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
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
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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
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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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