Dual-Doppler Wind Retrieval from two operational Doppler radars Yong Kheng Goh, Anthony Holt University of Essex, U. K. for CARPE DIEM, Bologna 29 Nov – 03 Dec 2004

Dual Doppler Wind Retrieval DDWR Procedure: ● Terrain analysis – establish area amenable to dual-Doppler analysis. ● Data gridding – interpolating polar data into Cartesian data. ● Wind field calculation. ● Verifying wind field

SMR Radar

SMR Radars ● Locations:- Po Valley, Italy – Gattatico (44 o 47' 30'', 10 o 30' 30'') – S. Pietro Capofiume(44 o 39' 17'', 11 o 37' 25'') ● C-band/5.5GHz ● Synchronised scan strategy every 15 minutes: 3 volumes – 4 – elevs (4 mins, Dual PRF, Nyquist 49 m/s, 0.5 deg, 1.4 deg, 2.3 deg, 3.2 deg) – 12 – elevs (8 mins, Single PRF, Nyquist 16 m/s) – 5 – elevs (3 mins, Long range, non-Doppler)

Height Profile

Terrain Analysis – Po Valley

Location of Po Valley study region

Data Gridding ● 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 e.g. Doppler Vel.

Calculating wind field ● Three Fundamental Equations: – Radial Velocities (from each radar) – Mass Continuity

Numerical procedures Iterative method: – horizontal components – vertical component Boundary conditions: – zero vertical velocity on ground – zero horizontal velocity gradient on ground (optional: simplify computation w/o loss of accuracy)

Case I: stratiform 2002 Dec 17 (19:15 hr) Examples of Retrieval

Radar: Gattatico Date: 2002 Dec 17 Elevation:1.4 deg Field:Z Threshold:0 dBZ

Radar: Gattatico Date: 2002 Dec 17 Elevation:1.4 deg Field:V Threshold:0 dBZ tangential

3D representation not very helpful

Wind travelling to the East-North-East at the centre Vel. (m/s)

Thresholded at 15 dBZ

Checking the wind field ● comparing with VAD ● calculating convergence factor for vertical wind component ● by reconstructing radial velocity PPI and comparing with original PPI ● comparing “along-track” components How do we know that the wind field is correct?

Velocity-Azimuth Display (VAD) Gattatico: minimum ~ 60 deg wind travel to East-North-East gat window spc window

Convergence factor ● Typical convergence factor

Comparison of reconstructed radial velocity field and radar measurement Gridded data Reconstructed Gattatico S. Pietro Capofiume Doppler wind measurement

“along-track” components v r1r1 r2r2 r 12

Magnitude of “along-track” component (height = 1.4 km) derived from measurement from calculation

L1 L2 L3 L4

Case II: Convective Storm 2003 Jul 31 (17:30 hr) Examples of Retrieval

Radar: S. Pietro C. Date: 2003 Jul 31 Elevation:1.4 deg Field:Z Threshold:0 dBZ

Radar: S. Pietro C. Date: 2003 Jul 31 Elevation:1.4 deg Field:V Threshold:0 dBZ

VAD is not very helpful in this case Seen from S. Pietro C. wind changes direction reflectivity Doppler vel. gat window spc window

Vel. (m/s) Thresholded at 0 dBZ

Triple-Doppler Analysis ● We have obtained data from Teolo. ● In theory, 3 radars should give 3 wind components, but... – 3 radars scanning at low elevation -> large error in vertical component. ● However, 3 radars allow us to do – Validate dual-Doppler technique – Improve horizontal wind extraction using along- track method.

A.D.Fontana (TEO): we are working with EEC to solve this problem, that seems to correlated to a bad functioning of unfolding processing of velocity.... Teolo data (positive vel. = away from radar) Gattatico (positive vel. = toward radar) Problem: Radial wind PPI do not agree between Teolo and Gattatico (or SPC).

Reconstruction from DDWR Observation Reconstruction (GAT + SPC) Gattatico S. P. CapofiumeTeolo

Wind fields from DDWR GAT+SPC SPC + TEO wind field wind speed

Along-track comparison DDWR using GAT + SPC Along-track using GAT+SPC+TEO wind direction wind speed

Reconstruction of 'AT' wind Gattatico S.P.Capofiume Teolo Observation Reconstruction

Summary ● Dual Doppler wind retrieval with operational weather radars. ● Useful and reliable in both uniform windfields and convective storms. ● Expect to extend the study to triple Doppler Analysis when suitable datasets from Monte Grande are available. ● Will be comparing results with output from NWP analysis by our Carpe Diem partners.