Precipitation formation from orographic cloud seeding

Fig. 2 shows radar reflectivity from the Packer John DOW radar over a 60-min period during case #1. Seeding began at 0000 Coordinated Universal Time (UTC) and continued for eight legs, concluding at 0130 UTC. EJ flares were released on all legs about every 30 s, corresponding to 3-km spacing along the flight track. BIPs were used only on flight legs 2 to 5 and leg 8. Before 0030 UTC, the DOW detected only a few small radar reflectivity echoes in excess of 10 dBZ, 30 km to 40 km east of Packer John. Between 0030 UTC and 0100 UTC, radar echoes with reflectivity between 15 dBZ and 25 dBZ began to appear 5 km to 35 km SE of Packer John. The first two lines farthest downwind at 0100 UTC clearly show individual isolated areas of enhanced reflectivity spaced at about 3 km, identical to spacing of EJ releases and distinctly different from a pattern that would be produced due to AgI released through BIPs or ice produced by the aircraft themselves, both of which should result in a continuous line. As these lines of enhanced reflectivity moved downwind, they broadened and merged. Subsequent lines of enhanced reflectivity with maximum values between 20 dBZ and 30 dBZ appeared between 0100 UTC and 0115 UTC, continuous due to the use of both BIP and EJ flares. Variations along lines were likely due to spatial heterogeneity in wind and cloud properties. A zig-zag pattern of continuous radar reflectivity lines was readily visible at 0115 UTC. Lines of isolated reflectivity maxima that began to appear at 0130 UTC resulted from the use of EJ flares only in seeding legs 6 and 7. The reflectivity lines continued to be visible until 0255 UTC, after which the area largely became devoid of radar echoes again.

Measurements from case #2 revealed enhanced radar reflectivity from two seeding lines from both EJ and BIP flares and provided a comprehensive set of observations from ground-based radar and from airborne in situ and radar measurements through the entire evolution of the seeding plume. In this case, the seeding aircraft flew a total of six legs. The first two legs were flown at 4 km above mean sea level (MSL), just at the top of cloud at that time. However, on all subsequent legs, the seeding aircraft flew at 4.5 km above MSL, about 1 km above cloud top. EJ flares released at this altitude burned out before entering the cloud, and thus these legs were too high to impact precipitation formation with AgI seeding material. A line of enhanced reflectivity with maximum value of about 20 dBZ first appeared roughly 45 min after completion of the first seeding leg (Fig. 3, 1710 UTC). The line appeared about 30 km downwind of the track of the seeding aircraft, and extended from near Packer John on its northwestern end to the southeast edge of the radar scan. At this same time, the vertical dimension of the line (Fig. 4) extended from cloud top, about 4 km above MSL, to just above the surface. The reflectivity return had a strong tilt due to a differential wind speed of about 5 m⋅s⁻¹ between 3 km and 4 km above MSL. Fifteen minutes later, at 1724 UTC, a second line was readily visible about 5 km downwind of the first. The appearance of this line, on the downwind side of the first line, resulted from the seeding aircraft shifting roughly 17 km downwind for its second flight leg. Subsequent scans from both DOW radars allowed the horizontal and vertical evolution of these two lines to be tracked. By 1740 UTC, both lines extended from cloud top to the surface, and their maximum reflectivity increased from 20 dBZ to >28 dBZ. As the reflectivity lines reached the northeastern portion of the study area, the radar signals in the lower portions became blocked by terrain.

Over a 75-min period, the WKA passed through or just below the seeding lines seven times. At 1629 UTC, the seeding aircraft passed 28 km directly upwind of Packer John radar on its first flight leg. Assuming a steady wind speed of 11 m⋅s⁻¹ based on WKA measurements, by 1651 UTC, the seeding material and any newly formed ice crystals would have advected 14.5 km downwind, to a location ∼13.5 km upwind of the radar. On its first leg at 1651 UTC, the WKA passed below an area of enhanced reflectivity, detected by the WCR, about 12.5 km upwind of the Packer John radar site (Fig. 5), within 1 km of the expected location of the seeded region. The WKA was at an altitude of 3.6 km MSL, 500 m to 1,000 m below cloud top, at a temperature of −11.5 °C. The region of enhanced reflectivity extended from flight level to the top of the cloud and had maximum reflectivity of 5 dBZ, about 20 dBZ to 30 dBZ greater than reflectivity at that altitude in the surrounding area. In situ probes on the WKA indicated that the cloud at this level outside the seeded regions was composed mostly of liquid water droplets, with mean diameter of about 20 μm, concentration of 25 cm⁻³, and a liquid water content of about 0.08 g⋅m⁻³ (Fig. 6). At flight level, ice water content was below the detection limit of the probe, less than 0.01 g⋅m⁻³.

