WMOD00530

• Project Name: High Plains Cooperative Program
• Title: Radar Observed Characteristics of Summertime Echoes in Eastern Montana
• Date: 3/28/1978
• State: MT
• Country: United States
• Weather Modification - Doc Type: Report

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The external days during the May through
July 1976 period produced 40% more tot&l esti-
mated rainfall than the local days. If the
rainfall estimates are normalized by day,
external days yielded approximately 20% more rain
than local days. However, these average values
are biased by a single echo on May 28 which
contributed 20% of the total rainfall for all
external days. It was almost equivalent to the
combined contribution of the four next largest
echoes on external days, or the three largest
echo rainfall estimates occurring on the local
days. Most of the days usually had one to three
echoes which produced an order of magnitude more
rainfall than the next smaller ones.

The top ten rain producing echoes in each
category were extracted for detailed examination.
It was found that only two of the maximum tops
were less than 12 km (lowest=10.2 km) for the
local category, while the lowest maximum top in
the external category was 12.6 km. Thus, the top
ten rain producing echoes in each category were
in about the upper two percentile for maximum
echo tops (see Fig. 1).

Only one of the ten local echoes had a
maximum volume less than 1000 km3, while the
smallest of the ten external echoes was 1800
km3. Reference to Fig. 2 indicates that the
top ten rain producers in each category were
also in about the upper two percentile wi~h
respect to maximum volume.

All of the echoes in both categories were
long lived; that is they existed for at least
two hours while several existed for more than 4
hours. This placed them in the upper 3 or 4
percentile for external and local convective
systems, respectively.

All the echoes in both categories exhibited
high maximum dBZ values which ranged from about
47 to 60. dBZ in each category. It might be noted
that rainfall estimates for these echoes should
be used with caution in light of these high dBZ
values (Hildebrand and Wildhagen, 1977).

In summary, less than one percent of the
total echoes in each category produced over half
the total radar estimated rainfall for all echoes.
These high rain producing echoes were character-
ized by high maximum tops, large volumes, long
lifetimes, and high maximum dBZ values. In
short, they were large complex systems that
likely produced hail as well as substantial
rainfall.

4. DISCUSSION

Radar characteristics of echoes with a
minimum equivalent reflectivity bhreshold of
20 dBZ have been computed for two categories of
summer convective cloud systems which produced
thunderstorms in eastern Montana during 1976.
Most of the results presented concern summaries
of maximum echo tops, maximum echo volumes, echo
lifetimes and radar estimated rainfalls from 2684
individual echoes. The relative frequency histo-
grams for echoes from all periods comprising the
two classifications were found to be similar.

Significant results for maximum tops are:
a) Most echoes had limited top heights with
only 7 and 14% of the echoes exceeding
10 km during local and external periods,
respectively.
b) Often one echo in each period exceeded
15 km.
for maximum volumes:
a) Most echoes had limited volumes with 84%
being less than 100 km3 (4.6 km cube).
b) The largest echo during each external
period generally had a greater volume
than the largest echo during each local
period.
for echo lifetimes:
a) Most echoes had brief lifetimes with
.63 and 68% of the echoes during local
and external periods, respectively,
lasting 30 minutes or less.
and for radar estimated rainfalls:
a) Two-thirds of all echoes in either
category produced less than 104 m3
(8 acre ft) of rainfall per echo.
b) Ninety percent of the rainfall came from
only 6% of the radar echoes; almost
two-thirds came from only 1% of the
echoes.
c) The largest rain producing cells had
high tops, large volumes, long lifetimes,
and high maximum dBz values. Thus, they
were quite complex systems.
d) The external periods yielded somewhat
more estimated rainfall than the local
periods.

Future research will be directed toward
obtaining the radar observed characteristics for
other storm categories described by Hartzell
(1977a), and also assessing the radar character-
istics for data collected during the summer of
1977 at Miles City.

5 . ACKNOWLEDGEMENTS

The helpful suggestions provided by
Dr. Arlin B. Super are gratefully acknowledged.
Personnel of the Montana Department of Natural
Resources provided the manual definition of the
radar echoes and Mr. Lee Brueni of the University
of North Dakota, ,Department of Aviation was
responsible for writing most of the computer
programs used to process the large quantities of
radar data.

6. REFERENCES

Boatman, Joey, Arlin Super and Edmond Holroyd, III,
1977: Summer rainstorm types and associated
rainfall characteristics in eastern Montana.
Preprints Sixth Conf. on Inadvertent and
Planned Weather Modification, Champaign-
Urbana, IL, Amer. Meteor. Soc., pp 327-330.

Hartzell, C. U., 1977a: Development of objective
forecasting techniques for the Montana
HIPLEX project area. Rpt. SR-849-59, Bureau
of Reclamation Contract No. l4-06-D-7665,
Western Scientific Services, Inc., Ft. Collins,
CO, 50 pp.

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