Retrieval of Cloud Microphysics During the Mt. Washington Icing Sensors Project (MWISP)

MWISP, the Mt. Washington Icing Sensors Project, was conducted in April, 1999, to assess the ability of remote sensing devices to measure the microphysics of inflight icing conditions, principally liquid water content, drop size and the presence of drizzle drops. An additional goal was to characterize the icing environment in complex terrain in a mid-latitude, early spring environment. Both goals required continuous, high quality in situ and remote measurements of cloud microphysical conditions, an opportunity afforded by a mountain summit observatory, and the close proximity of remote sensing devices near the base of the mountain. This was accomplished through use of a wide variety of instrumentation located at both the Mt. Washington summit (1916 m) and near the mountain base (811 m) including Particle Measuring Systems (PMS) probes, Rosemount ice detectors, an icing radiosonde system and rotating multicylinders.

CRREL operated three PMS probes at the summit, an FSSP (2-47 mm), a 2D cloud probe (0-800 mm) and a 2D precipitation probe (0-6400 mm). The three instruments were located on a mast extending about 2-m above the Mt. Washington Observatory 12-m high concrete observation tower to provide maximum exposure to winds. The instruments were operated during the full project, the entire month of April, and acquired approximately 150 hours of measurements.

Though PMS probes are typically exposed to harsh icing conditions when carried on aircraft, the mountain summit presented several unique challenges to acquiring high quality measurements. Air flow over an aircraft wing is generally consistent because angle of attack varies over only a few degrees. On Mt. Washington, wind direction changes over time, both because of geostrophic changes but also as a result of turbulence over time periods of seconds to a few minutes. Though probes can be rotated into the wind during large scale direction changes, they cannot track rapid fluctuations. Icing was a significant problem, and additional operational obstacles included vibration-loosened circuit boards, signal noise produced by commercial radio-frequency interference, and required periodic maintenance necessitating removal of probes in environments with wind speeds of 20 ms-1 or more.

Because of the ferocity of the operating environment, the FSSP probe calibration drifted considerably during the project. In addition, our older FSSP did not provide activity levels or total strobes to the data acquisition system for making corrections to the data. Comparisons of the PMS probe parameters with liquid water contents retrieved from rotating multicylinders and a Rosemount ice detector are providing verification of the PMS probe measurements. In addition, comparisons of summit measurements are being made with those of a radiosonde that measures supercooled liquid water content, and measurements made by NASA’s Twin Otter research aircraft during six flights over the summit. Results of these comparisons are presented with discussions of reasons for physical consistency or inconsistency of the measurements. Such instrument intercomparisons are generally not available in most field programs, thus affording the participants of MWISP an opportunity for more reliable cloud microphysical measurements, despite the operating difficulties presented by the Mount Washington environment.