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 anddeep 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 ?

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

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

MODIS (Terra, Aqua)AMSR-E

ERA-INTERIMHYSPLIT(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

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

Trajectories going over warmer waters

(SW in NENW 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

Probability distribution of the selected trajectories ending point (%)

NEA NEP

+ 6 days + 6 days

+ 6 days

SEA SEP

+ 6 days

* * ** * ** * *

* * ** * ** * *

* * * * * * *

* * * * ** * *

The average trajectory + CF MODIS Terra (10.30 am)NEA NEP

SEA SEP

<<

<

< <

< <

<

<<

<

Cloud fraction along the trajectories in the 4 zones

CF MODIS

Variables along the trajectories in the 4 zones (I)

SST LTS D

CF MODIS LWP AMSR-E PP GPCP

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?

composite- - - - streamline

Mean July trajectory (NEA)(from mean July day, mean July fields)

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 ?