Influence of tropopause-level disturbances on convection Geraint Vaughan

Simple picture: upper-level PV maximum After Hoskins et al 1985 Motion Ascent ahead of upper-level PV maximum Descent behind

Effect of lifting profile Stable Convectively unstable

Problems with simple model Upper-level PV anomaly isn’t just a basin Tropopause folds are dry, stable layers extending into the troposphere After Mel Shapiro, in Hoskins et al (1985)

Problems with simple model Potential instability relies on dry air aloft A moist profile becomes more stable if lifted!

Problems with simple model Ascending motion is in general slantwise /3/09

Convection isn’t always at the leading edge of the PV anomaly Leading Edge Beneath anomaly AVHRR Vis image AVHRR IR image 315K PV 2/3/09

Tropopause folds can cap convection Ozone sonde, 1700 Front Deeper convection Shallower convection

Dry intrusions can release convection Scalloped line: eastern edge of dry intrusion over-running cold front Colours: Meteosat IR image; orange < -28°C. A and B are regions of thunderstorms confirmed by sferic observations Browning and Roberts 1994

Tropopause folds can release convection Sequence of MT-SAT Water Vapour images Nov 2005: Tropical Plume 1833 UT 2333 UT 1033 UT 0233 UT

Cross-section in latitude of wet-bulb potential temperature (colour contours) derived from ECMWF analysis fields through the 129°E meridian at 0930 local time on 22 November Labelled black contours define relative humidity isopleths. The dashed white line highlights 14°S 0.5-degree clustered 4-day ECMWF back trajectories originating at 00 UTC on 20 November 2005, corresponding to the layer of potential instability at 900 mb centred at 129°E, 14°S. ECMWF-derived PV fields (colour contours) are over-plotted on the 314 K isentropic surface at trajectory origin. G. Allen et al 2009

Convection capped by fold remnant CSIP case 15 June Panel below contours sequence of radiosondes from Larkhill Dry stable layer 2.5 – 3 km Primary tropopause fold Positive CAPE 315 K PV, 12 h A. Russell et al 2008

Wind profiler observations UHF radar, near Larkhill – signal power shows capping of convection by lower stable layer MST radar, Aberystwyth – wind shear depicts descending tropopause fold A. Russell et al 2008

Origin of the lower stable layer Fold observed here in St Johns radiosonde profile Two clusters of 7-day back trajectories initiated at 1200 UTC 15 June 2005 from 725 hPa and 280 hPa above Larkhill. A. Russell et al 2008

Multiple tropopause folds 1645 UTC Rain radar 18/7/05 A. Russell et al 2009

1200 UTC ECMWF PV (shading and contours) and θ (dashed contours) A. Russell et al 2009

Conclusions The simple model has been accepted too readily Folds are integral to upper-level PV anomalies and can suppress convection as well as promoting it Fold remnants complicate the issue – thin layers of dry stable air advected thousands of kilometres