1. Observational study of the transition from unbroken marine boundary layer stratocumulus to the shallow cumulus regime Irina Sandu and Bjorn Stevens Max-Planck-Institut für Meteorologie KlimaCampus, Hamburg

2. Motivation Cloud regimes ranging from stratocumulus in the subtropics, to shallow cumuli and deep convective clouds toward the Equator (Fig. 1 Stevens, 2005b, following Arakawa (1975)). Aqua Images NE PacificSE Pacific

3. So far ? Observations: Albrecht, 1995, subsequent studies based on ASTEX (1992), Pincus et al. 1997 Theory and modeling: Bretherton et al., 1992, 1997,1999, Krueger et al. 1995, Wyant et al. 1997 (Bretherton et al., 1992)

4. So far ? Now ? Last generation satellites (MODIS, MSG, etc.) ECMWF ERA-INTERIM re-analysis Improved LES models (or at least more powerful computers) Aim Use satellite data and NWP reanalysis to develop a statistical view of the transition between stratocumulus and shallow cumuli Observations: Albrecht, 1995, subsequent studies based on ASTEX (1992), Pincus et al. 1997 Theory and modeling: Bretherton et al., 1992, 1997,1999, Krueger et al. 1995, Wyant et al. 1997

5. Our questions Do the data show a transition from Sc. to Cu.? How frequently does it occur? Is it different from one region to another? How is it related to the large scale factors?

6. Methodology MODIS (Terra, Aqua) AMSR-E ERA-INTERIM HYSPLIT (ERA-INTERIM) Trajectories + Re-analysis + Satellite data 2002-2007 (May to October in NE, July to December SE) Starting time: 11 LT, Duration: 6 days, Height: 200m How ? When ? Where ? Klein&Hartmann (1993) zones : NE/SE Atlantic, NE/SE Pacific NEA SEA NEP SEP

7. Data sets ERA-INTERIM: latest ECMWF reanalysis (from 2002) 1.5 X 1.5 degrees, every 6 hours SST, , q t, LTS, D, LE, H, CF, AOD MODIS: Terra (10.30 LT) and Aqua (13.30 LT) (from 2002/2003) L3 products:1 X 1 degrees Liquid Water cloud fraction, LWP, optical thickness, effective radius AMSR-E: Aqua (1.30 and 13.30 LT) (from 2003) 0.25 X 0.25 degrees LWP, TWP, SST, precipitation GPCP: daily means of precipitation rate 1 X 1 degrees

8. Some statistics Trajectories going over warmer waters (SW in NE NW in SE) For the subsequent analysis we consider the 30% of the total number of trajectories having the biggest CF (initially) and going over warmer waters 30% of the total number of trajectories, having the biggest initial CF, i.e a CF superior to Total number of trajectories

9. Probability distribution of the selected trajectories ending point (%) NEA NEP + 6 days SEA SEP + 6 days * * * * * * * * * * *

10. The average trajectory + CF MODIS Terra (10.30 am) NEA NEP SEA SEP < < < < < < < < < < <

11. Cloud fraction along the trajectories in the 4 zones CF MODIS

12. Variables along the trajectories in the 4 zones (I) SST LTS D CF MODIS LWP AMSR-EPP GPCP

13. Variables along the trajectories in the 4 zones (II) WATER VAPOR q t 700  700

14. Mean July trajectory (NEA) (from mean July day, mean July fields) Which is the difference between the transition along streamlines versus the transition composited over trajectories?

15. composite - - - - streamline Mean July trajectory (NEA) (from mean July day, mean July fields) SST LTS D

16. In summary There is a transition, between 1 and 3 days downstream of the maximum cloudiness The transitions are characterized by a sharp reduction in cloudiness, increased variability in cloud fraction among trajectories The transition is similar in the 4 basins, hence it makes sense to think of a generic transition Properties of the generic transition: increasing SST, constant divergence ?, constant  700, increasing q t 700 increased surface fluxes + decreased radiative cooling at cloud top, which supports Bretherton’s theory, the flow gradually becomes more surface driven Next: modeling study to explore these ideas, a possible intercomparison case

17. Questions ?

18. In agreement with Chellappan and Horváth JGR, 2009, subitted

19. Variables along the trajectories in the 4 zones (II) WATER VAPOR q t 700 Surface moisture flux (EI) Surface heat flux (EI)  700

20. Mean September trajectory (SEA) (from mean September day, mean September fields) Which is the difference between the average streamline trajectory and the composite trajectory?

21. Mean September trajectory (SEA) (from mean September day, mean September fields)

22. Probability distribution of the back-trajectories (at 200m) starting point (%) NEA SEA SEP NEP - 6 days * * * * * * * * * * *

23. Mean July 2002-2007 60W 30W 0 60N 40N 20N 0 Liq. cloud fraction MODIS Terra 60W 30W 0 Mean wind at 10 m (m/s) 60N 40N 20N 0 60N 40N 20N 0 60W 30W 0 Mean LTS (K) 60N 40N 20N 0 Mean SST (K) 60W 30W 0 Klein and Hartmann (1993) ECMWF ERA-INTERIM Starting point of trajectories (25N, 25W)