Transcript Slide 1

High Resolution UM and Orography:
Research at the Met Office
Richard Forbes
October 2004
Rachel Capon, Peter Clark, Humphrey Lean, Clive Pierce,
Nigel Roberts, Samantha Smith, Simon Vosper
© Crown copyright 2004
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Contents
 The Unified Model at high resolution
 Orography and the hi-res UM
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Orographic rainfall and smoothed orography
Rain advection and orography
Lee waves
MAP Case Study
Channel flow
Boscastle Flash Flood
 Summary
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Unified Model
 UM Configurations:
 Operational 12km 38 level model
 Research mode 4km 38 level and 1km 76 level models (High
Resolution Trial Model, HRTM)
 Relevant Model Parametrizations:
 Mixed-phase cloud and precipitation scheme (Wilson and
Ballard 1999, Smith 1990) + prognostic ice/snow/rain/graupel
option for high resolution
 Mass flux convection scheme (Gregory and Rowntree 1995)
+ CAPE dependent CAPE closure timescale for 4km
 Diffusion (targetted to high w regions) + Smagorinsky-Lilly type
turbulence parametrization (available soon)
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High Resolution Trial Model Configuration
1 km
76 levels
4 km
38 levels
Resolved convection
Prognostic rain
Initial 12km T+1
Mass-limited
convection
Prognostic rain
Initial 12km T+1
4 km
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1 km
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Orographic Rainfall
Humphrey Lean, Peter Clark
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Seeder Feeder Effect
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Seeder-feeder model
 Seeder-feeder model taking into account condensation, advection and
accretion of water cloud to form precipitation.
dqcl/ds = αC0 –(qcl/U)A0P0.68
dP/dz = –ρqclA0P0.68
 Equilibrium solution For infinitely long upslope:
Equilibrium rainfall rate: P(z)= P(h) + U C(h-z)
Equilibrium cloud water: q cl  = U C P(z)-0.68/A
Horizontal length scale for equilibrium
(distance for condensation to generate q cl ):
L(z) = (U/A)P(z)-0.68
(L ~50km for typical UK parameters)
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Non-dimensional solution:
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Surface Precipitation Along the Slope
Predicted from Simplified Model
h = 1.5km Ph=0.5mm/hr = 0.02
Red = without rain drift, black=with
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Does the seeder-feeder reach equilibrium ?
 For a long slope (e.g. >50km), the solution reaches
equilibrium and the total precipitation enhancement  hill
height
 For a short slope (e.g. <50km), the solution has not
reached equilibrium and the total precipitation
enhancement  hill volume
 This has implications for representation of orography in
models and the impact of smooth (filtered) orography on
the precipitation from the seeder-feeder process.
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Effect of Smoothing Orography
 Effect of smoothing orography depends on scale
compared to L (~50km)
 Recall for s<<L enhancement proportional to volume
of hill - hence smoothing on these scales just
redistributes rain (applicable to grid resolutions of
~10km and less).
 For s>>L enhancement proportional to height of hill - in
this case smoothing will reduce total amount of rain
(applicable to grid resolutions >10km).
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Effect of smoothing orography
12km Orography
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Smoothed 12km Orography
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Case Study: 29 Nov 2001
Frontal rainband with significant orographic enhancement
Met Office
analysis chart
for 00z on
29/11/2001
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Effect of smoothing orography on
total precipitation:
11-12Z 29th Nov 2001 Accumulated Rainfall
from 12km Unified Model with unsmoothed
orography (top) and smoothed (bottom).
Rain over South Wales mountains changes by
only ~1%.
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Seeder-Feeder Model Conclusions
• A simplified seeder-feeder model has been used to
investigate magnitudes and spacial dependencies in idealised
cases.
• This above model has been used to investigate scale
dependence for long and short hill limits.
• Smoothing the orography has no effect on the total rainfall in
the 12km model but a large effect in the 60km one as would
be expected from the limit arguments.
