dust and vapour cloud the view richard allan environmental systems science centre, university of...
TRANSCRIPT
Dust and vapour cloud the view
Richard Allan
Environmental Systems Science Centre, University of Reading, UK
Thanks to Tony Slingo, Ruth Comer, Sean Milton, Malcolm Brooks, and the GERB International Science Team
Introduction
• Climate and NWP models
• Model evaluation: – Top down/bottom up approach
• Diagnosing variability and feedbacks
• Nuts and bolts: model parametrizations and physical processes– fast feedbacks
• Limitations of the observing systems
Intro 3
• Clouds and climate
Bony and Dufresne (2005)
Clouds and Climate
Objectives
• Validation of new datasets (GERB/SEVIRI)
• Timely Model Evaluation• Understanding of physical processes
GERB July 2006 OLR Animation Model
Sinergee project: www.nerc-essc.ac.uk/~rpa/GERB/gerb.html
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Mean model bias: 2006
All-sky Clear-sky
All-sky Clear-sky
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Convective cloud
Surface albedo
Mineral dust aerosol
Marine stratocumulus
Cirrus outflow
All-sky Clear-sky
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Mineral dust aerosol
Dust
- Major dust source for Amazon
- Large component from March 2004 dust storms
March 2004: an interesting month
Loeb et al. (2007) J. Climate, 20, p.582
March 2006 was interesting too…
1200GMT, 6 March 2006
In these false-colour images, the dust appears pink or magenta, water vapour dark blue, thick high-level clouds red-brown, thin high-level clouds almost black and surface features pale blue or purple.
On 6 March, unusually strong northerly winds bring cold air at low levels over the desert, creating a broad front of dust as the air moves south.
The location of Niamey is marked by a cross.
RADAGAST project: http://radagast.nerc-essc.ac.uk
1200GMT, 7 March 2006
The shallow layer of cold air cannot rise over the mountains of the central Sahara (light blue in colour), so it is forced to follow the valleys. Streaks appear where it accelerates through gaps in the topography.
The dust reached Niamey at 0930 on 7 March.
In these false-colour images, the dust appears pink or magenta, water vapour dark blue, thick high-level clouds red-brown, thin high-level clouds almost black and surface features pale blue or purple.
RADAGAST project: http://radagast.nerc-essc.ac.uk
1200GMT, 8 March 2006
By 8 March, dust covers the whole of West Africa and is moving out over the Atlantic.
In these false-colour images, the dust appears pink or magenta, water vapour dark blue, thick high-level clouds red-brown, thin high-level clouds almost black and surface features pale blue or purple.
Animation available: http://radagast.nerc-essc.ac.uk
Sur
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et al…
Radiative transfer models underestimate the solar absorption in the atmosphere during March 2006 dust storm
Slingo et al. (2006) GRL, 33, L24817
Dust impact on longwave radiation
• Large perturbation to Met Office model OLR during summer over west Sahara
• Correlates with high mineral dust aerosol optical depth
Model minus GERB OLR: July 2006, 12-18 UTC
Consistent with calculations of dust longwave radiative effect
Clear-sky OLR bias (Wm-2) in 2003
Calculations:
Direct radiative effect
Direct plus shortwave feedback effect
Haywood et al. (2005) JGR 110, D05105
All-sky Clear-sky
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Marine stratocumulus
Convective cloud
Cirrus outflow Radiative biases in the Met Office global model
Marine Stratocumulus
• Curious banding structure– Transition across
model levels
• Cloud reflectivity bias
Changes in albedo bias (ocean)
• Model upgrade (March 2006) reduced but did not remove albedo bias– Compensating errors: ITCZ/stratocumulus
Stratocumulus composites
Cloud liquid water path
Bias: model minus GERB; SSM/I; SEVIRI
Albedo Liquid Water Path Cloud
Reduction in model bias from June to July 2006 - relates to cloud liquid water
LWPWentz: overestimate for low cloud fraction?
