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2305 DECEMBER 1978 R. E. CARBONE AND LOREN D. NELSON physical) information is output from the model. The authors will present only drop size spectra and param- eters thereof at cloud base, since these results provide a conceptual framework for understanding the aircraft observations of-spectral evolution, The aircraft observa- tions permitted input of measured state parameters, CCN, updraft radii and droplet concentrations. 3. Echo climatology The period 1 July~15 August 1975 exhibited con- siderable homogeneity with respect to environmental conditions and characteristics of initial echo formation and development. The majority of echoes first appeared at relatively low altitude and warm temperature and exhibited little or no vertical growth. Most echoes may be accurately characterized as single-cell, nonsteady showers with one cycle of updraft, precipitation forma- tion, and fall within a total lifetime of 30-45 min. Cloud bases were warm (:l0-180C) and vertical shear of the horizontal wind was weak ("-'2X 10-3 S-I). Boundary layer winds were typically south-southeast to southwest with cell movement toward the northeast. Cloud droplet spectra sanipled in August 1975 were quite continental in character. Fig. 2a shows the frequency distribution of first echo top temperature. The clear histogram includes all echoes, with 86% warmer than -lOoC and 49% warmer than Ooe. This distribution is highly suggestive of a warm rain precipitation-initiation mechanism. The shaded histogram in Fig. 2a is the so-called" shallow" first echo distribution. It was found that the 5 min scan cycle time was inadequate to observe first echoes in that many echoes were spread over 2-4 km vertical depth upon their initial detection. Clearly, substantial upward and/or downward growth had taken place within the 5 min interval and the echo top might not be indicative of the true precipitation initiation tempera- ture regime. The shallow:rrrst echoes are those which exhibited vertical depth < 1.5 km on detection. This reduced the sample population by roughly a factor of 6 and may have biased the sample. A qualitative review of the selection process revealed no obvious bias by date, time or growth rate, and it was concluded that the shallow echo distribution consisted of those echoes which developed by chance at a height and time favorable to early detection by the radar scan cycle. This distribution has higher frequency in the 0-50C interval with lower frequency at temperatures warmer than 50e. This shift may be due to the fact that 68% of the total sample population did not grow vertically but rather descended in height within 5 min after detection. Thus, it may be inferred that the shallow first echo population was detected prior to echo descent. Given this change in the distribution the result vis-a.-vis dominant precipitation mechanisms remains unchanged in that ice phase may not have been involved in the majority of first echoes during July and August. It 35 EARLY ECHO (a) TOP TEMPERATURE ... DISTRIBUTION 30 >-. <.;, 2: 25 LLI n Total sample (676) Shallow sample ( III) =, 01 ~120 LL. ~: 15 LLI <.;I a: LLI 10 A.. 5 o -40 -30 -20 -10 0 10 20 TEMPERATURE lOCI GROWTH TIME (min) 3.5 30 2~ ?O I~ 10 ~ 0 ECHO ( b) GROWTH DISTRIBUTIONS \ - .:'...:' 10 >- ~ 2~5 w ~ c ~20 lL. ~ 15 w u 0::: ~ 10 20 >- u z 30 ~ c w 40~ ~ z 50 t3 0::: W a.. . ....... 160 ~ 5 o o 100 200 300 400 500 GROWTH RATE (m/min) FIG. 2. Normalized frequency distributions for the July-August 1975 period: (a) early echo top temperature for the total sample population (clear) and shallow echoes < 1.5 km in depth (shaded) ; (b) echo growth rate (bottom) and echo growth time (top, inverted). Only 32% of early echoes grew subsequent to detection. should be noted that "first echo" definition in this study is threshold detectability, which ranges from 8 to 22 dBZe over the region of observation. It is possible that improved radar sensitivity could affect the results significantly in that these data could be dominated by a "bright-band" effect which results from melting ice particles. Improved sensitivity would resolve whether ice particles above the aoc level pro- duce stronger signals below due to the increased radio refractive index of liquid water.