Proceedings with Extended Abstracts (single PDF file) - Radio ...

Also Larsen and Röttger (1991) had shown that one needs interferometer measurements tocorrect the radial velocity for errors arising from tilted reflectivity structures. Incidence anglemeasurements are needed for this purpose, which can be done by a spaced antenna set up andthe analysis of the phase of the cross correlation functions as sketched in Fig. 5.It was shown that the velocities measured with the vertically pointing beam of 5 degreeswidth were mostly not the velocities in true vertical direction, since the real incidence of thesignal was not from the vertical direction. This was attributed to inclined surfaces ofreflectivity. The vertical velocity errors without correcting by the measured incidence angleswere between 5% and 200%. This method is not applicable in the Doppler beam swingtechnique unless the inclinations of the reflectivity structures have amuch larger horizontal extent than illuminatedFig. 5 Principle of phase by the radar beams. This usually is the case formeasurements with thesynoptic-scale disturbances but not for mountainspaced antenna technique. waves, which have much shorter scales.In practice, this problem is much more complicated as we will describe in the following bysketching three scenarios considering the inclination of reflectivity structures (so-calledreflectivity sheets or laminae), the position of isentropes and the streamlines of the air flow.Fig. 6 Sketch showing the main quantities in question:The incidence angle δ, which is equivalent to the inclination of athin scattering/reflecting layer (called sheet or laminae, but that also may holdfor thicker layers), the measured radial velocity Vr, the air velocity Uand the isentrope (surface of constant potential temperature Θ) in the x-z plane.In Fig. 6 the scattering/reflecting layer, i.e. a lamina of changes in radar refractivity, isaligned on an isentrope (constant level of potential temperature) and the streamline of airflowwith velocity U is along the isentrope. For any angle δ, the radial velocity Vr = 0.In Fig. 7 another scenario is sketched, which assumes that the reflectivity layer is not alignedon an isentrope and the streamline of airflow is in the plane of the isentropic surface. Whenthe angle δ 0, the radial velocity Vr 0, and the projection U’ of U along the measuredwave vector direction is the measured radial velocity Vr. The real velocity is U = Vr / sin δ.308