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i~ - .~~. ,. .[ ... 'I" Reprinted 1"rom Preprint Volume: 18th Conference on Radar Meteorology, Mar. 28-31, 1978, Atlanta, Ga. Published by the American Meteorological Society. Boston, Mass. RADAR OBSERVED CHARACTERISTICS OF SUMMERTIME ECHOES IN EASTERN MONTANA John R. Middleton High Plains Cooperative Program Bureau of Reclamation Miles City, Montana 1. INTRODUCTION The High Plains Cooperative Program (HIPLEX) is currently involved in the design of a randomized seeding experiment for rainfall augmentation from summer convective clouds in eastern Montana. A climatology of convective cloud characteristics is obviously one of the essential prerequisites for developing such a design. The purpose of this paper is to present a summary of several basic characteristics of convective clouds derived from radar observations made during the summer of 1976. 2. PROCEDURES 2.1 Radar Digitized radar data that were collected at Miles City, Montana, from May 1 through July 30, 1976, were' used for this study. The 5.4 cm radar had a one-degree beam which was swept through 360 degrees of azimuth in each of twelve one- degree steps from 1 through 12 degrees elevation angle. These "volume scans" were obtained once each 5 minutes whenever precipitation echoes (greater than 20 dBZ) were observed within 150 km of the radar between the approximate times 1200 and 0145 MDT. These radar data were processed by computer to produce a composite B-scan (range vs. azimuth) presentation which consisted of all echoes greater than 20 dBZ from all 12 elevation angles compressed onto a single plane. The 5-minute composite B-scans were then hand analyzed to identify range and azimuth boundaries of individ- ual echoes defined by the 20 dBZ contour, using minimum separations between echoes of 1 km in range and 1 degree in azimuth. Each radar echo was given a unique identifier the first time it was observed. This identifier was retained until the echo either. dissipated, propagated outside the observing area or merged into a larger echo. The individual boundaries were used for further computer processing to provide echo time histories of total volume, centroid location, maximum top, maximum dBZ, maximum rainfall rate, average rainfall rate, total rainfall and other parameters. Specific details concerning the radar characteristics and the data processing are given by Schroeder et al. (1976). The computer generated output for each ind- ividual 20 dBZ radar echo contains a summary of every tilt angle at which the echo existed. The tilt summary records contain the cell identifica- tion number, time, tilt angle, total area greater than 20 dBZ, centroid location, maximum dBZ value and location, total echo volume at the tilt angle, areas of echo in 5 dBZ steps ranging from 20 dBZ to > 65 dBZ, the average rainfall rate, maximum rai;fall rate and rain flux in km3/5 minutes. Rain flux is calculated for both the Marshall- Palmer Z-R relationship and the Z R relationship derived by Smith et al. (1975) using a 10 cm radar in the northern High Plains. There is also a volume summary record each 5 minutes which contains the summation of all tilt records in addition to the maximum top height and location. 2.2 Storm Period Classification Each of the two periods, 0900-1500 and 1500- 2100 MDT, of each operational day was post- stratified into one of the nine classes developed by Hartzell (1977a). This post-stratification was accomplished by Hartzell (1977b) who had access to both HIPLEX and National Weather Service (NWS) surface and upper-air observations, laser- fax satellite photos, and other HIPLEX data from radar, aircraft, and field notes. The nine classes ranged from non-precipitation periods to overcast periods with widespread precipitation. Two of the most interesting classes, as regards the HIPLEX experimental design, are: Class 6: Cumulonimbus and thunderstorms developed within the project area in add- ition to towering cumulus. Development was either random or in lines, and was sometimes associated with strong positive vorticity advection. The project area refers to all the area within 150 km radius of the Miles City radar. Class 8: Mesoscale system developed along a snyoptic feature such as a cold front or short wave trough and moved generally eastward across the project area as a line of thunderstorms or rain showers. 159