WMOD00419

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

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AUGUST 1984

DONEAUD, IONESCU-NISCOV AND MILLER,JR.

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scans (-10 min); 2) 1, 2 and 4 h periods; and 3)
growing and decaying periods. For all time intervals,
the sampling frequency was the radar scanning fre-
quency (-10 min). Only clusters with data from four
or more scans and with both identifiable growing and
decaying periods were selected. Of the 587 1981
Bowman clusters previously identified, 284 clusters
met the above criteria. Merging (or splitting) clusters
were classified separately.
The growing and decaying periods of storms were
determined in three ways: using maximum reflectivity
(ZMX), maximum echo height (MEH), and maxi-
mum areal extent (MAREA). On average, a typical
cluster reached its maximum growth after about 56%
of its lifetime. On a statistical basis, the maximum
area was reached first, followed by the maximum
reflectivity and then the maximum echo height. The
results regarding ATI-RERV relationships were about
the same for the growing or the decaying period as
determined by anyone of the three criteria (Table
2). The log-standard errors of estimate seem to be a
little larger during the decaying period. The average
rain rate is a little greater during the growing period
of the typical cluster compared to the decaying period.
The differences in means, according to a t-test, are
not significant (a > 0.05), except in the case where
ZMX was used to divide the cell duration. In that
case (Table 2), the average rain rate of the growing
period is -10% greater than that of the decaying
period. This result differs from what Griffith et al.
(1976) derived in south Florida from digitized Miami
WSR-57 observations. Using 5 min time increments,
the authors found rain rates of 15.60 and 7.92 mm
h-1 for growing and decaying periods, respectively,
giving an average of 11.76 mm h - I. The differences
between the average rain rates in North Dakota and
Florida for the entire cell duration may reflect a
climate difference between Florida and North Dakota.

Perhaps more significant are the differences in rain
rates during the growing and the decaying periods at
the two locales. In Florida, R for the decaying period
is hair of that for the growing period, while in North
Dakota R for the decaying period is only - 10% less
than that of the growing period.
The mode of the distribution (the 4-5 mm h-I
category) for both the growing and the decaying
periods of the North Dakota clusters represents ap-
proximately 50% of the observations (Fig. 7). The
A TI reflectivity threshold (the MART) in both situ-
ations is 25 dBz. If small clusters were discarded, th:c
average rain rate (R = 4.8 mm h-I) could be
considered independent of the A TI in more than two-
thirds of the cases.
A scan-by-scan calculation (-10 min intervals) of
the average rain rates was also performed. The rainfall
rates showed a similar overall frequency distribution
(unimodal with a right skewness). The rain volume
versus single-scan, area-time product correlation
dropp(:d to 0.94, and the (logarithmic) standard error
of estimate increased to 0.20 (Table 3). Table 3 is a
composite table displaying RER V versus A TI corre-
lations for the NDCMP and Quadra data sets. The
Quadra data were recorded in the tropical Atlantic
Ocean. The rain amounts were determined by mul-
tiplying the radar-determined rain area by a rain rate
averaged over a long period of time (Lovejoy and
Austin, 1979). In North Dakota, the average lifetime
of the clusters was somewhat longer than -1.0 h
(Fig. 2). The data were computed by averaging values
at 10 min time increments (Table 3). For the NDCMP
data, as the averaging time interval approaches storm
lifetimes, the RER V j A TI correlations become higher,
from 0.94 to 0.98. This strengthens the accuracy of
the rain volume estimates obtained using the recently-
developed RERVjATI technique (Doneaud et al.,
1981; Ionescu-Niscov, 1982), and the concept that

TABLE 2. RER V versus A TI 25 and rain rate regression analyses for the growing and the decaying periods of a cluster
(Bowman, North Dakota; 1981 Summer experiment, 284 clusters).

Split cluster criteria
Maximum area (25 dBz) ZMX MEH
Growing Decaying Growing Decaying Growing Decaying
period period period period period period
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Correlation coefficient 0.98 0.98 0.98 0.98 0.98 0.98
Slope 1.06 1.02 1.05 1.03 1.06 1.02
Intercept 3.56 . 4.06 3.88 3.92 3.69 ' 4.12
Logarithmic standard error
of estimate 0.15 0.18 0.16 0.17 0.16 0.17
Average rain rate (mm h-') 5.02 4.92 5.23 4.81 5.11 4.93
Standard deviation of rain
fates (mm h-') 2.10 2.72 2.32 2.20 2.43 2.24

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