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Wind Retrieval Simulation in Tropical Cyclone for FY-3 Dual-Frequency WFR
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摘要: 基于大气辐射传输理论分别建立Ku波段和C波段的降雨模型，模拟热带气旋降雨区洋面的雷达回波并反演了洋面10 m风场，用于研究降雨对测风的影响以及风云三号双频风场雷达 (WFR) 的测风能力。分析表明：回波的衰减或增强取决于降雨衰减项和后向散射项的相对大小；热带气旋的降雨使反演风速偏小，风向精度降低，Ku波段相对于C波段更易受影响，在高风速 (超过30 m·s-1) 条件下，可达5~20 m·s-1的负风速偏差。反演结果表明：双频反演的新方法能够结合Ku波段与C波段的优势，双频最大似然估计 (MLE) 方法在分辨率上优于C波段单频反演，相对Ku单频反演能降低降雨对测风的衰减作用，结合双频MLE方法和C波段单频反演优势的分区反演方法可以显著减小降雨偏差，提高风速反演精度，在有风云三号湿度计同步观测的条件下，是提高热带气旋降雨区测风精度的有效手段。Abstract: Tropical cyclone is one of the primary disastrous synoptic systems in China. With the continuous observation, global coverage and the ability to penetrate through precipitation layer, microwave sensors on polar orbit satellites can provide more precise observations of the tropical cyclone location and intensity for marine extreme weather forecasting, which will compensate for the shortage of conventional observations. The FY-3 satellite microwave scatterometer, named Wind Field Radar (WFR), is the only way to measure ocean vector winds (OVW).Compared with single-frequency scatterometer, dual-frequency scatterometer has advantages in higher spatial resolution and better response to winds in extreme weather conditions, where high winds are usually associated with high rain rates. The Jet Propulsion Laboratory (JPL) has developed a conceptual design for a Dual Frequency Scatterometer (DFS) in the Extended Ocean Vector Winds Mission put forward in 2007. The concept of Rotating Fan Beam Scatterometer which will be extended to dual-frequency mode has been studied under ESA. The WFR installed on FY-3 satellite which will be launched in 2016 is designed to use the dual-frequency and dual-polarization time-sharing observation pattern.Rain perturbations result from volume scattering and attenuation by precipitation in the atmosphere, as well as changes of sea surface roughness by impinging rain drops. Few studies which investigate effects of rain on Ku-band and C-band scatterometer data indicate that the impact of rain to sea surface is a complex phenomenon not yet fully understood, and it's not dominating in high wind and strong rain fall areas. Therefore, changes in surface roughness are not considered here.The purpose of this study is to investigate the potential of the WFR proposed to fly aboard FY-3 satellite to measure OVW in rain fall areas of Typhoon Ivan. A theoretical model based on radiation transfer equation including rain attenuation and scattering, has been developed to quantify the modification by rain of the measured backscatter. Based on the simulated normalize radar cross section (NRCS) dataset generated by forward model, the impact on the retrieved winds has been analyzed, and the dual-frequency retrieval algorithm has been firstly studied. Analysis shows that changes of contaminated NRCS briefly depend on relative size of volume scattering item and attenuation item. Wind vectors measured at Ku-band can be more severely altered by rain than those at C-band. Precipitation in tropical cyclone can significantly degrade the OVW accuracy. Retrieval results show that new methods combined Ku-band and C-band have higher spatial resolution than C-band retrieval, and better performance in rain fall region than Ku-band retrieval. Especially, partitioned wind retrieval technique can significantly reduce the rainfall error, is an effective way to improve the wind retrieval accuracy in tropical cyclone with the synchronous observation by microwave humidity sounder (MWHS) aboard FY-3 satellite.
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Key words:
- rain model;
- wind field;
- microwave scatterometer;
- radiation transfer
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图 1 RdB与C波段、Ku波段双向衰减系数、体散射项关系
Fig. 1 Two-way attenuation coefficient and volume-scattering factor versus RdB at C and Ku-band
图 2 不同风速条件下C波段、Ku波段A0，A2与降雨率的关系
Fig. 2 A0, A2 at C and Ku-band versus rainfall rate at different wind speeds
图 3 不同降雨率条件下C波段、Ku波段Δσ0与模式风速的关系
Fig. 3 Δσ0 at C and Ku-band versus wind speed at different rainfall rates
图 4 不同模式风速区间反演风速和模式风速之差与降雨率的关系
Fig. 4 The distinction between retrieved wind speed and simulated wind speed versus rainfall rate in different wind speed sections
图 5 加噪后两波段反演风速和模式风速之差与模式风速间的关系
(a) C波段加系统噪声，(b) C波段加地球物理噪声，(c) Ku波段加系统噪声，(d) Ku波段加地球物理噪声
Fig. 5 The distinction between noises added retrieved wind speed and simulated wind speed at C and Ku-band versus real wind speed (a) system-noise added to C-band, (b) geo-noise added to C-band, (c) system-noise added to Ku-band, (d) geo-noise added to Ku-band
图 6 传统的C波段单频反演风速 (a)、Ku波段单频反演风速 (b)、双频MLE方法反演风速 (c)、分区反演风速 (d) 与模式风速对比
Fig. 6 Comparisons between conventional C-band (a), Ku-band single-frequency retrieved wind speed (b), dual-frequency retrieved wind speed (c), partitioned retrieved wind speed (d) and simulated wind speed
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