Hail Experiment

Hail Experiment Description

HAIL EXPERIMENT

a) Background

Preliminary cloud physics studies on cloud turrets (feeder clouds) in the new growth zone of Alberta hailstorms and on the accompanying hailfall distribution patterns on the ground suggest that the cloud turrets represent a major source of hailstone embryos for Alberta hailstorms. Therefore, one of the overall goals of the controlled seeding experiments is to determine the growth trajectories of hailstones from their origin to final stages. In the summer of 1985, aircraft, radar, and ground based sampling systems will be used to reconstruct such trajectories and to determine the role of feeder clouds in hail formation.

Treatments to intervene in the natural hail process will be applied to these cloud turrets, and the effects of various treatments will be compared. These experiments will involve aircraft, radar, and ground based sampling systems. Tracer materials will be dispersed along with the seeding material in order to confirm that feeder clouds are the major source of embryos and to study the mixing and dilution of ice nucleants in the feeder clouds.

Based upon observational evidence gathered so far in Alberta hailstorms, three hail suppression hypotheses have been formulated to guide the controlled seeding experiments.

1. Precipitation initiation in the feeder cloud: If seeding can be carried out early enough in the life of the feeder cloud, then it may be possible to accelerate precipitation particle growth. Larger particles will be more efficient at sweeping out the cloud liquid water, and most of the available liquid water may be converted into particles which precipitate before reaching the storm hail growth zone (HGZ). The resulting reduction in embryos which can reach the HGZ should reduce the number of hailstones which are formed.

2. Embryo modification: Seeding material injected into the feeder cloud at a later stage of its development than as above could reduce the size and increase the number of hailstone embryos which are formed in the feeder cloud. In this scenario, the additional particles compete for the limited liquid water content available in the feeder cloud, thereby limiting their growth prior to the feeder cloud merging with the main updraft. The smaller embryos will follow higher trajectories in the updraft, thereby missing the optimal growth zone of warmer temperatures and higher liquid water contents. Should the seeding material be injected in sufficient quantity, it is possible that the size of the resulting particles will be reduced to such an extent that the updraft in the feeder cloud will carry them out to the cloud free environment and these particles will eventually sublime. This will prevent the feeder cloud from transporting hail embryos to the main updraft.

3. Beneficial competition in the main updraft: If the seeding agent is introduced into a feeder cloud where the resulting growth of particles is not limited by the available liquid water content, then it is hypothesized that more hailstone embryos could be formed than would have occurred naturally. When increased numbers of embryos are transported to the main updraft, then they will follow similar trajectories to the natural embryos, and the higher concentration will compete for the liquid water content, and a greater number of small hailstones will be formed. These hailstones would have a greater probability of melting prior to reaching the ground, and the total hail damage will be decreased.

In order to test these hypotheses and to determine the optimal seeding methodology, Figure 4.

Figure 4

The Conquest research aircraft and seeding aircraft will depart from the Red Deer Airport together and fly in formation towards the candidate hailstorm. The approach to the storm feeder clouds will be based upon the expected location of such clouds related to the storm as determined from radar information on storm structure and the environmental winds. When the region of feeder clouds has been found, the Conquest and seeder will assume a stacked formation with the research aircraft at the -10&#176C level about 5 km ahead of and about 150 m (500 ft) below the seeding aircraft. The choice of feeder clouds appropriate for controlled seeding experiments will be the responsibility of the Aircraft Mission Scientist (AMS). If a feeder cloud is relatively far away from the main hailstorm and at an early enough stage of development that precipitation can be induced before -5&#176C and -10&#176C, then the two research aircraft will descend to just below the cloud top level to conduct a precipitation initiation experiment. If the feeder cloud is closer to the hailstorm, then the penetration will be made at the -10&#176C level. The seeding altitude for hypothesis (a) is the -5&#176C level, while for hypotheses (b) and (c) it is the -10&#176C level (see above for definition of hypotheses).

The first penetration should be oriented orthogonal to the wind shear between the 0&#176C and -20&#176C levels, provided that clearance can be maintained from the main storm. The shear will be determined from the hodograph or by observing the structure of nearby clouds.

Feeder clouds will be accepted for experimentation based on the following criteria measured during the first penetration:

1. The cloud top temperature must be between -5&#176C and -20&#176C as estimated by the Aircraft Mission Scientist.

2. Liquid water concentration as measured by the Johnson-Williams probe or the FSSP must exceed 0.5 g m3 for a continuous 5 s (500m) period.

3. Ice concentration (based on the 2D-C shadow-or signal) should not exceed 1 L-1 for 5 continuous seconds. Allowance will be made for the real time interpretation of 2D-C images.

4. Vertical air velocity should be positive during the 5 s period.

5. The cloud penetration period (horizontal cloud dimension) must not exceed 100 s (approximately 10 km).

6. No echo from the experiment cloud should be detectable on the research aircraft radar (assuming 0 dBZ minimum detectable signal).

If the cloud fails on the first pass, the aircraft can test the cloud again to determine whether the liquid water content was too small or whether the cloud top temperature was too warm to meet the criteria.

If the cloud is accepted the Aircraft Mission Scientist will instruct the crew of the seeding aircraft to apply a treatment on their penetration of the test cloud. The crew of the seeding aircraft will determine the appropriate treatment from envelopes containing randomly ordered seeding instructions (seeding material and rate - AgI, Dry Ice, or Placebo).

