Estimation of Background Radiation Underneath Clouds for Wind Vector Height Assignment of Semi-transparent Clouds

Zhang Xiaohu; Zhang Qisong; Xu Jianmin

doi:10.11898/1001-7313.20170303

Vol.28, NO.3, 2017

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Article Navigation > Journal of Applied Meteorological Science > 2017 > 28(3): 283-291

Zhang Xiaohu, Zhang Qisong, Xu Jianmin. Estimation of background radiation underneath clouds for wind vector height assignment of semi-transparent clouds. J Appl Meteor Sci, 2017, 28(3): 283-291. DOI: 10.11898/1001-7313.20170303.

Citation:

Zhang Xiaohu, Zhang Qisong, Xu Jianmin. Estimation of background radiation underneath clouds for wind vector height assignment of semi-transparent clouds. J Appl Meteor Sci, 2017, 28(3): 283-291. DOI: 10.11898/1001-7313.20170303.

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Estimation of Background Radiation Underneath Clouds for Wind Vector Height Assignment of Semi-transparent Clouds

DOI: 10.11898/1001-7313.20170303

National Satellite Meteorological Center, Beijing 100081

Received Date: 2016-12-13
Rev Recd Date: 2017-04-10
Publish Date: 2017-05-31

Abstract

Abstract

Height assignment of semi-transparent clouds needs two variables:Radiation from the semi-transparent clouds themselves and radiation from the background underneath semi-transparent clouds. The background radiation is not directly observed by the satellite, but can be estimated with infrared/water vapor scatter diagrams of satellite measurements around tracer image boxes. According to the cloud amount and type observed, tracer image boxes can be divided into 4 types:With different levels of clouds inside, near the cloud edge, with only high or middle-high clouds inside and with only middle-low clouds or ground in tracer box. By studying characteristics of infrared/water vapor scatter diagrams of satellite measurements in 8 typical cases of 4 types which are variety, different estimation methods are discussed and a new estimation method for only high or middle-high clouds in tracer box is proposed. When there are cloud-free pixels in tracer boxes, infrared maximum brightness temperature may represent the background radiation for infrared channel; on the other hands, observation pixels with the highest infrared but relative lower water vapor brightness temperatures are used to estimate the background radiation for water vapor channel. Observation pixels with high infrared, but relative lower water vapor brightness temperatures are located in the clear sky region near the cloud edge, where atmospheric stratification conditions are relatively wetter than the region far away from clouds. The clear sky region near the cloud edge relatively better represent the condition underneath clouds better. Due to absorption and retransition processes, the water vapor brightness temperatures with wetter atmospheric stratification conditions in the clear sky region near the cloud edge are relative lower. When there are no cloud-free pixels in the tracer image box, the searching area should be expanded in the east-west direction to find cloud free pixels, until the background radiation can be estimated.Based on the new estimation method, FY-2 wind vector height assignment algorithm is improved, and effectiveness is analyzed by comparing with ECMWF reanalysis data, showing that FY-2 wind vector quality including biases and standard deviations is improved obviously.
- wind vectors;
- height estimation of semi-transparent clouds;
- background radiation underneath clouds

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References(16)

Figures and Tables

图 1 热带辐合带中混合着深对流的多层云典型案例

(a) 追踪图像块位于2.86°N，138.7°E，(b) 追踪图像块位于2.77°N，111.88°E

Fig. 1 Typical example analysis charts for multiple layers of clouds with deep convections in ITCZ

(a) tracer box located at 2.86°N, 138.7°E, (b) tracer box located at 2.77°N, 111.88°E

DownLoad: Full-Size Img PowerPoint

图 2 深对流云外围消散中的卷云典型案例

(追踪图像块位于12.59°N, 130.44°E)

Fig. 2 Typical example analysis chart for dissipating cirrus cloud outer of deep convective clouds

(tracer box located at 12.59°N, 130.44°E)

DownLoad: Full-Size Img PowerPoint

图 3 热带密实云区中的深对流云典型案例

(a) 追踪图像块位于18.48°S, 117.45°E, (b) 追踪图像块位于25.90°N, 112.84°E

Fig. 3 Typical example analysis charts for deep convective clouds in a tropical cyclone

(a) tracer box located at 18.48°S, 117.45°E, (b) tracer box located at 25.90°N, 112.84°E

DownLoad: Full-Size Img PowerPoint

图 4 追踪区内只存在中低云和地面典型案例分析

(a) 追踪图像块位于6.09°S, 131.98°E, (b) 追踪图像块位于14.51°S, 105.39°E, (c) 追踪图像块位于17.02°N, 90.38°E

Fig. 4 Typical example analysis charts for middle or low layer clouds

(a) tracer box located at 6.09°S, 131.98°E, (b) tracer box located at 14.51°S, 105.39°E, (c) tracer box located at 17.02°N, 90.38°E

DownLoad: Full-Size Img PowerPoint

Fig. 5 For deep convective clouds, expanding search area to seek back ground temperature underneath clouds (a) tracer box located at 18.48°S, 117.45°E, (b) tracer box located at 25.90°N, 112.84°E

DownLoad: Full-Size Img PowerPoint

图 6 扩大范围在深对流云外围消散中卷云搜索云下地面亮温效果

(追踪图像块位于12.59°N, 130.44°E)

Fig. 6 For dissipating cirrus cloud outer of deep convective clouds, expanding search area to seek background temperature underneath clouds

(tracer box located at 12.59°N, 130.44°E)

DownLoad: Full-Size Img PowerPoint

Table 1 Biases and standard deviations between FY-2E wind vectors and ECMWF analyses for height assignment algorithms whether using background radiation

统计量	未使用云下背景辐射	使用云下背景辐射
偏差/(m·s^-1)	-1.571	-1.012
标准差/(m·s^-1)	7.964	7.754
风矢量数	31507	34187

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