Rain Experiment

RAIN Experiment

a) Background

Preliminary controlled seeding experiments on towering cumulus clouds have suggested that cloud seeding can initiate the precipitation process and that droppable AgI flares may be more effective in enhancing precipitation than CO2 pellets. Further experiments are required to confirm these results and determine the range of conditions for which they apply.

In past experiments test clouds were required to have cloud top temperatures between -7&#176C and -15&#176C. The reasons for this temperature range are as follows. Clouds which do not extend above the -15&#176C level are not expected to rain naturally, so any rain produced by such clouds can be attributed to the treatment applied which simplifies the evaluation problem considerably. The lower temperature restriction is due to the fact that the nucleation characteristic of AgI is not thought to be effective at warmer temperatures. Besides, only clouds of a reasonable depth are likely to allow ice particles to grow to precipitation size. This summer, however, the cloud top temperature restriction will be relaxed and test clouds with cloud top temperatures between -7&#176C and -20&#176C will be considered.

Also this summer the South Dakota School of Mines and Technology T-28 research aircraft will participate in our projects during the period from June 20 to July 27. It is anticipated that this aircraft will collect micro physical observations on towering cumulus clouds reaching the -15&#176C to -20&#176C levels.

b) Objectives

The objectives of the controlled seeding experiments on towering cumulus clouds are:

1. to document the processes of precipitation development in natural and seeded cumulus clouds,

2. to determine the range of conditions under which clouds that would not rain naturally can be made of rain by cloud seeding,

3. to determine the most effective treatment for initiating rain in cumulus clouds,

4. to determine the feasibility of initiating the precipitation process using a cloud base seeding technique,

5. to determine if the precipitation from naturally raining clouds can be increased through cloud seeding.

c) Activities

1. Application of various treatments to isolated cumulus clouds and documentation of effect of treatment with the research aircraft and S or C-band radar data

2. Measurement of surface rainfall rates from test clouds when convenient and the collection of daily precipitation amounts by a network of volunteers.

d) Experiment Procedure

Seeding experiments should be carried out whenever isolated cumulus clouds or cumulus clouds embedded in a stratus cloud deck occur in the project area and when the research aircraft and a seeding aircraft are available.

The research aircraft and seeding aircraft are to fly in stacked formation with the research aircraft at about the -10&#176C environment level (or just below cloud top, whichever is lower) and about 7 km ahead of and about 150 m (500 ft) below the seeding aircraft. Potential test clouds are to be chosen by the aircraft mission scientist. The first penetration should be oriented orthogonal to the cloud top or -10&#176C level wind shear, whichever is lower. The seeding aircraft will follow the same heading. Clouds are accepted for the experiment based on measurements during the first penetration.

The criteria are:

1. Cloud top temperature between -7&#176C and -20&#176C as estimated by the Aircraft Mission Scientist, (Note: Scientist should have available a representative sounding indicating the altitude of the -20&#176C level).

2. The ice concentration (from 2D-C SHADOW-OR) must not exceed 1 L-1 for any 5 continuous seconds. Some allowance is made for real time rejection of artifacts.

3. The liquid water content (measured by the Johnson Williams hot-wire probe) must be greater than or equal to 0.5 g m-3 for any 5 continuous seconds. FSSP calculated LWC can be used in lieu of JW measurement.

4. The vertical velocity must exceed 0 m s-1 for five continuous seconds.

5. Cloud penetration length (horizontal cloud dimension) not to exceed 100 s (approximately 10 km).

6. No echo (assuming 0 dBZ minimum detectable signal) detectable on the research aircraft radar.

Otherwise, the cloud is rejected. If a cloud fails marginally on the first pass, it may be re-tested on a second pass.

If the cloud is accepted, the aircraft mission scientist will instruct the crew of the seeding aircraft to apply the appropriate treatment on their penetration of the test cloud. The crew of the seeding aircraft will determine the treatment from envelopes containing seeding instructions in random order. That is, the wind mounted flares are always the fourth treatment in a sequence of four clouds. The treatments are:

i. droppable 20-g AgI flares, one flare every 250 m. ii. 10 g dry ice every 100 m. iii. placebo (no seeding material dispensed during penetration). iv. wing mounted 150 g AgI flares (for a cloud base experiment).

After seeding, the seeding aircraft will maintain its altitude, and position itself approximately 5-10 km away to observe and photograph the test cloud. Observations documenting the size, structure, and evolution of the cloud will include time, altitude, aircraft heading and clinometer readings as well as narrative comments.

Successive penetrations by the research aircraft should be approximately perpendicular to the seeding track if possible. After seeding, the research aircraft will penetrate at the -10&#176C level (or cloud top, whichever is lower) for 8 minutes to allow for the nucleation and growth of ice crystals to riming size. Thereafter, penetrations will be at 300 m (1000 foot) intervals lower in the cloud, down to cloud base. Successive penetrations will be attempted within 3 minute intervals. The research aircraft "homing pigeon" should attempt to track the seeding curtain (if it can be identified), the position of maximum liquid water, or the maximum updraft. If time permits, the research aircraft will elevate the longer term effects of seeding by ascending to the -20&#176C level (or cloud top, whichever is lower) for further penetrations. A test cloud should be studied for at least 30 minutes following treatment or until radar echo exceeding 25 dBZ (as determined by the S or C-band radar) has formed. A schematic diagram describing the flight plan is predicted in the following figure.

On days when the test clouds are determined to approach -20&#176C, the T-28 will also participate in this experiment. While the experiment flight plan is being followed by the other two aircraft, the T-28 will make successive penetrations obtaining micro physical measurements and documenting the precipitation processes at the -15&#176C level.

h) Cloud Base AgI Seeding Technique

The research aircraft and the research seeder aircraft rendezvous at the cloud base altitude. Both aircraft are flying in formation with the research aircraft leading the seeder aircraft by about 7 km. When a cloud has been selected the research aircraft will climb top the -10&#176C level or just below cloud top, whichever is lower, for the initial test penetration. The penetration is oriented orthogonally to the cloud top or -10&#176C level wind shear. The same criteria are used to establish the seeding potential.

The seeding aircraft, meanwhile, approaches the potential cloud, or circles the cloud at cloud base until a seeding decision is given. If the cloud is accepted, 2 base flares will be ignited simultaneously. A racetrack pattern turning left orthogonal to the shear beginning on the upshear side of the cloud is the suggested seeding pattern.

If an area of inflow is detected by the seeder aircraft, treatment should be concentrated in that area. During the treatment, the seeder should remain at cloud base altitude. After treatment, approximately 5-10 km away, to observe, photograph and document the cloud from a mid-cloud altitude.

While treatment is being applied the research aircraft makes a sequence of penetrations parallel to the shear in the -7&#176C to -10&#176C region, and continues to make penetrations in this region until a seeding signature is detected. Thereafter, the research aircraft begins to descend at 300 m (100 ft) intervals per 3-minute pass until cloud base is reached. If the cloud is still active, penetrations through the cloud back to the -10&#176C level or cloud top, whichever is lower, should be made.

Amount of AgI to be Delivered

The seeding rate during a cloud top flare treatment calls for 20 g of AgI to be released every 250 m. A typical cloud length is 4 km or 16 flares (320 g) are released. To maintain the same seeding rate 2 base flares (150 g each) should be ignited simultaneously.