Some traditional methods, such as the velocity-azimuth display （VAD） and the velocity-azimuth processing （VAP）, have been widely used to retrieve the 3-D wind field from single-Doppler radar data because of their ...Some traditional methods, such as the velocity-azimuth display （VAD） and the velocity-azimuth processing （VAP）, have been widely used to retrieve the 3-D wind field from single-Doppler radar data because of their relative conceptual and practical simplicity. The advantage of VAD is that it is not affected by small-scale perturbations of the radial wind along the azimuth, to which the VAP method is very sensitive. Nevertheless, the spatial resolution of the VAD method is very poor compared to the VAP method. We show, in this study, that these two retrieval methods are actually related with each other and they are two special applications of a retrieval function based on the azimuthal uniform-wind assumption for a given azimuthal interval [01,02]. When using this retrieval function to retrieve wind fields, the azimuthal interval used in retrieval can be adjusted according to the requirement of smoothness or resolution. The larger （smaller） the azimuthal interval is, the coarser （finer） the horizontal resolution of retrieved wind field is, and the more insensitive （sensitive） the retrieval method is to small-scale perturbations. Because the full information within the azimuthal interval [01,02], instead of the information at two terminal points only, i.e., azimuths 01 and 02, is used to retrieve the wind fields, this method is referred to as the integrating VAP （IVAP） method, wherein the horizontal wind field is retrieved by using the Doppler velocity over the part of circumference, delimited by the given azimuthal interval times the scan radius. By contrast, the VAP method uses only the velocities at two terminal points of the given azimuthal interval. Therefore, the IVAP method has a filtering function, and the filtering rate can be controlled by adjusting the azimuthal interval. The filter such as that used in the pre-processing of the VAP method is no longer necessary for the IVAP method. When the retrieval azimuthal interval is as large as a whole circumference, the IVAP becomes the VAD. On the other hand, if only two neighboring azimuthal data are used, the IVAP becomes the VAP. The frequency response function of IVAP indicates that the IVAP method can filter out shortwaves, and a larger azimuthal interval leads to stronger filtering ability, therefore a smoother retrieved wind field. The shortwave filter function of the IVAP method is tested by an ideal experiment wherein the radar observations are artificially created by a uniform flow superposed with random disturbances. The VAP and IVAP methods are used in wind retrieval, respectively, and give different results for different azimuthal intervals （i.e., 6°, 12°, 24°, and 48°）. Because the VAP method is sensitive to small disturbances, the retrieved winds have larger errors for all different azimuthal intervals. However, the retrieved wind by the IVAP method has smaller errors when the azimuthal interval is larger due to its shortwave filter function. An experiment for an idealized linear wind field is also carried out to demonstrate the effect of the retrieval azimuthal interval on the IVAP method. The results show that a short interval gives the retrieval close to the ＂true＂ wind field with a linear distribution. When increasing the interval, the retrieval is smoothed and can represent only the mean wind field.展开更多
The error distributions of the wind fields retrieved from single and dual-Doppler radar observations are given in this paper.The results indicate that the error of dual-Doppler retrieval depends on the position in the...The error distributions of the wind fields retrieved from single and dual-Doppler radar observations are given in this paper.The results indicate that the error of dual-Doppler retrieval depends on the position in the scan region of the dual-Doppler radar.The error of single-Doppler retrieval by using velocity azimuth processing(VAP)technique de- pends on the angle between the directions of wind and the radar beam.Generally,the winds retrieved from single Doppl- er radar are close to those retrieved from dual-Doppler radar.But,the error distribution of the single-Doppler retrieval is different from the dual-Doppler retrieval.We simulate the retrievals of single Doppler observation by the use of the output wind data from a 3-D numerical model of severe convection.The comparison of the simulated single-and dual-Doppler retrievals shows that the VAP may be a suitable technique for the operational analysis of mesoscale wind fields.It can also be used as a supplement to wind field retrieval in the field experiment.展开更多
全极化数据可以获取比单极化数据更多的目标信息,研究发现C波段交叉极化数据同样可用于海面风速反演。针对RADARSAT-2 Fine Quad模式具有全极化成像的特点,以我国东部海域为研究区,结合同极化数据和交叉极化数据反演海面风速模型,探究...全极化数据可以获取比单极化数据更多的目标信息,研究发现C波段交叉极化数据同样可用于海面风速反演。针对RADARSAT-2 Fine Quad模式具有全极化成像的特点,以我国东部海域为研究区,结合同极化数据和交叉极化数据反演海面风速模型,探究各种极化数据的最优风速反演方法。对于同极化数据采用地球物理模型函数(GMF)和极化率模型(PR)组合的方式进行海面风速反演,对交叉极化数据采用C波段交叉极化海面散射模型(C-2PO)进行海面风速反演,反演结果与ERA-Interim风场数据进行比较分析;此外,对Scan SAR模式交叉极化数据的后向散射系数与海面风速的关系进行探索分析。研究结果表明,RADARSAT-2 Fine Quad模式四种极化数据选用合适的模型均可反演出高精度的海面风速,其中VH和HV极化数据的反演结果基本相同,交叉极化数据反演风速效果好于同极化数据,同时,Scan SAR模式交叉极化数据的后向散射系数随海面风速的增大表现出一定的线性变化趋势。全极化模式数据在海面风速反演上表现出比单极化模式数据较为明显的优势,将成为未来海面风速反演的发展方向。展开更多
基金Supported by the National Natural Science Foundation of China under Grant No.40405012.
