WMOD00488 - Laserfiche WebLink

Home Browse Search

1290 JOURNAL OF APPLIED METEOROLOGY VOLUME 21 Other possible reasons for the difference in L WC are that the FSSP sampled a limited spectrum of drop- sizes (2-30 JLm) and the accuracy of the JW probe (::!::50%). However, the significant point is that the air- craft verified the presence of supercooled liquid water detected by the ground-based instrument. Based upon the information from both the aircraft and the radi- ometer, the site director elected to perform a random- ized seeding experiment. In the randomized seeding mode, the seeding ma- terial is known only by the crew of the seeder aircraft. During a given seeding run, the dropped material will be either silver iodide flares, dry ice pellets or a pla- cebo, i.e., no material. For the placebo case, although no materials are actually dropped, the seeder aircraft is flown as if materials were being dispensed. After seeding, the crew of the cloud physics aircraft at- tempts to locate the seeded curtain and, by observing hydrometeors and other cloud characteristics, deter- mine a unique seeding signature from which the seed- ing material can be deduced. The numbered triangles in Fig. 2 show the locations of the three seeding runs performed on 3 March 1980. Note that the three seed- ing runs were conducted downwind from the radi- ometer beam. During each'ofthe three seeding runs, the crew of the cloud physics aircraft correctly iden- tified the seeding material from the microphysical sig- natures observed in the seeded plume. The following discussion briefly summarizes se- lected data collected by the cloud physics aircraft be- fore and after seeding; a detailed treatment of the microphysical observations is given by Stewart and Marwitz (1982). The cloud physics aircraft locates the seeded plume by using a computerized position-ref- erencing routine that uses the horizontal winds as an input and assumes that there is no vertical or hori- zontal shear. This assumption is probably valid in stratiform clouds with little convection as reported here. Microphysics data recorded after seeding, shown in Table 2, are averages of data recorded when the position-referencing routine indicated that the aircraft was within the seeded plume. The general decrease in L WC and the observed increase in ice particle con- centration after the first two seedings occurred, are considered to be verification that the aircraft is within the seeded plume. Since similar changes in the mi- crophysical signatures were not found following the placebo "seeding," the previous changes are believed to result from the artificial seeding. 7. Discussion Because the crew of the cloud physics aircraft cor- rectly identified the seeding agents from the micro- physical signatures, and since a decrease in LWC and increase in glaciation followed seeding with active agents, the 3 March 1980 seeding experiment must be considered to have had a positive outcome. The TABLE I. Comparison of average cloud liquid water content measured by radiometer and by probes on aircraft, 3 March 1980. Values within parentheses indicate maximum measured LWC. Average liquid water content in a vertical column (g m-3) Time (GMT) 1636-1640 1643-1647 FSSP JW Radiometer O. IO (0.4) 0.09 (0.3) 0.14 (0.5) 0.06 (0.25) 0.33 (0.36) 0.30 (0.33) fact that the radiometric instrument gave the first in- dication of a seeding opportunity, i.e., detected su- percooled liquid, when other ground-based sensors did not, clearly demonstrates the utility of the mi- crowave radiometer in cloud seeding research. A microwave radiometer offers the advantage of being able to detect small amounts of path-integrated liquid (.:;;;;0.5 mm) which appear to be characteristic of winter orographic clouds in the Sierra Nevada (Snider and Hogg, 1981). As was the case on 3 March 1980, such small amounts of liquid may go unob- served by radars operating at wavelengths ~ 3 cm. Therefore, we believe that ,continuous liquid data from a microwave radiometer, when used in con- junction with data from other systems, will continue to reveal seeding opportunities which would other- wise go undetected. A limitation to the receiver-radiometer technique is that a suitable microwave beacon may not always be available. In addition, the radiometric system used in the experiments reported here has a fixed-direction antenna beam. As a result, the ability to locate clouds with seeding potential is severely limited. However, use of a passive microwave radiometer with a steer- able antenna system will permit observation of clouds in any direction and at ranges up to -60 km from the radiometer. Although in the passive system one must employ an average temperature for the liquid (calculated from a suitable history of radiosonde data), it has been found that use of an average tem- perature results in little error. A two-frequency sys- tem, described by Guiraud et, al. (1979), was operated alongside the satellite receiver-radiometer for a three- month period at Denver, CO. The antennas were aligned so that clouds drifting through the antenna beams were observed simultaneously by each instru- ment. The two sets of measurements agreed with an rms error of 0.28 mm and a bias of -11 % (the two- frequency system being high). This result confirmed satisfactory accuracy for the passive method even though an independent measurement of the liquid temperature was not made (Snider et al., 1980b). A two-frequency steerable system was employed in the 1980-81 SCPP field season. It is expected that the steerable radiometric system will be used extensively in the future.