1. Inconsistencies between monthly mean longwave cloud forcing and dynamical fields from reanalyses
Richard Allan
ESSC, Reading University
Mark Ringer
Hadley Centre, Met Office

2. INTRODUCTION Clouds and Climate Advances:
Clear-sky fluxes from satellite (e.g. Ramanathan et al. 1989, Science)  “Cloud Forcing” LWCF=OLRc-OLR
Sub-sampling by “dynamical regime” (e.g. Bony et al. 1995, Climate Dynamics)
This Study: see Allan and Ringer (2003) GRL 30(9)

3. HadAM3 minus ERBS ( )
Clear-sky OLR differences > LWCF differences in some regions

4. Illustration of clear-sky sampling bias using HadAM3 3-hourly data

5. +ve w (mb/day) -ve OMEGA (mb/day) OLRc Higher SST OMEGA (mb/day)
Clear-sky OLR Composite with OMEGA and SST
OMEGA (mb/day)
More POSITIVE=more stable=drier/cloud-free=higher OLRc
+ve
w (mb/day)
-ve
More LW emission
OLRc
Higher SST
More moisture
OMEGA (mb/day)
More NEGATIVE=less stable=moister=lower OLRc

6. OLRc (Wm-2)

7. DOLRc (Wm-2)

8. Using ERA-40 Daily data to illustrate clear-sky sampling bias of CERES data

9. Using ERA40 clear-sky OLR to evaluate dynamical regimes
ERA40-CERES similar
ERA40 < CERES
ERA40 minus CERES clear-sky OLR
(January-August 1998)

10. dLWCF/dSWCF
CERES only:
-1.20
CERES/ERA40
-1.33

11. CONCLUSIONS Model Evaluation using satellite data
Sub-sampling of dynamical regimes
Clear-sky radiation
Clear-sky radiation measurements preferentially sample more stable, drier atmospheric profiles
Subsampling by vertical motion exagerates clear-sky sampling bias (~15 Wm-2)
Use of reanalysis data (ERA40) may improve interpretation of cloud radiative effect
Satellite LWCF may overestimate degree of cancellation between LWCF and SWCF