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Keywords:

  • Doppler radar;
  • wind field;
  • aliasing;
  • range folding;
  • discontinuous echo

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Algorithm description
  5. 3. Algorithm performance
  6. 4. Summary and prospects
  7. Acknowledgements
  8. References

A new automated velocity dealiasing method based on zero isodop searching has been developed primarily for linear wind fields. Its essence is to partition the radial velocity field obtained with a Doppler radar into two distinct regions of opposite velocity signs by two zero isodops. Zero isodops are searched point by point from the radar origin to the maximum detection range, and the accepted sign of each region separated by the zero isodops is determined. After that, the velocity sign at each gate is compared with the accepted sign of the region wherein the gate is. If they are consistent, the velocity is true; otherwise it is dealiased. Dealiasing results in real cases indicate that the new algorithm is practicable and effective, especially on aliasing lack of references affected by discontinuous echo or range folding, which is difficult for traditional methods of space continuity checking. Copyright © 2010 Royal Meteorological Society


1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Algorithm description
  5. 3. Algorithm performance
  6. 4. Summary and prospects
  7. Acknowledgements
  8. References

The velocity information received by Doppler weather radars has been widely used in weather analysis, wind field retrieval, data assimilation, etc. However, due to the restriction of pulse repetition frequency in sampling, velocity aliasing would appear in some severe weather and have a bad effect on data analysis and application. According to the Nyquist sampling theorem, the maximum unambiguous velocity (i.e. Nyquist velocity) is given by

  • equation image(1)

where λ is the Doppler radar wavelength and f is the pulse repetition frequency. If the true radial velocity is outside this range, the measured velocity will be aliased into

  • equation image(2)

where Vm and Vt are the measured and true velocity respectively, and integer n (positive or negative) is termed the Nyquist number which represents aliasing intervals the measured velocity deviates from the true value. The purpose of velocity dealiasing is to determine the correct Nyquist number and restore the measured velocity to the true value for each data gate.

There have been mainly two approaches to deal with the velocity aliasing problem. One focuses on the radar system to prevent the aliasing from occurring. For example, radar wavelength and pulse repetition frequency can be increased to raise the Nyquist velocity according to Eq. (1). Furthermore, aliasing can be reduced significantly by alternately using two different pulse repetition frequencies (Dazhang et al., 1984; Hildebrand et al., 1996). However, constrained by hardware techniques, they are not always compatible with the operational use of existing radar system, and may involve significant hardware costs. As a result, more studies refer to the ‘software’ approach that builds algorithms to achieve the true velocity from the aliasing on the basis of wind velocity field analysis.

Most software techniques are based on the assumption that the wind field is temporally and spatially continuous so aliasing is identified as abrupt change. One-dimensional methods along radials (Ray and Ziegler, 1977; Bargen and Brown, 1980), two-dimensional methods along radials and azimuths (Merritt, 1984; Eilts and Smith, 1990), three-dimensional methods along radials, azimuths and elevations (Bergen and Albers, 1988), and four-dimensional methods along radials, azimuths, elevations and time (James and Houze, 2001), all deal with the aliasing discontinuities of velocity fields. However, these methods need starting points as references to search the irregular gradients. For isolated areas, additional information provided by radiosonde, wind field model, and Vertical Azimuth Display (VAD) or modified VAD (MVAD) wind profile is required to serve as independent references (Hennington, 1981; Merritt, 1984; Eilts and Smith, 1990; Tabary et al., 2001). However, these auxiliary data can not usually be acquired or be available in operation due to their sparseness or mismatch in space and time compared with the radar data. Moreover, noise removal is usually needed in these techniques to correctly discriminate large gradients around aliasing boundaries from those related to noise and clutter, which might smooth real aliasing discontinuity and damage original information of the wind field.

