Temperature sheets and aspect sensitive radar echoes

Abstract

Abstract. There have been years of discussion and controversy about the existence of very thin and stable temperature sheets and their relationship to the VHF radar aspect sensitivity. It is only recently that very high-resolution insitu temperature observations have brought credence to the reality and ubiquity of these structures in the free atmosphere and to their contribution to radar echo enhancements along the vertical. Indeed, measurements with very highresolution sensors are still extremely rare and rather difficult to obtain outside of the planetary boundary layer. They have only been carried out up to the lower stratosphere by Service d'Aéronomie (CNRS, France) for about 10 years. The controversy also persisted due to the volume resolution of the (Mesosphere)-Stratosphere-Troposphere VHF radars which is coarse with respect to sheet thickness, although widely sufficient for meteorological or meso-scale investigations. The contribution within the range gate of many of these structures, which are advected by the wind, and decay and grow at different instants and could be distorted either by internal gravity waves or turbulence fields, could lead to radar echoes with statistical properties similar to those produced by anisotropic turbulence. Some questions thus remain regarding the manner in which temperature sheets contribute to VHF radar echoes. In particular, the zenithal and azimuthal angular dependence of the echo power may not only be produced by diffuse reflection on stable distorted or corrugated sheets, but also by extra contributions from anisotropic turbulence occurring in the stratified atmosphere. Thus, for several years, efforts have been put forth to improve the radar height resolution in order to better describe thin structures. Frequency interferometric techniques are widely used and have been recently further developed with the implementation of high-resolution data processings. We begin by reviewing briefly some characteristics of the ST radar echoes with a particular emphasis on recent works. Their possible coupling with stable sheets is then presented and their known characteristics are described with some hypotheses concernCorrespondence to: H. Luce (luce@kurasc.kyoto-u.ac.jp) ing their generation mechanisms. Finally, measurement campaigns that took recently place or will be carried out in the near future for improving our knowledge of these small-scale structures are presented briefly.

Aspect sensitivity in the VHF radar backscatters studied using simultaneous observations of Gadanki MST radar and GPS sonde

“…Luce et al (2001) have discussed contribution of temperature sheets to the aspect sensitive radar echoes. The above authors pointed out that the zenith and azimuth angle dependence of the echo power may arise either due to diffuse reflection from stable layers, or distorted/corrugated sheets or contribution from anisotropic turbulence occurring in the stratified atmosphere.…”

Evaluation of wind profiles from the NERC MST radar, Aberystwyth, UK

“…At VHF, the effects of anisotropic scatter must also be considered, since the strength of backscatter at angles closer to the vertical is usually enhanced (Luce et al, 2001a), causing the effective pointing angle of off-vertical beams to move closer to the zenith. Horizontal winds will be underestimated if the effective pointing angle θ eff is not used in Eqs.…”

“…ϑ s indicates the degree of aspect sensitivity (Hocking et al, 1986) and is defined as follows (Hooper and Thomas, 1995) using echo powers, P (θ ), from two beams at zenith angles θ 1 and θ 2 : (Hooper and Thomas, 1995). Aspect sensitivity tends to be strongest at the tropopause and in the lower stratosphere (Hocking et al, 1986(Hocking et al, , 1990Luce et al, 2001a), though strong aspect sensitivity has been observed throughout the free troposphere during, and after, the passage of synoptic fronts (Hooper and Thomas, 1995;Kawano and Fukao, 2001). It is likely that corrugated sheets of constant refractive index are responsible for the anisotropic echoes typically observed at VHF frequencies (Luce et al, 2001a).…”

MF and HF radar techniques for investigating the dynamics and structure of the 50 to 110 km height region: a review

“…The aspect sensitivity is parameterized as the half width of the backscatter polar diagram and given by θ S (see e.g., . A review of the methods for investigating thin sheets of temperature gradients in the lower atmosphere by Luce et al (2001) provides a useful review of techniques for measuring aspect sensitivity and of the relationship between atmospheric structure and aspect sensitivity.…”