© University of Reading 2006 www.reading.ac.uk

Anchoring of convective storms

Robert Warren

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Contents

• My background

• Introduction & Motivation

• Project aims and

methodologies

• Work so far– Background reading

• Boscastle Flood– Case studies

• 21/07/2010

• Work going forward

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My background

• Weather enthusiast

• Reading Graduate – 4 year MMet degree with a year in Oklahoma

• 2 Met Office summer placements:– 2009: Stratospheric

Sudden Warmings– 2010: Orographically

enhanced rain

• Storm chasing!

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Project motivation

• Flash flooding: one of the greatest natural hazards

• Results from specific combination of meteorological and hydrological conditions

• Precipitation proportional to rain rate, R, and duration, D (Doswell et al. 1996)

• Anchored convective storms:– Large R and relatively large D– Often involve echo training (quasi-stationary convective

systems)– Can cause flash flooding in just a few hours (e.g.

Boscastle)– Difficult to forecast

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Project aims and methodology

• Investigate mechanisms behind anchoring of convective storms in the UK

• Case studies:– Observational data (Met Office analyses, radar, rain

gauges, satellite imagery)– High-resolution UM simulations

• Idealised simulations – isolate mechanisms

• Ensemble model runs – determine predictability

• Automatic detection algorithm – build climatology

• End goal: ‘guidelines’ for forecasters

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Background reading

• High-resolution modelling of convection:– Bryan et al. (2003) – Use of Large Eddy Simulation– Lean et al. (2008) – Comparison of 12 km, 4 km and 1

km UM

• Rainfall verification from high-resolution models:– Roberts and Lean (2008), Roberts (2008) – Fractions Skill

Score

• Banded orographic convection:– Kirhsbaum and Durran (2005a,b), Kirshbaum et al.

(2007)

• Case studies of UK anchored convective storms (Weather):– Burt (2005), Golding et al. (2005) – Boscastle, 16 August

2004– Mayes and Winterton (2008) – Surrey, 13 August 2006

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The Boscastle Flood

• Devastating flooding in coastal settlements of Boscastle and Crackington Haven

• Echo training over high ground just inland of north Cornwall coast

• 200 mm of rain in under 4 hours

• Intense rainfall gradients (50 mm km-1) on SE flank

• Rain rates up to 400 mm hr-1

Rainfall totals from rain gauges (top) and radar (bottom). From Burt (2005).

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The Boscastle Flood, cont...

1200

16001500

14001300

1700

Rain rates from the Cobbacombe Cross radar in Devon. From Burt (2005).

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The Boscastle Flood, cont...

• High precipitation efficiencies due to large-scale ascent

• Storms forced by persistent narrow convergence line

• Combination of sea-breeze circulation and frictional backing in coastline-parallel low-level flow

Accumulated precipitation (mm) from 1200–1700 UTC

Surface wind vectors and convergence (s-1) at 11 UTC

UM simulations at 1 km grid-spacing: (a) Full simulation, (b) Flat orography, (c) Flat orography and no land-sea contrasts in surface heat and moisture fluxes and surface temperature . From Golding et al. 2005

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Case Studies

• Subjective identification:– Looked at last 2 summers– Examined looped animations of

composite radar images for each day

• Several (8+) cases found

• Some common features:– Echo training– Related to topography– Organised into flow-parallel

bands

• Some days have several, widely separated anchored storms

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Case 1 – 21/07/2010

• Remarkably similar to Boscastle storm

• Repeated convective development along North Cornwall/Devon coast

• Interesting evolution:– Early initiation (~08

UTC)– Organised into a line– Stationary until ~15

UTC– Sudden inland

propagation after this time, starting from the south

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Case 1 – 21/07/2010, cont...

• Radar maximum: 51.8 mm of rain in 3 hours (12–15 UTC)

• Event was not accurately captured by UKV modelUKV: 13 UTC (t+10h) – 18 UTC (t+15h)

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Case 1 – 21/07/2010, cont...

• NAE model analyses reveal some features of environment:– Ahead of vertically stacked

low– SSW–SW surface flow– Some evidence of

convergence line from surface winds

– Conditionally unstable with capping inversions at 900 and 750 hPa – moist at low levels

– Weak vertical wind shear– Approaching surface trough

may have forced eastward propagation after 1500 UTC

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Case 1 – 21/07/2010, cont...

• Satellite images reveal widespread cloud streets / bands

MODIS high-resolution image for 1233 UTC on 21/07/2010

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Case 1 – 21/07/2010, cont...

• Cloud tops up to 20,000ft (-18°C)

• Deep enough to generate heavy rain but shallow enough to allow development of closely-packed storms with weak downdraughts (c.f. Boscastle)

Derived cloud top temperature for 1400 UTC

Derived cloud-top height for1400 UTC

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Work going forward

• Focus on Cornwall case:– Continue examining analysis data– Environment Agency gauge data– Program to calculate CAPE and CIN across domain– High-resolution UM simulations of event

• Identify more cases (climatology?):– Program to automatically detect anchored convective

storms in past radar data

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References• Bryan, G. H., J. C. Wyngaard, and J. M. Fritsh, 2003: Resolution requirements for the simulation of

deep moist convection, Mon. Wea Rev., 131, 2394–2416.

• Burt, S., 2005: Cloudburst upon Hendraburnick Down: The Boscastle storm of 16 August 2004. Weather, 60, 219–227.

• Doswell, C., H. E. Brooks, and R. A. Maddox, 1996: Flash flood forecasting: An ingredients based methodology. Wea. Forecasting, 11, 560–581.

• Golding, B., P. Clark, and B. May, 2005: The Boscastle Flood: Meteorological analysis of the conditions leading to flooding on 16 August 2004. Weather, 60, 230–235.

• Kirshbaum , D. J., and D. R. Durran, 2005a: Observations and modelling of banded orographic convection. J. Atmos. Sci., 62, 1463–1479.

• Kirshbaum, D. J., and D. R. Durran, 2005b: Atmospheric factors governing banded orographic convection. J. Atmos. Sci., 62, 3758–3774.

• Lean, H. W., P. A. Clark, M. Dixon, N. M. Roberts, A. Fitch, R. Forbes, and C. Halliwell, 2008: Characteristics of high-resolution versions of the Met Office Unified Model for forecasting convection over the United Kingdom. Mon. Wea. Rev., 136, 3408–3424.

• Mayes, J., and J. Winterton, 2008: The west Surrey thunderstorm of 13 August 2006. Weather, 63, 50–55.

• Roberts, N. M., 2008: Assessing the spatial and temporal variation in the skill of precipitation forecasts from an NWP model. Meteor. Appl., 15, 163–169.

• Roberts, N. M., and H. W. Lean, 2008: Scale-selective verification of rainfall accumulations from high-resolution forecasts of convective events. Mon. Wea. Rev., 136, 78–97.