WMOD00419

• Title: Convective Rain Rates and their Evolution During Storms in a Semiarid Climate
• Date: 8/8/1984
• Weather Modification - Doc Type: Report

AUGUST 1984

. DONEAUD, IONESCU-NISCOV AND MILLER,JR.

1607

promise for a most appropriate ATI reflectivity
threshold (MART) for rain rate computation (if the
small clusters are deleted) seems to be 25 dBz [see
also Eq. (3) for b < 1]. For such .a threshold, if the
clusters up to 10 km2 h are deleted, the rain rate can
be considered in a first approximation to be indepen-
dent of the ATI, and roughly 4.0mm h-I. In such a
case, the accuracy of the estimates, according to the
standard deviation of the mean value (-1.55 mm
h-I), will range between :t40%. This accuracy is
comparable with that obtained using traditional Z-R
techniques for rain volume computations.
The summer 1981 Bowman data (587 clusters)
were used to check the 1980 findings. Frequency
distributions of R were computed on a monthly basis
using the 25 dBz A TI threshold (Fig. 5). The distri-
butions are unimodal. A stronger right skewness
occurs in August, generated by a small number of
outliers. As visible in Table 1 (1981 data), if the
clusters smaller than 1 km2 h A TI are discarded (four
clusters), the standard deviation decreases to less than
half of its initial value, remaining less than one-half
of the mean. The R range decreases from 98.20 to
16.44 mm h-I. The frequency distributions (Fig. 5)
show only two outliers in August with R exceeding
13 mm h -I. These cells were very unusual, being tall

and narrow with tight reflectivity gradients toward
the edges, and generating high rain volume estimates
for rather small area coverages. The removal of those
two clusters would bring the R range to not more
than 2.5 times the mean (-13.00 mm h-I). After
discarding the very small clusters (A TI up to 1 km2
h), R becomes -4.80 mm h-I (Table 1). A slight
increase is visible when clusters with A TI up to 25
km2 h are discarded. Investigating the A TI versus
RERV power-law relationship, Doneaud et al. (1984)
found a slope slightly greater than one, indicating
that the rain volume increased faster than A TI.
As mentioned earlier, the 1980 season was dry and
R was lower than for the 1981 wet season, suggesting
that adjustments for different seasonal conditions
may be necessary. The samples analyzed show an
increase in R of -20% for the wet weather season.
An analysis of the predictive power of the A TI
technique to estimate rain volume was made by
Doneaud et al. (1984). In that paper, the formula
derived from the 1980 NDCMP data was applied to
the 1981 echo cluster A TI values to estimate the
corresponding rain volumes. These estimates were
then compared with the radar-estimated rain volumes
computed on the 1981 radar data. Because of the
lack of an extensive raingage network in the NOCMP,

NDCMP, SUMMER 1981 BOWMAN DATA.

JUNE
NUMBER OF CLUSTERS: 72

60
(A)
40
20
~ 0
!!-
>- (B)
0
z
w 40
::l
a
w
IIC
II.
W 20
>
i=
cc
-'
w I>
IIC
.~ (C)
40
20

-,
I
I
,
,.
I
,
I r---i
\1 r-
I' I,
, "

MONTH
- JULY
h__ AUGUST

NUMBER OF CLUSTERS
391

124

TOTAL
NUMBER OF CLUST~RS: 587

1
,
,

o

5 10 15
AVERAGE RAIN RATE (mm hr-')

FIG. 5. Relative frequency distributions of R for 25 dBz ATI thresholds
calculated on monthly and seasonal bases.

o

20