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

where v R (θ,φ) is the radial component of velocity and w is the vertical component of velocity,which is taken as the radial velocity observed by a vertically directed beam. The DBStechnique implicitly assumes that the wind vector is homogeneous across the spatial scaleswhich separate the radar volumes for the different beam pointing directions and within thetime taken to complete a single cycle of observation. The value of v H (φ+180°), derived forthe complementary off-vertical beam, is therefore expected to be equal in magnitude andopposite in sign to v H (φ). This redundancy can be exploited by combing the two values as0.5×[v H (φ) - v H (φ+180°)] to give a better estimate of the horizontal component of velocityalong φ and as [v H (φ) + v H (φ+180°)] to give a measure of the reliability of this estimate; asmall value suggest high reliability whereas a large value suggests poor reliability. Thevalues of the complementary beam velocity variability factor shown in the fourth panel ofFigure 1 represent the square root of [v H (φ) + v H (φ+180°)] 2 + [v H (φ+90°) + v H (φ+270°)] 2 .It can be seen that the value of the complementary beam velocity variability factor istypically small, indicating high reliability of the corresponding horizontal velocity estimates.However, it rises to several 10s of m s -1 within specific time-altitude regions, most clearly theone associated with the extra-ordinary vertical velocity activity around 1310 UT. This isattributed primarily to the extremely rapid fluctuations of the vertical velocity which violatethe DBS assumption of constancy over the time scale for a single cycle of observation (andwhich could also broaden the spectral widths). Changes of a few m s -1 from one cycle to thenext at a particular altitude are not uncommon. As can be see from Equation 1, a change ofjust 1 m s -1 in the vertical velocity has approximately the same effect on v R (6°,φ) as a changeof 10 m s -1 in the component of the horizontal velocity along φ. By contrast, the verticalvelocities associated with mountain wave activity change much more slowly and they areassociated with small values of the complementary beam velocity variability factor.If attention remains confined to the event around 1310 UT, it can be seen that the extraordinaryvertical velocity activity is accompanied by enhancements in the radar return signalpower and spectral width (corrected for the effects of beam broadening) for the observationsmade by a vertically directed beam. Both of these signatures are consistent with the effects ofmoist convection transporting humid air into drier regions of the upper-troposphere andgiving rise to turbulent mixing. More will be said about both of these inferences shortly.Finally, it can be seen that this event is accompanied by reasonably heavy rainfall at thesurface which is again consistent with the a convective event.The combination of the above characteristics appears to be a good indicator of the presenceof convective activity and a number of events have been identified in this way. The clarity ofthe enhancement in vertical beam signal power depends largely on the altitude to which theconvection reaches. If the maximum altitude is less than 4 km, below which the signal powersignature of the humidity field is typically already strong, a convection effect can be hard todiscern. In general, both the maximum altitude and the duration of convection events arequite variable; the latter can extend to many hours.Another common characteristic at the lowest altitudes, albeit one which might not be obviousfrom the standard data products, is a contribution of Rayleigh scattering from hydrometeorsto the radar return signals. Figure 2 shows the normalised power spectra for vertical beamobservations made at 1323 UT; these 128 point spectra represent observations made over 22s. The Power Spectral Densities, PSDs, at each range gate are normalised to the peak PSD forthat gate. This is very similar to the pattern seen by Narayana Rao et al. (1999) forconvective precipitation at a tropical location. The bold lines superimposed on Figure 2indicate the limits of the spectral regions identified as containing signal using the standard336