• The drift of rainfall is expected to redistribute rainfall on
scales of order 10km
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Advection of Rain
Richard Forbes
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Advection of Rain in NWP Models
 For high resolution NWP models (< 5 km) the advection of rain by the
wind becomes more significant (smaller grid boxes, shorter
timesteps).
 Is it important to include this in NWP models ?
 Location of seeder-feeder rainfall over orography can be crucial for
river catchment rainfall accumulations (catchment boundaries are
associated with orography).
 Compare the Unified Model at 2km with and without rain advection
(prognostic vs. diagnostic rain).
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Impact of including rain advection on rainfall distribution. 10hr model
forecast.
Rainfall rate (mm/hr)
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Orography (m)
Rainfall rate difference
(advection-no advection)
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Vertical cross section across Dartmoor
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Vertical cross sections across Dartmoor
Snowfall rate
No Rain
Advection
Vertical cross section of
snowfall/rainfall rate across Dartmoor
Rainfall rate
Snowfall rate
With Rain
Advection
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Rainfall rate
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Verification
Dartmoor River Catchment Rainfall 9 Hour Accumulation
Model Forecast
(Diagnostic Rain)
Model Forecast
(Prognostic Rain)
NIMROD
(Radar Network)
Exe
Teign
Dart
Tamar
Avon & Erme
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Correlation between model and NIMROD radar-derived accumulated
rainfall for Dartmoor river catchments
No Rain Advection
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With Rain Advection
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Rain Advection Summary
 Rainfall forecasts for fast-response river catchments are
important for flood prediction.
 A difference in the location of orographically enhanced
rainfall of only a few km can result in a large difference in
the rainfall prediction for river catchments.
 Including the advection of rain in a 2km version of the
Unified Model significantly improved the spatial
distribution of surface rainfall over orography and
associated river catchments.
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Lee Waves
Simon Vosper
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Pennine Lee Wave Observation Campaign
 Region east of the Pennines in the
Vale of York, is known to have
significant forecasting problems
associated with orographic flows.
 Obs. field campaign for 1 year (2004).
 Study lee waves, rotors and downslope
windstorms.
 Compare models (3DVOM, BLASIUS
and UM) with observations.
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Pennines – Orography
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Pennine Lee Waves: Model Comparison
Plan view of vertical velocity on 2km model level
3DVOM
(linear model)
BLASIUS
UM
1km horizontal grid resolution
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Pennine Lee Waves: Radiosonde comparison
Radiosonde (black)
3DVOM (red)
BLASIUS (green)
UM (blue)
• UM has best phase but low
amplitude
• 3DVOM/BLASIUS have
good amplitude but poor
phase
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MAP Case Study
Samantha Smith
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MAP Case Study: 8 Nov
 How does the UM
represent flow over high
complex mountains,
especially for low
Froude number
problems ?
 MAP case 8 Nov 1999
 Northerly flow with
upstream flow splitting
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MAP Case Study: 8 Nov
 UM 4km reproduces large scale flow, flow splitting, cold pool in
Po valley, lee-side Foehn and Froude number well.
 Comparison with Payerne sonde.
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MAP Case Study: 8 Nov
 Comparison of model
vertical velocity with
aircraft at different
heights.
 Wavelengths are
predicted but
amplitude
underpredicted.
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Total surface drag as a function of model resolution
 Magnitude of surface
drag increases with
increasing resolution
 No sign of
convergence down to
4km
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Channel Flow
Rachel Capon
© Crown copyright 2004
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The Levanter Wind: Straits of Gibraltar
 Low level weak Easterly
flow capped by strong
inversion.
 Substantial acceleration
through Straits of
Gibraltar.
 Regular feature – quite
predictable given large
scale setup.
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Surface Observations Around Gibraltar
25 knots
25 knots
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‘Levanter’ Wind through
Straits of Gibraltar – Impact of Model Resolution
12 km L38 (part)
4 km L38 (part)
1 km L38
18 UTC 26/03/2002 from Global analysis 12 UTC 25/03/2002
30 Hour Forecast
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1 km Forecast 26/03/2002
 Peak 10 m wind
speed ~ 20 m/s
 Very steady
 Note ‘side bands’ –
are they realistic?
