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

THE SIGNATURE OF MID-LATITUDE CONVECTION OBSERVEDBY MST RADARDavid A Hooper 1 , Helen J Reid 2 and Ed Pavelin 31 Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, UK2 Department of Physics, University of Wales Aberystwyth, Ceredigion, SY23 3BZ, UK3 Department of Meteorology, University of Reading, PO Box 243, Reading. RG6 6BB, UKIntroductionFor MST radars operating at mid-latitudes, the magnitude of radial velocities observed by avertically directed beam (henceforth referred to as vertical velocities) is typically no morethan the order of 0.1 m s -1 . Occasional periods characterised by magnitudes of the order of 1m s -1 are usually associated with mountain wave activity. Convective events are recognised tobe a source of even more significant vertical velocity activity at equatorial latitudes, wherethe magnitudes can be of the order of 10 m s -1 (Jain et al., 2000). However, mid-latitudeconvection has received relatively little attention. The aim of the current study is to establishthe characteristics of convection as observed by the UK Natural Environment ResearchCouncil MST radar at Aberystwyth (52.4°N). It will be shown that these are not just ofscientific interest but have consequences for data reliability.Case study of 1st March 2003Data from the MST radar at Aberystwyth, and a co-located tipping bucket raingauge, for 1stMarch 2003 are shown in Figure 1. The crosses superimposed on 4 of the panels indicate thealtitude of the tropopause derived from the vertical beam signal power (top panel) using anobjective algorithm (Hooper and Arvelius, 2000). The time-altitude plot of vertical velocity(second panel) indicates that small magnitude fluctuations (< ±0.5 m s -1 ) as functions of bothtime and altitude are present throughout the day; positive vertical velocities correspond toupward movement. These fluctuations are consistent with the expectation that the surfacewinds (not shown) of between 5 and 10 m s -1 , which remain approximately south-westerlythroughout the day, will give rise to only weak mountain wave activity (Prichard et al.,1995). The fluctuations are easier to discern in the next panel which shows the time series ofvertical velocity observed at an altitude of 5.1 km. The vertical velocity behaviour around0950, 1310 and 1420 UT clearly does not fit the general pattern. It is not just the extraordinarilylarge peak values which distinguish this vertical velocity activity from thatassociated with mountain waves. It is the fact that the vertical velocities, at any givenaltitude, change much more rapidly and erratically as a function of time; there is typicallylittle coherence between the profiles of vertical velocity from one cycle to the next (~2.5minutes).With respect to the Doppler Beam Swinging (DBS) technique, for a radar beam directed at anangle θ from the vertical (6° in this case) and along an azimuth φ, the horizontal componentof velocity along the same azimuth, v H (φ), is given by:vHv( φ)=R( θ , φ)− wcosθsinθ(1)334