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[1] Eruption dynamics are sensitive to ash aggregation, and aggregates are commonly found in eruptive deposits. While ash dispersal and associated hazards are sensitive to aggregation, few experiments have been conducted on this phenomena using natural materials across the diverse range of conditions expected in volcanic flows. We have isolated two regimes, wet and dry, in which aggregation occurs due to two different forces, electrostatic and hydrodynamic. Using a closed chamber to create a controlled atmosphere, we found that relative humidity, residence time, and kinetic energy are the three variables necessary to define wet and dry flow regimes. A series of processbased equations defining the behavior of ash particles have been developed. We propose an aggregation model that can be used for ash dispersal forecasts across a range of conditions in an eruptive plume.

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Recent local-scale observations of glaciers, streams, and soil surfaces in the McMurdo Dry Valleys of Antarctica (MDV) have documented evidence for rapid ice loss, glacial thinning, and ground surface subsidence associated with melting of ground ice. To evaluate the extent, magnitude, and location of decadal-scale landscape change in the MDV, we collected airborne lidar elevation data in 2014-2015 and compared these data to a 2001-2002 airborne lidar campaign. This regional assessment of elevation change spans the recent acceleration of warming and melting observed by long-term meteorological and ecosystem response experiments, allowing us to assess the response of MDV surfaces to warming and potential thawing feedbacks. We find that locations of thermokarst subsidence are strongly associated with the presence of excess ground ice and with proximity to surface or shallow subsurface (active layer) water. Subsidence occurs across soil types and landforms, in low-lying, low-slope areas with impeded drainage and also high on steep valley walls. Glacier thinning is widespread and is associated with the growth of fine-scale roughness. Pond levels are rising in most closed-basin lakes in the MDV, across all microclimate zones. These observations highlight the continued importance of insolation-driven melting in the MDV. The regional melt pattern is consistent with an overall transition of water storage from the local cryosphere (glaciers, permafrost) to the hydrosphere (closed basin lakes and ponds as well as the Ross Sea). We interpret this regional melting pattern to reflect a transition to Arctic and alpine-style, hydrologically mediated permafrost and glacial melt.