Temperature sheets and aspect sensitive radar echoes

There have been years of discussion and controversy about the existence of very thin and stable temperature sheets and their relationship to the VHF r adar aspect sensitivity. It is only recently that very high-resolution insi tu temperature observations have brought credence to the reality and ubiqui ty of these structures in the free atmosphere and to their contribution to radar echo enhancements along the vertical. Indeed, measurements with very high-resolution sensors are still extremely rare and rather difficult to ob tain outside of the planetary boundary layer. They have only been carried o ut up to the lower stratosphere by Service d'Aeronomie (CNRS, France) for a bout 10 years. The controversy also persisted due to the volume resolution of the (Mesosphere)-Stratosphere-Troposphere VHF radars which is coarse wit h respect to sheet thickness, although widely sufficient for meteorological or meso-scale investigations. The contribution within the range gate of ma ny of these structures, which are advected by the wind, and decay and grow at different instants and could be distorted either by internal gravity wav es or turbulence fields, could lead to radar echoes with statistical proper ties similar to those produced by anisotropic turbulence. Some questions th us remain regarding the manner in which temperature sheets contribute to VH F radar echoes. In particular, the zenithal and azimuthal angular dependenc e of the echo power may not only be produced by diffuse reflection on stabl e distorted or corrugated sheets, but also by extra contributions from anis otropic turbulence occurring in the stratified atmosphere. Thus, for severa l years, efforts have been put forth to improve the radar height resolution in order to better describe thin structures. Frequency interferometric tec hniques are widely used and have been recently further developed with the i mplementation of high-resolution data processings. We begin by reviewing br iefly some characteristics of the ST radar echoes with a particular emphasi s on recent works. Their possible coupling with stable sheets is then prese nted and their known characteristics are described with some hypotheses con cerning their generation mechanisms. Finally, measurement campaigns that to ok recently place or will be carried out in the near future for improving o ur knowledge of these small-scale structures are presented briefly.