In fact, zero velocity isodops are very important for velocity field analysis when a Plan Position Indicator (PPI) display is given. Because the radar origin is certainly on zero isodops, when it is taken as a start point, there can be found two zero isodops extending to the maximum range with approximately opposite orientations. For a linear wind field as in most cases, when zero isodops are recognized, they separate a non-aliasing field into two regions. The sign of radial velocities in either region should be unique—positive or negative—while opposite to the sign in the other region. This simple concept can be introduced in dealiasing through zero isodop searching and velocity sign comparison. As a result, compared with the typical methods focusing on spatial continuity checking, a new automated dealiasing algorithm based on searching for zero isodop is proposed in this article. The new algorithm is described in detail in section 2. In section 3, the algorithm is applied to real cases. A summary follows in section 4.

2. Algorithm description

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Algorithm description
  5. 3. Algorithm performance
  6. 4. Summary and prospects
  7. Acknowledgements
  8. References

2.1. General description

The new algorithm is implemented on data fields at each conical scan which is dealiased separately. General steps are shown in Figure 1, and zero isodop searching is essential to the scheme. The zero-velocity region on a PPI velocity field could be a large connected area constituted of non-effective velocities gates (zero velocity, non-echo, range folding, etc). Two continuous zero isodops from the radar origin could be drawn through the zero-velocity region, partitioning the positive-velocity region and the negative-velocity region. After zero isodops and signs of the two non-zero velocity regions are determined, aliasing can be solved favourably through sign comparison of each data gate with the region wherein the gate is. There is no need to be concerned with other complicated circumstances.

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Figure 1. Algorithm flowchart for a PPI display.

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A linear and single-fold aliasing wind field is better suited to the new algorithm because in this situation the velocity field can be accurately divided into two distinct regions with opposite signs by zero isodops, and dealiasing can be simply implemented through sign comparison. Nevertheless, it may be more important that when zero isodops are discriminated, the algorithm also yields a basic wind field structure no matter what type the wind field is (linear or non-linear flow, single-fold or multiple-fold aliasing). That is a necessary complement to dealiasing methods based on space continuity checking, and is helpful for automated three-dimensional wind field analysis.

2.2. Zero isodop searching

If zero-velocity and non-zero-velocity regions in the velocity field are treated respectively as paths and walls, zero isodop searching can be referred to a maze-solving algorithm. However, the zero-velocity region has irregular widths, and a zero isodop can not intersect with itself. Additionally, zero isodops have a particular feature: the two regions separated by the zero isodops have opposite signs. Therefore, classic maze-solving algorithms such as turning right whenever the traveller reaches a junction can not be straightforwardly employed. Improvements and specific rules have to be produced for zero isodop searching. To make the algorithm easier to understand, a table of definitions is provided (Table I) before the following description.

Table I. Parameter definitions and their explanations.
DefinitionExplanation
search pointThe current point along the path from which the next point is to be determined
accepted signThe sign of a non-zero velocity region if not aliased deduced by the algorithm
detection directionDirection from the search point to probe the surrounding non-zero velocity gates
check directionDirection from the search point to check the sign of non-zero velocity gates on the left or right of the detection direction
check radiusA radius within which check directions are applied
driving directionDirection along which the search point moves to the next point

