WSR-88D Clutter Suppression and the VWP

Western Region Technical Attachment

No. 96-11

April 24, 1996

by Dave Sanders, SLC WFO and Steve Vasiloff, NSSL/WRH-SSD

1. Introduction

To most effectively use the WSR-88D requires consideration of potential impacts of various methods of clutter suppression on radar products. Most WSR-88D operators are aware that clutter suppression can remove slow-moving targets and cause under-estimation of precipitation estimates near the zero isodop.

This paper illustrates how the failure to use clutter suppression can herein affect the VAD wind profile (VWP). Specifically, it is shown that erroneous winds are generated if suppression is not applied to the entire domain. In the case presented herein, the by-pass map (the most commonly used suppression method) failed to eliminate much of the clutter picked up by the radar. The near-zero velocities associated with the clutter resulted in near-zero VAD wind estimates.

2. Case study

2.1 Effects of various filtering techniques

Data for this study are from the KMTX radar near the Great Salt Lake. The VWP from 2338 UTC 15 March 1996 to 0114 UTC 16 March 1996 is shown in Fig. 1.

Figure 1

Initially, the VWP indicates that the winds between 11,000 and 14,000 ft are generally from the northwest at 30-40 kt. In addition, there are few data below 11000 ft. At 0017 UTC, there is a dramatic increase in the vertical extent of the winds: data now extend from 7000 ft to over 20000 ft. Also, the wind speeds drop off to near 10 kt or less and shift to more westerly.

Deep and weak winds continue on the next volume scan. At 0046 UTC, the winds are again stronger and have a lesser vertical extent, closely resembling the winds prior to 0017. It is believed that the presence of the winds at 7000 and 9000 ft are not related to the clutter but were the result of an increase in non-clutter targets (e.g., birds, insects, and/or clouds).

2.1.2 VAD raw data

The 2.5 deg Doppler velocity field at 0017 UTC is shown in Fig. 2. Recall from Fig. 1 that the VWP wind speeds were strong at this time.

Figure 2

At the next volume scan (Fig. 3), there has been a large increase in the amount of data on the 2.5 deg tilt.

Figure 3

Also, most of the additional data are less than 5 kt, an indication that the data are from ground clutter. The reason for the increase in clutter is that the by-pass map was invoked whereas during the previous volume scan clutter suppression was on everywhere.

The 88D velocity-azimuth display (VAD) is a good indicator of how raw data are used in the determination of the horizontal wind. The VAD taken at 13000 ft from the 0017 UTC volume scan is shown in Fig. 4.

Figure 4

Although there are few data points, what little data do exist are fit very well by the sine curve. Thus, we have confidence that the 38 kt wind from 323 deg is valid.

The 13000 ft VAD at 0026 UTC is a different story (Fig. 5).

Figure 5

With the by-pass map being used, the zero-velocity clutter data dominate the VAD: the wind speed has dropped to 7 kt. In addition, there are fewer valid velocities having large values. This is caused when, within a range bin, sparse meteorological data are overwhelmed by the predominance of zero-velocity clutter data.

At 0046 UTC, clutter suppression was again turned on everywhere. The VAD (not shown), appeared much like the earlier VAD with suppression everywhere with no clustering of data about the zero line and a high wind speed (29 kt).

3. Conclusions

This paper demonstrates that excessive ground clutter can significantly degrade derived velocity products. This is particularly noticeable in the VWP product but will also be true for other velocity products since it is the base velocity data that are contaminated.

A cautious approach to the use of clutter suppression should be used since its overuse can also have adverse effects. In this case, degradation of the VWP winds might not have been as severe if more meteorological targets had been present. Thus, when velocity data are questionable, excess clutter should be considered as a possible cause.