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NOVEMBER 1983 HEGGLI ET AL. 1877 TABLE I. Description of sevFn precipitation echo types. Precipitation echo type Orographic (0) Area-wide (A W) Embedded band (EB) Major band (MB) Convective train (CT) , Cellular structure (>50% coverage, C2) Cellular structure (",,50% coverage, C I) Description An orographic echo (Fig. 2a) is a reasonably uniform etho tied to the higher topography. The bright band is often observed in this echo, as are generating cells and ~arying degrees of embedded convection. I This echo (Fig. 2b) is so named because it frequentlyl covers the entire radar scope and is of much larger scale than the other types, It is uniform in nature and is not tied to thd higher topography. The bright band is nearly always observed in this PET. : This feature I(Fig. 2f) is commonly observed in A W or 10 echoes as elongated regions of enhanced ireflectivity which generally move west to east through weaker surrounding echo, Motion is usually normal to' the major axis of the band. Reflectivities can reach 50 dB(Z) in the bright barld and average rainfall rates are higher thah these from any other PET, I The major b~nd (Fig. 2g) is a well organized mesoscale I feature observed as an elongated area of reflectivity with either strong reflectivit~ gradients or of sufficiently large scale to be easily identified. It generally moves normal to its major axis. The circulatiorls associated with a band often suppress rjldar echoes ahead and behind it. A convectivJ train (Fig. 2c) is a nonuniform echo orga~ized in a line withlcellular echoes tending to develop from a- single generatin~ point. The point mayor may not be fixed. Individual cells within CT tend to prbpagate downwind as they' grow along a ndarly stationary line, This echo pattern is pften found only on the mountain Ibarrier, but is sometimes observed to exist across the entire Sacramento Valley and onto the barrier. This is a pattern of convective elements that exist as individual cells or clusters of cells (Fig. 2d) and cover more than one half of an area. 4nlike the other types, coverage refers to the region of convective activity (as defined by radar echoes) ,and not the entire American River Basin. C2 often covers an ~rea similar to orographic echo but is less I uniform in appearance. I This echo tyPe is similar to C2 except coverage rhust be less than one half of the region where convection is occurring (Fig. 2e). : types were associated with significantly different lapse rates of equivalent potenti~ temperature (8e). Fig..3 presents the, mean lapse rates of 8 e for each PET. The profiles generally show incrJasing convective instability I I . I I i.",; i J ,~~~~~tl:'~~'0';"i\t~io;~'i;i.~5;j".., ~. ... from right to left as indicated by data taken from 1976- 77 through 1979-80. The various echo types have been observed in a variety of synoptic conditions, but a distinct sequence of echo types is often observed in well organized cy- clonic storms with their associated and well defined 700 mb trough passages. The occurrence of A W echoes was almost entirely pretrough with a peak 5-6 h ahead of trough passage. Major bands were widely distributed before and after trough passage with a major peak 2 . h ahead of the trough and minor peakS 1 and 3 h behind the trough. The distribution of embedded bands was centered on trough passage. The C 1, C2, and CT echo types were post-trough phenomena with peaks at 7, 4.5, and 5.5, respectively, behind the trough. Orographic echoes occurred mostly post-trough with a broad distribution peak 2 h after passage. Because the amount of aircraft data available for each PET was limited, it was decided to combine PETs into three major categories. The three combinations are shown in Table 2, with the respective number of days that each type and combinations were flown. Ta- ble 2 also shows the percentage frequency of occurrence of each echo type derived from radar from five winter field seasons, from 1976-77 to 1981-82, not including 1980-81 when there were no operations. The area-wide combination (0, A W, WB) represents a relatively stable subset of the seven echo types. The banded subset (MB, CT) is made up of two distinct echo features, which were combined based on their frequent simultaneous occurrence. The cr type is very unstable and the MB is a mixture of stable and unstable environs. The cellular combination (C1, C2) represents the unstable cumulus convective elements. 4. The L WC/ICC ratio Magnitudes and distributions of L WC and ICC in clouds can be used to help estimate opportunities for augmenting precipitation at the ground. Many other variables such as wind velocity and shear, cloud life- times, terrain influences, and precipitation trajectories need to be considered when making a complete as- sessment of seeding opportunities. However, if the candidate clouds have insufficient liquid water content and an overabundance of ice crystals initially, the re- maining considerations are academic. We believe the LWC/ICC ratio is a good measure of the microphysical potential for enhancing precipi- tation. It is possible that when our measure of seed- ability looks adequate in a given situation, other factorS may still negate attempts to increase precipitation at the ground.W e believe that these factors can be eval- uated independently at later steps in the precipitation process. It is understood qualitatively by those working in cloud physics and weather modification that when a cloud has a large L WC it is most likely to respond to