Climate

In the context of ROSA research the University of Camerino proposes an approach to use GPS RO data to study gravity waves and clear air turbulence. In order to perform this study we have to exploit the vertical resolution of the radio occultation signal. The target of the present study is the Mediterranean region, where sources of gravity waves are present because of the existing complex orography. The analysis will be performed on data coming from different radio occultation sources such as the Formosat-3/COSMIC, CHAMP, and ROSA missions. Gravity waves observed at scales within the resolving power of RO can be generated by various dynamical "unbalance" processes, typically interactions of atmospheric flows with mountains, and/or adiabatic processes and propagation in the troposphere. Atmospheric gravity waves are currently treated by applying only a statistical approach. An outstanding example is the so called wave-drag introduced a couple of decades ago in the global models (see Pulido and Thuburn 2005).

The Mediterranean area is rich of potential sources of gravity waves for the obvious reason of being surrounded by high and steep mountains and dominated by a thermal source like the Mediterranean Sea. It has long been suspected that gravity and gravity-inertia waves can play a deterministic role in the local Mediterranean dynamics, in particular during the occurrence of frontal precipitations in the area. If we consider the wave oscillations as small perturbations of the "background profile" (computed for example by using a filter at 8 km), the normalized temperature fluctuation can be assumed to follow a gravity wave dynamics dictated by the background Brunt-Vaisala frequency.

In order to study some gravity wave climatologies, we adopt the point of view of Smith et al. (1987) concerning velocity fluctuations successively extended by Allen and Vincent (1995) to temperature fluctuations. The analysis accounts for some limitations of radio occultation measurements, such as the real resolution of the observation given by the size of the Fresnel diameter (e.g., Hinson and Magalhaes 1991). If we would not consider this real resolution, phenomena appearing as gravity waves could instead be related to sub-Fresnel scale noise (Marquardt and Healy 2005).

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