Real-Time C-Band Radar Observations of 1992 Eruption Clouds from Crater Peak, Mount Spurr Volcano, Alaska

Repeated aircraft hazards in Alaska related to volcanic clouds have resulted in the use of a mobile C-band radar devoted to volcanic-cloud monitoring. The radar is located at Kenai, in range of several volcanoes in the Cook Inlet area. Three significant eruptions from the Crater Peak vent of Mount Spurr volcano (about 80 km from Kenai) in 1992 provided the first tests of the radar. The system constructs maps of the eruption columns and the drifting ash clouds for short periods (as long as 30 minutes) after eruption. The radar gives direct information about active eruptions in any weather conditions and allows estimates of the altitude of the column, which are useful for three-dimensional trajectory models of ash-cloud transport. It also allows an estimate of the eruption rate based on the ash-column height. However, such estimates may be lower than the true values because the very top of the eruption column, which may not contain coarse ash, may not be detected by the radar. We conclude that the radar detects mainly ash particles, sized from about 1 millimeter to a few centimeters on the basis of three sources-the brief duration of the reflected radar signal, data from independent ground observations on the mass and size of particles which fell out of the reflected cloud, and the intensity of the reflected signal. The most intense reflections come from ash clouds with particles that range from 2 to 20 mm in diameter and with a total particle mass concentration of less than .O1 to 1 g/m3. The data are useful for constructing models of ash columns and deposition of coarse tephra. Radars are not .useful for long-term volcanic cloud tracking because the large ash particles, that provide for strong radar signals fall out soon after an eruption. The radar does not detect smaller (<I-100 pm in diameter) ash particles in drifting volcanic clouds that can persist in the atmosphere for several days or more.