When the search point is at the radar origin from which it sets out, two original driving directions and accepted signs of the two non-zero velocity regions separated by two zero isodops should be determined. Firstly, all radials are arranged in a sequence according to the descending order of the distances from the radar origin to the first non-zero gates in radials. Secondly, signs of the first non-zero gates in radials within a window either side of each radial in that sequence are checked. If the signs are the same on either side and opposite to the other, the radial is chosen as the first original driving direction for the first zero isodop to be searched; otherwise, the next radial in the sequence is taken for checking until the first original driving direction is found. The sign on either side of the first original driving direction serves as the accepted sign of the region on the corresponding side of the first zero isodop. After that, the second original driving direction as well as the accepted signs is similarly singled out from the radials that have not been checked in the sequence for the second zero isodop to be searched, with additional requirement that the sign on either side of it should be opposite to the first original driving direction. Afterwards, the search point at the radar origin moves to the first gate along an original driving direction and the gate is recorded as the first point of the zero isodop. Figure 2 is a schematic diagram for selecting original driving directions, and Table II gives the analysing results. Although radial L ranks first in the sequence, the first non-zero gates in radials within the window on both sides of L are all positive, so L is excluded and then C comes to be checked. Because non-zero gates are positive in radials D, E and F on the left side of C while negative in B, A and X on its right side, C is selected as the first original driving direction. Consequently, the search point moves to C1 to search for the first zero isodop. Meanwhile, positive serves as the accepted sign of the non-zero velocity region on the left side of the first zero isodop, and negative on the right side. Afterwards, Q is selected as the second original driving direction because the first non-zero gates are all negative in radials R, S and T on its left side while they are all positive in P, O and N on its right side, and either is opposite to the sign on the corresponding side of the first original driving direction. The search point moves to Q1 to search for the second zero isodop. Negative serves as the accepted sign on the left side of the second zero isodop and positive on the right side.

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Figure 2. A sketch of selecting original driving directions when the search point is at the radar origin Or. Radials and gates in each radial are labelled with capital letters and Arabic numbers. The check window is set to 3 radials either side of a radial.

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Table II. Analysing results from Figure 2.
Radial sequenceDistance from the search pointCheck radials within theFirst non-zero gates and
 to the first non-zero gate in the radialwindow either side of thethe signs in check radials
  radial in the sequence 
L5 (L6)leftMM1 (+)
   NN1 (+)
   OO1 (+)
  rightKK3 (+)
   JJ1 (+)
   II1 (+)
C4 (C5)leftDD1 (+)
   EE1 (+)
   FF1 (+)
  rightBB1 (−)
   AA1 (−)
   XX1 (−)
W3 (W4)leftXX1 (−)
   AA1 (−)
   BB1 (−)
  rightVV1 (−)
   UU1 (−)
   TT1 (−)
R3 (R4)leftSS2 (−)
   TT1 (−)
   UU1 (−)
  rightQQ3 (−)
   PP1 (+)
   OO1 (+)
K2 (K3)leftLL6 (+)
   MM1 (+)
   NN1 (+)
  rightJJ1 (+)
   II1 (+)
   HH1 (+)
Q2 (Q3)leftRR4 (−)
   SS2 (−)
   TT1 (−)
  rightPP1 (+)
   OO1 (+)
   NN1 (+)
S1 (S2)leftTT1 (−)
   UU1 (−)
   VV1 (−)
  rightRR4 (−)
   QQ3 (−)
   PP1 (+)

When the search point arrives at a zero gate away from the radar origin to search for a zero isodop, a driving direction should be determined by the following means. Firstly, detection directions (e.g. 1° interval) from the search point are probed within a window (e.g. 90°) either side of the previously determined driving direction. They are arranged into a sequence according to the descending order of the distances from the search point to the non-zero gates they probe. The distances can be calculated with elementary geometric knowledge. If some detection directions reach straight to the maximum range without probing any non-zero gate, they are arranged according to the ascending order of the distances from the search point to the maximum range and placed in the sequence prior to other detection directions probing non-zero gates. Secondly, check directions (e.g. 1° interval) from the search point are probed within a window (e.g. 90°) either side of each detection direction in the sequence. If signs of the non-zero gates probed by check directions either side of a detection direction are all consistent with the accepted sign on the corresponding side of the zero isodop, the detection direction is chosen as the next driving direction; otherwise, the next detection direction in the sequence is taken for checking until the driving direction is found. If a check direction reaches the maximum range without probing any non-zero gate, the last zero gate along the check direction is regarded as the probed gate and its sign (0) is treated as consistent with the accepted sign. The requirement is called the sign check rule, and it makes the search point keep the correct orientation of its path, yielding a zero isodop. Afterwards, the search point moves to the next gate along the driving direction and the gate is recorded as a point of the zero isodop. Rounds of detection and checking like this will be repeated until the search point arrives at the maximum range, and the path traversed by the search point is just a zero isodop. Figure 3(a), Table III and Table IV show the general idea of choosing driving directions for a zero isodop. When the search point is at gate I4, all detection directions probe non-zero gates. Direction c ranks first in the sequence, but non-zero gates probed by check directions b, a and l on the left side of c are negative which is not consistent with the accepted sign (+) on the left side of the zero isodop, therefore c is not the driving direction. Then b and l come to be checked. Similar to c, non-zero gates probed by check direction a and l on the left side of b are negative, so b is also excluded. Signs of non-zero gates probed by all check directions on either side of l are consistent with those accepted, agreeing with the sign check rule, and l is thus chosen as the driving direction. Afterwards, the search point moves to J4, the next gate along l. When the search point is at gate QW, detection directions l, k, a and b probe the maximum range, and thus they are checked prior to other detection directions. Direction l is chosen as the driving direction since the distance from the search point to the maximum range along l is the shortest, and the sign check rule is obeyed at the same time. Afterwards, the search point moves to QV.

