The Dispersion of Silver Iodide Particles from Ground-Based Generators over Complex Terrain. Part I: Observations with Acoustic Ice Nucleus Counters

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VOLUME 53 J O U R N A L O F A P P L I E D M E T E O R O L O G Y A N D C L I M A T O L O G Y JUNE 2014 The Dispersion of Silver Iodide Particles from Ground-Based Generators over Complex Terrain. Part I:
VOLUME 53 J O U R N A L O F A P P L I E D M E T E O R O L O G Y A N D C L I M A T O L O G Y JUNE 2014 The Dispersion of Silver Iodide Particles from Ground-Based Generators over Complex Terrain. Part I: Observations with Acoustic Ice Nucleus Counters BRUCE A. BOE,* JAMES A. HEIMBACH JR., 1,# TERRENCE W. LULIN XUE, & XIA CHU,** AND JOHN T. MCPARTLAND 11,# * Weather Modification, Inc., Fargo, North Dakota 1 Department of Atmospheric Sciences, University of North Carolina at Asheville, Asheville, North Carolina, and Springvale, Krauss Weather Services, Inc., Red Deer, Alberta, Canada & National Center for Atmospheric Research, ## Boulder, Colorado ** Department of Atmospheric Science, University of Wyoming, Laramie, Wyoming 11 U.S. Bureau of Reclamation, Evergreen, Colorado (Manuscript received 25 July 2013, in final form 5 March 2014) ABSTRACT Part I of this paper presents the results from a series of plume-tracing flights over the Medicine Bow and Sierra Madre Ranges in south-central Wyoming. These flights, conducted during February and early March of 2011, were part of the Wyoming Weather Modification Pilot Project. Effective targeting of ground-based silver iodide plumes to supercooled clouds has long been a problem for winter orographic cloud-seeding projects. Surface-based ice nucleus (IN) measurements made at a fixed location near the Medicine Bow Range target area had confirmed the effective transport of IN plumes in many cases, but not all. Airborne plume tracing, undertaken to further illuminate the processes involved, provided additional insight into the plume behavior while providing physical measurements that were later compared with large-eddy-simulation modeling (Part II). It was found that the plumes were most often encountered along the flight paths set out in the experimental designs and, in the absence of convection, appear to be mostly confined to the lowest 600 m above the highest terrain. All passes above 600 m above ground level revealed IN concentrations greater than background levels, however. An estimate of IN flux measured over the Medicine Bow Range was approximately 85% of that produced by the five ground-based IN generators active at the time. 1. Introduction and background Wintertime orographic snow augmentation cloudseeding operations are common in much of the American West. During the winter of 2012/13, such operations were ongoing in California, Colorado, Idaho, Nevada, Utah, and Wyoming (see online at For the most part these programs are operational in nature; that is, they are conducted for effect and not for proof of concept. Sponsors include public utilities, municipalities, # Retired. ## The National Center for Atmospheric Research is sponsored by the National Science Foundation. Corresponding author address: Bruce Boe, Weather Modification, Inc., th St. N, Fargo, ND irrigation districts, and state agencies. Many have been ongoing for decades. Reynolds (1988) and Super (1990) suggested that a significant problem in seeding winter orographic clouds is the uncertainty of adequate targeting zones of supercooled liquid water (SLW). Some weather modification experiments may have failed because the targeting issue was not adequately addressed. There is a large body of transport and dispersion studies that have been conducted in the western United States. Among the earlier of these is the Bridger Range Experiment (BRE) of Montana (Super and Heimbach 1983), a randomized exploratory experiment that was conducted during the winters from 1969 to During the early winters, airborne silver iodide (AgI) sampling was done over the north southoriented Main Bridger Ridge in visual flight rules (VFR) flight conditions. The VFR conditions allowed sampling at near-surface altitudes. The Main Bridger Ridge crest line is approximately 2600 m above mean sea level (MSL), and the target area is approximately DOI: /JAMC-D Ó 2014 American Meteorological Society 1325 1326 J O U R N A L O F A P P L I E D M E T E O R O L O G Y A N D C L I M A T O L O G Y VOLUME m MSL, which allowed sampling over the target at the approximate elevation of the Main Bridger Ridge in 1985 (Super and Heimbach 1988). Two AgI generators in were located two-thirds of the way up the west slope of the Main Bridger Ridge. Plumes from these two generators were consistently traced over the Main Bridger Ridge toward the target. Plume widths for the southern generator site were Insufficient data precluded estimation of widths for the northern seeding site. In January of 1985 airborne physical measurements that included acoustic ice nucleus counter (AINC) data were taken in the same experimental area (Super and Heimbach 1988). Reynolds (1988) pointed out that at that time the BRE was the only statistical exploratory winter experiment that was verified with physically consistent measurements. In 1985, tracing flights were conducted under instrument flight rules (IFR) conditions over the north south-oriented target range about 17 km to the east of the randomized experiment s southern seeding site. By Federal Aviation Administration (FAA) regulations, IFR sampling is allowed 600 m above ground level (AGL) above the highest terrain within 9.3 km (5 nautical mi.). A special FAA waiver allowed sampling down to 300 m AGL. In 1985, plumes from the southern seeding site were tracked over the target area. In general, the bulk of the plumes remained below 300 m AGL. The plume widths ranged from 5 to 8 km over the target. The ice nucleus (IN) concentrations were sufficiently high to effectively seed winter orographic clouds ($10 L 21 ). Direct observations of surface-released AgI that are germane to this paper s design were made in February and March of Holroyd et al. (1988) and Super and Boe (1988) describe AgI plume tracing over the Grand Mesa of Colorado using an airborne AINC. Both aerial and ground releases were tracked, but the aerial releases are not discussed. Like the BRE, surface releases were from sites that were located more than midway up the windward side of the mesa. For the surface-released plumes, the median instantaneous width was 158 and the median meandering width was 388. The median plume height was greater than 0.5 km. The plumes were discernible up to 40 km downwind and 80 min after release. The character of AgI plumes and resulting IN was analogous to those of the BRE. Likewise, there was consistent delivery of seeding material to regions of orographic cloud SLW. Warburton et al. (1995) documented how inadequate targeting of seeding material degraded the detection of a seeding signal in two target areas of the central Sierra Nevada. Snow chemistry was used to define areas of surface-released AgI above a background of 2 parts per trillions (ppt) in and around the Truckee Tahoe and Lake Almanor target areas. Westerly winds produced statistical results that were in line with physical expectations; the southerly wind partition, in contrast, produced little silver in the Lake Almanor snow samples but gave evidence of upwind control sites being contaminated. This result suggests that improper targeting may explain why the Lake Almanor seeding did not produce a seeding effect. Direct observations of surface-released AgI were made in the early winters of 1991 and 1994 over the Wasatch Plateau of central Utah under the auspices of the National Oceanic and Atmospheric Administration/Utah Atmospheric Modification Program (Super 1999; Huggins 2007).Thetargetareawasthenorth southwasatch Plateau with an elevation of approximately 2900 m MSL, 1200 m AGL above the windward San Pete Valley. To the west of the valley are the San Pitch Mountains, about 800 m in elevation above the valley. Part of the field program involved aerial sampling of eight multiple operational surface seeding generators in the San Pete Valley during 1991 and There were two surface AINCs: one sampled at a high-altitude observation site near the crest of the plateau and the other was mobile, being in an instrumented van. A third AINC was mounted in an aircraft along with cloud physics instrumentation. A special exemption allowed IFR flights down to 300 m above nearby highest terrain. The high-altitude generators plumes traversed the plateau more reliably and with higher concentrations than were associated with the valley generators. Eighty percent of the valley plumes traversed the plateau, but the number of IN that could be activated at warmer temperatures were too few to be effective below 300 m AGL. a. The Wyoming program After being approached by conservation districts, the Wyoming Water Development Commission in 2004 decided to explore the potential for such operations in that state, beginning with a feasibility study. The results of that study, as reported by Weather Modification, Inc. (WMI 2005), suggested that potential existed, but the water development commission decided that the efficacy should be demonstrated before any state-sponsored operations were undertaken. Therefore, the Wyoming Weather Modification Pilot Project (WWMPP; Breed et al. 2014) was created to obtain additional information about the physical processes involved and to quantify the changes in precipitation thus produced. The WWMPP was funded for nine winter seasons, beginning with the winter of 2005/06, through the winter season of 2013/14. The first three seasons were used to complete environmental work, equipment deployment, and collection of physical data prior to the initiation of a JUNE 2014 B O E E T A L randomized statistical experiment (RSE). The WWMPP applied ground-based, remote-controlled AgI IN generators manufactured by WMI, operated upwind of three Wyoming mountain ranges (Breed et al. 2014). This winter orographic seeding strategy relies upon ambient winds to transport and mix the