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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.

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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


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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


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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)


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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-analysisImproved 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


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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?


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Methodology

Trajectories + Re-analysis + Satellite data

How ?

MODIS (Terra, Aqua)

AMSR-E

HYSPLIT

(ERA-INTERIM)

ERA-INTERIM

2002-2007 (May to October in NE, July to December SE)

Starting time: 11 LT, Duration: 6 days, Height: 200m

When ?

Where ?

Klein&Hartmann (1993) zones : NE/SE Atlantic, NE/SE Pacific

NEP

NEA

SEA

SEP


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  • 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


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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


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Probability distribution of the selected trajectories ending point (%)

NEA

NEP

* * *

* * *

* * *

* * *

* * *

* * *

+ 6 days

+ 6 days

SEA

SEP

+ 6 days

+ 6 days

* *

* *

* * *

* *

* * *

* * *


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The average trajectory + CF MODIS Terra (10.30 am)

NEA

NEP

<

<

<

<

<

<

SEA

SEP

<

<

<

<

<



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Variables along the trajectories in the 4 zones (I)

LWP AMSR-E

PP GPCP

CF MODIS

SST

LTS

D



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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?


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

composite

- - - - streamline

LTS

D

SST


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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