1. © Crown copyright Met Office CFMIP-GCSS Intercomparison of SCM/LES (CGILS) Results for the HadGEM2 SCM Mark Webb and Adrian Lock (Met Office) EUCLIPSE/GCSS BLWG Meeting, September 2010

2. © Crown copyright Met Office First round HadGEM2 SCM results: Recap Shallow convection (s6): (too deep) / little feedback Coastal Stratus (s12): very little cloud / small positive feedback Stratocumulus (s11): strong positive feedback with a deeper, permanently decoupled BL and less cloud / cloud water Suppressing evaporative entrainment/mixing term in the Lock boundary layer scheme changes positive s11 stratocumulus feedback to a negative feedback with an increase in cloud water path. The control simulation is largely unaffected. The Cloud Top Entrainment Instability (CTEI) mechanism is strongly implicated in the HadGEM2 SCM’s positive stratocumulus feedback at s11 But SCM stability drift is a concern – how relevant to GCM?

3. © Crown copyright Met Office 1 st round 2 nd round s6 Trade Cu s11 Stratocumulus s12 Coastal Stratus

4. © Crown copyright Met Office TCA 0.86 0.83 (- 0.03 / - 3%) LWP (kg/m 2 ) 0.025 0.020 (-0.005 / -20%) Precip (mm/day) 0.08 0.04 (-0.04/ - 50%) RH crit SH(W/m 2 ) 17.1 15.2 ( - 1.9 / - 11%) LH (W/m 2 ) 61.3 68.5 ( +7.2 / +11.7%) 100% well mixed s12 Coastal Stratus

5. © Crown copyright Met Office HadGEM2 s12 hypothesis: More evaporatively driven entrainment The Lock boundary layer scheme includes an evaporatively driven entrainment/mixing term which depends on the jump in specific humidity across the inversion (Cloud Top Entrainment Instability /CTEI ) This suggests an alternative hypothesis: q jump across inversion = q bl – q free is approx q sat (RH bl – RH free ) Relative humidity stays roughly constant in +2K experiment  Increasing temperature / q sat  Larger jump in specific humidity across the inversion  Evaporation term increases entrainment of warm/dry air  Drier cloud layer / less cloud water

6. © Crown copyright Met Office Standard HadGEM2 Coastal Stratus (s12) No Evaporatively Driven BL Entrainment /Mixing

7. © Crown copyright Met Office TCA 0.25 0.18 (- 0.07 / - 39%) LWP (kg/m 2 ) 0.005 0.001 (-0.004 / -80%) Precip (mm/day) 0.0 0.0 (0.0/ 0%) RH crit SH(W/m 2 ) 7.5 7.3 ( - 0.2 / - 3%) LH (W/m 2 ) 69.8 75.1 ( +5.3 / +7%) s11 Stratocumulus 40% Dec Sc/Cu 40% Cu capped 25% Well mixed 90% Cu capped 10% Dec Sc/Cu

8. © Crown copyright Met Office HadGEM2 s11 Hypothesis Increasing Shallow Cu Breaks up Cloud (based on Wyant and Bretherton, 1997) Increase in SST  Increased surface latent heat flux in decoupled boundary layer  Increase in shallow convection  Penetrates inversion entraining more dry air  Drier cloud layer / less cloud water / transition to trade cu In the Wyant and Bretherton lagrangian model surface latent heat flux increases as RH drops, but RH is not changing here However, LH = f ( q sat -q nearsurf ) = f ( q sat x ( 1- RH nearsurf ) ) So for fixed RH, a warming surface and surface layer will lead to an increasing surface latent heat flux TEST: force shallow cu to detrain below the inversion, inhibiting convectively driven cloud top entrainment ( To be done )

9. © Crown copyright Met Office CGILS 2 nd round HadGEM2 SCM results Summary and Conclusions SCM stability drift is resolved Shallow convection (s6): little feedback Coastal stratus (s12): strong positive feedback in well mixed convection free BL – reduced cloud fraction/LWP, little deepening Suppressing evaporative entrainment/mixing term in the Lock boundary layer scheme removes the positive feedback at s12 while having little impact on the control simulation Stratocumulus (s11): too little cloud / small positive feedback. Increasing surface LH flux, more shallow convection, transition from well mixed & decoupled Sc/Cu to trade cumulus s11 results consistent with decoupled to trade cu transition driven by increased surface evaporation, shallow convection and entrainment of dry air thought the inversion