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Figure 3. A sketch of choosing driving directions when the search point is at a zero gate. Velocities at gates with symbol ‘±’ at top right corners are aliased. Radials and gates in each radial are labelled with capital letters and Arabic numbers while detection and check directions are labelled with lower-case letters. 30° interval detection directions are probed within a 90° window either side of the previously determined driving direction a, and 30° interval check directions are probed within a 90° window either side of the detection direction. It is assumed that the accepted sign is positive on the left side of the zero isodop and negative on the right side. (a) The search point is at gate I4 or QW; (b) the search point is at gate M8 or QV. A small check radius (the circle around the search point) is additionally provided for check directions when the search point is at M8.

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Table III. Analysing results when the search point is at gate I4 in Figure 3(a).
DetectionDistance from the search point to the non-zeroCheck directions within theNon-zero gates and
directiongate probed by the detection directionwindow either side of thethe signs probed by
sequence detection directionthe check directions
c5.236 (B8)leftbG7 (−)
   aI7 (−)
   lK7 (−)
  rightdF4 (−)
   eF4 (−)
   fG3 (−)
b2.731 (G7)leftaI7 (−)
   lK7 (−)
   kK4 (+)
  rightcB8 (−)
   dF4 (−)
   eF4 (−)
l2.731 (K7)leftkK4 (+)
   jK4 (+)
   iK4 (+)
  rightaI7 (−)
   bG7 (−)
   cB8 (−)
a2.500 (I7)leftlK7 (−)
   kK4 (+)
   jK4 (+)
  rightbG7 (−)
   cB8 (−)
   dF4 (−)
d0.997 (F4)leftcB8 (−)
   bG7 (−)
   aI7 (−)
  righteF4 (−)
   fG3 (−)
   gI1 (0)
k0.740 (K4)leftjK4 (+)
   iK4 (+)
   hJ3 (+)
  rightlK7 (−)
   aI7 (−)
   bG7 (−)
j0.592 (K4)leftiK4 (+)
   hJ3 (+)
   gI1 (0)
  rightkK4 (+)
   lK7 (−)
   aI7 (−)
Table IV. Analysing results when the search point is at gate QW in Figure 3(a).
DetectionDistance from the search point to theCheck directions within theGates and the signs probed
directionmaximum range or non-zero gate probedwindow either side of theby the check directions
sequenceby the detection directiondetection direction 
l4.513 (maximum range)leftkRS (0)
   jRW (+)
   iRW (+)
  rightaOS (0)
   bMS (0)
   cOW (−)
k4.837 (maximum range)leftjRW (+)
   iRW (+)
   hRW (+)
  rightlQS (0)
   aOS (0)
   bMS (0)
a5.191 (maximum range)leftlQS (0)
   kRS (0)
   jRW (+)
  rightbMS (0)
   cOW (−)
   dPX (−)
b7.497 (maximum range)leftaOS (0)
   lQS (0)
   kRS (0)
  rightcOW (−)
   dPX (−)
   ePX (−)
c1.764 (OW)leftbMS (0)
   aOS (0)
   lQS (0)
  rightdPX (−)
   ePX (−)
   fQZ (+)
d0.909 (PX)leftcOW (−)
   bMS (0)
   aOS (0)
  rightePX (−)
   fQZ (+)
   gRX (+)
j0.742 (RW)leftiRW (+)
   hRW (+)
   gRX (+)
  rightkRS (0)
   lQS (0)
   aOS (0)

