The early-warning detection module within VARR algorithm is based on the temporal analysis of radar data in the area surrounding each volcano vent, giving as outputs the Probability of Ash Eruption onset (PAE) and a Spatial Identification map. The probability of the volcanic eruption onset is described in time by the PAE evolution. Its behavior is a indicator of lava fountains ejecting tephra in the surrounding of the volcanic vent. PAE ≥ 80% is associated to the presence of ash plumes, whereas PAE ≤ 60% are mainly due to meteorological targets. The other values lead to an uncertain due to mixed phase (hydrometeor and ash signatures) and/or other drawbacks. In the above figure in the left panel PAE value is directly displayed on VMI radar map at each acquisition time-step while the related spatial discrimination between tephra and meteorological targets is shown in the right panel for each pixel of the radar map. It is worth mentioning that VMI (Vertical Maximum Indicator) product gives a maximum value of reflectivity above each vertical column of radar volumetric data.

The Classification and Estimation modules in VARR algorithm aims at provide an automatic tephra categorization and concentration estimation making use of a synthetic dataset of the radar variables generated by a physical-electromagnetic model. These modules will be activated only if tephra has been detected, in this case PPI and VCUT maps will display the latest available images of reflectivity, tephra classification and related concentration estimation in g/m³. Tephra classification from radar echoes is made with respect to ash particle size: Fine Ash (FA) with average diameters of 0.01 mm; Coarse Ash (CA) with average diameters of 0.1 mm; Small Lapilli (SL) with average diameters of 1 mm; Large Lapilli (LL) with average diameters of 10 mm. It is worth mentioning that PPI (Plan Position Indicator) product it a slant-range display of radar reflectivity acquired from one antenna rotation with the antenna at a given elevation angle. On the other hand VCUT (Vertical CUT) product display radar reflectivity with height as the vertical axis and range as the horizontal axis, forming a vertical cross section of reflectivity in a given direction (azimuth angle clockwise from the North). The angle of elevation of the PPI and the direction of the VCUT will be set manually once the unrest volcano is known.