"REVIEWED" A PRIL2001 LONG

Review of Persistence Effects of Silver Iodide Cloud Seeding

by Alexis B. Long

P.O. Box 41,144 Jasper Road Bentleigh, Victoria, Australia 3204

Abstract. This paper is concerned with persistence effects of cloud seeding for precipitation enhancement. Effects may last for hours or days. Persistence of cloud seeding effects means that this environmental technology may affect the microphysical structure of clouds and the development of precipitation for a significant amount of time after the seeding has been completed.

According to Rottner, Brown, and Foehner (1975) persistence may complicate the evaluation of a cloud seeding experiment and reduce the perceived net effect of the seeding. The sensitivity of the experiment to the actual net effect may be reduced. Their work in Colorado and New Mexico demonstrated a smaller cloud seeding effect because of persistence. When no account was taken that cloud seeding material was incorrectly present in the control period part of the time, the seed:no-seed contrast in precipitation and the effect of seeding were smaller. When the incorrectly seeded parts of the control period were reassigned to the target period, the contrast and the effect of seeding were greater. There has been considerable post-analysis of precipitation data associated with cloud seeding experiments in Australia by Bigg and colleagues in a search for persistence effects. Unfortunately, there appears to be a flaw in some (but not all) of the analysis which exaggerates the time span (said to be up to two weeks) of the effects.

Artificial ice nuclei are generated by the cloud seeding apparatus and are injected in various ways into a cloud to increase precipitation. The surmised connection between higher nucleus concentrations and increased precipitation suggests that measuring and examining ice nucleus concentrations for a period of perhaps a few days after seeding may be worthwhile in a search for persistence.

Bigg and others have suggested that some silver iodide ice nuclei released in cloud seeding may be carried, presumably with precipitation, to the surface. There the nuclei are believed to stimulate chemical reactions, possibly on plants, that create products that are emitted into the atmosphere to function as persistent ice nuclei. An alternative scenario is that the deposited silver iodide modifies or otherwise causes bacteria on the plants to loft into the atmosphere and also act as persistent ice nuclei.

also be other effects extending into the 1. INTRODUCTION future some hours, days, weeks or months. These persistence effects complicate the Persistence effects of cloud evaluation of a cloud seeding operation or seeding with silver iodide mean experiment. The evaluation" normally that,while seeding may have immediate assumes cloud seeding effects essentially effects -- occurring perhaps 0.5 1 hr are turned on and off, except for a short after the seeding -- on the 0.5 -1 hr transport lag time, by executing microphysical structure of clouds and the the seeding or terminating it. The development of precipitation, there may complication may be such that the net 10 JOURNAL OF WEATHER MODIFICATION VOLUME 33 effect of cloud seeding is apparently compared. reduced and the perceived benefit is diminished. Emerging from this work was a recommendation that an ice nucleus The U.S. National Academy of concentration measuring network should Sciences (1973) reviewed the work be incorporated into any cloud seeding several investigators of persistence experiment using silver iodide. The effects. Although it was fairly well network should cover the target area accepted that: persistence, if present, since that is where silver iodide nuclei could have important effects on the are to be present in greatest evaluation of cloud seeding effects the concentrations and where seeding effects evidence for persistence itself as of the are to be most pronounced. The network NASreport date was ambiguous. Since that should also extend downwind of the target date, there has been further research. area since this may demonstrate where Both the early research and the later persistence is occurring and why research are examined here. downwind effects (treated in detail in companion review paper (Long, 2001)) 2. EARLY RESEARCH are observed. If the ice nuclei may be entrained in local circulations around Cloud seeding is usually and about the target area, the network characterized by the dispersal into the should also extend laterally and upwind of atmosphere near a cloud or into a cloud the target area and control areas. This itself of a smoke of ice nuclei composed of may demonstrate seeding effects in these the chemical silver iodide. These nuclei outlying areas occurring despite lead to the development of small ice confinement of any seeding to favorable crystals in ch)uds which, through a chain wind regimes.The network should operate of physical events, produce precipitation throughout a cloud seeding experiment, at the ground. In addition to any silver during off-seasons, and for perhaps one iodide ice nuclei produced by cloud year after the experiment has concluded. seeding there are natural ice nuclei The network should operate throughout occurring in relatively low any pre-determined quiescent, not-seeded concentrations throughout the periods inserted within the experiment to atmosphere. Low measured ice nucleus check for short-term persistent effects of concentrations suggest the nuclei are the seeding. naturally induced, whereas high concentrations may indicate an artificial 3. BOWENRESEARCH source such as cloud seeding. If the high concentrations occur for some time Bowen (1966) showed that in (hours, days, weeks, or months) after number of cloud seeding experiments the seeding then these elevated increase in precipitation calculated to be concentrations may reflect a persistence due to the seeding decreased with time. of the cloud seeding nuclei. According to Bowen (1966) these experiments (Smith, 1974) included those Early reports of such persistence in the Snowy Mtns, the New England came from Boucher (1956) and Grant region of New South Wales, the (1963) who both observed elevated Warragamba catchment, and the South "freezing" ice nucleus counts for days, Australia areas of Australia. Effects were weeks, or months after seeding was also observed in Israel and Arizona. discontinued. This evidence would be Either the