R. Greeley

Bio: R. Greeley is an academic researcher. The author has contributed to research in topics: Wind tunnel & Saltation (geology). The author has an hindex of 3, co-authored 4 publications receiving 222 citations.

Modeling the atmospheric dust cycle: 1. Design of a soil-derived dust emission scheme

Abstract: A soil-derived dust emission scheme has been designed to provide an explicit representation of the desert dust sources for the atmospheric transport models dealing with the simulation of the desert dust cycle. Two major factors characterizing the erodible surface are considered: (1) the size distribution of the erodible loose particles of the soil which controls the erosion threshold and the emission strength and (2) the surface roughness which imposes the efficient wind friction velocity acting on the erodible surface. These two parameters are included in a formulation of the threshold wind friction velocity by adapting a size-dependent parameterization proposed by Iversen and White (1982) and by applying to the rough erodible surfaces a drag partition scheme derived from Arya (1975). This parameterization of the threshold friction velocity has been included in an horizontal flux equation proposed by White (1979). This allows to attribute a specific production rate to each soil size range for each type of surface. The dust flux F is then considered as a fraction of the total horizontal flux G, the value of the ratio F/G being imposed, at this time, by the soil clay content. In summary, the computed mass fluxes depend on the soil size distribution, the roughness lengths, and the wind friction velocity. The different steps of this scheme have been independently validated by comparison with relevant experimental data. Globally, the agreement is satisfying, so that the dust fluxes could be retrieved with less uncertainties than those observed in previous simulations of the desert dust cycle.

The physics of wind-blown sand and dust

Abstract: The transport of sand and dust by wind is a potent erosional force, creates sand dunes and ripples, and loads the atmosphere with suspended dust aerosols This article presents an extensive review of the physics of wind-blown sand and dust on Earth and Mars Specifically, we review the physics of aeolian saltation, the formation and development of sand dunes and ripples, the physics of dust aerosol emission, the weather phenomena that trigger dust storms, and the lifting of dust by dust devils and other small-scale vortices We also discuss the physics of wind-blown sand and dune formation on Venus and Titan

A simple expression for wind erosion threshold friction velocity

Abstract: Threshold friction velocity u*t is the friction velocity at which wind erosion is initiated. While u*t is affected by a range of surface and soil properties, it is a function of particle size only for idealized soils. In this paper we present a simple expression for u*t for spherical particles loosely spread over a dry and bare surface. In this expression we consider the balance between the driving forces (aerodynamic drag and lift) and the retarding forces (cohesion and gravity) and assume that the cohesive force is proportional to particle size. It is found that u*t can be expressed as Y1d+Y21d, with Y1 and Y2 being empirical constants. The new expression is both simple and effective.

Simulation of eolian saltation.

Abstract: Saltation is important in the transport of sand-sized granular material by wind and in the ejection of dust from the bed both on Earth and on Mars. The evolution of the saltating population and all its characteristic profiles is calculated from inception by pure aerodynamic entrainment through to steady state. Results of numerical simulations of single-grain impacts into granular beds are condensed into analytic expressions for the number and speeds of grains rebounding or rejected (splashed) from the bed. A model is combined with (i) this numerical representation, (ii) an expression for the aerodynamic entrainment rate, and (iii) the modification of the wind velocity profile by saltating grains. Calculated steady state mass fluxes are within the range of mass fluxes measured in wind tunnel experiments; mass flux is nonlinearly dependent on the shear velocity. Aerodynamically entrained grains in the system are primarily seeding agents; at steady state, aerodynamic entrainment is rare. The time for the entire system to reach steady state is roughly 1 second, or several long-trajectory hop times.

Saltation threshold on Earth, Mars and Venus

Abstract: New formulations valid for wide ranges of particle diameter and density and gas density are presented for prediction of saltation threshold speed for small particles. A low-air-density wind tunnel was used to extend the range of previous investigations and to separate the effects of Reynolds number and interparticle forces of cohesion. The new formulations are used to predict saltation threshold for atmospheric conditions on the surface of the Earth, Mars, and Venus.