Comparisons between GERB and the Met Office NWP model

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Comparisons between GERB and the Met Office NWP
model
Richard Allan, Tony Slingo Environmental Systems
Science Centre, University of Reading Sean
Milton, Malcolm Brooks Met Office, Exeter
Thanks to the GERB International Science Team
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GERB Model
Sinergee project www.nerc-essc.ac.uk/rpa/GERB/ge
rb.html
Harries et al. (2005) BAMS Allan et al. (2005)
JGR
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Objectives

• Improve experience with satellite datasets
  including GERB
• Timely Model Evaluation
• using geostationary data independent of the
  assimilation system
• Understanding of physical processes

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All-sky Clear-sky
Mean model bias 2006
Shortwave Longwave
Allan et al. (2007) QJRMS
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All-sky Clear-sky
Radiative biases in the Met Office global model
Convective outflow
Shortwave Longwave
Surface albedo
Convective cloud
Marine stratocumulus
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All-sky Clear-sky
Mineral dust aerosol
Shortwave Longwave
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Dust impact on longwave radiation
Model minus GERB OLR July 2006, 12-18 UTC
All-sky Clear-sky dust aerosol

• Large perturbation to Met Office model OLR during
  summer over west Sahara
• Consistent with high mineral dust aerosol optical
  depth (see also Haywood et al. 2005, JGR)
• Used in planning of GERBILS

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Model-GERB OLR difference time series 15oW-5oE,
15-30oN
1800 1200 1200 CLEAR
2003 2004 2005
2006 2007
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Dust impact on longwave radiation
Model minus GERB OLR 12-18 UTC
June 18-28 2007 July 2006
All-sky Clear-sky dust aerosol
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19 June 2007
1800 1500 1200
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21 June 2007
1800 1500 1200
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Conclusions

• Model evaluation
• Satellite data independent of assimilation system
• Good feedback for modellers and satellite team
• Identified biases relating to cloud and surface
• Mineral dust aerosol over Sahara
• Monthly longwave radiative effect up to 50 Wm-2
• Large effect of single events (Slingo et al.
  2006, GRL)
• June 18-28 2007 smaller dust events than
  previous years but detectable OLR bias

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Marine Stratocumulus
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• Curious banding structure
• Transition across model levels
• (see Lock et al. 2001, MWR)
• Cloud reflectivity bias
• Model low-altitude stratiform clouds are too
  reflective

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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
but see also Horvath and Davies (2007) JGR
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Convective cloud
5th June 2006
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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