seeding effect actually more strongly supported if good decreased with time or else some factor climatological information on ice nucleus was masking the seeding effect which in concentrations existed which could fact remained approximately constant establish the natural background levels, with time. Bowen’s (1966) mathematical and variations from those levels, against analysis suggested a persistence model which measured artificial cloud seeding such that the seeding effect extended into ice nucleus concentrations can be successive subsequent periods of time APRIL2001 LONG I1 when the clouds were not to be seeded as photodeactivation is negligible), but required by the statistical controls rather the model implies a progressive applied to the experiment. decrease in the sensitivity of the experiment to the seeding effect with Bowen’s analysis implies that if time. The sensitivity is an issue because some fraction a of a percentage seeding nominally not-seeded data are effect s (increase in precipitation) in contaminated by seeding effects given seeded period extends into the overlapping from seeded periods which subsequent no-seed period, then the net cause a reduced seed/no-seed difference effect over the lifetime of a cloud seeding or ratio of precipitation amounts in the experiment is for there to be a seeding target area. An example of this reduced effect in the final no-seeded period that is sensitivity appears in Bowen’s analysis of a times as large as the seeding effect in the double ratio. This ratio is commonly the final seeded period. As a result, the used to measure cloud seeding effects in perceived seeding effect overall is an experiment. It reflects the relative degraded from about s to about s-a x s, say magnitudes of the seeded and not-seeded from 30 percent to 9 percent if a is 0.7. precipitation amounts in a target area. In This degradation means there is, Bowen’s specific example the double ratio inadvertently and misleadingly, an is shown to decrease from 1.2 (20 percent apparent decrease in the result of the increase in precipitation) to a perceived experiment with time. 1.1 with accumulated seed and no-seed.’: data sets even though the actual seeding From the cumulative properties of effect of 20 percent remains constant. the analysis this apparent decrease is greater for shorter, more frequent seeded It: should be noted that use of a and no-seeded periods of time and for single area or cross-over cloud seeding longer experiment lifetimes. There is a experimental design instead of the target- build-up of the degrading effect with time control-design considered by Bowen on all seeded and not-seeded precipitation (1966) will tend to exacerbate the effects and it becomes difficult to distinguish of persistence. In these two cases there is the actual seeding effect. The effect of either no not-seeded control area or one persistence, devolving from the non-zero of the two control areas is always seeded ...... factor a, will decay with time if seeding is Both circumstances work against..: terminated and restarted after some retaining part of the experiment in extended gap in time. In other words it is continual no-seed state and thus permit possible to avoid the effects of persistence persistence effects to invade the by careful design of an experiment. experiment. Persistence may be less if there is Bowen (1966) recommended photodeactivation of the silver iodide with measures be taken to negate the effects of time which would have occurred prior to persistence. First, good historical the early 1970’s when sodium iodide was precipitation records should be assembled combined with silver iodide in the prior to the experiment to form a base seeding solution from which the smoke against which to judge the experimental was made. After the early 1970’s (Dennis, precipitation amounts before persistence 1980) the solution contained ammonium effects occur. Second, the randomization iodide instead. There was then less should not be on the precipitation storm photodeactivation and presumably more or day, but rather a longer period such as persistence. the week, to counter the otherwise geometric increase of persistence effects. It should be noted that the Bowen Third, there should be a cohtrol area persistence model does not imply a which should never be seeded in order decrease in the effectiveness of post- that its precipitation not be influenced by 1970’s seeding with time in an experiment persistence. Fourth, the experiment (the seeding effectiveness remains as should be alternately opened and closed good as initially provided for long time periods such as a month or a 12 JOURNAL OF WEATIIER MODIFICATION VOLUME 33 year so that estimates can be made of the 4. EARLY- MIDDLE1970’S RESFARCEI build-up and decay of persistence. In contrast to the persistence Importantly, Bowen (1968) studies based on precipitation variables advanced one possible explanation for the of Bowen(196S, 1966, and 1968) and Gabriel persistence of cloud seeding effects. He , Avichai, and Steinberg (1967) was the believed that persistence is not due to a work of Reinking (1972). It was similar prolonged high concentration o.f silver the studies of Boucher (1956} and Grant iodide ice nuclei. He believed persistence (1963) and is now summarized herein. The is related to the presence of elevated rain horizontal and vertical air motions amounts in seeded areas. Bowen notes, present when silver iodide ice nuclei are first, that precipitation augmented by released are such that a fraction of the seeding leads to greater re-evaporation released nuclei do not immedia[ely reach into the atmosphere and a moister the cloud and are not used. These residual environment for cloud development. nuclei may be transported out of the Second, it appears (Twomey, 1960) that seeded area although some of them can be cloud condensation nuclei (CCN) are expected to remain. A fraction of the fewer over moist ground. residual nuclei may add on to the existing Accordingly,there is a conversion of population of natural nuclei and , if clouds from continental to maritime eventually ingested into c[oads, may character with a corresponding increase affect the precipitation in a way that in propensity to rain naturally. This persists beyond the initial precipitation conclusion is supported by the findings of pulse augmented by the initial seeding. Warner (1968) indicating that increased This additional