© Imperial College LondonPage 1 Cloud Forcing Studies using CERES and GERB(-like) data Joanna Futyan, Jacqui Russell and John Harries GIST 22 RMIB, Brussels, 09/11/2004

© Imperial College LondonPage 2 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

© Imperial College LondonPage 3 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?

© Imperial College LondonPage 4 Results – ERBE-like data from CERES on TERRA Distribution of 2.5 o 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

© Imperial College LondonPage 5 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

© Imperial College LondonPage 6 Monthly mean CRF for June 2004 GERB-like SEVIRI (GERB footprint resolution ~0.5 o ) LWCRFSWCRF net CRF CERES TERRA, ERBE-like (Ed 1, FM1 only), 2.5 o resolution LWCRFSWCRF net CRF

© Imperial College LondonPage 7 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

© Imperial College LondonPage 8 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 definitionHigh cloudsMid/ high cloudsLow clouds Convective?

© Imperial College LondonPage 9 CRF in the ‘convective region’ landocean netCRF - SWCRFLWCRF netCRF - SWCRFLWCRF CRF 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

© Imperial College LondonPage 10 Cloud forcing ratio land ocean 1 2 R 1 R Wm -2 limit removes some low clouds –Reduces R esp over ocean 40W0 40E 40S 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

© Imperial College LondonPage 11 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