Aiming to make the search point arrive at the maximum range as soon as possible, if all detection directions reach straight to the maximum range without probing any non-zero gate, the intersection of the previous driving direction with the maximum range is taken immediately as the last point of the zero isodop. It is termed a jump point. For example, when the search point is at gate QV in Figure 3(b), all detection directions probe the maximum range. Therefore, QS is a jump point and serves as the next and last point of the zero isodop.

Given that the zero-velocity region may be very sophisticated, or aliased velocity gates are exposed along check directions, the sign check rule may not be obeyed for all detection directions. To solve this problem, a check radius is adopted and checking is executed within that check radius. If a non-zero gate probed by a check direction is within the check radius, it is retained; otherwise the zero gate met by the radius along the check direction is regarded as the probed gate, and its sign (0) is treated as consistent with the accepted sign. In fact, the check radius for the first round of checking can be regarded as infinity. If all of the detection directions in the sequence can not obey the sign check rule within a larger radius, a smaller radius is given for a next round of checking until the driving direction is found. A rough draft and analysing results are given in Figure 3(b) and Table V. When the search point is at gate M8, neither of the detection directions from l to j in the sequence can obey the sign check rule (see Table V), and thus a small check radius (the circle around M8) is given for another round of checking. Consequently, l is chosen as the driving direction and the search point moves to N8.

Table V. Analysing results when the search point is at gate M8 in Figure 3(b).
DetectionDistance from the searchCheck directions within theGates and the signsGates and the signs probed
directionpoint to the non-zero gatewindow either side of theprobed by theby the check directions
sequenceprobed by the detectiondetection directioncheck directionswithin the small
 direction   check radius
l4.616 (PX)leftkQ9 (+)Q9 (+)
   jP8 (+)P8 (+)
   iO7 (+)O7 (+)
  rightaNY (−)NY (−)
   bLX (+)LY (0)
   cJY (+)KY (0)
c3.984 (JY)leftbLX (+)LY (0)
   aNY (−)NY (−)
   lPX (−)PZ (0)
  rightdJ9 (−)J9 (−)
   eK8 (−)K8 (−)
   fL7 (−)L7 (−)
b3.538 (LX)leftaNY (−)NY (−)
   lPX (−)PZ (0)
   kQ9 (+)Q9 (+)
  rightcJY (+)KY (0)
   dJ9 (−)J9 (−)
   eK8 (−)K8 (−)
k2.882 (Q9)leftjP8 (+)P8 (+)
   iO7 (+)O7 (+)
   hN5 (+)N5 (+)
  rightlPX (−)PZ (0)
   aNY (−)NY (−)
   bLX (+)LY (0)
a2.622 (NY)leftlPX (−)PZ (0)
   kQ9 (+)Q9 (+)
   jP8 (+)P8 (+)
  rightbLX (+)LY (0)
   cJY (+)KY (0)
   dJ9 (−)J9 (−)
d2.181 (J9)leftcJY (+)KY (0)
   bLX (+)LY (0)
   aNY (−)NY (−)
  righteK8 (−)K8 (−)
   fL7 (−)L7 (−)
   gL7 (−)L7 (−)
j1.856 (P8)leftiO7 (+)O7 (+)
   hN5 (+)N5 (+)
   gL7 (−)L7 (−)
  rightkQ9 (+)Q9 (+)
   lPX (−)PZ (0)
   aNY (−)NY (−)