precipitation may occur in CCNproduced by sugar cane fires leads to a nominally not-seeded period of time and less convective showery rain and reduce the seeded/not-seeded contrast in conversely reduced CCN concentrations precipitation and thus any perceived lead to more rain. The combined effect of seeding effect. The sensitivity of the greater water vapor and fewer CCNthen experiment is thereby reduced. Whereas would lead to increased precipitation on a a fraction (#1) of the residual nuclei no-seed day following a seed day. would act in approximately the above way it should be recognized that there is In response to Bowen (1965, 1966) another fraction (#2) of the residual Gabriel, Avichai, and Steinberg (1967) unused silver iodide ice nuclei which studied the precipitation data in the deposits from the atmosphere onto ground crossover Israeli experiment in a search surfaces, vegetation, and possibly in for persistence effects. The analysis snowpacks. These long-term nuclei (#2) considers progressively shorter periods may be released later into the atmosphere of time starting with season-to-season with effects on precipitation which can then turning to within-season be erratic and diluted. Reinking’s work persistence and finally treating day-to- did not deal with them but rather with the day persistence. The statistical analyses fraction (#1) of undeposited residual ice demonstrated no persistence effect in any nuclei which would be expected to have case. The authors believed the analyses an effect within cloud seeding seasons. could be adapted to other rainfall stimulation experiments to check for The data examined in Reinking’s persistence. It should be noted that study were collected with the Bigg- Gabriel, Avichai, and Steinberg (1967) Warner rapid expansion ice nucleus not comment on Bowen’s (1966) Figure counter at two sites in the Colorado which showed a reduced effect of seeding Rockies namely the High. Altitude with time in Israel. The persistence Observatory (HAO) and Rabbit Ears Pass shown by Bowen (1966) is not consistent (REP) . (Note that the Bigg attributed with their findings. the Bigg-Warner counter is the same individual whose extensive later work on persistence is reported below.) The nuclei measurements were for silver iodide A PRIL2001 LONG 13 seeding with 20 g per hour ground suggestions related to detecting and generators. A total of 10 years of IIAO and investigating the persistence effect in 4 years of REP measurements were seeding experiments of crossover design. obtained. The measurements were assigned to a succession of time periods Simpson and Dennis (1974) included with S denoting the time period of the a list of tentative causes for extended seeding itself, D denoting the remainder temporal effects of seeding. These causes of that day, and D+i denoting the ith day were said to be complex, widespread, and after D, The data extended out to i=8 or 8 subtle. They included days after seeding, in response to the a. physical transport of the seeding agent view that seeding effects may last up to b. physical transport of ice crystals about a week, Examination of the data produced by a seeding agent focussed on the median count since the c. changes in radiation and thermal mean of the counts is influenced by large balance, as for example, from cloud data outliers, and the mode of the counts shadows or wetting of the ground is near the detection limit of the counter, d. evaporation of water produced by The data show that for days D+ 1 and seeding D+2 the ice nucleus concentrations were e. changes in the air-earth boundary significantly above background but that such as vegetation changes over land this did not persist beyond two days, Still, f. advection or propagation of intensified many ostensibly not-seeded control storm cloud systems which subsequently : situations were subjected to this two interact with orography or .natural calendar day persistence of (#1) residual circulation silver iodide cloud seeding nuclei. It g. cold thunderstorm downdrafts setting should be noted that a residual ice nucleus off new convection cells elsewhere. concentration such as observed was about five times as high as the Rottner, Brown, and Foehner (1975) background concentration and is summarized a study of persistence based equivalent to the cloud top being about 2 C on data from the Colorado River Basin colder than normal based on the Fletcher Pilot Project (CRBPP) and the Jemez (1962) curves. The effect of a colder cloud Atmospheric Water Resources Research on precipitation was not determined. Project (JAWRRP). Silver iodide cloud seeding material was released from In Itess’ (1974) review of weather ground generators in both project areas. modification, Brier(1974) in Chapter 5 In the CRBPPthe concentration of silver the Design and Evaluation of Weather iodide ice nuclei was measured with an Modification Experiments and Simpson NCAR acoustic ice nucleus counter. and Dennis (1974) in Chapter 6 Concentrations were found to be factors Cumulus Clouds and Their Modification of 100 to 1000 higher than background concerned themselves with the subject of some hours after seeding had ceased. A persistence of cloud seeding effects. The similarly high concentration was following material summarizes their assumed in the JAWRRP.This persistence findings. of artificial seeding ice nuclei influenced the statistical results of the analysis of Brier (1974) notes Grant’s (1963) seeding effects on precipitation amounts. finding that freezing nuclei may 1) When the defined seed and no-seed remain high for many days after seeding, periods were considered there was no 2) alter the no-seed character of a target seeding effect. Whena 6 hr period of time area, and 3) reduce the adequacy of the was eliminated from the start of the no- controls to establish a seed/no-seed seed period because it may have been contrast for the experiment. Brier (1974) contaminated with persistent ice nuclei also noted that silver iodide nuclei may be the statistical test showed more trapped on vegetation in a target area and precipitation in the seeded period. When be blown into the air with nucleating the eliminated 6 hr periods were effect at a later time. Finally, Brier (1974) reassigned to the seeding period, there noted that Smith (1967) made several was an even greater seeding effect. These 14 JOURNAL OF WEATHER MODIFICATION VOLUME 33

