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

.... v.

Table II
Summary of causes of echo termination
4
Convective Class
Locally Externally 3
Developed Developed ,....
Dissipation ~2
within 150 km '51% 48% ><
of radar u
~ 10
Propagation 5-
~
beyond 150 km 12% 9% ..:
...
of radar 102 103 104 105 100 ~107
~
Merger with >
34% H
larger echo 39% j EXTERNAL
Radar turned ~ N=14l6
3% ..:
off 4%
---------------------- 2

While a breakdown of echo lifetime by the
categories of Table II is not presently avail-'
able, it seems likely that many echoes were
short-lived simply because they dissipated soon
after forming. However, merging may also have
been an important factor contributing to the
brief echo lifetimes usually observed.

3.4 Echo Rain Amounts

Estimates of rainfall were made using the
Marshall-Palmer relationship between rainfall
rate and equivalent radar reflectivity
(Ze=200Rl.6) for all of the echoes every 5
minutes at the lower tilt angle. All of the
estimated rainfall each 5 minutes was assumed
to fall to the ground, with the rainfall rate
respresenting the full 5 minute period. Total
rainfal~ estimates from each echo were calculated
by summing each 5 minute estimate. No correct-
ions were attempted for attenuation, evaporation,
or possible ice particles contributing to the
reflectivity values.

Figure 4 shows the relative frequency
histograms for both the combined local and
external periods. Over two-thirds of the echoes
in each category produced less than 104 m3 (8
acre ft) of rainfall, while only 2% of the
echoes in each category yielded 106 m3 or more
of rainfall. Obviously, the large majority of
echoes produced only trivial to limited rain
amounts.

The relative cumulative frequencies shown
in Fig. 5 illustrate the rainfall contribution
from those 12% of the echoes that produced the
largest rain amounts. Nearly two-thirds of the
estimated rainfall in each category came from
only 1% of the echoes, and over 90% of the rain
for both classes is attributable to only about
6% of the total number of radar echoes.

10
LOCAL
N=1268
,......
t)
><
u
Z
~
;::> U)
0- ....:I
~ ....:I
..: <(
... ~
~ H
> ;;i 2 4 6 8 10 12
H
E-<
j 0 10
~
8 u
::; ~
u ... 8 EXTERNAL
0
~
> N=1416
H 6
j
~
..: 4
2
2 4 6 8 10 12
RELATIVE CUMULATIVE FREQUENCY (%)
OF NUMBER OF ECHOES
Fig. 5 Relative Cumulative Frequencies of
Echo Rainfalls vs. Number for Larg-
est Rain Producing Echoes.

Fig. 4

162

102 10 3 104 105 10 6 ~107

RAINFALL (m3)

Relative Frequency Histograms of
Echo Rainfalls for Locally and
Externally Developed Convective
Periods.