2.3. Preprocessing

Considering that real wind fields are contaminated by noise and clutter, bringing much difficulty to zero isodop searching, PPI velocity data are preprocessed to facilitate the search for zero isodops. The running mean velocity averaged over every 10 gates by 5 radials area is calculated for effective velocities, and then divided by the Nyquist velocity for normalization. Normalized velocities below some threshold like 0.1 as well as non-effective velocities like non-echo and range folding are set to zero with the purpose of highlighting the zero-velocity region. As a result, when sign comparison is implemented for the original velocity field dealiasing, small velocities adjoining zero isodops whose signs are opposite to the accepted are possibly not aliased, therefore dealiasing is executed only for those whose normalized values are not very small (e.g. not smaller than the threshold). This is reasonable since small velocities are unlikely to be aliased on a single-fold velocity field. The threshold can be properly chosen depending on the velocity field. If zero isodops are very difficult to discriminate due to nearby disordered velocities, the threshold should be large; otherwise it can be small.

3. Algorithm performance

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Algorithm description
  5. 3. Algorithm performance
  6. 4. Summary and prospects
  7. Acknowledgements
  8. References

In order to interpret further the algorithm and verify its validity, the algorithm is applied to real cases covering various weather situations (Table VI). In general, searching zero isodops for a PPI tilt would be much easier as elevation increases, therefore dealiasing results at low elevations are presented in this section. The current velocity dealiasing algorithm (VDA) in WSR-88D Radar Product Generator (RPG) performs poorly in these cases due to discontinuous echo and range folding, giving low scores (see Table VI) according to the score penalties for different types of dealiasing errors (Witt et al., 2009). By contrast, the new algorithm gives very satisfying results. Two of the cases are illustrated in Figures 4 and 5. Given the comparison with VDA results, they are drawn with the same colour scale and the same scale dynamic range used in RPG. To avoid confusion, zero isodop 1 is defined as originating at the radar origin proceeding southwards and zero isodop 2 northwards. The region on the left of zero isodop 1 and right of zero isodop 2 is defined as region 1, and the other region is defined as region 2.

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Figure 4. Radial velocity field measured by Xiamen Doppler radar at an elevation angle of 0.5° at 0900 UTC on 17 May 2006. (a) Preprocessed velocity field and two zero isodops. (b) Original non- dealiased velocity field. (c) Dealiased velocity field by the algorithm; aliasing is nicely eliminated. (d) Dealiasing result given by the VDA on WSR-88D with default adaptable parameter settings; remarkable un-dealiased and wrongly dealiased areas are marked by three white circles. This figure is available in colour online at wileyonlinelibrary.com/journal/qj

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Figure 5. Radial velocity field measured by Nanjing Doppler radar at an elevation angle of 1.5° at 0003 UTC on 21 March 2009. (a) Preprocessed velocity field and two zero isodops. (b) Original non- dealiased velocity field; aliasing area in the east is marked by a white rectangle. (c) Dealiased velocity field by the algorithm; aliasing is nicely eliminated especially in the white rectangle. (d) Dealiasing result given by the VDA on WSR-88D with default adaptable parameter settings; un-dealiased and wrongly dealiased areas can be seen in the white rectangle. (e) Local enlarged view of the rectangle area in (d). This figure is available in colour online at wileyonlinelibrary.com/journal/qj

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Table VI. List of test cases and the performance scores of the current VDA in WSR-88D RPG. All times are in UTC.
Radar siteDateTimeElevationWeather situationVDA performance scores
Xiamen17 May 200609000.5Hurricane56
Xiamen28 Jul 200804050.5Hurricane24
Longyan22 Mar 200507260.5Squall line66
Nanjing16 May 200706221.5Thunderstorms80
Nanjing21 Mar 200900031.5Front70

