Q. J. R. Meteorol. Soc. (2004), 130, pp. 1195–1212 doi: 10.1256/qj.03.21 Convective mixing in a tropopause fold By H.J. REID and G. VAUGHAN ∗ Institute of Mathematical and Physical Sciences, University of Wales, Aberystwyth, UK (Received 31 January 2003; revised 19 December 2003) SUMMARY We present a case study of the passage of a tropopause fold over the UK behind a cold front, with emphasis on the mixing caused by convection extending into the fold. The event took place on 15–16 January 1999, and was the subject of intensive observations using the Met Office C130 aircraft and the mesosphere–stratosphere– troposphere (MST) radar at Aberystwyth. Here we concentrate on radar and satellite observations during the afternoon of 16 January, when the surface cold front had passed over the UK. A tongue of moist air moved north- eastwards over Wales at 700 hPa at this time, which, because of the very dry air in the fold above, resulted in potential instability. The resulting convection was clearly observed in NOAA satellite images. The MST radar depicted the passage of the cold front and tropopause fold as a layer of high-echo power and vertical wind shear ascending with time. Spectral widths showed the fold to be free of turbulence until 1200 UTC on 16 January, when convection was observed reaching into the frontal zone and generating turbulence. Eddy dissipation and diffusivity rates of 8.6 mW kg −1 and 8.5 m 2 s −1 , respectively, were derived for this event. To place these figures in context, they are compared with corresponding rates derived for sixteen other passages of tropopause folds over the radar, each resulting from shear rather than convective instability. The convective event is found to be comparable to the strongest shear events, and to correspond to moderate turbulence as experienced by an aircraft. This process is of potential importance for atmospheric chemistry because it mixes boundary layer air directly with stratospheric air over a timescale of 1–2 hours. KEYWORDS: Cold front MST radar Turbulence 1. I NTRODUCTION Stratospheric and tropospheric air possess very different chemical and dynamical properties, the former being dry and rich in ozone with high values of potential vorticity (PV), and the latter typically moister and lower in ozone and PV. Mixing of these two air masses can promote chemical reactions as well as transporting trace species from one region into the other (Bamber et al. 1984). Recent work has suggested that the chemical consequences of mixing depend on the nature of the mixing itself (Esler et al. 2001), particularly for short-lived species such as OH. There is also evidence that the chemical characteristics of a stratospheric intrusion into the troposphere are retained longer than the dynamical characteristics (Bithell et al. 2000). A thorough understanding of the way stratospheric air mixes into the troposphere is therefore needed if the dynamical and chemical impact of this mixing is to be properly evaluated. In this paper we examine measurements of turbulence in tropopause folds, as observed by the UK mesosphere–stratosphere–troposphere (MST) radar, for evidence of direct mixing between stratospheric and boundary layer air. Although a great deal of attention has been paid to the role of convection in the tropics for stratosphere–troposphere exchange (e.g. Holton et al. 1995; Stohl et al. 2003), the effect of convection in the extratropics has attracted much less attention. Convection in midlatitudes does not penetrate as far into the stratosphere as in the tropics, nor is convection the only (or indeed the major) mechanism for displacing air parcels vertically at mid and high latitudes. Observations of water vapour in the lower stratosphere show a rapid decrease with altitude above the extratropical tropopause, reaching typical ‘stratospheric’ concentrations on a scale of about 1 km (e.g. Oltmans et al. 2000). Mixing in the layer near the tropopause can be caused by a number of ∗ Corresponding author: Institute of Mathematical and Physical Sciences, University of Wales, Aberystwyth SY23 3BZ, UK. e-mail: gxv@aber.ac.uk c  Royal Meteorological Society, 2004. 1195