Dual-Doppler Wind Retrieval from two operational Doppler radars

Yong Kheng Goh, Anthony Holt

University of Essex, U. K.

for

ERAD04, Visby

Acknowledgment

Carpe Diem (EVG-CT-2001-0045) is supported by

European Commissionunder

5th Framework Programme

Thanks toP.P. Alberoni

A.R.P.A – SMRBologna,Italy

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 (44o 47' 30'', 10o 30' 30'') – S. Pietro Capofiume(44o 39' 17'', 11o 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)

Terrain Analysis – Po Valley

Location of Po Valley study region

Data Gridding

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

Calculating wind field

● Three Fundamental Equations:– Radial Velocities (from each radar)

– Mass Continuity

2,1,sin

sincoscoscossinˆ

iVWv

wvu

iitr

iiiii

i

vri

0

vvvtt

t

i

r

W

V

vi

wvu

i

velocity,terminal

velocity,radialnet

,radar at t measuremenlocity Doppler ve

,, vector,(velocity) wind v

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)

iterationth -,, 11 nwDCvwBAu nnnn

ln where,0dz

dw

z

w

y

v

x

u

Case I: stratiform2002 Dec 17 (19:15 hr)

Examples of Retrieval

Radar: GattaticoDate: 2002 Dec 17Elevation: 1.4 degField: ZThreshold: 0 dBZ

Radar: GattaticoDate: 2002 Dec 17Elevation: 1.4 degField: VThreshold: 0 dBZ

tangential

3D representation not very helpful

Wind travelling to the East-North-Eastat the centre

Vel. (m/s)

Vel. (m/s) Thresholded at 15 dBZ

How do we know that the wind field is correct?

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 degwind travel to East-North-East

gat windowspc window

Convergence factor● Typical convergence factor

030.0)1(

,,)(,,

)(,,

)1(,,

max

nkji

nkji

nkji

nkji

ww

ww

Comparison of reconstructed radial velocity field and radar measurement

Gridded data Reconstructed

Gattatico

S. PietroCapofiume

Doppler windmeasurement

“along-track” components

v

r1r2

r12

(obs)

(obs)(obs)

nd,Doppler wi measured

vector, windcalculated

where

1% deviation, relative Typical

0

12

ir

rr

v

vv

(cal)

12(cal)

1212(cal)

1212

v

rv

rrrv

rrrv

Magnitude of “along-track” component

(height = 1.4 km)

12(cal) rv

rr (obs)2

(obs)1 21 rr vv

12(cal)

12(cal)

rv

rv

derived from measurement

from calculation

L1 L2 L3 L4

Case II: Convective Storm2003 Jul 31 (17:30 hr)

Examples of Retrieval

Radar: S. Pietro C.Date: 2003 Jul 31Elevation: 1.4 degField: ZThreshold: 0 dBZ

Radar: S. Pietro C.Date: 2003 Jul 31Elevation: 1.4 degField: VThreshold: 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

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.