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| Titel |
A ‘Boscastle-type' quasi-stationary convective system over the UK Southwest Peninsula |
| VerfasserIn |
Robert Warren, Daniel Kirshbaum, Robert Plant, Humphrey Lean |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250072456
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| Zusammenfassung |
| Quasi-stationary convective systems (QSCSs) can produce extreme rainfall accumulations
and have been responsible for many devastating flash floods worldwide. An oft-cited case
from the UK is the ‘Boscastle storm’ which occurred on 16 August 2004 over the southwest
peninsula of England. This system produced over 200Â mm of precipitation in just four hours,
leading to severe flooding in several coastal settlements. This presentation will focus on a
QSCS from July 2010 which showed remarkable similarity to the Boscastle storm in terms
of its location and structure, but produced much smaller rainfall accumulations
and no flooding. First, observational data from the two cases will be compared to
highlight three factors which made the Boscastle case more extreme: (1) higher
rain rates, associated with a warmer and moister tropospheric column and deeper
convective clouds; (2) a more stationary system, due to slower evolution of the
large-scale flow; and (3) distribution of the heaviest precipitation over fewer river
catchments. Results from numerical simulations of the July 2010 case (performed
with convection-permitting configurations of the Met Office Unified Model) will
then be presented. A control simulation, using 1.5-km grid spacing, reveals that
convection was repeatedly initiated through lifting of low-level air parcels along a
quasi-stationary coastal convergence line. Sensitivity tests suggest that this convergence line
was a sea breeze front which temporarily stalled along the coastline due to the
retarding influence of an offshore-direction background wind component. Several
deficiencies are apparent in the 1.5-km model’s representation of the storm system,
including delayed convective initiation; however, significant improvements are observed
when the grid length is reduced to 500Â m. These result in part from an improved
representation of the convergence line, which enhances the associated low-level
ascent allowing air parcels to more readily reach their level of free convection. The
implications of this finding for forecasting convective precipitation will be discussed. |
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