TMIWentz/MODIS LWPOvercast boundary layer clouds: good agreement
Horváth and Davies (2007) JGR 112, D01202
Convective cloud
5th June 2006
Model evaluation: near-real time
• Change in model minus GERB flux differences
• Relate to change in model physics implementation
13th March | 14th March
Model SW albedo
2005 2006
Convective Decay Time-scale
• Unrealistically low levels of convective cloud
• On-off; common problem in models
• Simple fix…
Improved shortwave reflectivity
• Increased convective cloud cover
• But is the physics any better?
• Future work: Comparisons with CloudSat
Gulf of Guinea Model CloudSat
5th July 2006
19th July 2006
Clouds and water vapor
• Combine GERB/SEVIRI
• Diurnal changes in cloud and humidity
• Radiatively driven subsidence
• work by Ruth Comer
2-3 hr lag between tropical convection and
upper tropospheric water vapor
(Soden 2000, 2004)
above: central/South Americaright: Lagrangian tracking
Tracking over
Africa difficult?
Complex picture locally
due to propagating disturbances
Clouds and Water Vapor: Africa
~3-hour lag
Work by Ruth Comer
Clear-sky radiative cooling and the
atmospheric hydrological cycle
• Clear-sky radiative cooling: – radiative convective balance– atmospheric circulation
• Earth’s radiation budget– Understand clear-sky budget to
understand cloud radiative effect• Datasets:
– Reanalyses – observing system– Satellites – calibration and sampling– Models – wrong by definition
Links to precipitation
Tropical Oceans
1980 1985 1990 1995 2000 2005
Ts
CWV
LWc
SFC
ERA40
NCEP
SRB
HadISST
SMMR, SSM/I
Derived:SMMR, SSM/I, Prata)
Allan (2006) JGR 111, D22105
Surface LWc and water vapourdLWc/dCWV ~ 1.5 Wkg-1
ERA40 NCEP
dCWV/dTs ~ 3 kgm-2K-1
Allan (2006) JGR 111, D22105
Clear-sky OLR with surface temperature: + ERBS, ScaRaB, CERES; SRB
Calibration or sampling?
Tropical Oceans
Surface Net LWc
Clear-sky OLR
Clear-sky Atmos LW cooling
QLWc
ERBS, ScaRaB, CERES
Derived
ERA40
NCEP
SRB
HadISST
Allan (2006) JGR 111, D22105
Linear least squares fit
• Tropical ocean: descending regime
• Dataset dQLWc/dTs Slope
• ERA-40 3.7±0.5 Wm-2K-1
• NCEP 4.2±0.3 Wm-2K-1
• SRB 3.6±0.5 Wm-2K-1
• OBS 4.6±0.5 Wm-2K-1
ERA40 NCEP
Implications for tropical precipitation (GPCP)?
ERA40 QLWc
GPCP P
OBS QLWc
Pinatubo?
IPCC AR4 models: tropical oceans
• CWV
• Net LWc
• OLRc
• Q_LWc
IPCC AR4 models: tropical oceans
• QLWc
• Precip
Ongoing work…
Ongoing work…CMIP3 models
Also considering coupled model experiments including greenhouse gas and natural forcings
Conclusions• Top down-bottom up approach
– Good for feedback to modelers• Mineral dust aerosol
– Shortwave absorption; longwave radiative effect– Large effect of single events
• Marine stratocumulus– Reflectivity and seasonal variability: issues
• Deep convection– Intermittent in models; issues with detrainment
• Clear-sky radiative cooling– Links to atmospheric hydrological cycle– Need to understand before can understand changes in
cloudiness• Observing systems: capturing decadal variability
problematic
Spurious variability in ERA40
• Improved performance in water vapour and clear-sky radiation using 24 hour forecasts
• Reduced set of reliable observations as input to future reanalyses?
Clear-sky vs resolution
Sensitivity study
• Based on GERB- SEVIRI OLR and cloud products over ocean:
• dOLRc/dRes ~0.2 Wm-2km-0.5
• Suggest CERES should be biased low
by ~0.5 Wm-2 relative to ERBS
Links to precipitation