On the interpretation of the layered structures detected by mesosphere-stratosphere-troposphere radars in dual frequency domain interferometry mode

The frequency domain interferometry (FDI) technique has been developed for probing thin layered structures of the atmosphere. The position and thickne ss of a single layer embedded within the scattering volume can be deduced f rom the complex normalized cross correlation (coherence) of received signal s at two closely spaced frequencies. Applied in the vertical pointing direc tion, this technique identified layered structures ("FDI layers") of 50-200 m in thickness in the lower atmosphere. These structures are 1 order of ma gnitude thicker than observed temperature sheets (about 10-m thick) which a re very likely responsible for the main part of the VHF radar echoes in ver tical direction. In this paper, although the ambiguity of the dual FDI tech nique is well known, we emphasize that the FDI layers do not necessarily co rrespond to a single atmospheric layer; they can also be interpreted as a m ore complex structure of very thin atmospheric layers. A simple model, intr oduced as an example, shows that the FDI layer thickness can also approxima tely be interpreted as the vertical separation of two very thin atmospheric layers. This result can explain by itself the differences between the esti mated thicknesses by balloon and FDI radar techniques. Finally, we stress t hat comparisons with high-resolution in situ measurements are urgently need ed for interpreting the FDI layers.