VHF wind‐profiler data as a teaching aid

Abstract

Data from a VHF wind-profiling radar are being used as a teaching aid in the Department of Meteorology at the University of Reading. An example is given of data obtained during the passage of a train of baroclinic waves which shows how the radar, together with output from weather-forecast models, can vividly illustrate processes occurring over a range of scales.The Department of Meteorology at the University of Reading has, since its inception in 1965, been running weekly Current Weather discussions during term-time. The purpose of these has been to bring the current weather alive by synthesising observations and NWP model output illustrating the role of atmospheric processes in shaping the day-to-day weather. Many sources of data are used to give the students insight into the interplay of processes over a range of scales. A particularly useful tool for this purpose, which we have been using increasingly in recent years, is the VHF radar at Aberystwyth on the west coast of Wales, sometimes referred to as a MST (MesosphereStratosphere-Troposphere) radar (Vaughan 2002). This radar provides, amongst other things, continuous wind profiles from 1.8 km to more than 16 km. We find that time-height records from the radar, when accompanied by collocated time-height records from the mesoscale version of the Met Office's Unified (weather forecast) Model, provide an eye-catching impression of key structures in the vertical which can then be related to the passage of familiar atmospheric structures such as troughs and ridges associated with baroclinic waves as portrayed in plan view using the output from global models.We illustrate this approach with five-day time-height plots from the VHF radar. Figure 1 shows the wind direction (colours) and wind velocity (arrows) during the passage of a train of three baroclinic waves. Notice the abrupt veer of the winds during the passage of the troughs, especially at altitudes between 5 and 10 km. Figure 2 shows the associated pattern of echo power from which it is possible to infer the altitude of the tropopause (Vaughan et al. 1995). The returned echoes are due to a combination of Fresnel scattering from refractive-index layers on a scale of order 1 m and Bragg scattering from 3-m scale refractive-index fluctuations which are strong on the underside of stable atmospheric layers. The tropopause is marked in Figure 2 by the thin layer of intense echo (red and yellow) between 10 and 12 km altitude, which descends and becomes less distinct within major tropopause depressions and folds collocated with trough axes. It is instructive to relate these radar-detected features to the output from numerical weather prediction models.The concept of potential vorticity (PV) as a tracer of the stratospheric air descending within tropopause depressions and folds is well illustrated by the mesoscale model time-height plot in Figure 3. Potential vorticity is represented in Figure 3 by the coloured shading. Values of PV greater than 2 PV units (brown/red) are generally regarded as representing air of str...

Classifying fronts in data from a VHF wind-profiling radar

“…If so, could their variability be described? We would expect some success with this approach as VHF radars have previously been used to identify mid and upper tropospheric fronts (Neiman and Shapiro, 1989; Neiman et al , 1992, 1998, 2001; Spencer et al , 1996; Browning et al , 1998; Lucas et al , 2001; Pavelin et al , 2003). Indeed, Lucas et al (2001) automated an algorithm to detect fronts in data from a VHF wind profiler, but the signal‐to‐noise ratio of the Aberystwyth radar is significantly better than for the radar they used.…”