The seeding rates are:

1. one droppable 20 g AgI flare every 250 m, 2. one 10 g dry ice pellet every 100 m, 3. ten 10 g dry ice every 100 m, 4. placebo (no seeding material dispensed during penetration), 5. two 150 g AgI flares burned at cloud based (special seed cases).

Along with seeding treatments (1) to (5), the seeding aircraft will also dispense tracer material. Two tracer materials will be used this summer: powdered iron and indium pyrotechnic flares. The powdered iron is a new tracer material for an experiment designed by a visiting scientist, N. Knight, from NCAR. Indium has been selected by ARC as an additional tracer material because it is relatively inert as an ice nucleant and its natural occurrence is rare. Any indium which is found in hail, rain, or cloud water samples can be directly attributed to the seeding event. Each tracer will be added to only one feeder cloud per storm so as to avoid any ambiguity in the hail found at the ground. One feeder cloud will be treated with iron, the next one with indium and so on, as long as supplies last. The only restriction on this alternation of tracer materials is that iron powder is not to be applied to clouds that are being seeded below the -10&#176C level and a feeder cloud may not have any tracer material applied to it if chase vehicles are not in appropriate positions. Feeder clouds that are seeded below the -10&#176C level can be treated with indium. When indium is used, the seeder aircraft will dispense one 15 gram indium droppable pyrotechnic flare per second, to a maximum of 20 seconds, while simultaneously administering the seeding treatment.

When powdered iron is used, the powdered iron will be released at the same time as the seeding treatment material.

The research aircraft will collect one rime sample during each penetration of the test cloud. The rime samples will be analyzed for tracer materials and chemical composition. The T-28 aircraft will collect as many in-cloud hail samples as possible. Mobile units will deploy to the area ahead of the moving storm and will attempt to position themselves in a location amenable to collecting hailstones that originate in the feeder cloud in which the tracer material was dispensed. Project personnel will telephone the volunteer collectors of time-integrated hail samples that are in a position likely to receive hailstones that originated in the feeder cloud treated with the tracer material. These volunteers will be reminded to collect a sample.

After completing its seeding run, the seeding aircraft should break away from the cloud and position itself approximately 10-20 km away for photographing the test cloud and storm. Observations to document the size, structure and evolution of storm elements will include time, altitude, heading and clinometer data as well as narrative comments.

Successive penetrations by the research aircraft (ideally at 3 min. intervals) should be perpendicular to the seeding track if possible. Penetration levels should be chosen so as to maximize the likelihood of encountering precipitation particles which have been formed by the seeding. The dead reckoning homing pointer of the research aircraft computer system will be used to provide horizontal position keeping of the treated cloud, relative to the aircraft. A simple accretion growth model will be used for vertical position keeping of the aircraft relative to the growing hail embryos which are falling in the vertical wind field. The research aircraft will try to follow the developing embryos wherever they go, up to the limit when the feeder cloud merges with the main storm or until the radar echo exceeds 25 dBZ (as determined by the S or C-band radars), or until 30 min. has elapsed since the beginning of the experiment.

If the T-28 is available, then the AMS will notify the Research Director that it is required early enough so that it can be in position near the feeder cloud just before the feeder merges with the main hailstorm (see Figure 5). When the T-28 is ready, the Conquest will remain below the -13&#176C level. The T-28 flight track will be directed by the T-28 radar controller so as to penetrate the feeder and the main hailstorm along a line intersecting the top of the main updraft near the region of maximum hail size. All T-28 penetrations will be at the -18&#176C level or higher. The ground controller will guide the T-28 through a 180 deg. turn within the precipitation region of the hailstorm, and along a return path through the feeder. Similar penetrations will continue until the ground controller believes that hailstones which have grown from embryos formed in the feeder cloud have fallen below the -18&#176C level. At this time, the T-28 will position itself outside of the hailstorm in preparation for another experiment (if operational limits permit).

If a tracer material is to be dispensed, then the crew of the research seeder will immediately inform the Research Director of the exact time, location, and type of tracer, so that hail chase vehicles can be deployed into the appropriate region.

Storm chase is to occur on any medium to severe hailstorm. The priorities for storm chase activities are as follows:

1. If a tracer material is released in a feeder cloud during a seeding experiment then the chase crews must make every effort to position themselves so that they can catch hailstones that originate in the feeder cloud with the tracer material.

2. If no tracer experiment is carried out but a hailstorm seeding experiment is conducted, then the chase crews should be positioned so that they can catch hailstones originating in the treated feeder cloud.

3. If no tracer experiment and no hail seeding experiment is carried out but the research aircraft makes penetrations on a hailstorm, then chase vehicles are to be positioned so that they can catch hailstones originating in the portion of the storm being studied by the research aircraft.

4. If no research aircraft experiments are being carried out but hailstorms of medium to severe intensity occur that are not seeded, chase crews are to attempt to catch hailstone samples from natural (i.e., unseeded) storms.

5. If no research aircraft experiments are being conducted and no unseeded storms exist but hailstorms of moderate to severe intensity are being operationally seeded, then chase crews will attempt to catch hailstones from operationally seeded storms.