Topic Name: U of M researchers develop real-world measurements of "saltation" that could lead to better climate model
Category: Environmental Fluid Mechanics
Research persons: Nilton Renno, Jasper Kok
Location: University of Michigan, United States
Wind isn't acting alone in the geological process behind erosion, sand dunes
and airborne dust particles called aerosols.
The other culprit is electricity. By taking both factors into account,
researchers at the University of
Michigan have developed a new model that matches real-world measurements of
"saltation" better than the decades-old classical theory.
Saltation is the process of wind blowing grains of sand across a landscape,
sending them bouncing against the ground and each other. The bouncing motion of
the saltating grains on the soil bed kicks dust aerosols into the air.
This new knowledge could lead to better climate models because it helps
scientists understand how aerosols are released, U-M researchers say. Dust is
one type of aerosol. Burning fossil fuels releases another type. They are known
to affect Earth's climate by blocking and absorbing sunlight and seeding clouds.
Renno, associate professor in the Department of Atmospheric, Oceanic and
Space Sciences, and doctoral student Jasper
Kok have demonstrated that saltation creates a field of static electricity
that can be strong enough to double the concentration of bouncing sand
particles, compared to previous assumptions. A paper on their findings will be
published in the Jan. 11 issue of Physical Review Letters.
"The effect of aerosols is one of the most uncertain processes in climate
change modeling," Kok said. "We now know more of the physics of
how dust aerosols get into the atmosphere, so we should be able to improve on
the way that climate models account for their emission."
Saltation itself has never been fully understood. Only recently have detailed
measurements been made in nature, as opposed to in a wind tunnel. And those
natural measurements disagreed with classical theory.
Renno first noticed that electricity might be missing from the equation while
studying dust devils in Arizona years ago. The devils had a strong electric
"I was surprised at how large the field was," Renno said.
Others had suggested that electricity may be involved in saltation, but Renno
said no one determined the extent of that role and created a model to describe
the process including electricity, until now.
"What we discovered is as these particles bounce and rub against each
other, the surface of the ground gets a positive charge and the particles get a
negative charge," Renno said. "The electric field can become strong
enough to directly lift sand from the surface."
The surface of the ground acts as a conductor, Kok explains, because it has a
thin film of water on top.
The researchers say this model can accurately reproduce observations.
"It's a fundamental change in our understanding of the physics of
saltation," Renno said.
Renno, who is a co-investigator on NASA's Phoenix and Mars Science Laboratory
missions to Mars, speculates that these saltation electric fields get so large
on the Red Planet they produce ground-level sparks.
Note for Climate Model
Climate models use quantitative methods to simulate the interactions of the atmosphere, oceans, land surface, and ice. They are used for a variety of purposes from study of the dynamics of the weather and climate system to projections of future climate.
All climate models balance, or very nearly balance, incoming energy as short wave electromagnetic radiation (which in this context means visible and ultraviolet, not to be confused with shortwave) to the earth with outgoing energy as long wave (infrared) electromagnetic radiation from the earth. Any imbalance results in a change in the average temperature of the earth.
The most talked-about models of recent years have been those relating temperature to emissions of carbon dioxide (see greenhouse gas). These models project an upward trend in the surface temperature record, as well as a more rapid increase in temperature at higher altitudes.
Note for Electric Field
In physics, the space surrounding an electric charge or in the presence of a time-varying magnetic field has a property called an electric field (can also be equated to "Electric Flux Density"). This electric field exerts a force on other electrically charged objects. The concept of electric field was introduced by Michael Faraday.
The electric field is a vector field with SI units of newtons per coulomb (N C−1) or, equivalently, volts per meter (V m−1). The direction of the field at a point is defined by the direction of the electric force exerted on a positive test charge placed at that point. The strength of the field is defined by the ratio of the electric force on a charge at a point to the magnitude of the charge placed at that point. Electric fields contain electrical energy with energy density proportional to the square of the field intensity. The electric field is to charge as gravitational acceleration is to mass and force density is to volume.
Kok and Renno's research on the basic physics of saltation and its
implications to climate has been supported by the National
Science Foundation's Physical and Dynamic Meteorology Program.
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