Seven minutes later, at 1658 UTC, the region of enhanced reflectivity had descended through the WKA flight level and increased to 10 dBZ (Fig. 5). The maximum reflectivity was located 500 m above flight level and the same distance below cloud top. The enhanced reflectivity line was 7 km upwind of Packer John; the expected location of the seeding line assuming simple advection and a constant wind speed was 8.5 km upwind. In the line, at flight level, the liquid water was 0.04 g⋅m⁻³, reduced by half from the first pass. The ice water content, however, increased to 0.15 g⋅m⁻³. Measurements from cloud particle spectrometers indicate that the ice particles ranged in diameter from a few hundred microns to about 1 mm, in concentrations of about 4 L⁻¹, compared with concentrations outside of the line of 10⁻² L⁻¹. Particle images suggest that the crystals were moderately rimed. In the region a few kilometers downwind of the enhanced reflectivity, the cloud remained devoid of ice, and liquid water content ranged between 0.05 g⋅m⁻³ and 0.10 g⋅m⁻³.

During the time between the first and second pass, the region of enhanced reflectivity drifted about 5.5 km, consistent with WKA flight-level wind measurements. The location of the reflectivity region during each of these passes relative to the location (and time) that the seeding material was released match to within 1 km to 2 km, assuming a constant 11 m⋅s⁻¹ wind speed. Recall also that, at 1710 UTC, the first seeding line was detected by ground-based radar at an altitude between 3 km and 4 km above MSL about 3 km downwind of Packer John (Fig. 4). The enhanced reflectivity detected by the WCR during the WKA’s second pass at 1658 UTC drifted ∼10 km by 1710 UTC and was located just downwind of Packer John, confirming that this enhanced reflectivity echo was the seeding line detected by the ground-based radars.

At 1718 UTC, the WKA made a third pass through the first (upwind) seeding line. By this time, the radar signature from the WCR extended from cloud top down to the surface. The second seeding line was also detectable from the WCR reflectivity return, near the top of the cloud, 5.5 km downwind of the first line. For this pass and the subsequent one at 1730 UTC, the WKA ascended to 3.9 km above MSL, about 500 m below cloud top where the temperature was −13.5 °C. In the first seeding line, none of the in situ probes detected any supercooled liquid. Maximum ice water contents were 0.15 g⋅m⁻³, and in situ probes detected ice in a 2-km-wide area. Ice particles were mixed between moderately rimed and branched crystals, suggestive of growth through vapor deposition. Particle size distributions (Fig. 6, Bottom Right) indicate that the mean (maximum) diameter for the ice crystal population was about 1 mm (4 mm) within the seeding line at flight level. Concentrations of particles with diameters less than about 70 μm were small, at or below the detection level of the probes. For larger particles, concentrations were 0.8 L⁻¹. Conversely, outside of the seeding line, all of the particles detected were smaller than 100 μm and appeared to be liquid. In the second seeding line, which was first detected by the WCR at 1718 UTC, much of the water mass had not yet been converted to ice at flight level. Maximum cloud liquid water contents were 0.06 g⋅m⁻³, as were ice water contents.

By the time the WKA passed through the seeding lines again, at 1730 UTC, the lines had widened to 2.5 km to 3.0 km at flight level, and maximum ice water content had increased to 0.25 g⋅m⁻³ in the first line and 0.35 g⋅m⁻³ in the second. Little liquid water remained in either line, less than 0.05 g⋅m⁻³ at flight level. In the first line, most particles had grown to diameters in excess of 1 mm, and crystals as large as 8 mm in diameter were detected. Nearly all of the crystals appeared to be branched dendrites with only a light degree of riming. In the second line, crystals were smaller, with a mean diameter about 700 μm, and the crystals were heavily rimed. In both lines, reflectivity from the WCR had increased to a maximum of about 10 dBZ within about 500 m of the flight level of the WKA, and ice crystal concentrations for particles larger than 300 μm in diameter were 1 L⁻¹ to 5 L⁻¹. Concentrations outside of the seeded regions were less than 0.1 L⁻¹.

Between 1741 UTC and 1804 UTC, the WKA made three more passes as the seeding lines continued to drift downwind over the mountains. During these passes, the WKA flew at 3.6 km above MSL. By 1741 UTC, WCR reflectivity from both seeding lines extended from cloud top to the ground. No liquid was detected in either of the seeding lines, and maximum ice water contents had decreased to about 0.25 g⋅m⁻³. Particle size distributions indicate nearly all ice crystals were between 200 μm and 2 mm in diameter, with a mean size of about 500 μm, significantly smaller than earlier passes. During the last two passes at 1759 UTC and 1804 UTC, the >10-dBZ reflectivity region extended over nearly the entire vertical extent of the echo, with very little liquid water (<0.05 g⋅m⁻³) at flight level. Ice water content in excess of 0.45 g⋅m⁻³ was observed at flight level in the first seeding line at 1759 UTC, but it decreased to just over 0.10 g⋅m⁻³ in the subsequent pass. Below the level of the WKA, the WCR radar return from just above the surface suggests a region of hydrometeor growth with higher reflectivity extending over a broad area.