1. Analysis of Three Dimensional Wind Fields from Two Operational Radars Yong Kheng Goh* and Anthony Holt *email: kygoh@essex.ac.uk Doppler radar and wind-field retrieval Accurate wind-field information is important for numerical weather prediction. Whilst methods such as VAD or variational methods can extract useful wind-field coverage, it is difficult to recover three- dimensional information from a single Doppler radar. Although methods for using multiple Doppler radar to study wind fields have long been proposed, very few commercial radar operators adopt these methods which require the use of specific scanning strategies to allow the extraction of wind information. Locations/Terrain A collaborative study has been setup in Po Valley, Italy to use the existing operational radars for wind-field extraction. The radars are the two SMR radars (C-band Polarimetric) at San Pietro Capofiume and Gattatico. The height of the terrain around the radars determines the lowest elevation angle on which two radars can be operated. The Po Valley area is very flat with the plain to the north of the two SMR radars varying in altitude only from 10 m to 30 m above sea level. This enables us to use the lowest elevation scan (0.5 o ) data. We chose a square area of 30 x 30 km 2 for wind retrieval. The area has the advantage of being almost equal distant from both the GAT and SPC radars: thus both radars are looking at almost the same height. Methodology 1. Data Gridding 2. Constructing wind fields 3. Validating wind fields. Data Gridding a Radar data Polar data 4 elevation angles (deg) 0.5, 1.4, 2.3, 3.2 ray resolutions 0.25km x 440 bins, beam width: 0.9 deg Gridded data Cartesian data 4 layer altitudes (km) ~ 0.5, 1.4, 2.3, 3.2 horizontal resolutions cell spacing: 0.5 km, no. of cells: 60 x 60 Calculating Wind field Three fundamental equations: Radial velocities (one for each radar) Mass continuity These equations are solved by Iterative method: Boundary conditions: - zero vertical velocity on ground - zero horizontal velocity gradient on ground (optional: simplifies computation without significant loss of accuracy) Examples Case I: (2002 Dec 17, 19:15 hr, Stratiform) Gattatico: minimum ~ 60 deg wind travel to East-North-East gat window spc window The Doppler wind PPI shows the wind field is stratiform with inversion. Wind travelling towards the ENE at high altitudes and towards the WSW at low altitudes. Horizontal cross section of the wind field. The wind direction is towards the ENE at the centre of the region, in agreement with the PPI. Validation Several techniques have been use to check the extracted wind fields. VAD VAD diagram with range equal to the distance from radar to the centre of the retrieval area shows that the wind direction is towards the ENE. The extracted wind-field is used to reconstruct the PPI plot, from the perspective of the radar (right column). This is then compared with the original PPI plot (left column) and the PPI plot from the gridded data (middle column). PPI comparison Doppler Wind MeasurementGridded DataReconstructed GAT SPC Acknowledgement -This project is part of the European Carpe Diem project under contract no EVG1-CT-2001-0045. Along-track (AT) component comparison The AT (parallel to the line joining the radars) component of the wind vector can be calculated by using only the radar data (a) or by using only the extracted wind-field (b). Fig. (c) shows that the normalised differences of the AT components calculated from the two different methods are negligible. Case II: (2003 Jul 31, 17:30 hr, Convective) Note the hook-shape feature in the reflectivity PPI. This is usually an indication of a convective storm. Seen from S. Pietro C. wind changes direction gat window spc window The extracted wind-field shows the wind vortex, and the VAD plot shows that the abrupt change in the wind directions. Conclusions Operational radars can be used to perform Dual Doppler wind analysis and to provide accurate three-dimensional wind information.