Terminal settling velocity measurements of volcanic ash during the Etna eruption by an X-band microwave rain gauge disdrometer

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1 GEOPHYSICAL RESEARCH LETTERS, VOL. 32, L10302, doi: /2004gl022100, 2005 Terminal settling velocity measurements of volcanic ash during the Etna eruption by an X-band microwave rain gauge disdrometer Simona Scollo, 1,4 Mauro Coltelli, 1 Franco Prodi, 2 Marco Folegani, 3 and Stefano Natali 3 Received 27 November 2004; revised 14 April 2005; accepted 21 April 2005; published 19 May [1] This is the first report in the scientific literature of direct measurement of the terminal settling velocity of volcanic particles during an eruption. Field measurements using a continuous wave X-band disdrometer were carried out at Mt. Etna on 18 and 19 December 2002, when the explosive activity produced a 4 km high volcanic plume. These data allow the estimation of the intensity of the fallout and the measurement of the terminal settling velocities of the volcanic particles in real-time. The main results are: (1) the tested instrument detected coherent falling volcanic particles from 0.2 to 1 mm diameter; (2) measured terminal settling velocities were in agreement with both experimental and theoretical methods; (3) however, the measured velocities were clustered around few discrete values, rather than a range of velocities as would be expected if the particles were falling simultaneously and discretely. This new methodology has many new applications for local hazard mitigation and improved understanding of fallout processes. Citation: Scollo, S., M. Coltelli, F. Prodi, M. Folegani, and S. Natali (2005), Terminal settling velocity measurements of volcanic ash during the Etna eruption by an X-band microwave rain gauge disdrometer, Geophys. Res. Lett., 32, L10302, doi: /2004gl Introduction [2] Recent eruptions of Etna volcano (2001 and 2002) produced long-lived eruptive columns that rose through the lower troposphere up to 4 6 km (a.s.l.). Tephra fallout covered the most of the volcano, mainly on its south and east flanks, endangering the population living around the volcano and disrupting operations at the International Airport of Catania, only 35 km from the vent. The largest eruptions occurred in 2002 and produced widespread fine volcanic particles that drifted to central Italy, west Greece and the Libyan coast. The distance traveled by volcanic particles is a function of both their terminal settling velocity and atmospheric advection and diffusion. 1 Istituto Nazionale di Geofisica e Vulcanologia Sezione di Catania, Catania, Italy. 2 Istituto di Scienze dell Atmosfera e del Clima (ISAC), Consiglio Nazionale delle Ricerche, Bologna, Italy. 3 Nubila s.a.s., Research, Service and Instrumentation for Meteorology and Environment, Bologna, Italy. 4 Dipartimento di Scienza della Terra e Geologico Ambientali, Università di Bologna, Bologna, Italy. Copyright 2005 by the American Geophysical Union /05/2004GL [3] The terminal settling velocity is the result of the balance among the drag force and all externally applied forces (gravity in this case). It strongly depends on size, shape and density of the particles and the properties of the fluid medium. Walker et al. [1971] measured the terminal settling velocities for volcanic particles larger than 5 mm showing that their fall velocities were similar to those calculated for cylinders. Wilson and Huang [1979] reported experimental terminal velocities for pumice, glass shards, and feldspar crystals with a mean diameter between 30 and 500 mm. They determined that the experimental terminal settling velocity is lower than the theoretical velocity based on a sphere because of the particles irregular shapes. Riley et al. [2003] characterized the shape and size parameters of mm sized particles found in three distal fallout deposits of Fuego Volcano (basaltic magma), Mount Spurr Volcano (andesitic magma) and Ash Hollow Member (rhyolitic magma) and measured their terminal velocities using an air elutriation device. Their experiments suggest that the irregular shape greatly increases drag coefficient and produces aggregation during the fallout processes. [4] This work describes the first direct measurements of the terminal settling velocities carried out during an explosive eruption. During the Etna eruption, a new instrument, originally designed as a rain gauge disdrometer, was used to investigate the terminal settling velocity of volcanic particles. This instrument is an X-band continuous wave radar, based on the Doppler shift induced by falling particles on the transmitted electromagnetic wave [Prodi et al., 2000]. These new data allow improvements to the understanding of particle sedimentation processes and numerical dispersion modelling of volcanic clouds. 