The evolution of an MCS over southern England. Part 2: Model simulations and sensitivity to microphysics

Abstract

ReuseUnless indicated otherwise, fulltext items are protected by copyright with all rights reserved. The copyright exception in section 29 of the Copyright, Designs and Patents Act 1988 allows the making of a single copy solely for the purpose of non-commercial research or private study within the limits of fair dealing. The publisher or other rights-holder may allow further reproduction and re-use of this version -refer to the White Rose Research Online record for this item. Where records identify the publisher as the copyright holder, users can verify any specific terms of use on the publisher's website. TakedownIf you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. Simulations using the Met Office Unified Model at 1 km horizontal grid spacing of a Mesoscale Convective System (MCS) with a cold pool which propagated across southern England on 25 August 2005 are validated using detailed observations from the Convective Storm Initiation Project (CSIP). Early organisation of the system is not especially well treated, but the model goes on to form a system which developed qualitative and quantitative features remarkably similar to the observations.A sensitivity study suggests that the initial linear system is driven by the position of a low-level 'lid' and upper-level instability, the linear organisation being promoted by a weak rear-inflow jet forced by the upper-level warm anomaly in the cloud anvil. A weak cold pool develops in the absence of ice-phase processes, but this does not promote system propagation. Strengthening and descent of the rear-inflow jet, and acceleration of the system, is promoted by the additional heating through glaciation and cooling through snow evaporation. The surface cold pool and gust front are further strengthened by snow melting and rainfall evaporation. With ice-phase processes present, the cold pool strengthens as a result of the system development and its strength is broadly correlated with system propagation speed during the middle phase of the system's lifetime. Propagation enables the convective band to 'sweep up' any convective cells which trigger ahead of the system. The differing scales of microphysical processes means that it is difficult to form a steady-state system. The observed transition phase corresponds to an increase in the slantwise nature of the flow in the storm and in the low-level cooling by rainfall evaporation near the gust front, combined with a change in ambient conditions (advection over the sea) which eventually enable the cold pool to propagate ahead of the system. Key Words: convection; precipitation; cold pool; rear-inflow jet; cloud microphysics; mesoscale model

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

“…Convection and the RIJ formed during the model spin‐up period so it was not possible to investigate the initiation from these simulations, but tests showed that diabatic cooling enhanced the RIJ (consistent with Yang and Houze, ; Braun, ; Braun and Houze, ; Franklin et al ., ; Clark et al ., ). During the spin‐up period, diabatic cooling enhanced the undercurrent, and once the MCS formed the cooling enhanced the RIJ.…”

“…() and Clark et al . (), who all found that removing latent cooling either prevented or significantly weakened cold pool formation.…”

“…Diabatic cooling from microphysical processes is known to enhance the RIJ (e.g. Yang and Houze, ; Franklin et al ., ; Clark et al ., ). Latent cooling has also been shown to increase maximum vertical velocities, produce a surface gravity current, increase horizontal speeds and reduce upshear tilt (Trier et al .…”

A Study of Cloud Microphysical Processes Associated With Torrential Rainfall Event Over Beijing

“…Previous studies have shown that both WSP (Chong, ; Clark et al, ; Innocentini & Neto, ; Kingsmill et al, ; Lin et al, ) and CFP (Bodine & Rasmussen, ) are mainly produced by MCSs, which themselves are greatly affected by cloud microphysical processes. In this part, we begin by discussing the microphysical and thermodynamic vertical structures of MCSs in the WSP and CFP.…”

The evolution of an MCS over southern England. Part 1: Observations

“…The objectives of the article are to elucidate the observed structure of the MCS and how it changed in time, particularly focussing on a major transition period. Model simulations of the event are presented in Clark et al (2013), referred to as Part 2 hereafter, to gain some insight into the processes contributing to this structure, with particular emphasis on the contribution from different microphysical processes.…”