2010 Annual Research Report

During 2010 Metzger pursued three field research fronts; dust devil studies in the USA (with an MFR grant with PSI colleagues), dust devil studies in Chile (with the Geophysics Department at The University of Concepcion), and Nevada geoarcheology consulting (independent work on behalf of archeology clients). PSI Research As CoI with PI Matt Balme’s MFR project “In-Situ Studies Of Terrestrial Dust Devils And Ambient Meteorology: Application To Mars’ Climate”, my role is to contribute to general planning and analysis, provide the chase truck that directly samples dust devils in the desert, provide first aid gear & guidance to field operations, and evaluate the geologic environment of aeolian materials at the sites. Note, due to PI Balme’s parenthood that resulted in another future PSI Senior Scientist (it takes time; your patience is appreciated… ;), it trumped his participation onsite for the June 2010 field campaign and Matt remained in London. Therefore Oz and Metzger, in consultation with Matt, took over to organize, supply/equip, and conduct the field effort for our second, and final field season of this project. Our southern Nevada site has enjoyed the participation of additional professional colleagues since Metzger began in 1995 and this year was no exception. Fellow scientists from APL-John Hopkins, SETI-Ames, SwRI, Paris, London, Spain and Portugal volunteered to join in, for a total of 12 this summer.

Metzger  present a sample of initial findings from this recent field campaign to directly sample dust devil vortices in Eldorado Valley, NV, June 2010. Since 1995 the prime study area has been Eldorado Valley (EV), a closed playa basin (flat, hard, and dry, with zones of fine and coarse surficial material) outside Boulder City, in southern Nevada where climate is arid, hot and with consistent, strong insolation. Using a mobile vehicle to position a sensor array directly into the path of on-coming dust columns we had over 50 direct (symmetrical, through the core) and nearby indirect encounters, but considerably fewer than the 139 tagged in 2009. Strong regional wind, which can blow apart dust devil structure, and high daily temperatures made this a ‘challenging’ season. Although once thought of as interesting but inconsequential desert oddities, dust devils are now understood to be a major contributor to airborne dust loading in the arid regions of Earth and Mars. The vortex process(es) responsible for soil erosion, however,  remain poorly articulated. Dust devils have complex, intense, and highly variable wind fields at their base due to changes in boundary layer meteorologic, geologic, and surface aerodynamic conditions.

Dust devils are an effective aeolian erosion mechanism of small-scale, substantially particle loaded convective vortices driven by insolation, common on both Earth and Mars. These vortices are highly turbulent flows with dust entrainment that is dictated by the surficial material over which they pass, the ambient wind that push them, and the pockets of hot air that power their “engine”, thus they fluctuate second-to-second laterally and vertically. On Mars, dust devil activity might support the persistent dustiness in the atmosphere, although spacecraft observations appear to rule out the possibility that Mars’ global dust storms are triggered by dust devil activity. This interim report on field activities in the Eldorado Valley of Southern Nevada is part of a larger effort to understand dust devil formation under natural meteorological conditions and the resultant dust-lifting efficacy they achieve. Correct placement of any survivable instrumentation into the path of these dynamic phenomena is a non-trivial challenge and few instruments are available whose design can endure the conditions involved and provide the response time required. To this end we upgraded our sensors and in situ placement techniques (i.e. chase truck with extended boom). These include an Applied Technologies, Inc. 3-dimensional sonic anemometer with Vx probe that sampled U, V & W wind components, plus sonic temperature, at 10 Hz., A TSI DustTrak aerosol monitor was used to sample PM10 dust (0.1 to 10 um diameter) at 1 Hz. A Kestrel 4500 weather station was used to measure maximum pressure excursions (negative). Dust devil diameter was visually estimated by Metzger via direct observation near the columns’ base during encounters (Photogrammetric size determination via GIS techniques will be used to augment these estimates). All sensors were placed 2-m above the desert surface, 5-m in front of the chase vehicle on a rigid boom.

