INTERPRETATION OF VHF ST RADAR VERTICAL ECHOES FROM IN-SITU TEMPERATURE SHEET OBSERVATIONS

The interpretation of the radar aspect sensitivity observed at VHF fre quencies in the lower atmosphere is still a subject of controversy in the radar community. Indeed, scattering from anisotropic turbulence la yers and partial reflection from stable thin horizontally stratified l ayers are generally proposed without leading to a definitive conclusio n. A cause of this persistent discussion has been a lack of in situ hi gh-resolution observations which could identify and describe accuratel y the atmospheric structures at the origin of the radar aspect sensiti vity. The Radars, Scidar and Balloons (RASCIBA 90) campaign (February- March 1990, Aire sur l'Adour, France) was performed using simultaneous ly colocated VHF ST radars and balloon experiments. The objective was to obtain information about the small-scale structures of the lower at mosphere and then to identify the origin of radar echoes. The main res ult was produced by the high vertical resolution (20 cm) temperature m easurements. For the first time, very strong (positive) temperature gr adients within thin layers were detected in the lower atmosphere (at l east up to 27 km). Such an observation allowed then investigation of t he partial reflection interpretation using available bifrequency radar measurements: a 45-MHz radar for which typical 15-dB vertical enhance ment is observed just above the tropopause and a 72.5-MHz radar for wh ich an aspect sensitivity of the same order is presented for the first time at this frequency. A simple model considering flat and very exte nded sheets was assumed to estimate quantitatively the possible contri bution of these temperature gradients to the VHF radar vertical power. Radar and reconstructed power reflection coefficient profiles are in good agreement in shape and in level showing that partial reflection f rom atmospheric sheets is an important and generally dominant process at vertical incidence. Furthermore, the effect of sheets of limited ex tent is investigated using simple theoretical considerations, and this model shows that the correction to the infinite sheet approximation i s probably weak, The sheet distortion effect is also approximately eva luated applying Gaussian rough models. The power loss from the flat sh eet case at VHF in the vertical direction is weak for a mean roughness height lower than a few tens of centimeters and for larger heights if the correlation length of the irregularities is larger than a few hun dreds of meters. Sheet generation mechanisms are also briefly discusse d using two previously published models: the viscosity wave model (Hoc king et al., 1991) and ''sheet and layer'' model (Gossard et al., 1985 ). Neither of the models in their present forms seems to be able to ac count for the observations.