Evaluating the Met Office global forecast model using GERB data Richard Allan, Tony Slingo Environmental Systems Science Centre, University of Reading Sean Milton, Malcolm Brooks Met Office, Exeter
Objectives Improve experience with satellite datasets including GERB Timely Model Evaluation Understanding of physical processes
ShortwaveLongwave Mean model bias: 2006 All-sky Clear-sky
ShortwaveLongwave Convective cloud Surface albedo Mineral dust aerosol Marine stratocumulus Cirrus outflow
All-sky Clear-sky ShortwaveLongwave Mineral dust aerosol
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, UTC
18 UTC model-GERB OLR over West Sahara GERBILGERBIL – aircraft campaign (Jim Haywood) GERB Intercomparison of Longwave Radiation: June
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
- 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…
All-sky Clear-sky ShortwaveLongwave 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 –Model low-altitude stratiform clouds are too reflective
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 relates to cloud liquid water
LWP Wentz : overestimate for low cloud fraction? TMI Wentz /MODIS LWP Overcast boundary layer clouds: good agreement Horváth and Davies (2007) JGR 112, D01202
Convective cloud 5 th June 2006
Model evaluation: near-real time Change in model minus GERB flux differences Relate to change in model physics implementation 13 th March | 14 th March Model SW albedo
Convective Decay Time-scale Unrealistically low levels of convective cloud On-off; common problem in models Simple fix…
Increased convective cloud cover But is the physics any better? Future work: Comparisons with CloudSat
Improved shortwave reflectivity
Gulf of Guinea ModelCloudSat 19 th July th July 2006
Conclusions Top down-bottom up approach –Good for feedback to modellers 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