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

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Mean model bias: 2006

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Convective cloud

Surface albedo

Mineral dust aerosol

Marine stratocumulus

Cirrus outflow

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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, 12-18 UTC

18 UTC model-GERB OLR over West Sahara

GERBIL – aircraft campaign (Jim Haywood)

GERB Intercomparison of Longwave Radiation: June 2007 http://www.faam.ac.uk/public/campaigns/index.html#gerbil

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…

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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– 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 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…

• Increased convective cloud cover

• But is the physics any better?

• Future work: Comparisons with CloudSat

Improved shortwave reflectivity

Gulf of Guinea Model CloudSat

19th July 2006

5th 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