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SECTION 2 SUPERCOOLED LIQUID WATER INVESTIGATIONS 2.1 Introduction The importance of SLW to wintertime orographic weather modification potential is well recognized, see [2, 3J. As part of the assessment of wintertime seeding potential over the Grand Mesa, the CRADP has been monitoring liquid water and other parameters of interest on and near the Mesa during the winter months since early 1983. This section summarizes this work, some of which has appeared in conference pre- prints, and much of which is documented in Bureau of Re- clamation internal memoranda. Measurements of SL W have been made over the Mesa by aircraft-borne sensors, and on the Mesa by microwave radi- ometers and tower-mounted icing rate meters. 2.2 Instrumentation The primary sensor used in measurement of SLW over the Mesa has been the dual-channel microwave radiometer. Two similar units of the type discussed by Hogg, et al. [4J have been used at different times, as described in section 1. Both units operate on the 20.6- and 31.65-GHz bands Blowers were implemented to keep the radiometer reflectors clean. Both units were operated in the vertically-pointing mode near Island Lake, just south of and about 120 m below the Mesa top (fig. 1-1). Measurements represented the vertically inte- grated SLW from the surface to cloud top. The two units were operated side-by-side during May 1985. The resulting liquid water measurements agreed well and were consistent, even during periods of convection [5]. Airborne measurements of liquid water were made during portions of the months of January, February, November, and December 1983, by the Citation II cloud physics aircrah. Data from a J-W (Johnson-Williams) hot-wire sensor and a Rosemount icing rate meter were supplemented by integrated FSSP (Forward Scattering Spectrometer Probe) data. Additional SL W measurements were obtained by a tower- mounted Rosemount model 871CBl icing rate meter. ThiS meter consists of an exposed metal rod, upon which airborne supercooled water accretes [31]. When a given mass (deter- mined by wind tunnel calibration) has accreted, the sensor "trips", heating sufficiently to shed the ice. The meter was affixed at the 70-m level to a tower atop the Mesa, and yielded data throughout the winter of 1984-85. 2.3 Meteorological Conditions That Produce SLW Several dynamic and thermodynamic variables Were exam- ined to define those meteorological conditions most likely to result in the production of significant SLW [6J. Data obtained by NWS rawinsondes released from Walk~r Field (GJi) near Grand Junction, Colorado, were used to obtain the 70-kPa winds, temperatures, and dew points. Additional wind data collected at the 70-m level on the tower atop the Mesa, as well as temperature and precipitation data from near the GMO, were also used (fig. I-I), The release point of the GJT rawinsondes is 44 km west of the GMO. The distribution of winds recorded by the GJi soundings was examined for 411 hours when th,e average SLW recorded by a microwave radiometer was greater than or equal to 0.01 mm [7]. Virtually all the hours when SLW was detected had 70-kPa winds between 195 and 3150 true. Within this 120" window, 154 of the hours (37 percent) occurred with winds between 210 and 2400. Only 23 percent of the hours were associated with winds between 270 and 3150, The mean speed of the SLWprodudng winds was 10.4 m/s. A second important ingredient for SLW production was, not surprisingly, moistUre. Significant SLW was seldom observed when the GJT, 70-kPa, temperature-dew point depression was greater than 50 C. Virtually no SLW was observed when surface temperatures on the Mesa Were colder than ~ 140 C, although 18 percent of all hours during the 5-month period were at least this cold. The greatest SLW production was always coincident with Mesa top teniperatures in the -4 to -100 C range. In addition, SLW production on Grand Mesa Was linked to the passage of short-Wave troughs or low-pressure centers. Of the 23 heaviest SL W episodes recorded during the winters of 1983-84 and 1984-85, 17 were associated with short- wave trough passages, and the other 6 with low-pressure centers that were closed at the 70-I<Pa level. Also, 19 epi- 5