2. Eruption Synopsis [5] The Etna eruption began on 26 October 2002 with a complex system of eruptive fissures opening on the volcano s NE and S flanks (Figure 1). It ended on 28 January 2003, after three months of almost continuous explosive activity and more discontinuous lava flow emission [Andronico et al., 2005]. Until 5 November fire fountains and lava flows erupted from both the NE and S flanks of the volcano. After the 5th the eruption continued exclusively from the south fissure, with continuous explosive activity occurring from the vent at 2750 m elevation, and lava flow emission occurring from 13 November until the end of the eruption. A pulsing fire fountain at the southern vent produced a long-lived eruptive column between 4 and 6 km altitude that was continuously present above the volcano during the three-month long eruption. 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2 16 m s 1 integrating the backscattered signal over a period of 60 seconds with a 1 second sampling rate. The received signal is then analyzed with different algorithms for both time variability and precipitation type, and inverted to generate a hydrometeor size distribution into 21 bands of mean diameter between 0.8 and 7 mm. Data are recorded in real time and there are dedicated software procedures to handle both connection and data processing [Prodi et al., 2000]. [8] Based on the instrument specifics, Pludix is potentially able to detect falling volcanic particles. The main difference between hydrometeors and volcanic particles is the variations in the intensity of backscattered radiation due to different dielectric characteristics of volcanic ash, and shape-surface characteristics of the particles. Volcanic ash is a factor of 2.4 less reflective than liquid water at the same size and wavelength [Adams et al., 1996] but the backscattered radiation is still detectable. Figure 1. Sketch map of Mt. Etna volcano: in grey the sedimentary basement, in white the volcanic rocks and in black the lava flows formed from the NE and S eruptive fissures. SC: Summit Craters, VdB: Valle del Bove. Triangles indicate the two measurement sites and a picture of X-band microwave disdrometer is shown in the left down corner. Lapilli and ash covered all of the volcano s flanks, but was most copious in the east sector due to the prevailing wind direction. Fine ash reached the Aeolian Islands, central Italy, western Greece and Libya, as far as 500 km from the volcano. 3. Measurement of the Terminal Settling Velocity of Volcanic Particles [6] Radar systems have been common in meteorological studies since the early 50s [Clift, 1985] and they are starting to be used also in volcanic particles monitoring. The first observations of volcanic clouds with a meteorological radar detected volcanic plume and measured volcanic jet velocity [Harris et al., 1981; Harris and Rose, 1983; Rose et al., 1995; Delene et al., 1995; Seyfried and Hort, 1999; Rose et al., 2001; Dubosclard et al., 2003; Lacasse et al., 2004]. In this work we tested an X-band radar system named Pludix [Prodi et al., 2000] to measure the terminal settling velocities of volcanic particles. This instrument is presently used to investigate the space and time variability of rainfall, together with the total mass of rain accumulated on the ground. [7] Pludix is a rain gauge disdrometer that analyses the signal backscattered by hydrometeors [Prodi et al., 2000; Battaglia et al., 2001]. It is an upward-looking, bi-static system (i.e. the transmitting antenna is separated from the receiving one) that consists of a sensor, and a signal processing and data communications unit. The active sensor is a low power (10 mw) Continuous Wave (CW) Doppler radar operating at a frequency of 9.5 GHz. Pludix has been designed to measure the Doppler shift in the Hz range with a spectral resolution of 1 Hz from hydrometeors which are supposed to randomly cross the radar volume. In this configuration it can measure velocities between 0 and 4. The Field Tests [9] Sensitivity field tests were done on 18 and 19 December, when the Etna volcanic activity produced plumes rising to 4 km that drifted S-SE. The first day, measurements were performed at Piano del Vescovo, roughly 5 km from the eruptive vent (Figure 1), and lasted about 90 minutes. The radar instrument was placed 1 m above the ground along the dispersal axis of the volcanic plume. During the field test the volcanic particle fallout was high and continuous and a sample was collected nearby. Data were processed in real time, with each stored value integrated every 60 seconds and shown on a monitor four evident Doppler shift peaks. On the second day, the measurements were performed at Rifugio Sapienza, about 