Dust and vapour cloud the view

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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
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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

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Intro 3
Clouds and Climate

• Clouds and climate

Bony and Dufresne (2005)
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Objectives

• Validation of new datasets (GERB/SEVIRI)
• Timely Model Evaluation
• Understanding of physical processes

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GERB July 2006 OLR Animation
Model
Sinergee project www.nerc-essc.ac.uk/rpa/GERB/ge
rb.html
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All-sky Clear-sky
Mean model bias 2006
Shortwave Longwave
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All-sky Clear-sky
Mineral dust aerosol
Cirrus outflow
Marine stratocumulus
Shortwave Longwave
Surface albedo
Convective cloud
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All-sky Clear-sky
Mineral dust aerosol
Shortwave Longwave
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Dust
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- Major dust source for Amazon - Large component
from March 2004 dust storms
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March 2004 an interesting month
Loeb et al. (2007) J. Climate, 20, p.582
March 2006 was interesting too
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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
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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.
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.
RADAGAST project http//radagast.nerc-essc.ac.uk
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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.
By 8 March, dust covers the whole of West Africa
and is moving out over the Atlantic.
Animation available http//radagast.nerc-essc.ac
.uk
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et al
Surface Top of Atmosphere
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Radiative transfer models underestimate the solar
absorption in the atmosphere during March 2006
dust storm Slingo et al. (2006) GRL, 33, L24817
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Dust impact on longwave radiation
Model minus GERB OLR July 2006, 12-18 UTC

• Large perturbation to Met Office model OLR during
  summer over west Sahara
• Correlates with high mineral dust aerosol optical
  depth

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Consistent with calculations of dust longwave
radiative effect
Haywood et al. (2005) JGR 110, D05105
Clear-sky OLR bias (Wm-2) in 2003
Calculations Direct radiative effect Direct plus
shortwave feedback effect
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All-sky Clear-sky
Radiative biases in the Met Office global model
Cirrus outflow
Marine stratocumulus
Shortwave Longwave
Convective cloud
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Marine Stratocumulus
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• Curious banding structure
• Transition across model levels
• Cloud reflectivity bias

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Changes in albedo bias (ocean)

• Model upgrade (March 2006) reduced but did not
  remove albedo bias
• Compensating errors ITCZ/stratocumulus

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Stratocumulus composites
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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
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LWPWentz overestimate for low cloud fraction?
Overcast boundary layer clouds good agreement
TMIWentz/MODIS LWP
Horváth and Davies (2007) JGR 112, D01202
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Convective cloud
5th June 2006
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Model evaluation near-real time

• Change in model minus GERB flux differences
• Relate to change in model physics implementation

Model SW albedo
2005 2006
13th March 14th March
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Convective Decay Time-scale

• Unrealistically low levels of convective cloud
• On-off common problem in models
• Simple fix

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Improved shortwave reflectivity
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• Increased convective cloud cover
• But is the physics any better?
• Future work Comparisons with CloudSat

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Gulf of Guinea
Model CloudSat
5th July 2006
19th July 2006
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Clouds and water vapor

• Combine GERB/SEVIRI
• Diurnal changes in cloud and humidity

• Radiatively driven subsidence
• work by Ruth Comer

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2-3 hr lag between tropical convection and upper
tropospheric water vapor (Soden 2000, 2004)
above central/South Americaright Lagrangian
tracking
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Tracking over Africa difficult?Complex picture
locally due to propagating disturbances
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Clouds and Water Vapor Africa
3-hour lag
Work by Ruth Comer
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Clear-sky radiative cooling and the atmospheric
hydrological cycle

• Clear-sky radiative cooling
• radiative convective balance
• atmospheric circulation
• Earths radiation budget
• Understand clear-sky budget to understand cloud
  radiative effect
• Datasets
• Reanalyses observing system
• Satellites calibration and sampling
• Models wrong by definition

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Links to precipitation
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Tropical Oceans
ERA40 NCEP SRB
Ts CWV LWc SFC
HadISST
SMMR, SSM/I
DerivedSMMR, SSM/I, Prata)
1980 1985 1990 1995 2000 2005
Allan (2006) JGR 111, D22105
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Surface LWc and water vapour
dLWc/dCWV 1.5 Wkg-1
ERA40 NCEP
dCWV/dTs 3 kgm-2K-1
Allan (2006) JGR 111, D22105
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Clear-sky OLR with surface temperature
ERBS, ScaRaB, CERES SRB
Calibration or sampling?
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Tropical Oceans
ERA40 NCEP SRB
Surface Net LWc Clear-sky OLR Clear-sky Atmos
LW cooling QLWc
HadISST
ERBS, ScaRaB, CERES
Derived
Allan (2006) JGR 111, D22105
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Linear least squares fit
ERA40 NCEP

• Tropical ocean descending regime
• Dataset dQLWc/dTs Slope
• ERA-40 3.70.5 Wm-2K-1
• NCEP 4.20.3 Wm-2K-1
• SRB 3.60.5 Wm-2K-1
• OBS 4.60.5 Wm-2K-1

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Implications for tropical precipitation (GPCP)?
GPCP P
ERA40 QLWc
OBS QLWc
Pinatubo?
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IPCC AR4 models tropical oceans

• CWV
• Net LWc
• OLRc
• Q_LWc

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IPCC AR4 models tropical oceans

• QLWc
• Precip

Ongoing work
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Ongoing workCMIP3 models
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Also considering coupled model experiments
including greenhouse gas and natural forcings
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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

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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?

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Clear-sky vs resolution
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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

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Links to precipitation