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;. Submitt~d to AMS Conference on Cloud Physics, San Francisco, CA, July 1990 OROGRAPHIC PRODUCTION OF SUPERCOOLED LIQUID WATER AND PRECIPITATION OVER THE MOGOLLON RIM OF ARIZONA by William D. Hall National Center for Atmospheric Research 1 Boulder, Colorado 80307 and David Matthews and Arlin Super Bureau of Reclamation Denver, Colorado 1. INTRODUCTION Wintertime orographic clouds over the Mogollon Rim of Arizona. are known to produce significant qua.ntities of supercooled liquid water which may be suitable for precipitation enhancement. To assess any weather modification strategy it is desirable to understand the airflow and precipitation formation characteristics during these winter storm events. The present paper discusses preliminary numerical simulations of airflow over the Rim and subsequent development of supercooled liquid water and precipitation of an event observed during the 1987 field season of the Arizona Snowpack Augmentation Project conducted by the Bureau of Reclamation and the Arizona Department of Water Resources and described by Super et. al. (1989). 2. MODEL DESCRIPTION: The Clark (1984, 1990) non-hydrostatic three- dimensional, nested grid model with realistic orography was used to study the evolution of orographically forced flow. The model ice microphysical processes are parameterized following the works of Koenig and Murray (1976). Local mesoscale rawinsondes and aircraft soundings were used to initialize the model. The present results are from a two domain simulation using the sounding information from the morning of 28 January 1987. Figure 1 shows the orography of the Mogollon Rim in domain 1. Observations were centered around the Happy Jack Ranger Station (HJK) located along the top of the rim and is marked by the X on the map. The corner brackets mark domain 2. 3. OBSERVATIONS: Conditions on 28 January 1987 represented strongly forced orographic cloud with light natural precipitation and significant quantities of supercooled liquid water (SLW). This case was chosen because it represented conditions in which significant quantities of supercooled liquid water were produced in a stable orographic cloud. Radar observations indicated that a cloud echo pattern developed over the Rim from 0400 to 1000 MST. Aircraft data and satellite imagery indicated that the cloud extended 40 km upwind from HJK to the crest of the Rim. In the present observations the cloud base was higher than typical conditions for winter storms where the cloud bases are usually near the Rim crest. Strong southwesterly winds of 15- 25 m s-1 at 850-700 mb veered to westerly at 20- 30 m s-1at 500 mb levels. These winds were nearly perpendicular to tbe barrier resulting in a cloud deck that thickened downwind from the base of the Rim to maximum depths just upwind of the crest at ElJK. It reached its maximum echo top elevation near 9 km by. 0530 MST with a base at 2.9 km until 1000 after which the cloud was no longer detectable on radar as shown in Figure 2. Bases of echoes reached their lowest level at 2.9 km (500 m above ground level) from 0600 to 0830 MST, but no precipitation was measured within the precipitation gauge network on this date. Visual observations indicated trace amounts of precipitation at HJK and Flagstaff from snow showers. The University of Wyoming King Air cloud pbysics aircraft made observations within the cloud over the HJK area from 0820 to 1010 MST. Observations were made along-the-wind at 2350 true heading marked by the line A-B in figure 5 at five levels from below cloud base at the minimum safe altitude at 3 km MSL to cloud top at 5.2 km. Figure 3 shows the vertical cross- section of the liquid water content observed along the flight track through HJK from 0820 to 1010 MST. This composite of the data observed within 20 km of this track provides a summary of the liquid water observed by the CSIRO King liquid water instrument. The vertical profile of the terrain within the same 20 km wide zone is shown below the aircraft track. Note that the origin for this plot is set at 1.0 km MSL. The cloud was primarily composed of supercooled liquid water with peak values from 0.2 to 0.7 g m -3 from the cloud top at 5.18 km MSL (-130C) to 3.6 km (-50C) near cloud base at the flight time. A region of ice crystals were observed near the base and below cloud creating a snow shower to the lee side of the crest. Each of three passes near cloud base over HJK encountered high ice particle concentrations (IPC) over the highest terrain in the -4.0 to -6.50C temperature range. Most of the crystals were needles with some aggregates of needles and occasional small graupel. The peak IPC exceeded 60 particles L-1 with a mean value near 30L-1 in a small shower that extended to about 500 m below the visible cloud base (Supf7'r et al., 1989). Note the region of supercooled liquid water production upwind of the crest marked by SLW in Figurt~ 3, and the region of high ice particle concentrations marked by IPC to the lee of the crest. In the IPC region the 2D-C cloud particle spectrometer indicated significant particle concentrations with diameters larger than 100 ILm. The 1 The National Center for Atmospheric Research is sponsored by the National Science Foundation