On the Retrieval of Volcanic Ash Bulk Parameters Using Polarimetric Weather Radars

This work describes recent advancements in the field of ground-based radar monitoring of volcanic ash plumes. As a matter of fact, the detection and quantitative retrieval of ash bulk parameters is crucial due to the environmental, climatic, and socioeconomic effects of ash fallout that might cause hardship and damages in areas surrounding volcanoes, representing a serious hazard to aircrafts.

Real-time monitoring of such phenomena is crucial for this reason and for initializing ash dispersion models. Ground-based and space-borne remote sensing observations provide essential information for scientific and operational applications. Satellite visible-infrared radiometric observations from geostationary platforms are usually exploited for long-range trajectory tracking and for measuring low-level eruptions. Their imagery is available every 10–30 min and suffers from a relatively poor spatial resolution.

Moreover, the field of view of geostationary radiometric measurements may be blocked by water and ice clouds at higher levels and the observations’ overall utility is reduced at night.

In this contest, ground-based microwave weather radars represent an important tool for detecting and, to a certain extent, mitigating the hazards presented by the ash clouds. The possibility of monitoring in all weather conditions at a fairly high spatial resolution and every few minutes after the eruption is the major advantage of using ground-based microwave radar systems.

Ground-based weather radar systems can also provide data for estimating the ash volume, total mass, and height of eruption clouds. Previous methodological studies have investigated the possibility of using ground-based single- and dual-polarization radar system for the remote sensing of volcanic ash cloud providing the

In the present work, the estimation technique was revised to overcome some limitations related to the assumed microphysics.

Additionally, a new methodology for the estimation of the ash mass eruption rate (MER) based on the combination of radar and a thermal camera is presented. Indeed, its quantitative retrieval is crucial for the initialization of the transportation models. The methodology is based on the exact calculation of the mass flow using radar-derived ash concentration and particle diameter at the base of the eruption column and the exit velocity estimated by the thermal camera.

The proposed procedure was tested on four Etna eruption episodes that occurred in December 2015 as observed by the available network of C and X band radar systems. The results are congruent with existing empirical methodologies deriving the mass eruption rate by the plume top height. The agreement between the total erupted mass derived by the retrieved MER and the plume concentration can be considered as a self-consistent methodological assessment. Interestingly, the analysis of the polarimetric radar observations allowed to derive some features of the ash plume, including the size of the eruption column and the height of the gas thrust region.

Starting by this paradigm, a new self-consistent radar approach is also evaluated to overcome the limitation of the thermal camera measurements in case of poor visibility conditions and/or limited field of view.