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
Sinergee project: www.nerc-essc.ac.uk/~rpa/GERB/gerb.html
GERB Model
Harries et al. (2005) BAMS; Allan et al. (2005) JGR
Objectives
• Improve experience with satellite datasets including GERB
• Timely Model Evaluation– using geostationary data independent of the
assimilation system
• Understanding of physical processes
Sh
ort
wav
eL
on
gw
ave
Mean model bias: 2006
All-sky Clear-sky
Allan et al. (2007) QJRMS
All-sky Clear-sky
Sh
ort
wav
eL
on
gw
ave
Marine stratocumulus
Convective cloud
Radiative biases in the Met Office global model
Convective outflow
Surface albedo
All-sky Clear-sky
Sh
ort
wav
eL
on
gw
ave
Mineral dust aerosol
Dust impact on longwave radiation
• 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
Model minus GERB OLR: July 2006, 12-18 UTC
All-sky Clear-sky dust aerosol
1800
1200
120
0 C
LEA
R
2003 2004 2005 2006 2007
Model-GERB OLR difference time series 15oW-5oE, 15-30oN
Dust impact on longwave radiationModel minus GERB OLR: 12-18 UTC
All-sky Clear-sky dust aerosol
Jun
e 18
-28
2007
Ju
ly 2
006
19 June 2007
180
0
1500
1200
21 June 2007
180
0
1500
1200
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
Marine Stratocumulus
• Curious banding structure– Transition across model
levels (see Lock et al. 2001, MWR)
• Cloud reflectivity bias– Model low-altitude
stratiform clouds are too reflective
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
Convective cloud
5th June 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