WISP90: Winter Icing and Storms Program: 1990

Summary

The Winter Icing and Storms Project (WISP) utilized nineteen PAM stations and four CLASS systems during February and March, 1990. It was the first in a series of deployments sponsored principally by NCAR's Research Applications Program, NOAA, and the FAA, with a variety of university support. It's primary goals are to investigate the production and depletion of supercooled liquid water in winter storms and thereby enhance aircraft safety through improved forecasts of icing conditions. PAM stations were deployed in a variety of configurations during WISP. Fifteen had Rotronic humicaps to permit humidity measurements below freezing when wet bulb data are lost; six had a combined rain and snow gauge instead of standard tipping buckets. Rotronic performance was good for humidity measurements but their temperature accuracy was insufficient despite several efforts to improve the readings with enhanced aspiration and various air-filters and radiation shields. The PAM dry bulb remained the primary measurement of air temperature. Other attempts were made to improve psychrometer performance below freezing by using a 10\% alcohol solution instead of straight distilled water. This proved ineffective because temperatures quickly dropped below the solution's freezing level and an even higher alcohol content would have disrupted sensor accuracy. Overall PAM data recovery was good however, averaging 95\%, although freezing and icing became a problem on several occasions effecting sensors, radios and batteries. Seven sites were operated with commercial A/C power which was required for the heated snow gauges. Snow gauge performance was mixed. During WISP a computerized data-base was utilized by PAM technicians for the first time to help document station visits and intercomparison readings. Approximately 300 soundings were taken from the 4 CLASS sites during WISP. Overall data quality and recovery was quite good. Freezing rain and icing of the ground-based equipment did cause some degradation of the data as well as missing flights. In severe cases of icing the hatches on the CLASS trailers would freeze closed, making balloon launches impossible. Ice on the ground antennas had a tendency to short the received signal to ground, causing periods of poor reception. It was noted during the project that winds would often be lost when the radiosonde entered storm clouds. Atmospheric noise increased dramatically when this happened, causing poor LORAN reception, subsequently poor wind data. This phenonema is well documented when releasing sondes in or near thunderstorms but has not been as noticeable when operating in the winter in stratiform clouds.