AN INTEGRATIING VAP METHOD FOR SINGLE-DOPPLER RADAR WIND RETRIEVAL
Liang Xudong 1,2Wang Bin1
1 LASG,Institute of Atmospheric Physics,CAS, Beijing 100029
2 Laboratory of Typhoon Forecast Technique, Shanghai Typhoon Institute, CMA, Shanghai 200030
Abstract
Some widely-used methods have relative conceptual and practical simplicity.Among them, the simplest one is the VAD (velocity-azimuth display) technique which is based on the assumption that the horizontal wind field varies linearly in space. Another method is VAP (velocity-azimuth process) which assume that the horizontal wind vectors of two neighboring azimuths are uniform. The advantage of VAD is that it isn't affected by small perturbations of the radial wind along the azimuth, to which VAP method is very sensitive. Nevertheless, the VAD method has a very poor spatial resolution compared with the VAP method.
However, as shown in this paper, the relationships used to retrieve the wind fields in the VAD and VAP methods are all special cases of the retrieval function based on the azimuthal uniformwind assumption for a given azimuthal interval ［θ1，θ2］. When using this retrieval function to retrieve wind fields, the azimuthal interval used in retrieval can be varied according to the requirement of smoothness or resolution. The larger/smaller is the azimuthal interval, the coarser/finer the horizontal resolution of retrieved wind field, and the more insensitive/sensitive is the retrieval method to small scale perturbations. Because the whole information within the azimuthal interval ［θ1，θ2］, instead of only the information at two terminal points, i.e. azimuthsθ1， andθ2, are used to retrieve the wind fields, this method is referred to as the integrating VAP (IVAP) method, wherein horizontal wind field is retrieved using the Doppler velocity over the circular arc delimited by the given azimuthal interval times the scan radius. It is different from the VAP method wherein only the velocities at two terminal points of the given azimuthal interval are used. Therefore, the IVAP method has a filtering function, and the smoothness of the retrieved wind field can be obtained by adjusting the azimuthal interval. The filter which is required in the preprocess of VAP method is no longer necessary in the IVAP method. When the retrieval azimuthal interval is as large as a whole circle, then the IVAD method becomes the VAD one, and on the other hand, if only two neighboring azimuthal information is used, then the IVAP method becomes the VAP one.
The frequency response function of IVAP indicates that the IVAP method can filter out short waves, and a larger azimuthal interval used leads to stronger filtering ability, and then a smoother retrieved wind field.
The shortwave filter function of the IVAP method is demonstrated by an ideal experiment wherein the radar observations are artificially constructed with a uniform flow superposed with random disturbances. The VAP and IVAP methods are used in wind retrieval, respectively. The VAP and IVAP methods 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 error 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 demonstr
ate the effect of retrieval azimuthal interval in the IVAP method. The results show that the shorter interval gives closer retrieval to the real wind field, and the larger interval gives the smoother retrieval which is closer to the mean wind field.
Key words: Doppler radar, IVAP, Wind retrieval.