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

Classes 6 and 8 are hereafter referred to as
"local" for locally developed and "external" for
externally developed convective periods, .respect-
ively, where local (external) is within (outside
of) the 150 km radius circle centered on Miles City.

It should be noted that another important
class of externally developed mesoscale cumulo-
nimbus systems exists in southeastern Montana that
is not considered in this paper. This is the
mountain-generated system discussed by Boatman
et al. (1977).

Table I

Summary of available radar data
with locally 9r externally peveloped
convective echoes within 150 km of
Miles City, MT

Local
1976 Echo times (MDT) It of 20 dBZ
echoes
May 20 1427-0147 203
May 31 1407-0152 173
June 24 1342-0132 366
July 2 1032-1717;1822-0017 241;71
July 19 1342-2207 164
July 23 1637-0152 50
External
1976 Echo times (MDT) II of 20 dBZ
echoes
May 25 1152-1952 522
May 28 1717-0207 168
July 1 1737-0147 75
July 12 1312-1632;1757-0147 46;134
July 18 1607-0142 133
July 21 1637-0152 126
July 24 1722-0142 212
-------------------------

3. RESULTS

3.1 Maximum Echo Tops

Maximum echo tops were derived for each 20
dBZ echo from the radar volume scans where the
antenna was stepped from 1 through 12 degrees
in elevation angle. The approximate 1 degree
vertical beam width corresponds to 0.4 km at
25 km range and 2.6 km at 150 km.

Maximum echo tops were occasionally higher
than the radar beam at the 12 degree tilt,
especially at closer ranges. Approximately 3%
of the echoes from both the local and external
periods had radar returns at the 12 degree tilt.
Thus, the maximum top observed by radar may have
been somewhat low in these few cases whi~h have
been included.

Figure 1 presents a relative frequency
histogram of maximum echo tops for all local and
external periods combined. Some 1268 echoes were
available from local convective periods while

,~

1416 existed from externally developed convection.

It can be seen that the distributions are
rather similar for the two classes of convections.
Over 74% of the maximum echo tops were between 4
and 10 km msl in both cases, and both exceeded
35% in the 6 to 8 1~ interval. Thus, the large
majority of echoes had limited vertical extent.
Some echoes were less than 4 km high, especially
during external periods, probably due to the
splitting of decaying systems. New convection
in the study area is unlikely to develop precip-
itation sized particles at such low altitudes
(McPartland et al... 1977).

Only 7 and l4:~ of all echoes exceeded 10 km
for the local and external periods, respectively.
However, four days in each category had at least
one maximum top exceeding 15 km. The maximum
top observed was 17 km during an external period.

40
30
20
,......
iN1
'-' 10
><
u
z
~
g 4 6 8 10 12 ~14
~
p.: 40
~
~
;> 30
H
Eo-<
j
~ 20
p.:
10
4 6 8 10 12 ~14
ECHO TOP (km-msl)
Fig. 1 Relative Frequency Histograms of
Maximum Echo Tops for Locally and
Externally Developed Convective
Periods.

3.2 Maximum Echo Volumes

Relative frequency histograms for the maxi-
mum echo volumes for both the combined local and
external periods are presented in Fig. 2. The
maximum volume occasionally may have been under-
estimated when echo existed above the 120 tilt
angle.

Over one-third of all echoes in both cate-
gories had a maximum volume less than 10 km3,
while at least 84% were less than 100 km3. These
volumes correspond to cubes of 2.2 and 4.6 km on

160