This work, which is sponsored by the Qatar National Research Foundation, is a collaboration with the Weill-Cornell Medical College in Qatar, the Universite de Rennes, the Universite de Nantes and the Faculte des Sciences de Nouakchott. So far, we have carried out measurements in the Sahara desert on barchan sand dunes in Mauritania, and in Qatar.
Our latest field campaigns took place in April 2012 in Qatar. Data from earlier field campaigns (March 2010, January 2011, March 2011, June 2011, November 2011, and January 2012) are found below.
Top to bottom and left to right: Michel Louge and Alexandre Valance interrogate the ThermoWorks RTR-53 humidity/temperature recorder buried in the dune. Alexandre Valance holds the Young rain gauge that he deployed high above his flux "saltation meter" to avoid sand contamination. Renee Richer sets up the LiCor 8100A for CO2 flux measurements on the dune surface. Anthony Hay sets up the AMS soil gas sampling conesfor CO2 analysis. Alexandre Valance stands on the dune. A group picture.
Top to bottom: data for H2O mass fraction and temperature at 15 depths; net radiation sgtriking the dune. A serendipitous rainstorm occurred on April 13, 2012, causing massive changes in humidity near the surface, which took several days to dry up.
On November 21, 2011, Sara Abdul Majid's presentation on this research at the Qatar Foundation's 2011 Annual Research Forum won the "Best Environment Research Program of the Year," which carries a $100,000 award encouraging us to pursue this work, see this article in the Qatar Tribune, and this one written by Susan Lang for the Cornell Chronicle. The New York Academy of Sciences also wrote a report on the conference, available by clicking on the tab "Meeting Report > Soil Science" of this web site.
Left: Sara Abdul-Majid and Michel Louge pose behind the weather station that they installed in November 2011. The station includes a Nomad2 Wind Data logger that acquires signals from a Young tipping bucket rain gauge, two SecondWind C3 anemometers, a SecondWind NRG 200P wind vane, and a Young combined HMP155 temperature/humidity sensor from Vaisala with radiation shield.The station is powered by a REC10-12 batteryand a solar panel. Right: Michel Louge installs a fence in January 2012.
From right to left: Said Al-Hajri, one of his camels, and Michel Louge pose for a photo in January 2012. Mr. Al-Hajri supports our research by providing essential logistical support near his farm.
Top, left to right: Anthony Hay, Renee Richer, Sara Abdul-Majid, Michel Louge. Bottom: system tested in the lab; Renee and Sara test the LiCor; Michel and Renee raise the weather station with Nexgen anemometer and wind vane acquired with a Second Wind NOMAD 2 data logger, as well as two ThermoWorks temperature and humidity USB gauges. Data from the capacitance instrument, the temperature probe, and the Kipp & Zonen net solar radiation detector was acquired with a National Instruments cRIO, which, together with a 20Ah LiPo battery from Tenergy, allowed us as long as 60 hours of autonomy. Matthew Blair wrote a tutorial on developing the LabView-based software for the cRIO.
The movie below shows variations of diurnal sand temperature (red) and humidity (blue) below the surface at the toe of a barchan sand dune taken on March 19-21, 2011 in Qatar. The dashed red line is a numerical model of temperature time-history obtained by solving the unsteady heat equation in the sand bed with input from the measured Kipp & Zonen net radiation flux on the surface and the turbulent heat transfer flux at the surface arising from the measured wind speed. The vertical arrow indicates the direction and strength of humidity gradient at the surface. The blue line is a spline fit to the data that is used to evaluate the gradient. Note a gradual drying of the bed over the 44 hours of this field experiment.
Top, from left to right below the group photos: Michel Louge inserts his 15-sensor capacitance probe made of a PCB designed using PCB Artist through the surface of a barchan dune in the Sahara desert near Akjoujt (19N 50.637' 014W 08.869') in January 2011; close-up of the PCB capacitance probe that records depth profiles of volumetric water content < 1% per mass accurately by multiplexing the sensors to single-channel capacitance processing electronics; close-up of the temperature probe made of 15National Semiconductor LM235 sensors deployed along the capacitance instrument. Middle: data acquisition system based on aNational Instrument USB-6259 BNC acquisition board driven by an Acer Aspire ONE 532h-2588 laptop of low power consumption (< 1 amp at 14 V); the mutiplexing electronic box shown at the upper left was built by Patrick Chasle at the Université de Rennes. Bottom, from left to right: two ThermoWorks TW-USB-2-LCD+USB data loggers recorded diurnal variations of ambient temperature and relative humidity in the turbulent boundary layer above the surface, along with two anemometers recording wind speed and direction using a Nomad data logger; a Kipp & Zonen NR-Lite 2 recorded simultaneously irradiation from the sun minus radiosity from the sand surface. Patrick Chasle shows the 100 W solar panel, which allowed us, together with a 30 Ah Lithium Ferro Phosphate Ion rechargeable battery donated by Dr. Chun-Chieh Chang, complete energy self-sufficiency in the field.
