## 1 Introduction  
Tephra injected into the atmosphere during explosive eruptions poses a direct threat to aviation and can have primary and secondary impacts at different spatial and temporal scales on surrounding communities [1; 2; 3]. Current monitoring procedures necessary to mitigate the potential impact of tephra dispersal and fallout are mostly expert-visioned, resulting into a delay ranging from a few minutes to a few hours from the occurrence of an event [4]. Automatic systems for the detection of explosive activity based on volcanic tremor [5] and infrasound [6; 7] are essential for early warning, but they do not provide eruptive source parameters (ESPs), such as the top plume height, size, and the mass and/or volume eruption rate at the source. Infrasound arrays can be used to detect the onset of the eruption in near-real-time up to distances of a few hundreds of kilometers. The infrasonic signals are acoustic waves produced by explosive gas expansion (mainly water vapor) in the atmosphere, with frequencies less than 20 Hz. The analysis of this signal is fundamental for both volcano monitoring purposes and for a better understanding of eruption dynamics [8; 9]. Volcano observatories around the world routinely use infrasound networks aimed at detecting, locating, and characterizing volcanic activity. These systemshave shown promising results in forecasting paroxysmal activity at open-vent systems and providing new insights on eruption dynamics and volcanic processes [10; 11]. An automatic warning system of explosive eruptions, based on the infrasound system (IS), has also been developed at Etna [6; 12; 13]. The infrasonic observations are collected by two small-aperture arrays, showing how the eruptive episodes are marked by a smooth increase in amplitude preceding the proper paroxysmal phase, sometimes also hours before the formation of a volcanic plume. Today, the link between acoustic pressure and eruptive dynamics of Strombolian eruptions at the Etna volcano is well-understood [8], as well as the correlation between the eruptive phase and other sensors' measurements [14; 15]. Thermal cameras (TC) are usually employed to monitor the explosive activities generating lava fountains [10; 16; 17]. Indeed, the analysis of these images allows to identify the lava fountains as the bi-dimensional temperature-saturated region, named the incandescent jet region (IJR) [18]. The height of the IJR can be converted into exit velocities of pyroclastic material at the volcanic vent and, in turn, into the erupted volume [16; 19]. TC can work both in good weather conditions and during cloudy days, albeit with reduced sensitivity. The weather radar (WR) has shown great potential in evaluating real-time features of the explosive activity [20]. In fact, WR visibility is not hindered by weather clouds. Applying the Volcanic Ash Radar Retrieval (VARR) approach [14; 21; 22] to WR observables makes it possible to quantify the main ESPs in near-real-time [23; 24; 25]. The early-warning system proposed in this work combines the ability of infrasound and the thermal camera in detecting and timing the onset of the explosive activity with that of the weather radar to probe the volcanic plume and retrieve key ESPs, e.g., the mass eruption rate and the plume height [14]. We have developed a new algorithm, named paroxysmal early warning (PEW), based on the combination of IS, TC, and WR. The method was tested on nine cases of Etna lava fountains: (1) 10 April 2011, (2) 12 April 2012, (3) 23 November 2013, (4) 3-5 December 2015, and (5) 16 and 23 February 2021. The work is organized as follows: Etna paroxysmal activities are briefly described in Section 2, the methodology for each sensor is presented in Section 3, the applications to several Etna explosive events are discussed in Section 4, the combined approach for PEW is presented in Section 5, and the discussion and concluding remarks are provided in Section 6.