1. Introduction
Comments related to the need for improved vertical resolution:
Comments related to the need for improved temporal resolution:
Comments related to the need for improved horizontal resolution:
2. Characteristics of VCP 11 and VCP 21
3. Technique for optimizing VCPs
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Fig. 7. Schematic of the process used to compute optimized VCPs. See discussion in the text for details. |
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Fig. 10. Same as Fig. 5, except for optimized VCP 11. | Fig. 11. Same as Fig. 1, except for optimized VCP 11. |
4. VCPs having improved vertical and temporal resolution
a. Strategies for new VCPs
b. Development of VCP A
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Fig. 12. Same as Fig. 1, except for optimized VCP A. | Fig. 13. Same as Fig. 5, except for VCP A. |
c. Development of VCP B
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Fig. 14. Same as Fig. 1, except for optimized VCP B. | Fig. 15. Same as Fig. 5, except for VCP B. |
d. Testing improved vertical resolution
e. Testing improved temporal resolution
5. Improving horizontal resolution
a. Improved tornado detection
b. Improved mesocyclone detection
6. A look at an initial elevation angle of 0.3o
7. Recommendations
1. Optimized VCPs (Section 3). We developed a technique for computing "optimized" VCPs. The maximum height underestimate (expressed as a percentage) owing to finite increments between elevation angles is essentially the same at all heights and all ranges. Besides providing a logical increase in the separation of elevation angles with increasing elevation angle, the technique assures adequate sampling of distant storms. We recommend that all future VCPs be developed using this optimization technique.
2. Flexible VCPs (Section 4). To improve temporal resolution without sacrificing vertical resolution, the VCPs used by the WSR-88D should be flexible. That is, there should be an algorithm that ends a volume scan after detecting two consecutive elevation angles having no radar return. We recommend that all future VCPs be flexible, so that, when conditions merit, a VCP can be ended before it reaches the maximum elevation angle.
3. Faster VCPs (Section 4). The evolution of some severe weather phenomena (such as tornadoes and downbursts) is so rapid that forecasters are at a disadvantage in issuing warnings based on 5- and 6-minute VCPs. Faster VCPs will improve the warning process. By the time that fast VCPs are implemented, the communication links will have been upgraded so they can handle the increased flow of information. In addition to using flexible VCPs for decreasing volume scan time, we recommend that all future VCPs use maximum antenna rotation rates that are consistent with specified maximum Doppler velocity and reflectivity estimates of error.
4. Improve horizontal resolution (Section 5). Simulations of WSR-88D data collected in model mesocyclones and tornadoes show that the range of mesocyclone and tornadic vortex signature detection increases by 50% when data are collected at 0.5o instead of the conventional 1.0o azimuthal increment. This increase is equivalent to doubling the detection area. We recommend that all VCPs developed for convective storm detection collect data using a 0.5o azimuthal increment.
5. Increase the maximum elevation angle (Sections 3 and 4). The current VCPs 11 and 21 do not tilt higher than 19.5o. The volume of space between 19.5 and 90o is literally a "cone of silence", where the radar cannot detect storm features. The narrow band of data that appears on a PPI display at high elevation angles is difficult for a human to interpret. Also, Doppler velocity measurements at higher elevation angles contain a significant bias from precipitation fall speeds. However, the higher elevation angles provide a wealth of additional reflectivity information to the algorithms that ultimately benefits the users. Therefore, we recommend that all optimized VCPs include elevation angles up to 60o, which is the mechanical upper limit for the antenna.
6. Decrease the lowest elevation angle (Section 6). A theoretical study suggests that the WSR-88D antenna can be lowered to 0.3o without degrading WSR-88D reflectivity and Doppler velocity measurements. Decreasing the lowest elevation angle from 0.5o to 0.3o will increase the range of detection by 15-20 km. Also, at a given range, it will lower the height of the lowest data points by 0.4 km at 115 km range and by 0.8 km at 230 km range. Though these improvements appear to be minimal, they can be significant in dealing with convective phenomena that are only 1-3 km deep. We recommend that the lowest elevation angle be decreased to 0.3o.
7. Make algorithm developers/modifiers aware of upcoming VCP changes. At the present time, new VCPs cannot be added to the WSR-88D system until changes are made to some of the algorithms. Users have been requesting VCP changes for years, but nothing has happened because there is no easy way to modify the algorithms. We are proposing changes that likely will affect all of the WSR-88D algorithms. In preparation for these changes, we recommend that algorithm developers and modifiers be mandated to add flexibility to their algorithms that will easily allow changes to be made to such parameters as elevation angle (from negative angles for mountaintop radars up through 60o), number of elevation angles, horizontal azimuthal increment, rotation rate, VCP recycle time, etc.
8. Replace VCP 11 and VCP 21. VCPs 11 and 21 do not do a very good job in resolving nearby and far-range convective storms. Using the optimization technique developed for this study, we prepared new VCPs A and B that include the types of changes that forecasters have requested to help them with issuing more timely and accurate warnings. We recommend that VCP 11 and VCP 21 be replaced with optimized VCPs, such as those typified by VCP A and VCP B (fast rotation rates, elevation angles from 0.3o to 60o for non-mountaintop radars, horizontal azimuthal increment of 0.5o, ability to end the VCP partway through the sequence).
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