In this paper, we present high-resolution observations obtained with the Middle and Upper Atmosphere (MU) radar (Shigaraki, Japan, 34.85 degreesN, 136.10 degreesE) using the frequency radar domain interferometric imaging (FII) technique. This technique has recently been introduced for improving the range resolution capabilities of the mesosphere-stratosphere-troposphere (MST) radars which are limited by their minimum pulse length. The Fourier-based imaging, the Capon method have been performed with 5 equally spaced frequencies between 46.25 and 46.75 MHz and with an initial range resolution of 300 m. These results have been compared firstly to results obtained using the frequency domain interferometry (FDI) technique with Deltaf = 0.5 MHz and, secondly, to results from a classical Doppler beam swinging (DBS) mode applied with a range resolution of 150 m. Thin echoing structures could be tracked owing to the improved radar range resolution and some complex structures possibly related to Kelvin Helmholtz instabilities have been detected. Indeed, these structures appeared within the core of a wind shear and were associated with intense vertical wind fluctuations. Moreover, a well-defined thin echo layer was found in an altitude range located below the height of the wind shear. The radar observations have not been fully interpreted yet because the radar configuration was not adapted for this kind of study and because of the lack of complementary information provided by other techniques when the interesting echoing phenomena occurred. However, the results confirm the high potentialities of the FII technique for the study of atmospheric dynamics at small scales. (C) 2001 Elsevier Science Ltd. All rights reserved.