Measurement of airborne volcanic ash using millimeter-wave radars

Meteorological radars are frequently used for the measurement of volcanic ash plumes. Of particular interest to the volcanological community is the monitoring of the mass of volcanic ash being emitted into the atmosphere by a particular eruption. Currently S-, C- and X- band radars are used quite successfully to monitor ash plumes, but are less suited to measuring finer ash particles or lower mass loadings. Simulation results are presented which strongly suggest that millimeter-wave radars of appropriate frequencies could provide data on volumes containing finer particles and lower concentrations that would complement those obtained with lower frequency radars.

Millimeter-wave radars can also be smaller, more portable and, depending on the type of architecture used, lower power than their lower-frequency counterparts, allowing for rapid deployment even in remote regions with poor infrastructure. Additionally, they potentially can be used to image the plume from angles difficult to achieve with existing radars.

The particle size distribution (PSD) within volcanic eruption plumes remains largely unknown. Using a dual-frequency millimeter-wave radar it would be possible to determine the ash PSD using a technique similar to that proposed by Goldhirsh and Katz for rainfall. This method would allow the independent retrieval of the ash concentration which is of considerable value for atmospheric transport modelling. The potential effectiveness and limitations of this technique are explored.

In addition to measurements of the ash plumes, at least one report currently exists of the detection of volcanic ash in the atmosphere far from a volcanic eruption using a millimeter-wave radar (Madonna et al, GRL vol. 37, 2010). Whilst it is likely that lidar would generally be more suitable for making this type of measurement, under some circumstances millimeter-wave radars could also be used, offering complementary information. The potential of such a technique is also discussed here.