Browning, KA;
Smart, DJ;
(2018)
Invigoration of convection by an overrunning diabatically modified cloud-top layer.
Quarterly Journal of the Royal Meteorological Society
, 144
(710)
pp. 142-155.
10.1002/qj.3190.
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Abstract
This paper investigates the factors that invigorated an outbreak of warm-sector convection which was instrumental in transporting high momentum downwards to give damaging surface winds. We have re-examined a wintertime extratropical cyclone from the Fronts and Atlantic Storm-Track EXperiment for which an earlier study had shown the warm-sector convection to be in the form of a series of arc-shaped rainbands. Using a 5 km grid version of the WRF (Weather Research and Forecasting) model, we show that this convection was enhanced as a result of the properties of an airstream at the base of the dry intrusion that overran the shallow moist zone (SMZ) of the warm sector. This airstream, which we refer to as the Diabatically Modified Cloud-Top Layer (DMCTL), is shown to have originated several hours earlier in part of an ana-cold frontal cloud layer where a region of previously ascending air began systematically to descend. Air from the DMCTL descended from heights of 2–3 km to heights of 1–2 km over a roughly 5 h period during which sustained evaporation occurred and its potential temperature dropped by up to 5 °C. This substantially enhanced the convective instability where this airstream overran the SMZ, leading to vigorous boundary-layer convection. The same region of evaporative cooling is also shown to have generated positive potential vorticity (PV) in the upper parts of the DMCTL, with values reaching about 3 PV units where it overran the SMZ. This layer of high PV, although we have not quantified its impact, may have increased the convective instability further by inducing differential advection below it. The cyclone in the present study later underwent frontal fracture and developed a cloud head but the processes diagnosed in this study were underway before the transition occurred.
| Type: | Article |
|---|---|
| Title: | Invigoration of convection by an overrunning diabatically modified cloud-top layer |
| Open access status: | An open access version is available from UCL Discovery |
| DOI: | 10.1002/qj.3190 |
| Publisher version: | http://dx.doi.org/10.1002/qj.3190 |
| Language: | English |
| Additional information: | This version is the author accepted manuscript. For information on re-use, please refer to the publisher’s terms and conditions. |
| Keywords: | extratropical cyclone, FASTEX, sting jet, damaging winds, WRF model, evaporative cooling |
| UCL classification: | UCL UCL > Provost and Vice Provost Offices > UCL BEAMS UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Earth Sciences |
| URI: | https://discovery.ucl.ac.uk/id/eprint/10038224 |
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