Environmental Thermal Radiation Interference on Atmospheric Brightness Temperature Measurement with Ground-based K-band Microwave Radiometer

Wang Zhenhui; Li Qing; Chu Yanli; Zhu Yayu

Vol.25, NO.6, 2014

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Article Navigation > Journal of Applied Meteorological Science > 2014 > 25(6): 711-721

Wang Zhenhui, Li Qing, Chu Yanli, et al. Environmental thermal radiation interference on atmospheric brightness temperature measurement with ground-based K-band microwave radiometer. J Appl Meteor Sci, 2014, 25(6): 711-721.

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Environmental Thermal Radiation Interference on Atmospheric Brightness Temperature Measurement with Ground-based K-band Microwave Radiometer

Wang Zhenhui^{1,2
,
,},
Li Qing^1,2,
Chu Yanli³,
Zhu Yayu²

1.
Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, CMA Key Laboratory for Aerosol-Cloud-Precipitation, Nanjing University of Information Science & Technology, Nanjing 210044
2.
School of Atmospheric Physics, Nanjing University of Information Science & Technology, Nanjing 210044
3.
Institute of Urban Meteorology, CMA, Beijing 100089

Received Date: 2014-04-06
Rev Recd Date: 2014-09-10
Publish Date: 2014-11-30

Abstract

Abstract

Effects of operating environment thermal radiation interference on atmospheric brightness temperature measurement with ground-based K-band microwave radiometer especially for channels near 28.0 GHz and 30.0 GHz are studied. A model for simulating antenna temperature which expresses the energy received by the radiometer based on radiative transfer is derived and used to calculate the response of the brightness temperature measurements to parameters such as antenna specifications, radome, surrounding temperature and emissivity. Results show that the equivalent main beam efficiency (η_e) defined by 3 dB points is only 73.17% for a typical antenna, of which the half-beam half width α=3.1° and the gain G=30 dB. The value of η_e would be even smaller if factors like aperture radiation effect, shape-error, and occlusion and so on are taken into account. The brightness temperature would fluctuate by 4.0 K in case that η_e=70%, the surrounding temperature and emissivity would change by ΔT_g=10 K and Δε=0.05 around T_g=280 K and ε=0.85 if the radome can be neglected. The fluctuation would increase up to 9.6 K if the size of the opening in the radome is just for the main beam. Therefore, if the equivalent main beam efficiency determined by the antenna gain and 3-dB beam width for the current radiometer system is not large enough, variation of the operating environment must be taken into account during the correction of K-band brightness temperature measurement even though LN calibration of the radiometer system can be performed as manual-required. For this, a brightness temperature correction method for operating environment variation is suggested according to the theoretical relationship and the result from application to observations. Over one year application after LN calibration shows that the fitness and correlation between the observed brightness temperature after correction and the calculated brightness temperature with radiative transfer equation is obviously better than before, especially for channels of 28 GHz and 30 GHz.
- ground-based microwave radiometer;
- K-band brightness temperature correction;
- antenna specifications;
- operating environment interference

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References(31)

Figures and Tables

Fig. 1 Schematic of radiometer antenna directivity function

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Fig. 2 Time series of clear sky environment temperature change T_g and the observed brightness temperature T_BM, the corrected brightness temperature T_BO and the simulated T_BC for 4 typical channels in K-band at 0800 BT and 2000 BT from 22 Dec 2010 to 31 Dec 2011

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Table 1 Antenna performance and analysis for two typical types of K-band radiometers

行号	条件	参数	A类辐射计	B类辐射计
行号	条件	参数	22~31 GHz	22 GHz	30 GHz
1	厂家提供	G/dB	33.2	30	32
2		X/dB	< -30	< -23	< -24
3		W_HPB/(°)	3.3~3.5	6.3	4.9
4	取α= 0.5W_HPB	α/(°)	1.7	3.1	2.5
5	γ=X_max	γ/dB	-30.0	-23.0	-24.0
6	γ=X_max	η_e /%	18.04	13.13	10.64
7	γ由G和α决定	γ/dB	-35.9	-35.7	-38.1
8	γ由G和α决定	η_e /%	45.98	73.17	75.42
9	由需求η_e=90%决定	γ/dB	-46.2	-40.8	-42.8
10	由需求η_e=90%决定	α/(°)	2.38	3.44	2.73

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Table 2 Comparison of the K-band downward brightness temperature and antenna temperature for type-B radiometer and analysis on the influence of environment under different sky conditions (let T_S=238 K, ΔT_S=22.5 K and other parameters for T_B calculation are the same as Reference [31])

行号	条件	计算参数	22.2 GHz (η_e=73.17%)	25.0 GHz (η_e=73.98%)	28.0 GHz (η_e=74.84%)	30.0 GHz (η_e=75.42%)
1		晴空T_B/K	31	28	17	16
2		云天T_B/K	42	33	29	30
3		雨天T_B /K	61	53	51	54
4	β=1	晴空T_A/K	59	55	45	43
5		云天T_A /K	68	60	55	56
6		雨天T_A /K	85	77	75	77
7		δT_B/K	3.5	3.4	3.2	3.2
8		晴空T_B/δT_B	8.9	8.3	5.3	5.1
9		云天T_B/δT_B	12.0	9.8	9.0	9.5
10		雨天T_B/δT_B	17.5	15.8	15.8	17.1
11	β=0	晴空T_A/K	87	83	73	71
12		云天T_A /K	95	86	82	81
13		雨天T_A /K	108	101	98	99
14		δT_B/K	8.3	7.9	7.6	7.3
15		晴空T_B/δT_B	3.8	3.5	2.2	2.2
16		云天T_B/δT_B	5.1	4.2	3.8	4.1
17		雨天T_B/δT_B	7.4	6.7	6.7	7.4

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Table 3 Coefficient c for calibrating the environment temperature influence on K-band brightness temperature measured by type-B radiometer and the statistics to show the efficiency of calibration

通道	频率/GHz	订正系数c	订正前拟合直线 T_BM=aT_BC+b	拟合度 R_MC²	订正后拟合直线 T_BO=aT_BC+b	拟合度 R_OC²
1	22.23	0.215088	y=1.1112x-2.4141	0.9537	y=0.9862x-0.2928	0.9763
2	22.50	0.224503	y=1.1119x-1.1412	0.9571	y=0.9792x+0.9970	0.9763
3	23.03	0.352293	y=1.1879x-0.9221	0.9194	y=0.9700x+2.1907	0.9680
4	23.83	0.354430	y=1.2161x-0.7508	0.8861	y=0.9634x+2.4390	0.9563
5	25.00	0.329222	y=1.2402x-0.9890	0.8174	y=0.9577x+2.0860	0.9395
6	26.23	0.265974	y=1.2885x-2.6649	0.8019	y=0.9890x+0.6199	0.9431
7	28.00	0.350330	y=1.0714x+0.3436	0.3846	y=1.0091x+0.0591	0.9138
8	30.00	0.457644	y=0.1129x+12.659	0.0049	y=0.9314x+2.2196	0.7931

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