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<br />VOLUME 29
<br />
<br />318
<br />
<br />JOURNAL OF APPLIED METEOROLOGY
<br />
<br />05 FEB 1986
<br />
<br />natural ice feature that had ICC greater than in the
<br />seedlines. This feature was associated with a mesoscale
<br />band which developed upwind of S 10 as evidenced by
<br />the echo observed there. This region, several kilometers
<br />upwind of S I 0, had ICC similar to the seeded plumes
<br />and was 3 to 4 times broader. Particle images indicated
<br />the ice crystals were needles, columns, and small grau-
<br />pel. Considering the similarities of the seeded and nat-
<br />ural ice crystal regions in particle concentration, im-
<br />ages, and size distributions, the primary reason to con-
<br />clude that seeded plumes were intercepted is the
<br />increased ICC observed at the appropriate places.
<br />
<br />(ii) Radar and precipitation measurements
<br />
<br />To compare seeding effects with radar and ground
<br />observations the OTM was used to obtain a mean ad-
<br />vection from cloud to ground for the seeded ice crystals.
<br />The result was 2650 at 7 m S-1 in rough agreement
<br />with echo motion. Only scattered, weak echoes at low-
<br />elevation angles were observed. Seeding was done pri-
<br />marily in a non-echoing cloud. Based on the concept
<br />of the fixed target experiment a radar seeding signature
<br />may be apparent in this situation, and echoes were
<br />observed to form and dissipate in three of the seedlines.
<br />Figure 20 presents a time sequence of PPls with the
<br />advection of seeding material superimposed. Echoes
<br />can be observed to form and dissipate in a portion of
<br />S2, 2220-2235; S3, 2230-2245; and S6, 2250-2305.
<br />The genesis of these echoes is ambiguous. Since seed-
<br />lines covered most of the area where echoes formed
<br />naturally, echoes would likely appear in seedlines. Also,
<br />the echoes do not fill in a large portion of the seedline,
<br />nor do they orient themselves only along the seedline,
<br />as was observed in S2 in the 18 December 1986 case
<br />study. However, in all three cases the echo appeared
<br />30 minutes after seeding material was released, grew
<br />slightly in the next 5 minutes, then dissipated or de-
<br />scended below the 1.70 beam 10 min after it appeared.
<br />During this period the echoes moved with the seedline.
<br />These observations of echo formation and dissipation
<br />in the seed line are consistent with trajectory calcula-
<br />tions for the seeded ice crystals moving through the
<br />1. r beam.
<br />Three natural echoes, labeled NI-N3 in Fig. 20, were
<br />present at the seed point as seeding material for S 1,
<br />S4, and S 10 was released. These echoes provide a check
<br />on the advection scheme used for the seedlines since
<br />they could be tracked through part of the precipitation
<br />network. The precipitation rate from affected stations
<br />is shown in Fig. 21, annotated with the PPE for S I-
<br />S 10 and the arrival of N 1- N3 on the 1.7 degree PPI.
<br />Seeding material for S 1 was released into N 1 west of
<br />Yuba Gap (YUB). This echo could be tracked through
<br />the precipitation network from Blue Canyon (BLU)
<br />to YUB, Plavada (PLA), KGV, and Castle Valley
<br />(CAS), with an average advection of7.6 m S-I. Note
<br />that except for Snow Mountain (SNO), NI arrived at
<br />
<br />scpp 5 MINUTE PREClP.
<br />8
<br />
<br />S8:MLLE CNlI'ON (1609) , ,
<br />6 _ _ _ _ _ _ _._ _ _ _ _ _ _ _.Uf'~INp !If_ S.E~Dl,IN~S_ _ _ . . .
<br />
<br />N. Nz IN' I
<br />4 - - - ~ - "+ -:- - - - - - to: - - - - - - - : - - - - - - -
<br />2 - - - -
<br />
<br />
<br />!f 2
<br />
<br />....
<br />
<br />GMT ( HaJR )
<br />
<br />FIG, 21. Precipitation-rate measurements annotated with the pre-
<br />dicted arrival times ofSI-SIO, using 2650 at 7 m S-I, and with the
<br />arrival of NI-N3 as observed on the SHR radar. The parentheses
<br />around some natural echoes indicate inferred arrival times. Precip-
<br />itation gauge elevation (m) is shown following the gauge name.
<br />
<br />each of the gauges coincident with the onset of precip-
<br />itation at that location. At 2215 as NI arrived at PLA
<br />(Fig. 20a) N2 was observed beneath S4 (not shown),
<br />and at 2255 (Fig. 20d) N3 can be seen behind SIO. N2
<br />and N3 can both be tracked through the precipitation
<br />network and can be used as a window to bracket the
<br />arrival of seeded precipitation from S5-S10. Coinciding
<br />with the PPE of S5-SIO, the gauges at SNO, PLA,
<br />KGV, and CAS showed increases of approximately 0.2
<br />mm, while BLU and YUB, both upwind of the fallout
<br />of seeded precipitation, recorded no precipitation.
<br />These gauges did, however, record precipitation during
<br />the passage of the natural echoes, except for N3 at BLD.
<br />Although it is not possible to claim that the precipi-
<br />tation measured during the PPE for S5-S 1 0 resulted
<br />exclusively from seeding, the measurements do provide
<br />
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