Wintertime Cloud Liquid Water Observations over the Mogollon Rim of Arizona

Abstract

Liquid water, wind and other meteorological observations were made over the Mogollon Rim of Arizona from mid-January through mid-March 1987. A dual-channel microwave radiometer provided cloud liquid water (CLW) measurements. Winds were sampled by a variety of devices including a doppler acoustic sounder, tower-mounted anemometer and vane, rawinsondes and an aircraft equipped with an inertial navigation system. Temperature observations indicated that the bulk of the CLW was supercooled. Distributions of vertically-integrated CLW are examined from the thirteen synoptic scale storms that occurred during the observational period. It is shown that most hourly means were less than 0.1 mm, implying limited liquid water contents. While some diurnal variation in CLW occurrence was found, the early morning maximum indicates it was not solar-forced and may have been the result of the random passage of storms. The durations of CLW episodes are shown to have varied from an hour to over a day. The majority of CLW occurred with southwest winds although a secondary maximum was apparent with northeast flow. Both are upslope for the observing site. The horizontal flux of CLW was estimated hourly over the crestline of the Rim, just upwind of the lee subsidence zone. Total flux per storm varied widely and three large storms produced three-quarters of the total (two month) flux. However, it is shown that the low hourly values of CLW produced much of the total flux because of their frequent occurrence. The cumulative frequency distribution of the 260 h with flux estimates is shown to be similar to that previously reported for the Grand Mesa, CO, 600 km to the north. The total CLW flux for the two month sampling period is estimated to have been roughtly half the mean annual streamflow from the same area. This suggests that significant potential may exist for winter precipitation augmentation through cloud seeding on the Mogollon Rim.