Thursday, March 5, 2009
The generation of giant dunes explained
Ever climbed up a 200+ ft dune (I know some of you have from your memes ...) and wondered how it formed?
Well, a new paper in Nature explains the controls on giant dune size. Andreotti et al. (2009) use a combination of field measurements and aerodynamic calculations to provide evidence that giant dunes are built of smaller dunes and their terminal height is controlled by an atmospheric boundary layer.
They show that large dunes grow by the amalgamation of superimposed dunes. Dune growth stops (assuming it's not the result of climatic change) by interaction with an inversion layer. Once the dune has grown large enough, it begins interacting with the inversion layer which confines airflow to move around the dunes rather than over them. The height of this inversion layer correlates to the variation in annual temperature. The greater the variation in annual temperature the higher the altitude of the inversion layer and the taller the giant dunes can grow. Continental dunes with larger annual temperature changes grow larger than coastal dunes which have smaller annual temperature changes moderated by their proximity to the ocean.
There is also a short piece on Ralph Bagnold and his seminal book "Physics of Blown Sand and Desert Dunes." And in case you missed it in the Carnival of the Arid #1, Through the Sandglass' Michael Welland describes the fascinating life of Ralph Bagnold.
Posted by Mel at 9:56 PM