ice nuclei upward into clouds bearing supercooled liquid water, where snow formation occurs. The primary purpose of this paper is to present observations of AgI plumes produced by the WWMPP groundbased IN generators, as measured above the Medicine Bow Range (MB). The airborne observations provide a basis for validation of high-resolution large-eddy simulation (LES) (Xue et al. 2014, hereinafter Part II) and also provide some physical evidence of the generator IN output and transport and dispersion to the regions above the MB that would be occupied by supercooled cloud during seeding. b. The ice nucleus generators The IN generators used for the WWMPP burn an AgI solution that after combustion yields a nucleus of AgI 0.8 Cl 0.2 -NaCl formulation (DeMott 1997), shown to function effectively by the condensation freezing mechanism. All generator components through which the solution flows are corrosion resistant, being either stainless steel or appropriate plastics. Solar panels provide the power. Connection for activation and monitoring is done via satellite. When a generator is activated, the propane burner is ignited within the wind shroud, open at the top and bottom. Once flame temperature confirms propane ignition, solution flows through an Intek, Inc., Rheotherm flowmeter, up to the burner head, where it passes through a nozzle and is atomized into the propane flame and burned. Combustion occurs at 5.7 m AGL. When the seeding is complete, the solution flow ceases, and the solution line is purged with nitrogen. When the purge is complete, the propane flow ends, and the burner goes out. Real-time flow rates are provided by the Rheotherms, and flame temperatures are measured by thermocouples. In addition, system pressure (to push solution up to the burner) and battery voltage are monitored. Four of the generator sites also have Vaisala, Inc., WXT-510 weather stations that add surface weather observations to the generator data streams. Thus, ignition is always confirmed, and flow rate known for each generator, in each seeding event. c. The ice nucleus counters One AINC was flown and another operated simultaneously at the surface. These two units were previously run side by side while sampling from a common source (Heimbach et al. 2008). The comparative tests showed reproducible and quantitative agreement. Using ratios of total IN counts for each test adjusted for the difference in sample rates gave ratios of the airborne AINC to the ground-based AINC ranging from 1.04 for condensation freezing nuclei to 1.66 for contact nuclei. The IN produced by the WMI generators and solution used in Wyoming function by the condensation freezing process (DeMott 1997). 2. Measurements with the AINC Detailed specifications of the two AINCs used in this study are presented in Table 1 of Heimbach et al. (2008). Following their convention, unit 1 herein refers to an AINC built by Langer, the inventor of the instrument (Langer 1973), and unit 2 refers to one that was built by the second author. Sample air is drawn into the AINC s humidifier at approximately 10 L min 21 and a side stream of sodium chloride (NaCl) cloud condensation nuclei (CCN) are added to the humidified air prior to entering the cloud chamber. The sample, now fortified with additional moisture and CCN, flows into the cloud chamber where a supercooled cloud is maintained at 2208C, as measured in the lower chamber. There, those IN active at the cloud s temperature or warmer nucleate ice crystals that grow to detectable size: ;20-mm diameter. Ice crystals exiting the base of the cloud chamber rapidly accelerate as they pass through a glass Venturi tube, producing an audible click. The sound is detected by a microphone connected to an electronic signal processor. Each legitimate count triggers a transistor transistor logic (TTL) signal that is sent to a data system for real-time counting, display, and archiving at 1 Hz. In this effort, the AINC chambers were operated at temperatures of 2188 and 2208C in AINC unit 1 and unit 2, respectively. These temperatures are easily cold enough to grow ice particles nucleated from AgI, which can be activated at 288C. These crystals have ample time to grow to detectable sizes (20 mm) before exiting the chamber through the sensor (Heimbach et al. 2008). Most natural IN activate at significantly colder temperatures, however, and often do not have time to grow to detectable sizes in the AINC chamber. To verify that this is indeed the case in the MB and Sierra Madre Range (SM) of Wyoming, the ground-based AINC (unit 1) was operated at 2208C for many hours in the absence of seeding during several winters ( ), and the airborne unit likewise was operated more or less continuously from 2188 to 2208C while in flight. In all cases, the background was observed to be very low, well less than 0.1 L 21. Many times, tens of minutes passed with nothing detected at all. Thus, IN plumes as reported herein are 1328 J O U R N A L O F A P P L I E D M E T E O R O L O G Y A N D C L I M A T O L O G Y VOLUME 53 anything more than single isolated IN