This report covers 50 encounters that directly bisected the dust devil columns, using a mobile chase vehicle. These data were collected in late June 2010. We are not aware of any other study that has measured, in situ, this range of thermal vortex parameters at such high-resolution, spatially and temporally. The following results from direct dust devil sampling will be integrated into the next phase of this project; the determination of the consequences of various weather conditions on vortex formation, size, longevity, and dust flux. The typical boundary layer thermal vortex laden with dust (dust devil) was sampled at 13:51 LST, rotated at 9.8m/s (with a peak max 13.5), experienced buffeting upward vertical flow of 3.6 m/s and downward vertical flow of -1.8 m/s, a sonic temperature of 3.2 °C above ambient (at 2-m ht), a pressure drop below ambient of 0.8 mbar, and a resultant dust load of 11.3 mg/m^3. The highest observed values were a maximum rotation of 25.5 m/s, minimum rotation of 3.3 m/s, maximum upward lift of 7.1 m/s and maximum downward “lift” of –4.3 m/s, maximum pressure depression of 1.9 mbar from ambient and a minimum of 0.3 mbar, sonic temperature excursions over ambient ranging from 0 to 8.2°C, and dust loading as high as 139 mg/m^3 yet as low as 0.4 mg/m^3 while ambient dust remained below 0.02 mg/m^3. Chilean Research Metzger was invited onto a project by Michael Kurgansky, a Visiting Professor at the University of Concepcion, Chile, to assist in writing the grant proposal and serve as the “veteran field guy”. This was the first-ever dust devil campaigns conducted in the Atacama Desert of northern Chile (January & November-December, 2009). At present, field studies for this project are completed, we have one published paper, and as lead author of the vortex-sampling effort, we are readying another paper for submission & review (likely to Aerosols & Air Quality or the Journal of Boundary-Layer Meteorology) In addition to dust devil activity surveys and ambient meteorology monitoring with 2 Met Stations, we successfully chased and directly sampled more than 35 vortices, with approximately half being ideal symmetrical core penetrations. Field studies were sited outside of Huara (80 km from the city of Iquique, Chile) adjacent to a broad flat dry lakebed and inter-fingered mudflows, composed of finely layered clay and silt, at the base of a very long slope leading from the Andes Mountains. The meteorology stations were equipped to measure at several heights, thus enabling calculation of wind shear across the surface, and temperature gradients. The chase instrumentation included state-of-the-art 3D sonic anemometers as well as dust and pressure sensors that Metzger supplied. Dust devil vortices, including those successfully chased by truck, were recorded in thermal images taken by a University of Concepcion student. To date, no thorough analysis of dust devils has been published using thermal imaging, despite their role as a convective transfer of surface heating into the atmospheric boundary layer.

Metzger and the Chilean university are trying to coordinate that student’s participation in desert fieldwork in Nevada, July 2011, and perhaps bring the thermal camera. From this, not only will we get an excellent assistant (who gets a good look at science, culture and grad school options in the USA) but also allows us to systematically utilize a valuable resource and build collaboration between PSI, University of Concepcion, and the University of Kentucky (home to Jeff Moersch, a new collaborator who, accompanied by his grad student, acquired dust devil thermal imagery with us in 2009, but has not had opportunity to analyze them).

Much of Metzger's academic focus (and income) is centered on consulting around central and northern Nevada. The archeology clients are tasked with identifying and recording artifacts from early settlers and ancient inhabitants prior to expansion of mining operations (although some focus is directed toward infrastructure and transportation construction around urban areas). He gets called in for cases where geology plays an influential role in a specific site to uses my background in geomorphic surface processes, sedimentology, and paleoclimate reconstruction to determine the mechanisms that work to modify a site’s surface, the environmental conditions at the time of ancient habitation, and the fate (weathering, transportation and placement) of artifacts.

Publications

Pathare, A.V., M.R. Balme, S.M. Metzger, A. Spiga, M.C. Towner, N.O. Renno, and F.

Saca, 2010, Assessing the power law hypothesis for the size-frequency distribution of

terrestrial and martian dust devils, Icarus; DOI 10.1016/j.icarus.2010.06.027

Kurgansky, M.V., A. Montecinos, V. Villagran, and S.M. Metzger, 2010, Micrometeorological

Conditions for Dust-Devil Occurrence in the Atacama Desert, Boundary-Layer

Meteorololgy; DOI 10.1007/s10546-010-9549-1