Time evolution of Land Surface Temperature (LST) in active volcanic areas detected via integration of satellite and ground-based measurements: the Campi Flegrei caldera (Southern Italy) case study.

Thermal features of environmental systems are increasingly investigated after the development of remote sensing technologies; the increasing availability of Earth Observation (EO) missions allows the retrieval of the Land Surface Temperature (LST) parameter, which is widely used for a large variety of applications (Galve et al., 2018). In volcanic environment, the LST is an indicator of the spatial distribution of thermal anomalies at the ground surface, supporting designed tools for monitoring purposes (Caputo et al., 2019); therefore, LST can be used to understand endogenous processes and to model thermal sources.

In this framework, we present the results of activities carried out in the FLUIDs PRIN project, which aims at the characterization and modeling of fluids migration at different scales (https://www.prinfluids.it/). We propose a multi-scale analysis of thermal data at Campi Flegrei caldera (CFc); this area is well known for hosting thermal processes related to both magmatic and hydrothermal systems (Chiodini et al., 2015; Castaldo et al., 2021). Accordingly, data collected at different scales are suitable to search out local thermal trends with respect to regional ones. In particular, in this work we compare LST estimated from Landsat satellite images covering the entire volcanic area and ground measurements nearby the Solfatara crater.

Firstly, we exploit Landsat data to derive time series of LST by applying an algorithm based on Radiative Transfer Equations (RTE) (Qin et al., 2001; Jimenez-Munoz et al., 2014). The algorithm exploits both thermal infrared (TIR) and visible/near infrared (VIS/NIR) bands of different Landsat missions in the period 2000-2021; we used time series imagery from Landsat 5 (L5), Landsat 7 (L7) and Landsat 8 (L8) satellite missions to retrieve the thermal patterns of the CFc area with spatial resolutions of 30 m for VIS/NIR bands and 60 m to 120 m for TIR bands. Theoretical frequency of acquisition of the Landsat missions is 16 days that is reduced over the study area by cloud cover: Landsat images with high cloud cover were in fact discarded from the time series.

In particular, we process both the daily acquisitions as well nighttime data to provide thermal features at the ground surface in the absence of solar radiation. To emphasize the thermal anomalies of endogenous phenomena, the retrieved LST time-series are corrected following these steps: (i) removal of spatial and temporal outliers; (ii) correction for adiabatic gradient of the air with the altitude; (iii) detection and removal of the seasonal component.

Regarding to the ground-based acquisitions, we consider the data collected by the Osservatorio Vesuviano, National Institute of Geophysics and Volcanology (OV- INGV, Italy, Naples); the dataset consists of 151 thermal measurements distributed within the 2004-2021 time-interval and acquired inside the Solfatara and Pisciarelli areas at a depth of 0.01 m below the ground surface. Similarly, we process this dataset following corrections (i) and (iii).

Finally, we compare the temporal evolution of thermal patterns retrieved by the satellite and ground-based measurements, highlighting the supporting information provided by LST and its integration with data at ground.