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Index.Hydrometeor Systems |
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One unique feature of the T-28 instrumentation system is its ability to measure the numbers and sizes of hydrometeors over almost the entire size spectrum present within a storm. The particles may range from cloud droplets a few micrometers in diameter to hailstones several centimeters in diameter. Various sensors cover different portions of the size range in an overlapping fashion. Somewhat comparable measurements can be obtained for each subrange from two different sensors, again affording a useful degree of redundancy. The sensors applicable to each particle size category are listed below. The values in parentheses indicate the approximate sampling volume per unit distance along the flight path for each instrument; the sampling volume per unit time can be estimated by multiplying by the nominal T-28 true airspeed of 0.1 km/s.
The sampling volumes tend to increase for instruments designed to sample larger particles to compensate for the smaller concentrations of such particles. The particle camera and hail spectrometer cannot be carried simultaneously because both require the same mounting points under the left wing of the aircraft. The above allocation of instruments to particle size categories is arbitrary to some extent. For example, the two-dimensional probe (2D-C) provides partial images of particles considerably larger than 1000 um, while the particle camera can photograph centimeter-size hailstones. However, the instrument sampling volumes can impose serious limitations on the representativeness of the data. It is also generally recognized that all of the available instruments are deficient in the 50-150 um size range. Our data system can accept data from a PMS 2D-P probe (covering the size range from ~200 um to ~6.4 mm) and the T-28 has, in fact, carried a 2D-P on many projects. As currently configured, however, it can carry only one PMS imaging probe at a time. Normally no 2D-P is available for use on the T-28, but one can sometimes be borrowed on a project-by-project basis. A variety of computer techniques have been developed to process the two-dimensional image data to determine particle sizes and crystal habits. A preliminary capability to automate the processing of foil impactor data has been developed but additional work is needed to make this routine. Information about particle size distributions can be obtained from the PMS probes, the particle camera, the foil impactor, and the hail spectrometer. Particle phases (ice or water) can be determined unequivocally from the particle camera data and frequently can be identified from the foil and PMS two-dimensional images as well. (Attempts to identify phases from the foil impactor data occasionally can be suspect, as shown by Knight et al. , 1977.) Particles larger than 5 mm, which are measured mainly by the foil impactor and hail spectrometer, are normally assumed to be ice because raindrops of these sizes break up very quickly in nature due to dynamic instabilities. The hailstone spectrometer, developed at the South Dakota School of Mines and Technology, operates on a "shadowgraph" principle similar to that employed in the PMS probes. It uses 128 phototransistors spaced at 0.9 mm intervals in a linear array to count, size, and image hailstones as they pass through a planar beam of laser light perpendicular to the flight path. Shadows smaller than about 4.5 mm are not counted, and the data are usually analyzed with the assumption that all particles larger than this are hail. A device has been developed by NCAR scientists to capture hailstone samples inside the thunderstorm. Frozen particles are decelerated and captured in a chilled receptacle for later analysis in the laboratory. This device is available for use with the T-28. |