Anthony Hay observes live microbes with fluorescence microscopy using a Paralens kit, and explains students at the Faculte des Sciences de Nouakchott the significance of the microbiology in sand dunes.
Movies below show variations of diurnal sand temperature and humidity below the surface at the toe of a barchan sand dune taken on January 19-21, 2011.
The graph below also provides wind speed at two elevations above ground, ambient temperature and humidity, and net radiation flux on the Mauritania barchan. Kipp & Zonen, manufacturer of the radiation instrument, published a short article on our work in its July Newsletter.
In March 2010, we carried out field experiments near Akjoujt, as well as in the Banc d'Arguin National Park of Mauritania (19N 29' 31.6"; 016W 24' 50.9"). Left: Alexandre Valance records the location of the relative humidity/temperature profile measurements. Center: Michel Louge connects instruments before carrying out measurements. Right: Ahmed Ould el Moctar and Michel Louge enjoy breakfast after a night of measurements on a barchan sand dune.
Our main publications on this subject include:
This paper reports measurements of solid volume fraction at the surface of sand ripples using our capacitance technique. By recording variations of sand elevation at the capacitance probe (see picture below), we find that sand ripples have compaction at troughs near random jammed packing (~64%) and near the minimum packing for a stable solid at crests (~54.5%). The paper discusses these results in the light of models of aeolian transport. It also describes a seepage flow of air that is driven by the Bernouilli effect as streamlines contract and expand on ripples. A short movie demonstrates that ripple crests cannot sustain much stress, as explained in the paper.
Measurements of the ripple profile with a diode laser and solid volume fraction with a capacitance probe. From left to right, Michel Louge, Alexandre Valance, Dah Ould Ahmedou, Ahmed Ould el Moctar, and Ahmedou Ould Mahfoudh, February 2006.
This paper reports measurements of solid volume fraction and humidity in sands just below the surface of a barchan dune in the Sahara using our the same probe we had deployed in snow packs. The paper also shows that the seepage flow through the sand surface acts forces dust particles to penetrate the ripple surface through troughs and guides water vapor through crests. The suction of dust into ripple troughs, which causes it to accumulate under a protective sand surface, may explain why desert air is so clear under normal winds, while dust only resuspends during unusually high winds.
A short erratum clarification can be downloaded here. It corrects a minor parenthetical point on Darcy shear stresses on p. 4, shortly after equation (10).
(a) Channeling of moisture through crests and (b) dust penetration in troughs driven by seepage on sand ripples;
for details, see Louge, et al (2009).
Ryan Musa, Brian Mittereder, and Mike Berberich designed and built a unique setup to record pore pressure within a porous rippled plastic sheet reproducing seepage flow within desert ripples. They used CNC milling machines to create waves on the free surface. Once they demonstrated feasibility of these measurements in Cornell's Environmental Wind Tunnel, Smahane Takarrouht took an extensive data set for mean and fluctuation pore pressure by multiplexing pressure taps to a single MKS Baratron 10 mTorr differential pressure transducerwith a Scanivalve in the summer 2011. In the Fall 2011, Amin Younes constructed another bed with twice the ripple amplitude.
Left: Brian Mittereder in front the Okuma CNC milling machine. Right: the porous rippled bed being machined.
The design team and their realization. Left to right: Brian Mittereder, Michael Berberich and Ryan Musa.
Data for the 3 mm ripple amplitude setup have been reduced and compared with theory, as shown in graphs above. We are now completing a new facility with 6 mm amplitude for comparison.
Amin Younes and his latest rippled bed with 6 mm amplitude,
which he manufactured on the Architecture department CNC with the help of Frank Parish.
This paper extends our analysis of the aeolian transport on sand ripples. It predicts the threshold shear velocity, and compares aeolian transport on Earth and Mars.
A movie shows wind transport on a small barchan. A movie requiring longer download and only playing on QuickTime (.mov extension) shows various modes of avalanching on the downwind face of barchan dunes.
Alexandre Valance (right) and Michel Louge (left) record the volume fraction on sand ripples near the route de l'Espoir in Nouakchott.
Current work involves the water balance in sand dunes of hyper-arid regions, see for example simulations of water penetration through a mobile dune after modest desert rain:
You can generate barchan dune profiles in the figure below using this Matlab program, which implements dune dimensions reported by G. Sauermann, P. Rognon, A. Poliakov, H. J. Herrmann, The shape of ....
You can do similar calculations with this Mathematica notebook.