A comparison of field- and satellite-derived thermal flux at Piton de la Fournaise: implications for the calculation of lava discharge rate
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We present thermal measurements made by high spatial resolution ground-based (a hand-held thermal camera) and low spatial resolution space-based (MODIS) instruments for a lava flow field active during the last phase of the May–July 2003 eruption at Piton de la Fournaise (La Réunion). Multiple oblique ground-based thermal images were merged to provide full coverage of the flow-field. These were then corrected for path length attenuation and orthorectified, allowing the at-surface radiance emitted by the flow-field to be estimated. Comparison with the radiance recorded by the MODIS sensors during the eruption reveals that, for clear-sky conditions and moderate-to-low viewing angles (satellite zenith <40°), the satellite measurements represent ∼90% of the at-surface radiance, and thus represent valuable data for quantifying volcanic thermal anomalies. Nevertheless, extreme viewing geometries and the presence of clouds strongly affect the radiance reaching the sensor and affected data from 94% of the overpasses. Ground-based thermal data were used to investigate an empirical relationship between the radiant heat flux and lava discharge rate during the emplacement of pahoehoe flows. While the average radiation temperature for flow surface that were 6–24 h old ranged between 500 K and 625 K, the ratio between radiative heat flux and Time-Averaged lava Discharge Rate (TADR) ranged between 1.5 × 108 J m−3 and 3.5 × 108 J m−3. This relationship was used to estimate TADR values from optimal MODIS data and produced results in line with those obtained from GPS surveys (Coppola et al. 2005). Our results underscore the importance of ground-based thermal analysis for the interpretation of satellite measurements, particularly in terms of calculating discharge rate trends.
KeywordsPiton de la Fournaise MODIS Field-based thermal imaging Lava discharge rate
The research was funded by the Italian Ministry for Universities and Research (MIUR). Additional funds to D.C. were provided by the European Research Training Network (EU-RTN). MRJ was supported by the Royal Society and Stuart Robson is thanked for his ongoing support with VMS. We acknowledge Edouard Kaminsky, Alain Bonneville, Fabrizio Ferrucci and Barbara Hirn for useful discussions on the interpretation of field and satellite thermal data, Rob Wright for the discussions about MODVOLC data and the MODIS Alert Team (http://modis.higp.hawaii.edu/) for providing the basis of this work. MODIS level 1 products were obtained from NASA (http://ladsweb.nascom.nasa.gov/). We thank Matthew Patrick and an anonymous reviewer for their very constructive comments, Davide Piscopo for assistance in processing MODIS data, and Philippe Catherine and Philippe Kowalsky for support during fieldwork. Instrumental and logistic support was kindly provided by the Observatoire Volcanologique du Piton de la Fournaise (http://ovpf.univ-reunion.fr/).
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