three sequentially different statistical on days that were suitable for seeding but analyses suggested that a seeding effect were not seeded. These Ski days were was occurring during the first 6 hr of the compared with the main sequence of days no-seed day because of contamination of (SS) that were suitable for seeding and that period with ice nuclei remaining were seeded. Altogether, there were 211 from the previous seeded day. This work is SU days and 202 SS days. a particularly graphic demonstralion of a) how ice nuclei can remain in a seeding Bigg (1985a) applied project area after seeding has ceased, of "superposition" method of analysis to the b) how seeding effects can be masked if precipitation data.. To apply this method seeding occurs even in parts of no-seed all seeded days (SS) are counted as day control periods, and of c) how seeding zero (0). Bigg states that all target-area effects become clearer if there is a clear rainfalls on those days are summed to assignment of precipitation data to the give a total E T(0) and the mean of north seeded-category if warranted. and south control rainfalls is )" C(0). Similarly, target and control rainfalls are 5. BIGG RESEARCH summed for each day n to give )- T(n) Bigg (1985a, 1985b, 1985c, 1988, 1995), and ~ C(n). The ratio Bigg and Turton (1986, 1987, 1988) and Mather, Bigg, and Renton (1990) ( Y T(n)/ Y C(n))SS addressed the question of persistence seeding effects. The papers are largely is calculated. concerned with rainfall and ice nuclei data from precipitation enhancement The rainfall sequence subsequent to a projects in Australia. The data are particular seeded day was terminated at incorporated in several different kinds of the next seeded day. This is a particularly analysis aimed at demonstrating a important point since it ensures that the persistence effect if one is present.The precipitation nominally on an n-th day is material that follows critically examines actually on the n-th day after a seeded day this potentially important body of work. and not the rainfall on some smaller-n day after an intermediate seed day. This a) Precipitation effects termination of the rainfall sequence prevents confusion of the day and mixing The Tasmanian I experiment up of data from different n-value days. (Smith , Adderley, Veitch, and Turton, The purpose of emphasizing these 1971) was specifically designed to reduce considerations is to reflect in advance on accumulating, persistence effects of cloud the fact that later analyses by Bigg did not seeding by alternating entirely not- so terminate the rainfall sequence at the seeded (odd-numbered) years between next seeded day and hence used confused randomly seeded (even numbered) years. data. In my view, this seriously Seeding was conducted one-half of the compromised Bigg’s later work. time throughout the latter years on a randomized basis. The primary controls Note that changes in the ratio were areas to the north and south of the target , the overall control rainfall C ( I~T(n)/ Y C(n))SS being the average rainfall of the two. The first year of the experiment was 1964 and from n to n+l and so on are an indication the last year 1970. (Additionally, more of systematic changes in precipitation intense seeding was conducted in 1971.) that follow seeding. It should be noted subsidiary area to the east of the target that this ratio is not an ideal measure of was used for studies of downwind effects, persistent effects of seeding, for three discussed in Long (2001). reasons according to Bigg (1985a): 1) seeded days form a meteorologically An important feature of the 1964-70 biased selection, so that changes with experiment was the inclusion of rainfall time may be reflecting natural changes "REVIEWED" APRIL2001 LONG 15 in meteorology rather than any effect of ratios were not terminated at the next seeding, seeded day but instead carried out forward 2) the probability of encountering for n = 24 days and backward for n -- -12 another seeded day within n days days. It was argued that by going beyond diminishes in summer, so that there is the the next seeded day one uses more of the possibility of seasonal bias in changes in data, but it is equally clear that one is the ratio as n increases, confusing the data by calling some datum 3) the level of the n-th datum when in fact it may be for the zeroth or first day past the next seeded ( ~ T(n)/ ~ C(n))SS day. Thus, any composite effect such as greater precipitation ratio when undisturbed by cloud seeding can only be determined historically, but ( E T(n)/ ~NWC(n))SS climatic shifts may occur that cause historical data to be an unreliable guide. attributed to the n-th day may, in fact, The biases above may be removed by simply exist because it is near, within a making use of the SU days mentioned few days of, a seed day. Bigg and Turton above which should have a range of (1986) also argue that additivity meteorological conditions very similar to seeding effects means that one can the SS group. One then calculates consider ratios at negative times, although the physical meaning of ( ~-T(n)/ C(n))SU seeding effects before seeding has occurred is questionable. to go along with Whereas Bigg and Turton (1986) ( E T(n)/ Z C(n))SS start by considering single ratios

and forms the double ratio ( Y T(n)/ Y NWC(n))SS ( 7 T(n)/ Y C(n))SS/( 7 T(n)/~- C(n))SU they recognize there can be biases in using them and choose to use the double The double ratio decreases from values ratio instead. This is consistent with the greater than unity for n ~- 5 as expected double ratio choice in Bigg (1985a) and but then rises to values of perhaps 1.2 for Bigg (1985b). From Bigg and Turton (1986), n > 19. This suggests seeding effects may the double ratio varies with number of persist much longer than the usually days after a seeded day, and is high (1.41) accepted limit of 24 hours. But still, for on day zero . This was likely not due to large n, there are less data and they are chance. A rerandomization is used to no longer scattered throughout the gauge the statistical significance of the experiment. Thus, the results for n > 19 double ratios . Bigg and Turton (1986) may be questionable. claim from the double ratio values versus day curve a 41 percent positive seeding A subsequent paper by Bigg and effect on day zero and double that for day Turton (1986) also examined the nine. There is no discussion on how the persistence effects in Tasmania. The basic procedure of calculating the ratio physical layout of the target and control beyond the next seeded day affects the areas was as noted above but with a ratio. This is a major worry and potential northwest control (NWC) area beyond the flaw in this procedure. north area (N). The superposilion method described in Bigg (1985a) and (1985b) In a later study of persistence, Bigg again employed in Bigg and Turton (1986) and Turton (1988) considered but the sequence of combination of the data from seven different Australian precipitation enhancement experiments. Three of the ( [ T(n)/ ~NWC(n))SS experiments were of the target-control design, and the other four were 16 JOURNAL OF WEATHER MODIFICATION VOLUME 33

