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Observational study of the transition from unbroken marine boundary layer stratocumulus to the shallow cumulus regime

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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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slide1

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
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

slide3

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)

slide4

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

slide5

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?

slide6

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

slide7

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
slide8

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

slide9

Probability distribution of the selected trajectories ending point (%)

NEA

NEP

* * *

* * *

* * *

* * *

* * *

* * *

+ 6 days

+ 6 days

SEA

SEP

+ 6 days

+ 6 days

* *

* *

* * *

* *

* * *

* * *

slide14

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?

slide15

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

composite

- - - - streamline

LTS

D

SST

slide16

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

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