3.5 km from the vent (Figure 1). Unfortunately, only 20 minutes of measurements were possible owing to adverse weather conditions. The integration period of the instrument, usually set to 1 minute with a time resolution of 1 second, was set to 2 minutes in order to integrate a more significant portion of the signal. During the two acquisitions no significant shifts of the velocity peaks were observed, however, in the second day a more intense volcanic particle fallout was recorded for about six minutes and the radar spectrum showed a well defined increase of the signal amplitude centred around 32 Hz. 5. Data Analysis [10] The Electromagnetic (EM) Doppler effect can be described as the frequency shift induced by a moving object that interacts with existing EM radiation. The Doppler shift value depends on the relative velocity of the target in respect to the source. The velocity v of the moving object is given by: n ¼ Df l=2 Where Df is the Doppler frequency shift, and l is the wavelength (constant) of the EM source. The intensity of the backscattered signal depends on the particle size and concentration inside the analyzed volume. [11] The collected data set consisted of a series of 1 or 2 minute integration spectra on the entire frequency range ð1þ 2of5

3 Figure 3. Grain-size distribution of the samples collected during the measurements (black for the sample collected on 18 December 2002 and grey for the sample collected on 19 December 2002). Figure 2. Integrate spectra over the entire data set measured during (a) 18 and (b) 19 December 2002 (frequency range between 0 and 1023 Hz). Df, plotted on the x-axes, is the Doppler shift (Hz), a function of the relative velocity of the target with respect to the source; the intensity of the measured signals (db), plotted on the y-axes, depends on the particle size and concentration inside the analyzed volume. The graphs show four welldefined peaks on the spectrum of each day. ( Hz). For each day, all data were integrated over the entire period and the resulting graphs (Figure 2) show four well-defined peaks on the spectrum. Both days display the same general pattern of three peaks around Hz, and a fourth near 70 Hz. [12] The relative error in frequency measurements, dðdf Þ jðdf Þj, is 3% (±1 Hz). Because the terminal settling velocity is the l product of a constant 2 multiplied by the frequency shift, its relative error will be dv jj v ¼ dðdf Þ ; i.e. the same of the jðdf Þj frequency shift. Numerical results of the terminal settling velocities obtained by applying the equation (1) to the central frequency of each peak are shown in Table 1. They range between 0.45 and 1.09 m s 1, having a total error of 0.01 m s 1. It is notable that they were similar on both days, as might be expected, because the explosive activity of Etna volcano did not change significantly between 18 and 19 December. [13] During the field test, volcanic particles were collected near the instrument and then a grain-size analysis was made with a set of sieves with mesh sizes spaced at one-phi. Figure 3 shows that the volcanic particles were composed of ash with high percentages of particles between 0.5 and mm in diameter. The sample collected during the second day was composed of coarser grains because the measurement site was 1.5 km closer to the eruptive vent (Figure 3). Particle features collected on 18 December were investigated by the scanning electron microscope (SEM) at INGV facility in Catania. They were mostly composed of poorly vesicular sideromelane and tachilite, usually elongate with contours from smooth to sharp. Lithic fragments and free crystals were present in small quantities. An accurate grain-size distribution based on image analysis was performed (Figure 4). [14] The terminal settling velocities measured by radar disdrometer are in the range of these experimentally obtained by Wilson and Huang [1979] for poorly vesicular volcanic particles of similar density and aspect ratios (density > 1800 kgm 3 and aspect ratios of 1 1.9, from SEM analysis). Terminal velocities smaller than 0.4 m s 1 (corresponding to particles with diameter <0.2 mm) were not detected by the instrument. This could be caused by the instrument s lower cut off (which is 0.8 mm for raindrops). [15] Bonadonna et al. [1998] proposed a theoretical model based on the Reynold s number to calculate the terminal settling velocities of the volcanic particles. We applied this model to estimate the terminal settling veloci- Table 1. Frequency Peaks (Column 2) Measured From X-Band Microwave Disdrometer and Relative Falling Velocities (Column 3) Extracted From Integrated Spectra During the Field Test of 18 and 19 December 2002 Df (Hz) v (m s 1 ) 18/12/02 30 ± ± ± ± ± ± ± ± /12/02 29 ± ± ± ± ± ± ± ± 0.01 Figure 4. Grain-size distribution based on the SEM images analysis of 2065 grains of the sample collected on 18 December of5