crossover. For lhese latter experiments it double ratio series. Bigg {1985a, 1985c) was necessary to create new control data discussed several double ratio sets. A double ratio of seeded to not-seeded formulations that may be used when a precipitation in target and control areas cross-over experiment is being evaluated. n days after a seed or no-seed day appeared to imply a prolonged after effect b) Ice nucleus effects of seeding peaking at perhaps 10-15 days after seeding. The study was repeated just Whereas the Bigg work presented for the winter season and a similar effect so far has been concerned with was found. As before, the superposition precipitation amounts which are the function was not well-defined and bottom line of a precipitation represents a confusion of the number of enhancement experiment, additional the day for which a precipitation amount understanding of the persistence effect is being allocated. Also of concern is how can be gained from consideration of ice the superposition function for no-seed nucleus information. Bigg (1985b) days takes into account data on the farside presents a time-series of the ice nucleus of an intervening seed-day. Overall, Bigg concentration northwest of the northwest and Turton (1988) consider the control area. The time-series shows a precipitation for all Australian maximum in the concentration some ten precipitation enhancement experiments days after seeding, suggesting some through 1983. persistence of the seeding effect. Yet, these concentrations are measured Although Bigg and Turton (1988) upwind of the seeding area , and it is lay out the reasons for preferring the therefore difficult to understand how double ratio analysis they still resort to silver iodide reached the sampling site. the single ratio analysis. Reference in These results are for up to 2 weeks after this review is also made to Mather, Bigg, seeding. Longer term ice nucleus effects and Renton (1990) and Bigg (1995). will now be considered. first paper compares the single ratio from Bigg and Turton (1988) with that found In both the New England and South Africa ~,here a hail suppression Warragamba experiments and in an project apparently resulted in more rain. operation in a wheat-growing area of The ratios show what appears to be a western Victoria a decrease in the ice persistence of seeding effect occurring nucleus concentration by a factor of 2 or in both South African and Australian data 3 was observed after the cessation of cloud sets 10-15 days after a seeded day. Bigg seeding activity with the decrease (1995) is concerned with single ratio occurring over a period of perhaps 6-12 precipitation superposition series for the months. Both decreases are toward a Melbourne Water target and control areas background ice nucleus concentration for both seeded and not-seeded days. The thereby implying that the higher initial seeded series is found to correlate concentration was due to persistence of reasonably well with the single ratio seeding effects. series of the 7 experiments through 1983 considered in Bigg and Turton (1988). Bigg Bigg (1985b) derived a cumulative (1995) concluded there was an apparent seeding index which appears to have persistence effect in the Melbourne some value in predicting seeding effects Water experiment. That this was not from seeding material amounts . It is coincidence was supported by a very low assumed that the concentration N of ice correlation between the MWnot-seeded nuclei from seeding at time t= t(n~, series and the 7-experiment seeded series. namely N(t(n)), is related to the initial It is not clear why the Melbourne Water seeding concentration N(t{0)) at time or the ?-experiment results of their t--t(O) and to the amount of seeding respective series did not involve the the material S dispensed at time t(n) by the relatively unbiased double ratio equation series.Bigg (1985a) and Bigg and Turton {1986) both noted this feature of the "P~,VI~EWED" APRIL 2001 LONG

t=t(n) chemically, and not be reemitted from the N(t(n)) = N(t(O))+ surface as a secondary ice nucleus. The t=t(0) iodine naturally present in soil would be much more abundant than that from S(t(n)-t(O))exp(- c~ (t(n)-t(O))) seeding such that the iodine in silver iodide would not be relatively active to S is an accumulating seeding material any extent. The deactivation of silver factor while the exponential is a decay iodide in sunlight would work against it term with a time constant alpha. Bigg acting as a secondary ice nucleus. Bigg’s related the target to control precipitation point of view was also published in Bigg ratio in the Tasmania I and New England (1988) and eventually elicited a response projects separately to the summation term from Rosinski (1987). (the cumulative seeding index) and found a reasonably linear relation in each case. Rosinski (1987) argued that the The time constant alpha was 1/75 day for iodine present naturally in the Tasmania and 1/36 day for New England. environment is not present in compounds This finding indicates a connection which nucleate ice at warm (but subzero) between ice nucleus concentration and temperatures and that it is present in any the precipitation seeding effect. What is case in quantifies lower than produced by particularly interesting is that if the plants on which silver iodide is deposited. nucleus concentration only slowly decays The iodine from silver iodide takes part in with time after some seeding there will be a photochemically activated reaction with a corresponding slow decay in seeding terpenes with the solid or liquid reaction effect - hence, a persistence effect. product (ice nucleus) reaching the atmosphere by simple evaporation from c) Chemical and biological effects the plant surface.

