Transcript Factors controlling the transition from unbroken marine boundary

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
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 Pacific
SE Pacific
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)
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
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
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?
Methodology
How ?
Trajectories
HYSPLIT
(ERA-INTERIM)
+
Re-analysis
ERA-INTERIM
+ Satellite data
MODIS (Terra, Aqua)
AMSR-E
When ? 2002-2007 (May to October in NE, July to December SE)
Starting time: 11 LT, Duration: 6 days, Height: 200m
Where ? Klein&Hartmann (1993) zones : NE/SE Atlantic, NE/SE Pacific
NEP
NEA
SEP
SEA
Data sets
ERA-INTERIM:
 latest ECMWF reanalysis (from 2002)
 1.5 X 1.5 degrees, every 6 hours
 SST, , qt, 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
Some statistics
Total number of
trajectories
Trajectories going
over warmer waters
(SW in NE
NW in SE)
30% of the total number of
trajectories, having the
biggest initial CF, i.e a CF
superior to
For the subsequent analysis we consider the 30% of the total number of
trajectories having the biggest CF (initially) and going over warmer waters
Probability distribution of the
selected trajectories ending point (%)
NEA
NEP
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SEA
SEP
+ 6 days
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The average trajectory + CF MODIS Terra (10.30 am)
NEA
SEA
NEP
SEP
Cloud fraction along the trajectories in the 4 zones
CF MODIS
Variables along the trajectories in the 4 zones (I)
CF MODIS
SST
LWP AMSR-E
LTS
PP GPCP
D
Variables along the trajectories in the 4 zones (II)
WATER VAPOR
qt700
700
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?
Mean July trajectory (NEA)
(from mean July day, mean July fields)
composite
- - - - streamline
SST
LTS
D
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 qt 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
Questions ?