For the case on 17 May 2006, zero isodop 1 proceeding southwestwards and zero isodop 2 proceeding northeastwards are paths of the search point taking 681 and 726 paces from the radar origin to the maximum range respectively, either of which has a jump point at the end. The accepted sign is negative for region 1 and positive for region 2 (see Figure 4(a)). For the case on 21 March 2009, zero isodop 1 and zero isodop 2 are constituted by 738 points and 423 points respectively, and both of them have jump points. The accepted sign is positive for region 1 and negative for region 2 (see Figure 5(a)). Afterwards, the zero isodops and the accepted signs are implemented for the original velocity fields before preprocessing (Figures 4(b) and 5(b)). If the sign of a velocity whose normalized value is not smaller than the preprocessing threshold is opposite to the accepted sign of the region, it is considered single-fold aliasing and restored to the true value (Figures 4(c) and 5(c)).

It is noted in Figures 4(b) and 5(b) that due to disconnected echo and range folding, many aliasing areas have a lack of references in radials or azimuths to compute velocity gradients, and are difficult to dealias by continuity checking techniques. Researchers have already pointed out this confusing problem (Bargen and Brown, 1980; Bergen and Albers, 1988; Desrochers, 1989; Jing and Wiener, 1993; Gao and Droegemeier, 2004). That is also the very reason that the VDA on WSR-88D gives disappointing results with remarkable un-dealiased and wrongly dealiased areas for these cases (Table VI; Figures 4(d) and 5(d)). By contrast, the new algorithm overcomes this problem and provides very satisfying results.

4. Summary and prospects

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Algorithm description
  5. 3. Algorithm performance
  6. 4. Summary and prospects
  7. Acknowledgements
  8. References

Previous algorithms typically exploit spatial continuity of the distributed weather echoes, and use abrupt changes or gradient maxima in velocity fields as an indication of aliasing. However, real data are often impacted by noise, clutter, isolated echo and range folding so that only a fraction of the field is well sampled, and aliasing is usually difficult to discriminate by continuity checking. By contrast, a new automated velocity dealiasing method is presented in this article. The new proposed algorithm gets around this troubling problem and surveys the velocity field from another point of view.

The essence of the new algorithm is zero isodop searching. For a PPI conical scan, the search point sets out from the radar origin in two paths through the zero-velocity region, and moves finally to the maximum range. The final paths obtained are just two zero isodops. Velocity aliasing is discovered and dealiased according to sign comparison in either of the regions separated by the two zero isodops. If the sign of a velocity at a gate is opposite to the accepted sign of the region wherein the gate is, the velocity is considered single-fold and then unfolded to the true value. Applied to real cases, the algorithm is verified to be available for aliasing which can not be solved by traditional methods. It is a useful alternative to currently used dealiasing algorithms.

The algorithm is a new idea in velocity dealiasing schemes. Theoretical illumination and practical verification show it is a good prospect for application. However, the algorithm in its present form is suited to a linear wind field that can be divided into two regions, with no patches of one region within the other. Thus advecting vortices or multiple (intersecting) fronts, boundaries etc. would present a problem that the algorithm could not handle. Therefore, future work is to improve the technique, with emphasis on extending it to more weather situations like non-linear wind fields and multiple-fold aliasing.

Acknowledgements

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Algorithm description
  5. 3. Algorithm performance
  6. 4. Summary and prospects
  7. Acknowledgements
  8. References

The authors are grateful to the referees for their constructive criticism and suggestions regarding an earlier version of this paper. This research was supported by the National Natural Science Foundation of China (60674074), College Graduate Student Research and Innovation Programme of Jiangsu province (CX09B_227Z), Meteorology Industry Special Project of CMA (GYHY (QX) 2007-6-2) and projects BK2009415, 20093228110002, 2007AA061901, 2008LASW-B11 and 2009Y0006.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Algorithm description
  5. 3. Algorithm performance
  6. 4. Summary and prospects
  7. Acknowledgements
  8. References
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