2006/4/13 中尺度氣象學 ii real-time retrieval of wind from aliased velocities measured by...
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2006/4/13 中尺度氣象學 II Introduction Doppler radar measurements provide the radial wind within an unambiguous interval due to the limited value of the sampling frequency ( pulse repetition frequency )TRANSCRIPT
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2006/4/13 II中尺度氣象學
Real-Time Retrieval of Wind from Aliased Velocities Measured by Doppler radars
Hochin Chang
Tarary, P. and, G. Scialom, 2001: Real-time retrieval of the wind from aliased velocities measured by Doppler radars. J.Tech, 18, 875-881
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2006/4/13 II中尺度氣象學
Outline
• Introduction– Aliasing– Nyquist velocity
• Method– VAD– Modified VAD
• Result• An operational application of the algorithm• Concluding Remarks
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2006/4/13 II中尺度氣象學
Introduction
• Doppler radar measurements provide the radial wind within an unambiguous interval due to the limited value of the sampling frequency ( pulse repetition frequency )
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2006/4/13 II中尺度氣象學
Introduction
• unambiguous range and unambiguous velocity
2
4
8
sa
as
a a
cTr
v Tcr v
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2006/4/13 II中尺度氣象學
Introduction
• There are two directions to overcome this difficulty, that is , to dealias velocity measurements.– It is possible to increase nyquist velocity by alte
rnatively using to different PRF.– To build algorithms in order to put a posteriori
Vr in the right interval by continuity on the measurements.
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2006/4/13 II中尺度氣象學
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2006/4/13 II中尺度氣象學
Method
• VAD (Velocity Azimuth Display)
00 0
00 0
u uu u x yx y
v vv v x yx y
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2006/4/13 II中尺度氣象學
Browning and Wexler, 1968
0 1 1
2 2
cos sincos 2 sin 2
rV a a ba b
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2006/4/13 II中尺度氣象學
cos sincos cos
x ry r
T
x y zVr u v w vr r r
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2006/4/13 II中尺度氣象學
00 0
00 0
u uu u x yx y
v vv v x yx y
cos sincos cos
x ry r
Tx y zVr u v w vr r r
0 1 1
2 2
cos sincos 2 sin 2
rV a a ba b
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2006/4/13 II中尺度氣象學
0 1 1
2 2
cos sincos 2 sin 2
rV a a ba b
2
00 0
1 0
1 0
2
20 0
2
200
cossin2
cossin
cos2
cos2
Tr u va w v
x y
a vb u
r u vax y
r u vby x
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2006/4/13 II中尺度氣象學摘自 Deng (1985)
時間
高度
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2006/4/13 II中尺度氣象學
Modify VAD
• In the classical VAD analysis, a major requirement is that radial velocities have to be corrected for alising.
• This is the reason why to propose a modified version of the VAD analysis that can be applied to raw, that is , possibly aliased data.
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2006/4/13 II中尺度氣象學
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2006/4/13 II中尺度氣象學
Modify VAD
1 1
1 0
1 0
cos sin
coscos
rV a b
a ub v
2
00 0
1 0
1 0
cossin2
cossin
Tr u va w v
x y
a vb u
0 1 1 2 2cos sin cos 2 sin 2rV a a b a b
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2006/4/13 II中尺度氣象學
Modify VAD
0 0
2
0 021
,
1 cos cos sin minqN
rq q q
q q
J u v
V u v
1 1
1 0
1 0
cos sin
coscos
rV a b
a ub v
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2006/4/13 II中尺度氣象學
Modify VAD
• Select all data Mq of the sweep with the same gate number as M and with azimuth between θq+Δθ,and θ-Δθ,where Δθ is the half width of the angular search window.
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2006/4/13 II中尺度氣象學
Modify VAD
• Reject all data Mq with radial velocity outside the Vr-ΔVr and Vr+ΔVr where ΔVr is the nyquist velocity.
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2006/4/13 II中尺度氣象學
Modify VAD
• If the number of remaining data is sufficient, fit the resulting dataset to a straight line ( Vr =αθ+β)
• The slope is rV
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2006/4/13 II中尺度氣象學
Modify VAD
• Estimate the quality of the fit σ by calculating the dispersion of the data around the straight line
0 0
2
0 021
,
1 cos cos sin minqN
rq q q
q q
J u v
V u v
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2006/4/13 II中尺度氣象學
Results
• 0300 UTC 20 Sep 1999 • Mesoscale Alpine Program IOP#2• French C-band Doppler radar Ronsard • San Pietro Mosezzo in the southern forela
nd of the Alps.
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2006/4/13 II中尺度氣象學
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2006/4/13 II中尺度氣象學
Vertical profiles of the horizontal wind direction (a) and intensity (b).Diamonds mean classical VAD analysis to previously dealiased radial velocities and stars mean application of the method to raw radar data.
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2006/4/13 II中尺度氣象學
An operational application of the algorithm
• Implementation– A similar VAD algorithm is operational at the th
ree radar sites of MeteoSwiss since August 1998
– It relies on exactly the same idea, but the calculation of derivative is somewhat simpler.
• Quality control
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2006/4/13 II中尺度氣象學
Concluding remarks
• A method based upon a modified VAD analysis that provides an estimate of the wind profile directly from the raw( possibly aliased) radial velocities.
• The basic idea of the method is to estimate empirically the local azimuthal derivative of the radial velocity at each measurement and to perform a VAD analysis with that parameter instead of the radial velocity.
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2006/4/13 II中尺度氣象學
Concluding remarks
• An illustration of the performance of the method has been carried out on a real case of the MAP Experiment ( 0300 UTC 20 Sep 1999) using data from the French C-band Ronsard radar.
• Difference in terms of wind speed and direction between the modified and classical VAD analysis are smaller than 10%.
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2006/4/13 II中尺度氣象學
Concluding remarks
• A similar method has been implemented on Swiss C-band radar network in August 1998.
The End