84. In addition, the accuracy of the wind flow from land is low, while the accuracy of the wind flow from sea is higher. In most China nearshore regions, the applicability of ASCAT coastal wind product is good, but in Bohai Sea, the effect of topography on ASCAT coastal wind product is apparent.
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Key words:
- coastal area;
- ASCAT;
- buoys
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图 1 用于ASCAT近岸风场产品检验的美国西海岸近岸浮标站分布
Fig. 1 Location of 7 nearshore buoys off US West Coast used to evaluate ASCAT coastal wind product
图 2 用于ASCAT近岸风场产品检验的中国近岸浮标站分布
Fig. 2 Location of 8 nearshore buoys off China Coast used to evaluate ASCAT coastal wind product
图 3 2012年ASCAT近岸风场与美国西海岸近岸浮标风速和风向相关分布
Fig. 3 Correlation coefficients between ASCAT coastal wind products and US West Coastal buoy wind observations in 2012
图 4 ASCAT近岸风场与美国西海岸近岸浮标风速和风向偏差直方统计图 (a) 风速偏差, (b) 风向偏差
Fig. 4 differences between ASCAT coastal wind products and US West Coastal buoy observations (a) wind speed, (b) wind direction
图 5 ASCAT近岸风场与美国西海岸近岸浮标风速和风向均方根误差风玫瑰图
(a) 风速均方根误差 (单位：m·s-1), (b) 风向均方根误差 (单位：(°))
Fig. 5 Wind rose diagram of root mean square error between ASCAT coastal wind products and US West Coastal buoy observations
(a) wind speed root mean square error (unit:m·s-1), (b) wind direction root mean square error (unit:(°))
图 6 2012年1—6月ASCAT近岸风场与中国渤海海域气象浮标平均偏差的风玫瑰图 (单位：(°))
Fig. 6 Wind rose diagram of wind direction mean error between ASCAT coastal wind products and nearshore buoy data of Bohai Sea in China from January to June in 2012(unit:(°))
表 1 NDBC浮标信息列表
Table 1 Information list of the NDBC buoys
浮标号 纬度 经度 测风高度/m 离岸距离/km 46013 38.242°N 123.301°W 5 22 46014 39.235°N 123.974°W 5 17 46027 41.850°N 124.381°W 5 13 46028 35.741°N 121.884°W 5 41 46041 47.349°N 124.708°W 5 34 46042 36.785°N 122.469°W 5 35 46086 32.491°N 118.034°W 5 85 
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表 2 中国近岸浮标信息列表
Table 2 Information list of China nearshore buoys
浮标号 纬度 经度 测风高度/m 离岸距离/km 54558 39.25°N 120.58°E 10 57 54641 38.85°N 118.55°E 10 8 58573 29.75°N 122.75°E 10 34 58768 27.53°N 121.40°E 10 39 58951 25.50°N 120.32°E 10 45 59334 23.64°N 118.20°E 10 54 59515 22.33°N 117.34°E 10 110 59765 20.75°N 111.66°E 10 85
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表 3 ASCAT近岸风场与美国西海岸近岸浮标对比
Table 3 Comparison between ASCAT coastal wind products and US West Coastal buoy wind observations
浮标号 匹配样本量 风速平均偏差/(m·s-1) 风向平均偏差/(°) 风速相关系数 风向相关系数 46013 298 0.82 22.69 0.97 0.78 46014 282 1.59 25.11 0.93 0.57 46027 178 2.14 31.72 0.90 0.55 46028 349 0.77 15.02 0.97 0.87 46041 334 0.92 25.70 0.96 0.67 46042 328 0.74 16.52 0.96 0.78 46086 354 0.68 21.86 0.94 0.74
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表 4 根据风速等级划分的ASCAT近岸风场和美国西海岸近岸浮标观测风场对比
Table 4 Comparison between ASCAT coastal wind products and US West Coastal buoy wind observations according to wind speed
类型 样本量 风速平均偏差/(m·s-1) 风向平均偏差/(°) 风速相关系数 风向相关系数 全部样本 2123 1.0 21.89 0.94 0.71 风速不大于3 m·s-1 548 1.15 50.8 0.26 0.39 风速大于3 m·s-1 1575 0.96 11.83 0.94 0.84
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表 5 ASCAT近岸风场与中国近岸浮标对比
Table 5 Comparison between ASCAT coastal wind products and China nearshore buoy wind observations
浮标号 样本量 风速平均偏差/(m·s-1) 风向平均偏差/(°) 风速相关系数 风向相关系数 54558 204 1.33 136.10 0.94 0.06 54641 141 8.37 53.63 0.42 0.55 58573 274 2.48 36.27 0.87 0.77 58768 196 1.12 30.18 0.96 0.95 58951 173 1.27 38.04 0.97 0.78 59334 124 1.60 40.86 0.96 0.83 59515 137 1.15 26.92 0.98 0.93 59765 160 1.06 29.64 0.96 0.92
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表 6 根据风速等级划分的ASCAT近岸风场和中国近岸浮标对比
Table 6 Comparison result between ASCAT coastal wind products and China nearshore buoy wind observations according to wind speed
分类 样本量 风速平均偏差/(m·s-1) 风向平均偏差/(°) 风速相关系数 风向相关系数 全部样本 1409 1.70 32.73 0.92 0.63 风速不大于3 m·s-1 253 2.00 65.80 0.30 0.40 风速大于3 m·s-1 1156 1.55 18.20 0.93 0.73
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