The TITAN software processes volume scan data from the SOAR 5cm radar recording a full volume scan in 3 minutes. The data allows analysis of different variables such as storm identification, location, area, volume, mass of precipitation, Vertically Integrated Liquid (VIL) as well as rates of variation of these parameters.
Titan provides a tool for an appropriately trained meteorologist to quantify a seedable cloud appropriately. The meteorologist usually undergoes a series of decisions that may be best characterized as: Nowcasting, decision time, qualification, treatment, maintenance and termination.
1) Nowcasting is when the meteorologist is monitoring the atmospheric conditions desirable for deep convection and seedable clouds to form. This decision is taken after studying the meteorological model output that forecast the prevailing thermodynamic conditions. This process is usually a routine analysis of upper air conditions and surface conditions. Nowcasting of the possibility of thunderstorms follows and weather modification pilots are briefed accordingly.
2) Decision time is when the meteorologist decides to launch a seeding operation based on his/her observations of the current and forecast atmospheric conditions. This decision is usually taken after observing cloud echoes on TITAN and the echo development trend. Sometimes an operation is launched after watching clouds grow visually or by observing the surface temperature reach a threshold when convection is expected to initiate or intensify, referred to as the convective temperature. For an operation with good timing, decision time should be preceded by qualification.
3) Qualification is when a cloud becomes seedable. This decision can be made visually by the pilot observing a cloud before it is detected by radar. Most frequently, a cloud is observed on radar before seeding occurs. In the SOAR target area, a seedable cloud echo usually reaches a VIL of 10 kg/m2 and continues rising. The volume of the cloud echo should be in the order of 200km3 with cloud tops above 8 km. The development trend of other clouds outside the target area is usually observed to determine the growth characteristics and the lifetime of the clouds. A short lifetime does not allow much opportunity for seeding. On TITAN, a seedable cloud usually shows a pocket of about 15% of the echo volume with a higher reflectivity at or slightly above 40 dBZ reflectivity at an altitude ranging from 6 to 10 km. This is characteristic of a cloud with weak coalescence and with a loading of supercooled liquid water above the freezing level early in its lifetime. Such a case is presented in Figures 5, 6 and 7.
4) Treatment is the time after initial seeding. Occasionally, treatment may be preceded by qualification in isolated cases. In most cases, however, a cloud qualifies as seedable and the meteorologist instructs the pilot to start seeding. The seeding starts when the pilot encounters, locates or is directed to the updraft portion of the cloud where the agent is released. The updraft usually has to exceed 200 feet per minute.
5) Maintenance is when a constant rate of seeding is established with continued observations of growth in the echoing volume. During this period the cloud echo has not reached it’s half-life time. Careful analysis of the dynamic variables of the cloud echo and their trend is necessary to define the half-life of the cloud. The pilot usually continues to experience updrafts and the meteorologist is able to locate areas of new growth within the cloud structure.
6) Termination is when seeding is stopped. A seeding operation is usually terminated either due to the absence of updrafts and/or due to the cloud echo exceeding its half-life time. When the National Weather Service issues a warning on the seeded cloud, the seeding is terminated.
When Top Seeding is favorable, the correct place to disperse the agent would be where updrafts are found. During top seeding operations three conditions must exist. Cloud top temperatures must be between -5 and -15 degrees Celsius. There must be a very high concentration of supercooled water. Updrafts should be no less than 200 feet per minute. On the ground at the radar facility the Meteorologist will assign the pilot the cloud that appears to be the most workable as far as cloud height and supercooled liquid concentration. The aircraft will find the altitude where the temperatures are between -5 and -15 C. Next the aircraft will make several penetrations in to the cloud to find sufficient super cooled water. When the pilot is satisfied with the liquid water content, updrafts will be measured accordingly. When 200 feet per minute (fpm) updrafts or a greater value is observed on the vertical speed indicator, the pilot will induce the agent as required by the Meteorologist. This is usually 1 flare per 200 fpm of updraft.
After the agent is dispersed the cloud should show significant growth. This event can be described as a cauliflower structure, shiny white in color while growing significantly. If this is observed the seeding agent is taking its course. Keep in mind that when the aircraft makes these penetrations both up and downdrafts are encountered making controllability of the aircraft difficult at times. In some situations downdrafts may exceed 2000 fpm. If the aircraft is only in the cloud for the required amount of time to displace the agent, generally a few seconds, a full recovery is made without hazard. However, if the aircraft is getting to deep into the cloud an out heading that was previously planned prior to penetration will be used. The Meteorologist will also assist the aircraft with any complications of finding clear sky conditions.
The other type of seeding operation is base seeding where pilots look for the inflow or updraft in areas of new convective development. When base seeding the aircrafts position is very important to make sure the seeding agent is properly dispersed. Generally, the altitude of 500 feet below the cloud base is used as a seeding altitude. However, it is very important that the aircraft is trimmed accordingly and the power is set depending on which type of aircraft is used. Power settings will vary as different aircraft can be used for cloud seeding operations. After the aircraft is properly set up for base seeding operations it is the pilot’s job to look for updrafts. The rain shafts are blue in color to almost black depending on the rate of rainfall. The sky conditions are dark blue in color making the targets easily identifiable. With the rain shaft already located the aircraft is then directed by the Meteorologist to the areas of updrafts. The Meteorologist has radar which can detect areas containing updrafts. This is a nice feature because sometimes sustained updrafts can be extremely difficult to locate. The aircraft should be underneath the base overhang and just outside the area where precipitation is falling. When flying through precipitation or to close to the rain shaft the aircraft will almost always encounter downdrafts. In this case the pilot would adjust the aircrafts position by simply moving in an outward direction from the rain shaft. Each cloud seeding aircraft has a standard instrument called a vertical speed indicator. Its purpose is to make the pilot aware of the aircrafts climb and descent rate. Again, properly trimmed when updrafts are encountered it is possible to measure updrafts in hundreds of feet per minute. When updrafts of 200 feet per minute are sustained the pilot will monitor that particular area. Usually 1 flare is fired per 200 fpm of updraft if sustained. The Meteorologist will calculate how much agent should be dispersed for a particular cloud.
Weather Modification Pilots go through very thorough training programs to satisfy requirements that must be met by the state of Texas. Our Pilots hold the appropriate Certificates such as FAA Commercial Pilot: Airplane Single and Multiengine Land Ratings with Instrument privileges. Weather Modification training programs and work shops are attended each year by both Pilots and Meteorologists.
|SANDY LAND UNDERGROUND WATER CONSERVATION DISTRICT PO BOX 130 PLAINS TX 79355 email@example.com|