Radar-observed Lahar at Redoubt Volcano on 4 Apr 2009

During the spring of 2009, Mt Redoubt Volcano in the Cook Inlet region of Alaska erupted explosively 19 times during a 14 day period spanning March 22 to April 4. In addition to the obvious hazard of significant ash production during an eruption, historical observations have shown that eruptions at Redoubt usually include fast-moving currents of hot gas and rock known as pyroclastic flows. Because Mt Redoubt is heavily glaciated, these pyroclastic flows rapidly melt large sections of the upper Drift glacier, sending large mudflows of pyroclastic material and water – known as a lahar - down the north side of the volcano and into the Drift River valley. These lahars have been observed to extend up to 22 miles down the valley to the shores of Cook Inlet.

Approximately 50 miles east/northeast of the summit of the volcano lies the Kenai, Alaska WSR-88D Doppler radar (PAHG). Installed in the mid 1990s, this is the first major volcanic eruption at such a relatively short distance from the radar to be sampled in real-time. In addition, because the terrain increases so dramatically on the west side of Cook Inlet, under normal atmospheric conditions the lowest (0.5 degree) elevation slice of the PAHG radar is located very close to the actual terrain on the north/northeastern flanks of the volcano.

On the morning of April 4, 2009, Redoubt had its 19th and final major explosive eruption. With very little precipitation in the vicinity of the volcano, the PAHG radar was able to sample not only the large ash cloud produced during the eruption, but also a smaller, progressive feature on the north side of the volcano. As observed on both radar reflectivity as well as radar radial velocity, this feature descended the north flank of the volcano at a speed between 20 and 25 kts, and turned approximately 80 degrees to the right – following the terrain of the Drift River Valley – and spread out and slowed down as it approached Cook Inlet. This element that was picked up by the PAHG radar has been identified as a lahar - as its timing, path, and other traits are consistent with historically observed lahars during eruptions at Redoubt volcano.