Interpretation of observed microwave signatures from ground dual polarization radar and space multi-frequency radiometer for the 2011 Grímsvötn volcanic eruption

Open Access Atmospheric Measurement Techniques Atmos. Meas. Tech., 7, 537–552, 2014 www.atmos-meas-tech.net/7/537/2014/ doi:10.5194/amt-7-537-2014 © Author(s) 2014. CC Attribution 3.0 License. Interpretation of observed microwave signatures from ground dual polarization radar and space multi-frequency radiometer for the 2011 Grímsvötn volcanic eruption M. Montopoli1,4,5 , G. Vulpiani2 , D. Cimini3,5 , E. Picciotti5,6 , and F. S. Marzano4,5 1 Department of Geography, University of Cambridge, Cambridge, UK 2 Department of Civil Protection, Rome, Italy 3 IMAA-CNR, Tito scalo, Potenza, Italy 4 Dep. of Information Engineering, Electronics and Telecommunications, Sapienza University of Rome, Rome, Italy 5 CETEMPS, University of L’Aquila, L’Aquila, Italy 6 Himet srl, L’Aquila, Italy Correspondence to: M. Montopoli () Received: 9 May 2013 – Published in Atmos. Meas. Tech. Discuss.: 9 July 2013 Revised: 7 January 2014 – Accepted: 8 January 2014 – Published: 19 February 2014 Abstract. The important role played by ground-based microwave weather radars for the monitoring of volcanic ash clouds has been recently demonstrated. The potential of microwaves from satellite passive and ground-based active sensors to estimate near-source volcanic ash cloud parameters has been also proposed, though with little investigation of their synergy and the role of the radar polarimetry. The goal of this work is to show the potentiality and drawbacks of the X-band dual polarization (DPX) radar measurements through the data acquired during the latest Grímsvötn volcanic eruptions that took place in May 2011 in Iceland. The analysis is enriched by the comparison between DPX data and the observations from the satellite Special Sensor Microwave Imager/Sounder (SSMIS) and a C-band single polarization (SPC) radar. SPC, DPX, and SSMIS instruments cover a large range of the microwave spectrum, operating respectively at 5.4, 3.2, and 0.16–1.6 cm wavelengths. The multi-source comparison is made in terms of total columnar concentration (TCC). The latter is estimated from radar observables using the “volcanic ash radar retrieval” algorithm for dual-polarization X-band and single polarization C-band systems (VARR-PX and VARR-SC, respectively) and from SSMIS brightness temperature (BT) using a linear BT–TCC relationship. The BT–TCC relationship has been compared with the analogous relation derived from SSMIS and SPC radar data for the same case study. Differences between these two linear regression curves are mainly attributed to an incomplete observation of the vertical extension of the ash cloud, a coarser spatial resolution and a more pronounced non-uniform beam-filling effect of SPC measurements (260 km away from the volcanic vent) with respect to the DPX (70 km from the volcanic vent). Results show that high-spatial-resolution DPX radar data identify an evident volcanic plume signature, even though the interpretation of the polarimetric variables and the related retrievals is not always straightforward, likely due to the possible formation of ash and ice particle aggregates and the radar signal impairments like depolarization or non-uniform beam filling that might be caused by turbulence effects. The correlation of the estimated TCCs derived from DPX or SPC and SSMIS BTs reaches approximately −0.7. 1 Introduction The ability to recognize the signature of volcanic ash clouds on remote sensing data, and therefore to retrieve quantitatively their physical parameters, is of significant importance. The volcanic ash dispersed in the atmosphere after an eruption may have an impact on the environmental, climatic, and socio-economic effects (Cadle et al., 1979). Regular monitoring of volcanic emissions can provide information on the Published by Copernicus Publications on behalf of the European Geosciences Union. 538 M. Montopoli et al.: Interpretation of observed microwave signatures underlying volcanic processes, and it can serve as an input source for modelling trajectories of airborne ash (Sparks, 2003). Many recent research efforts have been focusing on the characterization of volcanic plumes and their dynamics into the atmosphere as for example those of Herzog and Graf (2010) and Denlinger et al. (2013). Investigating the ash dispersion in the atmosphere from remote also offers the practical advantage of monitoring it in near-real-time, thus avoiding impractical or even dangerous conditions of in situ sampling. In this perspective, remote sensing observations provided by visible, infrared, and microwave remote sensors on either ground or satellite platforms are of particular interest. When the observation is close to the volcano vent, remote sensing instruments can be used to estimate the near-source eruption parameters. The most important near-source parameters are the plume height and the tephra eruption rate and mass (Mastin et al., 2009; Marzano et al., 2011; Vulpiani et al., 2011; Maki et al., 2012). The retrieval of these parameters represents an important input for Lagrangian ash dispersion models, which are used to predict the geographical areas likely to be affected by significant levels of ash concentrations (Webley and Mastin, 2009). Sensors from geostationary earth orbit (GEO) platforms are exploited for long-range trajectory tracking and for measuring eruptions with low ash content (Rose et al., 2000). GEO imagery is available every 15–30 min at 3–5 km spatial resolution. When GEO radiometric measurements at visible– infrared wavelengths are used, water and ice clouds above the ash plume may partially block the sensor field of view, thus making the observations less useful for ash tracking. This feature becomes problematic especially at night, when the lack of visible observations does not allow for ash/water cloud discrimination. Compared to GEO, sensors in low earth orbits (LEOs) have a longer revisit time (more than 12 h) but enhanced spatial resolutions, which vary from several kilometres down to metres, depending upon the sensor and wavelength used (e.g. Grody and Basist, 1996; Marzano et al., 2013a). As a general rule, the smaller the sensor’s wavelength is, the higher the horizontal spatial resolution. Ground-based instruments usually have spatial and temporal resolutions higher than GEO–LEO sensors, though their areal coverage may reach few hundreds of kilometres at most. Either from ground or space, remote sensors operating at infrared and visible wavelengths suffer from strong ash cloud opacity (mixed with water cloud at times) due to the significant radiation extinction, which is often the case in the proximity of the volcanic source. In this respect, the exploitation of passive microwave sensors represents a good opportunity to probe ash clouds, despite some inherent limitations (Delene et al., 1996; Grody and Basist, 1996; Marzano et al., 2012b; Montopoli et al., 2013). Atmos. Meas. Tech., 7, 537–552, 2014 On the other hand, active microwave sensors have the capability (...truncated)