Dual-Doppler Wind Retrieval from two operational Doppler radars Yong Kheng Goh, Anthony Holt...
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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.