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Second Levanter Case 21st May 2003
1 km forecast
08 UTC (T+14)
Side Bands
14 UTC (T+20)
Side Bands
How do we verify?
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Some serendipitous data!
AVHRR Visible Image 13:45 pass on 21/05/2003
Sun Glint
Dark=low reflection
=more waves
=higher wind speed
Greatly Contrast Enhanced
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Sun Glint Anemometry
14 UTC (T+20)
AVHRR Visible Image
13:45 pass on 21/05/2003
Greatly Contrast Enhanced
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Comparison of Dover and Gibraltar Straits
 Straits of Gibraltar Jet occur with a strong capping
inversion to the boundary layer.
 Gibraltar appears to be genuine ‘gap flow’.
 Note scale of channel flow - ~100 km, and some
upwind impact reflecting stable flow dynamics.
 “Side bands” represented in the model. Cause ?
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Boscastle Flash Flood
Peter Clark, Humphrey Lean,
Clive Pierce, Nigel Roberts,
Richard Forbes
© Crown copyright 2004
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Boscastle Flash Flood
 Boscastle (South-West
England), 16th August
2004
 Large amount of precip
>60mm over a few hours
 Small river catchment
 ~£500,000,000 damage
 Fortunately, no one killed
 No warnings (even 2 hour
warning would be useful)
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Radar actual at 1600UTC 16/8/04
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Predicted catchment accumulations
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Rainfall Accumulations
12-18 UTC 16th August 2004
12 km
Forecasts from 03 UTC
4 km
NIMROD radar
20 km radius
from Boscastle
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Peak Accumulations >60mm
On 4 km grid
Positional error and false alarm
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Comparison of 00 UTC and 03 UTC
4 km forecasts
12-18 from 03 UTC
12-15 from 03 UTC
12-15 from 00 UTC
T+15 run from 00 UC does not cover full period of actual rain
00 UTC run better than 03 UTC
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Boscastle: Interpreting the forecast
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Mechanism for 16th August 2004 Storm Triggering
10 m wind convergence
11 Z
10 Z
Persistent Convergence
Due to coast and orography
Orography in white
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Boscastle Summary
 Explicit convection in 4 km gave better peak
accumulations than parametrized in 12 km
 00 UTC run better than 03 UTC
 High predictability but not high enough to focus
attention on individual catchments
 Triggering mechanism appears to be orography +
land/sea roughness + surface heating
 Forecast would have justified flash flood warnings for
N Devon & N Cornwall 6-9 hours ahead
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Summary of Research Results for the High Resolution UM
and Orographic Case Studies
 Seeder-Feeder
 Smoothing orography should not have much impact on the
total rainfall for grid resolutions <10km.
 Rain Advection
 Should include prognostic rain for grid resolutions <10km
 Lee Waves
 UM (1km) able to represent lee waves but amplitude low
 MAP Case Study
 Orographic drag not converged at 4km
 Channel Flow
 UM (1km) captures details of Levanter wind
 Boscastle Flash Flood
 High accumulations in the region of Boscastle predicted by
4km (but not 12km) UM
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Comparison of 03 UTC 12km , 4km and 1km
forecasts
12-18 from 00 UTC
1km better location, but accumulations low
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Comparison of 03 UTC 12km , 4km and 1 km
forecasts
12-18 from 03 UTC
T+15 run from 00 UC does not cover full period of actual rain
00 UTC run better than 03 UTC
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Comparison of 03 UTC 1km and 4km forecasts
12-18 from 03 UTC
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12-15 from 03 UTC
12-18 from 03 UTC
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Low level convergence
16th August 2004 Storm
4 km
12 km
10 m wind convergence at 11 UTC (at convection triggered)
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Contribution of Surface Sensible Heat Flux to
triggering
10 Z
11 Z
Probably promoted coastal convergence
but no strong local signal
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Vertical cross sections across Dartmoor
Along-section
wind speed
~ 20m/s
Vertical wind
speed
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Vertical sonde Profile from Gibraltar
26/03/2002 12 UTC
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