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.. " " .. a side. Thus, most 20 dBZ echoes were rather lillr.i.ted in size. The external periods showed a tendency to have more echoes with large volumes than the local periods. The largest volume occurred on one of the external days and was over 25,600 km3. All but one of the external days had at least one echo with a volume greater than 3000 km3, with the maximum being 4800 km3. Fig. 2 6 5 4 3 ~ ~ ...... 2 ;H CJ z' ~ 1 p 0- ~ p:: '" ~ :> H H 50 j gj 40 30 20 10 100 101 let 103 ~104 VOLUME (km3) Relative Frequency Histograms of Maximum Echo Volumes for Locally and Externally Developed Convective Periods. 3.3 Cell Lifetimes The calculated cell lifetimes should be used with some caution because bias exists at both ends of the distribution. By definition, an echo was required to exist for at least three volume scans (15 min) before it could be con- sidered with the exceptions of echoes which formed and merged into other echoes in less than 15 minutes, or those which split from existing echoes. (About one-fourth of all echoe$ in each category formed by splitting.) Thus, relatively few echoes could have durations less than 15 minutes. Also, cell lifetimes were sometimes limited because echoes were still present on the last radar scan for the day. This occurred on all but one day in each of the two classifications. As mentioned earlier, one day in each category had approximately 1 hour of missing data. For both of these days, the echoes were considered terminated at the beginning of the radar data outage and the second set for each day was essentially independent. Also, some echoes may not begin or complete their lifetimes within 150 km of Miles City. The relative frequency histograms for both the local and external days are presented in Fig. 3. The two distributions are quite similar. Sixty-eight percent of the echoes on the external days existed 30 minutes or less compared with 63% of the echoes on the local days. Only about 13% of the echoes in either category lasted longer than one hour. While both classes had one day on which an echo existed over 7.7 hours, the large majority of 20 dBZ echoes existed as isolated entities for rather brief periods. 40 LOCAL 30 20 ~ ~ 10 ...... ;H CJ Z ~ 0 P 0- ~ p:: 40 '" ~ :> 30 H ~ H ~ 20 p:: 10 DURATION Fig. 3 Relative Frequency Histograms of Cell Lifetimes for Locally and Externally Developed Convective Periods. As discussed previously, echoes could term- inate by dissipating, by moving outside the 150 km radius observational area, by merging with a larger echo, or by the radar being intentionally (i.e., end of shift at ~ 0145 MDT or inadvert- ently (e.g., power failure) turned off. Table II lists the percentage of total echoes that were in each of these categories for locally and external- ly developed convection. It can be seen that about half of all echoes terminated by dissipating, while over one-third of them merged into larger echoes. The remainder were terminated due to propagation beyond the observing area or, in a few cases, due to radar shut-down. 161