WMOD00425

• Title: Demonstration of Improved Operational Water Resources Management Through Use of Better Snow Water Equivalent Information
• Date: 9/1/2002
• Weather Modification - Doc Type: Report

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6.3.1 Bright Band Characteristics

An illustration of beam geometry in the presence of a bright band is given by figure 7. Figure 7a
shows the altitudes ofthe lowest beam's bottom, center, and top over flat terrain. The 0 C)
Centigrade (C) altitude is indicated at 2.36 km above the radar, while the +4 oC altitude is at
1.68 km above the radar. Figure 7a also indicates beam width with sample circles and the
portion of the beam affected by bright band. Figure 7b graphs the portion of the beam
contaminated by the bright band versus range. The curve is asymmetric because of the
expanding beam diameter and loss of resolution with distance. Most rain studies show a Ze
enhancement of 5-10 dB in the bright band, leading to precipitation rates up to five times too
large there (Austin 1987; Joss and Waldvogel, 1990).

The case study for figure 7 (section 6.6.3) suitably illustrates the bright band effect. The study
employed data from KMPX (Minneapolis, Minnesota) between 1200 UTC on 5 November 2000
and 1200 UTC on 7 November 2000, especially during the middle 24 hours ofthat period.
Precipitation was widespread. Local rawinsonde balloons gave temperature profiles at 12-hour
intervals. The melting level (0 OC) was at about 2.4 km AGL for most of the period, associated
with a strong bright band. At 0000 UTC 7 November, there was an inversion that resulted in
three different melting levels. This became one level at about 1.9-km AGL 12 hours later.
Unfortunately, rawinsonde temperature soundings at intermediate hours are unavailable.

For a quantitative representation of the bright band effect versus range, we use the SAA. Figure
7c depicts the ratio of SAA-ca1culated precipitation and actual observed rainfall for two 24-hour
periods (5-6 and 6-7 November 2000) versus range. All precipitation observations at the surface
were of rain. At near ranges, rain was also observed by the radar, and the SAA underestimated it.
Only snow was observed by the radar at far ranges, and the precipitation was also underestimated
there, probably because the large beam was only partly filled by the shallow precipitation. At
intermediate ranges, melting snow ("slush" in the figure) created a bright band, enhancing the
SAA calculation of precipitation and erroneously increasing the plotted ratio up to 2.5 times, in
accord with previously cited bright band research. The large ratios are offset in range from the
curve of figure 7b, probably because of changing bright band elevations during the 48-hour
period.

Hourly VPRs (section 6.2 and figure 8) were produced for this case at 50-km range intervals for
altitudes of 0-5 km above the radar. At 50-km range, these VPRs resembled a classic bdght band
profile with good resolution. As the range increased, bright band effects broadened verti.cally
and weakened as the radar beam increased in diameter and resolution was lost. At still longer
ranges, beyond 150 km, only the first tilt contributed to the profiles, and resulting data were from
the mid-troposphere (we restricted data gathering to the first 5 km above the radar). The beam
was above the bright band beyond 200 km.