WMOD00419

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

1602

MONTHLY WEATHER REVIEW

VOLUME 112

Convective Rain Rates and their Evolution during Storms in a Semiarid Climate

ANDREA DONEAUD, STEFANO IONESCu-NISCOyI AND JAMES R. MILLER, JR.
Institute of Atmospheric Sciences, South Dakota School of Mines and Technology, Rapid City, SD 57701-3995
(Manuscript received 5 July 1983, in final form 10 May 1984)

ABSTRACT

Rain rates and their evolution during summertime convective storms were analyzed for the semiarid
climate of the northern High Plains. Radar data from a total of 750 radar echo clusters from the 1980 and
1981 summer cloud seeding operations of the North Dakota Cloud Modification Project (NDCMP) were
used. The analysis suggests that the average rain rate R among storms is, in a first approximation, independent
of the total rain volume if the entire storm duration is considered in the averaging process. This average rain
rate depends primarily on the reflectivity threshold considered in calculating the area coverage integrated
over~the lifetime of the storm, the area-time integral (ATI). For the 25 dBz reflectivity threshold used in the
, ATI computations, R was 4.0 mm h-I with a standard deviation of 1.55 mm h-', being -20% higher for
wet season conditions.
The evolution of rain rates during storms was analyzed by dividing each storm lifetime into 10 min, I, 2
and 4 h, and growing and decaying periods. A 10 min time increment was used in computing the parameters
for all time intervals. A storm cluster reached its maximum growth after an average of 56% of its lifetime.
The average rain rate for the growing period exceeded that for the decaying period by about 10%. As the
time interval used in computations approached the storm lifetime, the scatter of the average rain rates was
reduced, thus increasing the accuracy of rainfall estimates using the area time integral. The value of R
remained independent of the total rain volume when the growing or decaying periods of storms were
considered separately. The total rain volume was also well correlated with the maximum single-scan rain'
volume. These findings suggest the possibility of estimating total storm rain volume at its maximum stage of
development.
It is hoped that improvements in rainfall estimation over areas using satellite data may result from further
studies, since the precipitating part of a cloud picture can be more accurately defined for the growing period
of a cloud's history.

1. Introduction

It is generally recognized that the average rain rate
considered over short periods of time (i.e., minutes)
exhibits large variations during convective storms.
However, if computed over longer time intervals or
particularly ever the--lifetime ofa storm, the average
rain rate shows much less variability. A recently
developed technique for estimating rainfall amounts
on the basis of tjme-resolved area coverage informa-
tion (Doneaud et al., 1981; Liu, 1982; Ionescu-
Niscov, 1982) implied a slight increase in the average
rain rate (computed over the lifetime of a storm)
when the area coverage was integrated over the rain
duration. The key element of that technique was the
very high linear correlation found between rainfall
area integrated over the rain duration, called the
area-time integral (A TI), and the radar estimated
rain volume (RERV). The ratio of the RERV (km2
mm) to this A TI (km2 h) gives the average rain rate
R (mni h-1). The slope of the linear fit was found to

1 Present affiliation: Atmospheric Science Center, Desert Research
Institute, P.O. Box 60220, Reno, NY.

@ 1984 American Meteorological Society

be a little higher than 1. The close relationship
between the rain volume and the A TI producing it
indicates that the reflectivity factors and the associated
rainfall rates likely follow some well defined statistical
distribution for a given locale and the degree of
_ orga~jzati~n of a convective syst~!D. (Stout and
Mueller, 1968). Lopez et al. (1983) studied similar
relationships between the areal.extent of precipita~on
and the corresponding rain volumes in south Florida--
using FACE II data. They found correlation coeffi-
cients as high as 0.92 and 0.90 for radar-estimated
and gage-derived rain volumes, respectively.
Byers (1948) was apparently the first to find a slight
variability in storm duration average rain rate, and
emphasized the close relationship between the amount
of rain falling from a shower and its size and duration.
The echo area-rain volume relation was extensively
employed as a tool for estimating rain volumes from
satellite data. Stoutet al. (1979) described similarities
in rain 'fate evolution during tropical storms using
GATE satellite and radar data. They also stated that
rain rate varied depending upon storm type. Griffith
et al. (1976, 1978) developed a technique for rain
volume estimation over areas. The original Griffith-