Observations of dual slantwise circulations above a cool undercurrent in a mesoscale convective system

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A region of large-amplitude Kelvin-Helmholtz billows, of area 2000 km2 , was observed in a region of moderate rain at the rear of a mesoscale convective system on a day during the Convective Storm Initiation Project in southern England. The mesoscale convective system (MCS) was characterized by elevated convection above a layer of cool air, separated by a sloping statically stable layer characterized by strong wind shear. Doppler radar observations showed that the large patch of billows was situated within this shear layer and that it persisted over the 2 h period of observation, maintaining its position with respect to the MCS and producing surface pressure perturbations of ± 0.3 hPa. Potentially unstable air above the sloping shear layer reached its level of free convection while ascending above this layer. The resulting elevated convection was in the form of cells whose spacing appeared to be influenced by the underlying billows. Results from large-eddy model simulations are shown to support the hypothesis that, although the large-scale ascent alone would have triggered the elevated convection, the billows exerted a secondary forcing effect on the convection.

Section: Effect Of Deepening Undercurrent On Elevated Convectionmentioning

confidence: 96%

Section: Location and Extent Of The Billowsmentioning

confidence: 97%

Section: Relationship Of the Elevated Convection To The Billowsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

A case study of a large patch of billows surmounted by elevated convection

Browning¹,

Marsham²,

White³

et al. 2012

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Convective updraught evaluation in high‐resolution NWP simulations using single‐Doppler radar measurements

Nicol

Hogan

Stein

et al. 2015

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This study presents an evaluation of the size and strength of convective updraughts in high‐resolution simulations by the UK Met Office Unified Model (UM). Updraught velocities have been estimated from range–height indicator (RHI) Doppler velocity measurements using the Chilbolton advanced meteorological radar, as part of the Dynamical and Microphysical Evolution of Convective Storms (DYMECS) project. Based on mass continuity and the vertical integration of the observed radial convergence, vertical velocities tend to be underestimated for convective clouds due to the undetected cross‐radial convergence. Velocity fields from the UM at a resolution corresponding to the radar observations are used to scale such estimates to mitigate the inherent biases. The analysis of more than 100 observed and simulated storms indicates that the horizontal scale of updraughts in simulations tend to decrease with grid length; the 200 m grid length agreed most closely with the observations. Typical updraught mass fluxes in the 500 m grid length simulations were up to an order of magnitude greater than observed, and greater still in the 1.5 km grid length simulations. The effect of increasing the mixing length in the sub‐grid turbulence scheme depends on the grid length. For the 1.5 km simulations, updraughts were weakened though their horizontal scale remained largely unchanged. Progressively more so for the sub‐kilometre grid lengths, updraughts were broadened and intensified; horizontal scale was now determined by the mixing length rather than the grid length. In general, simulated updraughts were found to weaken too quickly with height. The findings were supported by the analysis of the widths of reflectivity patterns in both the simulations and observations.

Simulations of an observed elevated mesoscale convective system over southern England during CSIP IOP 3

White

Blyth

Marsham

2016

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Simulations of an elevated mesoscale convective system (MCS) observed over southern England during the Convective Storm Initiation Project (CSIP) provide the first detailed modelling study of a case of elevated convection occurring in the UK. The study shows that many factors can influence the maintenance of elevated deep convection, from large‐scale flow through to surface heating processes and diabatic cooling within the convective system. It is also shown that interactions and feedback mechanisms between a stable layer and the storm can act to maintain deep convection. The simulation successfully reproduced an elevated MCS above a low‐level stable undercurrent, with a wave in the undercurrent linked to a rear‐inflow jet (RIJ). Convection was fed from an elevated (840 hPa) source layer with CAPE of about 350 J kg−1. The undercurrent in the simulation was approximately 1 km deep, about half that observed. Unlike the observed MCS, a transition from elevated to surface‐based convection occurred in the simulation due to the combined effects of a pre‐existing large‐scale θe gradient, advection and surface heating causing the system to encounter increasingly unstable low‐level air and a shallower stable layer that was more susceptible to penetration by downdraughts. The transition to surface‐based convection was accompanied by the development of cold‐pool outflow and an increase in system velocity from about 6 to 10 m s−1. Diabatic cooling from microphysical processes in the simulation enhanced the undercurrent and strengthened the RIJ. This strengthened the wave in the undercurrent and led to more extensive convection. The existence of a positive feedback process between the convection, RIJ and stable layer is discussed. Uncertainty in the synoptic scale generating errors in the undercurrent is shown to be a major source of error for convective‐scale forecasts.