7 oktober 2009

Challenge the future

DelftUniversity ofTechnology

Clouds and Climate

KNMI Climate Course 2011

A. Pier SiebesmaKNMI & TU DelftMultiscale Physics DepartmentThe NetherlandsContact: siebesma@knmi.nl

2Climate modeling

Clouds play a crucial role in weather and climate

3Climate modeling

Central Questions:

How do clouds respond to a perturbed climate and affect this climate (feedback)?

Perturbations:

• Increased Temperature (due to enhanced Greenhouse Gases)

• Increased aerosol amounts

5Climate modeling

1.What is a cloud and how do they

form?

6Climate modeling

What is a Cloud?

“just” water !

28-06-2006 ; 12:00 Amsterdam

7Climate modeling

Saturation specific humidity qsat

Clausius-Clayperon

• Because of the presence of Cloud Condensation Nuclei (CCN’s) in the atmosphere condensation takes place if

qv >qsat(p,T)

• Usually through cooling that results from rising motion.

CCN’s are hygroscopic aerosols (salt, dust, etc)

8Climate modeling

Rising air cools with 1 K per 100m ………..

Until it becomes so cold that it starts to condensate…

and a cloud is born!!!

Cooling through rising air

9Climate modeling

2.What makes air to rise?

10Climate modeling

1. Orography

Lenticularis above Mount Etna seen from Taormina, Sicily Italy.

11Climate modeling

2. Convection

•The sun heats the soil so that…..

•Thermals are formed….

•that rise because of buoyancy….

•And a cloud forms as a wig on top of an invisible man

24-07-2006 12:30 Amsterdam:

cumulus humilis or “fair weather” cumulus

12Climate modeling

•Humidity condensates into cloud water…..

•And produces latent heat

•Which serves as onboard fuel that allows the cloud to rise further…..

•With ~5 m/s….

•Until the cloud is stopped by a temperature inversion.

24-07-2006 Amsterdam: cumulus mediocris. 15:30

•Wolken basis (~1km)

•Wolken top (~3 km)

But what if the cloud breaks through the inversion?????????

13Climate modeling

Then the cumulus can rise to the tropopause and reach the stage of a socalled cumulonimbus

•With vertical velocities over 10m/s

•Up to a height of 5~15 km

•So that the water becomes ice

•which gives the fluffy appearance of the top of the cloud

•and strong precipation is on the way

08-02-2006 Amsterdam: cumulonimbus.

Moist Convection occurs all over the globe but is predominant in the tropics and over the subtropical oceans.

ijs

•Wolken top (5~8 km)

14Climate modeling

3. Large Scale Lifting through fronts

Occuring at mid-latitudes

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Different Cloud Types

16Climate modeling

3.A global view on clouds amount and

cloud dynamics

17Climate modeling

Clouds as seen by geostationary satellites (infrared)

July 1994

18Climate modeling

Clouds as seen by geostationary satellites (infrared)

January 1994

19Climate modeling

Monthly global cloud cover for the period 1983-2008 (source ISCCP)

•Mean global cloud cover : ~66%

•No clear trend observed yet….

20Climate modeling

4.Importance Clouds for Climate

21Climate modeling

Radiative Effects of Clouds

2 main effects:

• Shortwave Reflection

(cooling) “umbrella effect”

• Longwave Emission

(warming) “blanket effect”

0)1(4

0)1(

4

TS

FFR lwsw

swF

Top of the atmosphere

lwF swF : planetary albedo = 0.3

22Attacking the cloud feedback problem

Cloud Radiative Forcing

0),(,, ccldclrLWobsLWclrLW TFFaFFC

04,, cldclrSWobsSWclrSW

SaFFC

0 SWLWnet CCC

Clouds have a net cooling effect

Many factors matter:

•Cloud amount: a

•Cloud top height: Tc

•Cloud optical depth: cld

Strong correlation between cloud forcing and low clouds !

31 W/m2

-44 W/m2

-13 W/m2

23Attacking the cloud feedback problem

Latent Heating by Cumulus Convection

24Climate modeling

5.Clouds in Climate Models

25Climate Modelling

1.How did I get here?

~1m - 1m

~107 m ~105 m

~103 m

The planetary scaleCloud cluster scale

Cloud scaleCloud microphysical scale

The climate system : A truly multiscale problem

10 m 100 m 1 km 10 km 100 km 1000 km 10000 km

turbulence Cumulus

clouds

Cumulonimbus

clouds

Mesoscale

Convective systems

Extratropical

Cyclones

Planetary

waves

Large Eddy Simulation (LES) Model

Cloud System Resolving Model (CSRM)

Numerical Weather Prediction (NWP) Model

Global Climate Model

No single model can encompass all relevant processes

DNS

mm

Cloud microphysics

27Climate modeling

Grid-box size is limited by computational capability

Processes that act on scales smaller than our grid box will be excluded from the solutions.

We need to include them by means of parametrization (a largely statistical description of what goes on “inside” the box).

Similar idea to molecules being summarized statistically by temperature and pressure, but much more complex!

Parametrization

28Climate modeling

Examples for processes that need to be parametrized in the atmosphere

Parametrization

29Climate modeling

As parametrizations are simplifications of the actual physical laws, their (necessary) use is an additional source of model uncertainty.

Parametrization

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6.Clouds in a Future Climate

31Climate modeling

Uncertainties in Future Climate model Predictions with different climate

models

2.5-4.3°CIPCC 2007

Past FuturePresent190

0

32Climate modeling

Climate Model Sensitivity

temperature radiative forcing

Water vapour

With feedbacks:

Snow albedo

clouds

33Climate modeling

Dufresne & Bony, Journal of Climate 2008

Radiative effects only

Water vapor feedback

Surface albedo feedback

Cloud feedback

Cloud effects “remain the largest source of uncertainty”in model based estimates of climate sensitivity IPCC 2007

2XCO2 Scenario for 12 Climate Models

34Climate Modelling

Primarily due to marine low clouds

“Marine boundary layer clouds are at the heart of tropical cloud feedback uncertainties in climate models”

(duFresne&Bony 2005 GRL)

Stratocumulus

Shallow cumulus

35Climate Modelling

• Definition: temperature change resulting from a perturbation of 1 Wm -2

• Radiative forcing for 2XCO2 3.7 Wm-2 (R)

• Temperature response of climate models for 2XCO2 2~4.3 K (T)

• Climate model sensitivity: 0.5-1.2 K per Wm-2 (T/R)

• The climate model sensitivity is not (very) dependent on the source of the perturbation (radiative forcing)

• Main reason for this uncertainty are the representation of (low) clouds

• Reducing uncertainty of climate models can only be achieved through a more realistic representation of cloud processes and is one of the major challenges of climate modelling

Climate Model Sensitivity

36Climate modeling

7.Clouds and Aerosols

38Future Climate

Radiative Forcing Components (Source IPCC 2007)

39Future Climate

Droplet concentration and Radiation:

"Indirect" aerosol effect

40Future Climate

Direct and Indirect Aerosol effects

41Future Climate

•More info : Pier Siebesma (siebesma@knmi.nl) ; siebesma@knmi.nl