The rapid detection of explosive volcanic eruptions and accurate determination of eruption-column altitude are critical factors in initializing models that are used to forecast the movement of volcanic clouds that can pose a hazard to aviation. The U.S. Geological Survey deployed a transportable Doppler radar during the 2009 eruption of Redoubt Volcano, Alaska, and it provided valuable information during subsequent explosive events. We describe the capabilities of this new monitoring tool and present data it captured during the Redoubt eruption. A MiniMax-250C (MM-250C) radar was installed 82 km east of Redoubt at the Kenai Municipal Airport and provided 45 degree sector scan volumes of Redoubt every 70-90 s at altitudes of 3-20 km ASL. The MM-250C operates in the C-band (5.36 cm wavelength) and has a 2.4 m parabolic antenna with a beam width of 1.6 degrees, a transmitter power of 250 W, and a maximum effective range of 240 km. The system was controlled remotely from the Alaska Volcano Observatory office in Anchorage, and data was integrated into the routine geophysical monitoring streams from seismic and pressure sensors, web cameras, and satellite images during the eruption. Radar data from the WSR-88D NEXRAD radar located about 10 km north of the MM-250C were primarily used as the authoritative source of cloud height information.

The MM-250C system became functional roughly a day before the first explosive event at Redoubt but was able to detect the eruption column and proximal cloud from the seventeen largest explosive events between March 23 and April 4, 2009. Maximum eruption column heights for these events ranged from 9-19 km above sea level, with almost all of them in excess of 13 km above sea level. Vertical rise rates for these ash columns ranged from 25-60 m/s and reached 10 km (jet cruise altitude) within 3-5 minutes of eruption onset. Radar reflectivity values of the central core of the eruption column core were high (50-60 dBZ) and are interpreted to be the result of rapid formation of volcanic ash-ice aggregates. The formation of these aggregates removed fine-grained volcanic ash from the column resulting in a dispersed volcanic cloud that was prematurely depleted in small particles. This process appears to be common during eruptions of Redoubt and may contribute to the poor thermal infrared brightness temperature difference discrimination of the observed volcanic ash clouds. Furthermore, the scrubbing of fine-grained ash needs to be accounted for when initializing volcanic ash transport and dispersion models. The relative proportion of fine-grained ash is a critical model parameter used in forecasting how long volcanic ash remains in the atmosphere, and radar observations may prove to be useful in constraining this eruption source parameter.