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<br />Printed January 30, 1990 <br /> <br />The two types of new sensors which have had the greatest impact on cloud seeding in the past <br />decade are the particle measuring probes and the microwave radiometers. They will now be <br />described briefly; examples of their applications are found in later sections. <br /> <br />Particle Measuring Probes. Particle measuring probes are devices that record cloud and <br />precipitation particles as the particles are carried past the probes (slides). The passing cloud <br />particles are illuminated by laser beams; the light passing around or scattered from the particles is <br />recorded by photodetectors and associated electronic circuitry. Knollenberg (1981) has described <br />the principles of operation of these probes and the accuracy of measurements obtained with them. <br />Interpretation of data from the probes requires consideration of many complicating factors, such <br />as edge effects and simultaneous passages of more than one particle through the illuminated <br />volume. <br /> <br />Four probes are needed to cover the range of particles encountered in a typical precipitating cloud. <br />The small cloud particles are recorded by the forward scattering spectrometer probe (FSSP), which <br />is often set to cover the range from 2 to 30 fLm, classifying the particles in that range into 15 size <br />categories. The optical array probes (OAP), which make use of shadowing techniques as <br />hydrometeors pass between the light source and a bank of photo diodes, are used to record larger <br />particles. The OAP technique is widely used in a 20- to 300-fLm version (the cloud probe) and in <br />a 300- to 4500-fLm version (the precipitation probe). The first OAP probes recorded only one piece <br />of shadow information per diode from each particle passing through the beam. From this <br />information, only one particle dimension can be calculated, so such instruments are called I-D <br />probes. The newer 2-D probes contain high speed front-end memories, enabling each <br />photo detector element to encode many bits of shadow information about each particle. A particle's <br />transit scans the array and image slices are recorded across the shadow to develop a two- <br />dimensional image. Some probes make use of polarizatiolll effects to distinguish between solid and <br />liquid particles. <br /> <br />Microwave Radiometers, Microwave radiometers (slides) are sensitive to all forms of matter in the <br />antenna beam. Unlike radar sets, which illuminate their targets with pulses of radiation, they rely <br />on black-body radiation, which is emitted by all objects at temperatures above absolute zero. A <br />radiometer reading represents an integration of radiation from all substances distributed along <br />the beam, so there is no range information in it. <br /> <br />As the radiation from water vapor, liquid cloud droplets, and ice particles varies with frequency in <br />quite different ways, it is possible to build dual-frequency radiometers which distinguish between <br />water vapor and liquid cloud droplets, while not responding to ice particles at all. The two <br />frequencies of 20.6 GRz and 31.6 GRz are sometimes employed for this purpose (Ragg et aI., <br />1983). Dual-frequency radiometers are proving particularly useful in detecting supercooled liquid <br />water (SLW) in clouds. For this purpose, a ground-based radiometer must be located above the <br />DoC isotherm; otherwise, the signal from the SLW would be masked by radiation from the cloud <br />water at temperatures above OOC. SLW, which is considered an important indicator of seedability <br />(WMO,1982), nearly always occurs in the form of small cloud droplets, which are undetectable by <br />ordinary weather radar sets, <br /> <br />3 <br /> <br />__. __J <br />