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Volcanic plumes are challenging to detect and characterize rapidly, but insights into processes such as hail formation or ash aggregation are valuable to hazard forecasts during volcanic crises. Global Navigation Satellite System (GNSS, which includes GPS) signals traveling from satellites to ground receivers can be disturbed by volcanic plumes. To date, two effects aiding plume detection from GNSS observations have been described: (a) ash-rich plumes scatter the signal, lowering the signal-to-noise ratio (SNR), and (b) some plumes refract and thus delay GNSS signals. Using GNSS data from the VEI 4 2011 Grimsvotn eruption, we show that tephra and water contents of plumes distinctly affect SNR and phase residuals. The signals suggest high-altitude freezing of plume water into volcanic hailcorroborated by 1-D modeling and volcanic hail deposits. Combining GNSS SNR and phase residual analyses is valuable for detecting processes that rapidly scrub fine ash out of the atmosphere.

Plain Language Summary Explosive volcanic eruptions eject hazardous volcanic plumes into the atmosphere, threatening aircraft and downwind communities. Rapid detection and characterization of ash clouds are valuable components of volcano monitoring and hazard assessment. We demonstrate that it is possible to distinguish ash-rich from vapor or ice portions of volcanic plumes using Global Navigation Satellite Systems (GNSS, of which GPS is one constellation) data for the 2011 Grimsvotn eruption in Iceland. Our analyses, combined with insights from 1-D plume modeling, corroborate that GNSS phase residuals and signal-to-noise ratios (SNRs) are sensitive to different plume compositions. Ash-laden plumes generally decrease the SNR at high-elevation angles. Water-vapor-rich plumes, on the other hand, have a greater impact on phase residuals. We combine phase residual and SNR observations and demonstrate that this technique can detect condensation or ice formation in different parts of the plume. Our analysis is validated by models of volcanic plume rise and water phase changes and by field observations of volcanic hail deposits. This finding is significant because it provides a new approach for characterizing plume processes that are relevant to the transport of hazardous ash as hail formation scrubs particles out of the atmosphere.