Terrestrial and Martian Dust Devils: study of the translational motion and resolution of the size/distance degeneracy of the meteorological time series

Dust devils are convective vortices able to lift sand and dust grains from the surface. They are common not only in the terrestrial deserts, but also on Mars, where they give a substantial contribution to the planetary dust budget (∼50%). Dust devils are characterized by a central pressure drop that generates the rotation and translate advected by the wind background.
One of the problems related to their monitoring is due to the impossibility to directly separate the translational and rotational motion components from the study of the wind speed and direction time series. This means that it is not possible to directly retrieve information on their translational speed and direction and on the maximum rotational wind speed using only a meteorological station. 
In addition, there are other fundamental parameters that are not directly measurable, such as the distance of passage of the vortex from the station, its sense of rotation and its diameter.
These limitations lead in general to a size/distance degeneracy of the results, i.e. the acquired meteorological signatures of a smaller dust devils passing near the station could not be distinguished from the ones of a bigger and farther event. This, in turn, leads to several problems in the study of their physics.
To avoid these issues, the monitoring meteorological station can be equipped with an imaging camera system with a sufficient acquisition rate, resolution and field of view. However, this is not always possible, in particular for the planetary space missions.
Here, we want to present two simple methods for the characterization of the wind speed and direction time series of dust devils that allow an easy solution for the measure of the vortex translational wind speed and direction, distance of passage and sense of rotation.
The knowledge of these parameters allows to completely characterize the measured dust devils encounter just using the meteorological station acquisition.
In order to test the methods, we performed a field campaign in the Sahara desert, deploying a fully equipped meteorological station coupled with a camera system. We compared the results of the meteorological analysis with the ones obtained from the images, confirming the effectiveness of our methodology.
This methodology can give a substantial improvement in the interpretation of the past and next martian dust devils data. For example, the ESA/Roscosmos ExoMars 2020 mission will host on its lander a meteorological station (METEO package) and the Dust Complex, a suite of specific instruments devoted to the study of the primary airborne dust. These instruments can be used in tandem for the characterization of the local dust devils activity.