4 Figure 5. Terminal settling velocities distribution calculated with the algorithm from Bonadonna et al. [1998]. Terminal settling velocity was carried out from the full range of volcanic particle sizes collected on 18 December for an average density of 2450 kg m 3 measured on 21 particles. The terminal velocities are grouped to 0.1 m s 1 classes. ties of the full range of volcanic particle sizes collected on 18 December (Figure 4). The velocity distribution (Figure 5) presents a continuous spectrum that differs from the only four discrete velocities measured with the radar disdrometer during the sedimentation process (Figure 2). Nevertheless, the measured velocities (between 0.47 and 1.09 m s 1 )fall within the interval calculated from the model that reach the peak value at 0.82 ± 0.01 m s 1 (Figure 5). 6. Discussion and Conclusions [16] Measurements performed on both the first and the second day show that the radar rain-gauge disdrometer efficiently detects volcanic particles between 0.2 and 1 mm diameter. Similar Doppler shift peaks were obtained on both days (Figure 2), which we believe reflects the almost constant explosive activity of Etna volcano during 18 and 19 December. Moreover, the amplitude of the peaks with a lower Doppler shift (corresponding to the smaller grains) of the first day is bigger than the second day, consistent with the grain-size distributions (Figure 3). We measured only four terminal settling velocities, which fall within the range obtained from Wilson and Huang s [1979] experiment for particles of similar size, density and aspect ratios. We also applied the analytical model of Bonadonna et al. [1998] to calculated the corresponding terminal settling velocities of the full range of volcanic particle sizes collected on 18 December (Figure 4). The results suggest that the terminal settling velocities measured by instrument, fall in the interval analytically determined but theoretical velocities presents a continuous spectrum (Figure 5). The presence of four distinct velocity peaks in both days of observations is a remarkable result. This pattern could represent aggregation into preferential aerodynamic diameters, as already observed by Riley et al. [2003] for smallersized particles (i.e., mm). Alternatively, cascade fallout from the volcanic plume, caused by settling-driven instabilities [Holasek et al., 1996], was observed during Etna eruption and could have produced the measured fall velocity pattern. Future observations could help confirm and explain if these observed peaks are a common feature of the volcanic sedimentation process. [17] In conclusion, an X-band radar system was tested for the first time to explore the terminal settling velocities of the particles during a volcanic eruption. It was not only able to detect them but also measures their velocities and then, potentially, the sedimentation rate during a volcanic particle fallout event. Presently the instrument is calibrated to retrieve and characterize meteorological precipitation including rain, snow and hailstones. Nevertheless, it will be modified to improve its spectral resolution from 1 to 0.25 Hz to better detect the volcanic ash characteristic dimensions and to reduce the instrument cut-off. The test reveals that the terminal settling velocities measured during an eruption are in agreement with those experimentally measured [Wilson and Huang, 1979] and analytically determined from an accurate theoretical model based on the Reynold s number [Bonadonna et al., 1998]. Further measures of the terminal settling velocities of volcanic particles have to be carried out to enhance the physical modelling of the sedimentation process and to improve the numerical dispersion models of the volcanic cloud. Finally, the radar disdrometer could be a helpful tool for the realtime monitoring of the particles fall rate in the airports near to active volcanoes. It may reduce airport operational disruptions caused by ash fallout, addressing a faster runway clean-up that needs timely and precise information on size and quantity of the particles covering the runways. [18] Acknowledgments. We are grateful to Costanza Bonadonna, Gregg J. S. Bluth and Marco Casazza for the suggestions and discussion of the results. We also thank Lucia Miraglia for useful support in SEM analysis and Simone Mantovani for field measurement assistance. This research was supported by GNV of Italy. References Adams, R. J., W. F. Perger, W. I. Rose, and A. Kostinski (1996), Measurements of the complex dielectric constant of volcanic ash from 4 to 19 GHz, J. Geophys. Res., 101, Andronico, D., et al. 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