文摘Some traditional methods, such as the velocity-azimuth display （VAD） and the velocity-azimuth processing （VAP）, have been widely used to retrieve the 3-D wind field from single-Doppler radar data because of their relative conceptual and practical simplicity. The advantage of VAD is that it is not affected by small-scale perturbations of the radial wind along the azimuth, to which the VAP method is very sensitive. Nevertheless, the spatial resolution of the VAD method is very poor compared to the VAP method. We show, in this study, that these two retrieval methods are actually related with each other and they are two special applications of a retrieval function based on the azimuthal uniform-wind assumption for a given azimuthal interval [01,02]. When using this retrieval function to retrieve wind fields, the azimuthal interval used in retrieval can be adjusted according to the requirement of smoothness or resolution. The larger （smaller） the azimuthal interval is, the coarser （finer） the horizontal resolution of retrieved wind field is, and the more insensitive （sensitive） the retrieval method is to small-scale perturbations. Because the full information within the azimuthal interval [01,02], instead of the information at two terminal points only, i.e., azimuths 01 and 02, is used to retrieve the wind fields, this method is referred to as the integrating VAP （IVAP） method, wherein the horizontal wind field is retrieved by using the Doppler velocity over the part of circumference, delimited by the given azimuthal interval times the scan radius. By contrast, the VAP method uses only the velocities at two terminal points of the given azimuthal interval. Therefore, the IVAP method has a filtering function, and the filtering rate can be controlled by adjusting the azimuthal interval. The filter such as that used in the pre-processing of the VAP method is no longer necessary for the IVAP method. When the retrieval azimuthal interval is as large as a whole circumference, the IVAP becomes the VAD. On the other hand, if only two neighboring azimuthal data are used, the IVAP becomes the VAP. The frequency response function of IVAP indicates that the IVAP method can filter out shortwaves, and a larger azimuthal interval leads to stronger filtering ability, therefore a smoother retrieved wind field. The shortwave filter function of the IVAP method is tested by an ideal experiment wherein the radar observations are artificially created by a uniform flow superposed with random disturbances. The VAP and IVAP methods are used in wind retrieval, respectively, and give different results for different azimuthal intervals （i.e., 6°, 12°, 24°, and 48°）. Because the VAP method is sensitive to small disturbances, the retrieved winds have larger errors for all different azimuthal intervals. However, the retrieved wind by the IVAP method has smaller errors when the azimuthal interval is larger due to its shortwave filter function. An experiment for an idealized linear wind field is also carried out to demonstrate the effect of the retrieval azimuthal interval on the IVAP method. The results show that a short interval gives the retrieval close to the ＂true＂ wind field with a linear distribution. When increasing the interval, the retrieval is smoothed and can represent only the mean wind field.
文摘The error distributions of the wind fields retrieved from single and dual-Doppler radar observations are given in this paper.The results indicate that the error of dual-Doppler retrieval depends on the position in the scan region of the dual-Doppler radar.The error of single-Doppler retrieval by using velocity azimuth processing(VAP)technique de- pends on the angle between the directions of wind and the radar beam.Generally,the winds retrieved from single Doppl- er radar are close to those retrieved from dual-Doppler radar.But,the error distribution of the single-Doppler retrieval is different from the dual-Doppler retrieval.We simulate the retrievals of single Doppler observation by the use of the output wind data from a 3-D numerical model of severe convection.The comparison of the simulated single-and dual-Doppler retrievals shows that the VAP may be a suitable technique for the operational analysis of mesoscale wind fields.It can also be used as a supplement to wind field retrieval in the field experiment.
文摘全极化数据可以获取比单极化数据更多的目标信息,研究发现C波段交叉极化数据同样可用于海面风速反演。针对RADARSAT-2 Fine Quad模式具有全极化成像的特点,以我国东部海域为研究区,结合同极化数据和交叉极化数据反演海面风速模型,探究各种极化数据的最优风速反演方法。对于同极化数据采用地球物理模型函数(GMF)和极化率模型(PR)组合的方式进行海面风速反演,对交叉极化数据采用C波段交叉极化海面散射模型(C-2PO)进行海面风速反演,反演结果与ERA-Interim风场数据进行比较分析;此外,对Scan SAR模式交叉极化数据的后向散射系数与海面风速的关系进行探索分析。研究结果表明,RADARSAT-2 Fine Quad模式四种极化数据选用合适的模型均可反演出高精度的海面风速,其中VH和HV极化数据的反演结果基本相同,交叉极化数据反演风速效果好于同极化数据,同时,Scan SAR模式交叉极化数据的后向散射系数随海面风速的增大表现出一定的线性变化趋势。全极化模式数据在海面风速反演上表现出比单极化模式数据较为明显的优势,将成为未来海面风速反演的发展方向。