Bigg and Turton (1987) are Bigg’s (1985b) discussion of the concerned with mechanisms whereby biological origin of ice nuclei centers on silver iodide can interact with the the ice-nucleating abilities of two plant environment and lead to persistent cloud or soil-dwelling bacteria found in leaf seeding effects. The mechanisms fall into litter known as pseudomonas syringae two categories: chemical or biological. and erwinia herbicola. (Note: The former Rosinski and Parungo (1966) and Rosinski bacterium is used to nucleate water in ski (1987) have been concerned with the snow-making. It is marketed as Snomax chemical mechanism while Bigg (1985b, by Genencor, Inc.) These two bacteria 1988) and Bigg and Turton (1986, 1987) possess ice-nucleating abilities which have discussed possible biological vary amongst members of the same mechanisms. Both mechanisms are bacteria. It has been shown that the treated here. emission of ice nuclei occurs mainly from the plant canopy. Bigg speculates that the Rosinski and Parungo (1966) nuclei are the bacteria themselves. The proposed that persistence could occur if ice-nucleating ability may originate in the iodine in silver iodide reacted with the outerqayer of the bacterium in ice- terpenes in the plants on which it was nucleating sites mimicking those on deposited after release. A possible example silver iodide deposited on plant material was a seeded pine forest in which ice and present in the vicinity of the nuclei were found for some months bacterium. It is further speculated that subsequent to silver iodide release. ice-nucleating bacteria , compared to non-ice-nucleating bacteria, have a Bigg (1985b) was dubious about the propensity to multiply and di.sperse and above mechanism since the amounts of propagate. If this is true persistence of silver iodide (1 microgram per square seeding effects may be conducive to meter) deposited annually were so small. further persistence. In his view the silver iodide would terminate in the soil, be bound there Bigg and Turton (1986) conducted 18 JOURNAL OF WEATHER MODIFICATION VOLUME 33 field experiment, involving the (1987) and with bacterial stimulation application of silver iodide to plant life on silver iodide and transport, but still there the ground, in order to discover what ice is the possibility that the measured nuclei developed and whether they might nuclei were being (re)emitted by primary be suitable for promoting the persistence silver iodide particles lodged on the of seeding effects. Two complete .plastic ground. enclosures surrounding growing grass 6. GENETICENGINEERING were prepared. Silver iodide solution was added to the grass in one of the enclosures Levin , Yankofsky, Pardes, and and the ice nucleus concentrations in Magal (1987) have explored the question both enclosures were measured of bacterial ice nucleation. They do not simultaneously and daily over a consider it to occur after silver iodide is succeeding 220 day period. About 2-3 dispensed into clouds. Rather, they treat times as many ice nuclei were found in ice nucleation as an inherent property of the atmosphere in the seeded enclosure. some bacteria. Their discussion begins This result is believed to demonstrate that with a recitation of bacteria which may application of silver iodide leads to a promote condensation- followed -by - persistent enhanced concentration of ice freezing. A selection of the bacteria are nuclei. The nature of the nuclei was genetically-engineered to increase the studied by culturing. Approximately five proportion of bacteria which may times as many bacterial ice nuclei were promote these processes. The first process measured in the seeded enclosure as in is condensation and the bacteria are the not-treated one. From these results it highly effective in promoting it. Freezing is hypothesized that a delayed effect of process activity appears rarer involving cloud seeding is an enhanced perhaps 0.1 percent of all bacteria. Efforts concentration of airborne ice-nucleating are made to increase this percentage to bacteria induced by silver iodide added to 100 percent such that every particle vegetation. Bigg (1985b) concludes that contains a freezing nucleus. cloud seeding experiment should be accompanied by an extensive ice-nucleus A cloud model was used to measuring network. Lengthy not-seeded investigate quantitative particle growth periods in the experiment as in Tasmania by condensation and freezing. Simulated I may aid in detection of persistent cloud not-seeded clouds as well as clouds seeded seeding effects. with silver iodide or bacteria were considered. More rain was found to fall Bigg’s (1988) second field from the bacteria-seeded cloud than from experiment involved deposition of a silver either of the other clouds. In the iodide solution on a field of newly bacteria-seeded cloud art ice-process had growing wheat, and subsequent developed at temperatures as warm as -5C measurement of the concentration of ice whereas in the silver iodide seeded cloud nuclei at various downwind or other temperatures colder than -10 C were directions from the treated field. One of required. the filter sites was largely upwind of the treated field and displayed a lower 7. RELATEDWORK nucleus concentration. Overall, the directions of the high nucleus Ryan and Sadler (1995) argued concentrations were consistent with a that allowance in the past for persistence nucleus source in the sprayed field and effects would have implicitly increased travel of the nucleus with the surface seeding effects though there is still a winds. The highest concentrations in the need for development of new statistical sprayed area occurred within 24 hr of the tests that explicitly take into account the treatment but peaks in the concentrations possibility of persistence. Since were also observed at 10, 20, and 40 days persistence appears associated with the afterward as well. The observation of use of silver iodide as the particular ice high concentrations was consistent with nucleant efforts may be required to terpene-iodine reactions of Rosinski identify an alternative material such as APRIL 2001 LON G 19 bacterial nuclei. Ryan and King (1997) employed should