Utilizing the smoke plume of Volcano Villarrica to examine the dynamics and detailed structure of its wave and rotor

The smoke plume of Villarica volcano (2850m) in Chile offers a unique opportunity for the study of stratified and hydraulic airflow over a conical mountain. This smoke serves as a semi passive, visible marker that is injected into the predominantly westerly flow that passes directly over the crest of this conical mountain. Roughly constant yet mild venting activity of this volcano produces a plume often visible for 10-20km. In addition to the smooth venting, individual slightly buoyant puffs exit the crater on the order of 1-2 per minute. This introduces a desirable heterogeneity to the smoke plume. Individual puffs remain distinguishable for up to 3km downwind of the crater and can be utilized for tracer anemometry. In the first two pilot phases, Jan-Mar and Dec 2007, of the Chilean Wave Study, CHIWAS, this flow was documented via time lapse photography. Wind and temperature logging was conducted on transects originating at the summit of the volcano. The temperature transect had 250m vertical resolution from 300m ASL to 2800m ASL on the north, crosswind side. Three anemometers were placed on both the west and the east side at ~300m vertical increments. These surface observations along with sounding data from Puerto Montt are used to examine the characteristics of the flow that are apparent in the time lapse films. These films are created from high resolution digital photographs taken with a polarizing filter at five second intervals. Wave, rotor and subrotor behavior is clearly identifiable in the smoke plume at 10:30AM local time on December 19, 2007. The boundary layer separation point is witnessed to remain over the same geographical location for an hour and a half before migrating up the lee side. Synchronous with this migration the amplitude of the wave diminishes and eventually just before 1:15PM the laminar flow in the lee transitions to a state of intermittent horizontal vortex shedding in the immediate lee of the summit. Wind speed and stability both decrease during this transition. In the lee, surface wind speeds are progressively stronger with decreasing altitude until boundary layer separation is reached. Lee-side maximum wind speed of over 10m/s is twice as large as the ambient incoming winds of 5m/s. Under the rotor, wind speeds are found to be half of the upwind wind speeds. Greatest gustiness is found at the first lee side station, 300m below the summit while the greatest shifts in wind direction are found at the third lee station under the rotor at 900m bellow summit level. We will also address a potential future phase of this field campaign that will use radiosondes to obtain upwind soundings so numerical models can be initialized and tested.