Orographic-Convective Precipitation 
as Revealed by Radar 
BERNICE ACKERMAN 
Department of Meteorology, University of Chicago, Chicago, Illinois 
Abstract—Summer cloud systems in the arid and mountainous region around Tucson, 
Arizona, are predominantly convective in nature. Extensive radar observations of these 
systems have been made by the Institute of Atmospheric Physics, University of Ari- 
zona, using height-finding radar. A study of the level of formation of radar echoes, based 
on data collected during the summer of 1956, indicates that an all-water process, as well 
as one involving the ice phase, was effective in initiating precipitation. Moreover there 
appeared important day-to-day differences in the efficiency of the water mechanism. 
Introduction—For the past four or five years 
the Institute of Atmospheric Physics, Univer- 
sity of Arizona, has been investigating the cloud 
and precipitation characteristics of arid regions. 
During the course of this investigation extensive 
radar observations have been made using the 
AN/TPS-10, a height-finding radar. Radar data 
obtained during the summer rainy season in 
1956 have been used to study precipitation fea- 
tures around Tucson. In particular, this report 
is concerned with the heights at which echo 
clouds first formed; the significance of such in- 
formation, of course, lies in what may be learned 
about the mechanisms initiating precipitation. 
The AN/TPS-10 radar has a 3-em wavelength 
and an elliptical beam with widths of 0.7° in the 
vertical and 2° in the horizontal. The radar sys- 
tem was monitored and power levels maintained 
at 47 dbm and —79 dbm for transmitted and 
minimum detectable returned power, respec- 
tively. Automatic photography of the radar 
scope gave a permanent record. There was ap- 
proximately a three-minute interval between 
successive observations at a given azimuth. 
The radar, which is located on the campus 
of the University of Arizona, scanned the area 
within sixty miles of Tucson. As can be seen in 
Figure 1, this area is of interest because of its 
topography as well as its aridity. The terrain 
varies in elevation from 2000 to almost 10,000 ft 
msl. Several small but distinctive mountain 
ranges are in the area; most of these are fairly 
well defined by the 5000 ft contour. 
The summer rainy season starts rather sud- 
denly in late June or the early part of July and 
continues through August. During these months 
there are alternating dry and wet periods, the 
=I 
We) 
latter characterized by fairly general Cumulus 
activity and convective rain. 
Although association between clouds and 
mountains is usually observed, the location of 
the major convective activity varies consider- 
ably from day to day. On few days are clouds 
associated with all of the mountain ranges, and 
as a corollary, a given range does not give rise 
to cloud systems every day. This variability is 
illustrated in Figures 2 and 3 in which are shown 
the locations of precipitation echoes on two of 
the days studied. (In Fig. 2 and 3 the division 
of the precipitation areas into groups was made 
for purposes of a detailed description of the echo 
patterns given in a more complete report of this 
work [Ackerman, 1959]). On only two of the 
seven days studied were the echo patterns simi- 
lar. It is evident that processes considerably 
larger in seale than the cloud are factors in de- 
termining the location of cloud development, 
even in a region as small as the one being con- 
sidered. 
Analysis and discussion—Because of the time- 
consuming nature of the data reduction, the 
analysis covered only seven days of the season. 
The criteria used in the choice of days were 
sufficiently objective to insure a random sample. 
They were (1) that convective precipitation, as 
indicated by radar, has occurred; (2) that radar 
data be available; and (3) that the days be scat- 
tered through the summer. The resulting sam- 
ple was composed of nearly 300 first echoes, 
that is, first appearances of echo clouds. For all 
of these cases the area was known to be free of 
an echo three minutes earlier. 
In Figure 4 are shown the frequency distribu- 
tions of the temperatures at the bases and tops 