be applicable to daily have also commented on the question of sequences of precipitation measured at a persistence. number of stations . The methods should establish the places and seasons where Warburton (1973) described the precipitation may persist after a seeded Pyramid Pilot Cloud-Seeding Project. day. It will be important to understand the Radar data in the Sierra Nevada showed way in which a maximum of that in some locations radar echoes may precipitation develops from the data and persist while in other locations echoes the kind of data that are conducive to are transient. If echoes persist several development of the maximum. It would be days after seeding then greater important to understand what may be precipitation may occur over those days. unique in a set of precipitation data that If this persistence also occurs in a implies such a maximum in the reasonable fraction of a project target precipitation when data elsewhere may area then higher amounts of not demonstrate it. There should be precipitation may be found in the target provision for using both Lurid and during some time period after seeding. It Grantham’s approach and methods more should be noted that persistence of echoes current with existing probabilistic and may be connected with a combination of statistical methodology. wind flow and topographic features in the target area. Hence, the kind of Super, McPartland, and Heimbach persistence proposed by Bigg may (1975) measured the deactivation originate in wind flow interacting with activity of silver iodide released from a topography and producing radar echoes ground generator . (The generator in a part or parts of a target area for a solution included the silver iodide as well lengthy period of time over which there as ammoniumiodide, water, and acetone.) is a persistent accumulation of The plume from the generator was precipitation. tracked downwind with an aircraft carrying an ice nucleus counter.Nucleus Lund (1973) made a study of the concentrations were converted to persistence of cloudy and cloud-free downwind fluxes in the plume, and lines- of -sight at a location in Missouri, changes in fluxes with downwind .... U.S.A. It was found that if cloudy or cloud- distance indicated the persistence of the :~ free conditions of some degree prevail at nuclei. Persistences were calculated some starting time then those conditions amounting to a factor of two deactivation tend to continue during daylight hours. of nuclei and possibly none at all per The implications are that if cloudy hour of evolution of the nuclei . (This conditions are present when seeding is deactivation is to be compared with that of accomplished there is likely to be cloud a factor of 10-100 when generator later and, therefore, precipitation later solution includes sodium iodide instead of with a persistence effect occurring. The ammonium iodide.) The minimal whole sky camera, if used in numbers in a deactivation of the silver iodide solution network, could be the basis of a study of in the current tests suggests the silver cloud probability of occurrence as a iodide may be active for some time after function of spatial location in target and generation and thus persist. control areas. This would indicate whether cloud amount is greater during Persistence was treated by Vali, or after seeding, where seeding ought to Koenig, and Yoksas (1988) in a study occur, and where seeding effects would regions of cloud seeding potential in a likely occur. broad variety of clouds in the Duero Basin of Spain in three winter and three spring Lurid and Grantham (1977) seasons. Such regions contained for 10 considered persistence, runs, and rain or more, a supercooled liquid water recurrence of precipitation. Although content above 0.1 g per cubic meter over they focussed on time spans of 12 hr or distances exceeding 10 km or above 0.3 less, the probabilistic methods they grams per cubic meter over smaller 20 JOURNAL OF WEATHER MODIFICATION VOLUME 33 distances. The view taken was that if such responsible for effects on precipitation in liquid water content was observed then a turn. Ice nucleus concentration region of cloud seeding potential was measurements in at least one experiment persistently present. Whereas Bigg were about five times background values believes that the presence of secondary for two days after seeding (Reinking, ice nuclei is necessary for large amounts 1972). of precipitation 10-15 days after a seeding day there may be other Possible persistent effects on necessary conditions for this precipitation data were found in Colorado precipitation to develop. Other necessary and New Mexico. These studies conditions are a) the existence of demonstrated a) how ice nuclei can updrafts to lift moist air until it has cooled remain in a seeding project a~-ea after sufficiently for supercooled liquid water seeding has ceased, b) how seediaxg effects to form, b) a minimum supercooled liquid can be masked if remanent seeding water content, c) an upper bound on the occurs in parts of no-seed control number concentration of natural ice periods, and c) how seeding effects particles (say less than 10 particles per become clearer if there is a clear liter), d) microphysical colloidal stability assignment of precipitation data to the of the clouds simultaneously with little or correct seed or no-seed category if no precipitation, e) a liquid water warranted. content in excess of ice water content, and f) the above conditions prevailing The Bigg research showed an over an economically significant target apparent peak in the precipitation two area. weeks after a seeded day. A superposition method was used to extract the mean Deshler and Reynolds (1990) have precipitation n days after a seed day. In described a field experiment in the Sierra early analyses the number n was limited Nevada in which airborne generated by the next seed day but later analyses silver iodide ice nuclei and microphysical considered n larger than that of the next effects on hydrometeors persisted 90 re_in seed day. This later choice is viewed in the downwind and 100 km away from the present paper as confusing the analysis original seedline. and is not desirable.

