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Cloud Forcing Studies using CERES and GERB(-like) data

Joanna Futyan, Jacqui Russell and John HarriesGIST 22RMIB, Brussels, 09/11/2004

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Contents

• Background

• Results - ERBE-like data from CERES TERRA– Comparison with western tropical Pacific– Limitations of monthly mean gridded data

• Results – GERB(like) data– Comparison with CERES– Cloud forcing by cloud type

• Summary

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Background

• CRF - difference in flux between clear and cloudy sky conditions

• Balance between LW and SW depends on macro- and micro-physical properties

• Quantify via:

• ERBE - R~1 in tropical convective regions– i.e. monthly mean LW and SW CRF ~ equal and opposite– Studies tend to focus on the western tropical Pacific– How does behaviour in African/ Atlantic regions compare?

LWCRF

SWCRFR

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Results – ERBE-like data from CERES on TERRADistribution of 2.5o grid-box monthly mean R values for the Pacific, African and Atlantic convective regions for 2001 & 2002

• Pacific & African regions show high degree of cancellation

• Lower degree of cancellation over Atlantic

• Spread in part due to seasonal variations– least cancellation for

Atlantic in summer– SWCRF >20Wm-2 larger

than LWCRF– Similar to eastern

Pacific

Seasonal variations in R for each region

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Results 2.• Variability over Africa can be explained by

combination of cloud and surface albedo effects• SWCRF low over bright surfaces

– +ve netCRF (low R) where high cloud occurs over the desert

• Some regions have a mixture of convective and low non convective clouds during the month – Low cloud increases SWCRF but not

LWCRF -ve netCRF (high R)

– Need higher time resolution data to separate convective cloud

– GERB & SEVIRI

A

Futyan, J. M, Russell, J. E. and Harries, J. E., J. Climate, 17 (16), 2004

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Monthly mean CRF for June 2004 • GERB-like SEVIRI (GERB footprint resolution ~0.5o)

LWCRF SWCRF net CRF

• CERES TERRA, ERBE-like (Ed 1, FM1 only), 2.5o resolutionLWCRF SWCRF net CRF

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Using SEVIRI to select the convective region

• EUMETSAT MPEF ‘CLA’ cloud classification– 3x3 SEVIRI pixel scale, 3 hourly– meteorological cloud types – simplified here to high-,

mid-, low-

LW fluxSW flux Cloud type

• Gridded to GERB scale and used to select data on an instantaneous basis

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Cloud forcing by cloud type• Calculate instantaneous CRF (clear sky flux – obs flux)

– Use time-step mean estimate for clear

• High cloud CRF = instantaneous CRF if flag as high cloud

= zero otherwise

LWCRF

SWCRF

Standard definition High clouds Mid/ high clouds Low cloudsConvective?

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CRF in the ‘convective region’

land ocean

netCRF

- SWCRFLWCRF

netCRF

- SWCRFLWCRF0

100

-60

60

0

-20C

RF

CR

F

• Select footprints with LWCRF>30Wm-2 and find average CRF

• GERB-like agrees reasonably well with CERES• Esp over ocean have significant contribution to netCRF from low and mixed level clouds• For ocean netCRF is always –ve• Over land - high clouds have a +ve net forcing

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Cloud forcing ratio

land

ocean

1

2R

1

R

0

0

• 30Wm-2 limit removes some low clouds– Reduces R esp over ocean

40W 0 40E40S

0

40N

• If low clouds are excluded explicitly R falls further – No longer need LWCRF limit to select region

• Over land R~1 in spatial average– Not at higher spatial scales

• Over ocean R>1 even when low clouds are excluded

3

0.5

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Summary & future work

• Monthly mean CRF averages the effects of all cloud regimes present during a month– Hard to attribute differences between regions

• Synergy between GERB and SEVIRI allows calculation of cloud forcing by cloud type– Separate effects of low stratocumulus cloud from active convection

• Cloud forcing ratio for ‘convective’ clouds ~1 over African land (on average)

• Significant departures over Atlantic (-ve net CRF) even when low clouds are excluded

• Future work:– investigate behaviour at daily and higher timescales