detections. The discernment of plumes defined in this way is not difficult [e.g., see the total counts column of Table 3 (described below in more detail) in which raw counts (individual IN detections not converted to IN per liter) are reported]. The weakest plume observed in this case study produced 118 counts in a single pass, which, corrected for sample rate, translated to a peak concentration of 87 L 21. The WMI IN generators have not been directly calibrated for yield of IN because of the unavailability of a suitable U.S. testing facility such as the Colorado State University isothermal cloud chamber (ICC) previously used for this purpose (e.g., DeMott et al. 1995). Although other, similar generators had been tested prior to the decommissioning of the ICC, the AgI-complex solution and generators presently being used in the WWMPP were not. A nearly identical solution was tested at the ICCwhenburnedwithanairborne-styleINgenerator (DeMott 1997), however, although not with wind-tunnel drafts at speeds commensurate with those typical of ground-based generators. Neither the ICC nor the AINCs precisely mimic typical winter orographic clouds (Boe and DeMott 1999). Liquid water content (LWC) within the ICC was generally reported for two values, 0.5 and 1.5 g m 23, with corresponding cloud droplet concentrations of about 2100 and 4300 droplets per cubic centimeter, respectively (Garvey 1975). New cloud droplets were continuously introduced to maintain LWC, and ice crystals were frequently collected on microscope slides for up to 50 min after aerosol introduction. The ICC droplet concentration and LWC values were consistently greater than most winter measurements within orographic clouds of the Intermountain West (e.g., Rauber and Grant 1986). Greater droplet concentrations are required within AINC cloud chambers to enhance the probability of nucleation and ice crystal growth within the limited chamber residence time typically about 1 min. Table 2 of Langer (1973) indicates that, for cloud and humidifier temperatures employed in this effort, LWC varied from about 1.5 g m 23 at the cloud-chamber top inlets to about 2gm 23 by the bottom exit. Calculations and observations suggested that typical droplet concentrations were in the range from to cm 23. The AINC functions by forcing nucleation by whatever process to maximize detection of AgI aerosol concentrations. These and other differences from natural clouds suggest some caution should be used in applying ICC or AINC results to winter orographic clouds. Despite these limitations, AINCs have been successfully used in many studies to document the presence and extent of AgI plumes (Holroyd et al. 1988, 1995; Super et al. 1975). Although the AINC does not provide the quality of IN measurement afforded by more sophisticated instrumentation such as the continuous-flow diffusion chamber (Rogers et al. 2001; DeMott et al. 2011), it is small enough to be flown effectively in a small aircraft. When operated with the complete knowledge of their principles and implementation of proper procedural safeguards, it provides a reliable and reproducible means by which AgIcomplex IN plumes can be consistently measured. The interested reader is referred to Langer et al. (1967) and Langer (1973) for further details of the design, functionality, and operation of the AINC. Before ambient air sampling began in each airborne experiment, phloroglucinol mist, an organic nucleant, was injected into the sample airstream to verify the functionality of the AINC. This procedure was repeated at the conclusion of each sampling period. As shown by Langer et al. (1978), phloroglucinol is an excellent surrogate for AgI. It has a very short persistence time and therefore affords minimal contamination potential. The AINC operators continuously monitored all instrumentation parameters throughout each flight, thus quickly identifying problems if and when they occurred. 3. Ice nucleus measurements in the WWMPP Physical measurements and numerical modeling are part of the WWMPP evaluation (Breed et al. 2014). The IN output produced by the generators was established (DeMott et al. 1995; Super et al. 2010), but additional verification of the desired transport and dispersion of these plumes was also deemed necessary to verify targeting in these ranges. Of specific interest was the time required for residual seeding plumes to vacate both the target and control areas in the randomized, crossover design. The most affordable avenue was to measure the IN in the MB, which sometimes is downwind of the SM, which was also targeted. Surface-based IN measurements were conducted initially, because they allowed continuous sampling over many hours. Table 1 provides the relative elevations of the key project facilities. All of the airborne-plume-tracing efforts reported herein used at least some IN generators sited at relatively high elevations some less than 400 m in elevation below that of the mean crest line (3180 m). Some sites are farther removed from the crest (MB09, MB10, MB11
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