8. CONCLUSIONS Bigg and others have addressed chemical and biological origins of ice Persistence of cloud seeding effects nuclei. The general opinion is that silver means the microphysical structure of iodide being deposited on plants leads to clouds and the development of chemical reactions or biological precipitation may be affected for a developments that result in the release of significant amount of time (say days) chemical reaction products or after the seeding has been completed. microorganisms into the atmosphere with Persistence may complicate the ice nucleating capability. Rosinski, Levin evaluation of a cloud seeding experiment and coworkers have made contributions and reduce the perceived net effect of the to these topics. seeding. The sensitivity of the experiment to the actual net effect may be reduced. A range of probabilistic and Studies of persistence have been ongoing statistical methods may be applied to a by various investigators for about 40 precipitation data set or cloud cover data years with mixed results as to how long it set ( the clouds controlling the occurs in any given situation. precipitation) to search for maxima in the precipitation and clouds so many days Ice nuclei and precipitation have after a seeded or not-seeded day. been the primary focus of persistence studies. Elevated concentrations of ice It should be noted that since the nuclei introduced into the atmosphere by early 1970’s photodeactivation of silver cloud seeding may persist and be iodide no longer appears to be a factor APRIL2001 .LoNo 21 reducing persistence given the versus the single ratio variable, and ii) ammoniumiodide now being used in place the number of days after a seed day for of sodium iodide in the seeding solution. which the method is valid. Develop the Hence, the silver iodide may now more probabilistic-statistical method of Lund readily persist. and Grantham (1977) and others for this work. Also, consider current methods for Although persisting ice nucleus extracting delayed seeding effects due to concentrations are viewed as being persistence drawn from the discipline of important for persistent cloud seeding time-series analysis. effects it is known that there are other f. In addition to ice nucleus necessary conditions which must prevail concentration as an experimental if clouds with significant potential covariate for precipitation incorporate as precipitation are to exist. These conditions other experimental covariates cloud are related to cloud formation dynamics, updrafts, boundary layer vapor content, minimum supercooled liquid water, excess supercooled cloud liquid water content, liquid water over ice water, and minimum natural ice particle concentration, cloud cloud depth and area. colloidal stability, satellite cloud top temperature, cloud top area, cloud volume, 9. RECOMMENDATIONS and cloud form g. Evaluate ice nucleus measurement a. Incorporate design and analysis technology (including networks) and its features into a cloud seeding precipitation suitability for measuring persistence enhancement project experiment which with respect to natural, silver iodide, permit testing for and estimating chemical, and biological ice nuclei. persistence effects in addition to, and Develop a network of whole sky cameras insofar as they influence, the traditional and associated data processing and cloud seeding effects. analysis hardware and software for b. As part of the design develop a assessment of cloud persistence. Apply comprehensive long-term precipitation meteorological radar for measurement of climatology for the experimental areas precipitation persistence in target, including the target, control, and control, and surrounding areas. surrounding areas to support a range of h. Devise further laboratory and field historical tests for experimental results.. experiments focusing on the surface Include in the design appropriately long, deposition of silver iodide on plants, the closed non-experimental periods during ground, and snowpacks and its the experimental seasons to allow nucleation, chemical and microbiological persistence to decay, to reduce effects. Consider inherent bacterial ice persistence accumulation, and to reduce nucleators as well. Investigate current persistence influences on experimental knowledge in aerochemistry and results. Include a control area in the aerobiology. Explore genetic engineering design but never seed it (i.e., do not use of cloud seeding bacteria and large single area or cross-over designs). quantity releases into the atmosphere. c. Evaluate the persistence fraction a (see Bowen (1966)) and develop measures ACKNOWLEDGEMENTS of the persistence-modified sensitivity of the main experimental tests. Mr. Ian Searle and the Hydro-Electric d. Make a clear exposition of all data Corporation supported this work. and analyses used in the persistence, main, and subsidiary experimental studies REFERENCES so other investigators can repeat the analyses or better understand how to Bigg, E.K., 1985a: Persistent effects of make similar analyses of their own data. cloud seeding. Search. 16; 40-42. e. Evaluate the precipitation Bigg, E.K., 1985b: Unexpected effects of superposition method of Bigg (1985a, cloud seeding with silver iodide. 1985b, 1985c) and Bigg and Turton (1986, ,|. Wea. 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