Sofia, May 17, 2006 S. A. Buehler Institute of Environmental Physics University of Bremen CIWSIR, a Mission to Study Cirrus Clouds

Sofia, May 17, 2006

S. A. Buehler

Institute of Environmental Physics

University of Bremen

www.sat.uni-bremen.de

CIWSIR, a Mission to Study Cirrus Clouds in the Sub-mm Spectral

Range

Stefan Buehler, Sofia, May 17, 2006

2

OverviewIce clouds in the earths radiation balance

Existing ice cloud observations

CIWSIR mission idea

Summary

(Picture by Claudia Emde)


3

Overview

Ice clouds in the earths radiation balance


CIWSIR mission idea

Summary



4

Earths Radiation Balance

Outgoing Longwave Radiation OLR

Incoming Shortwave

RadiationSun

Earth


5

Earths Radiation Balance

Wavelength [μm]

λEλ

[nor

mal

ized

]

(Wallace und Hobbs, `Atmospheric Science', Academic Press, 1977.)

Radiative equilibrium temperature: -18°C

Global mean surface temperature: +15°C

34 K natural greenhouse effect


6

Clear-Sky OLR Spectrum

Water vapor and CO2 are the most important greenhouse gases.


7

But what about Clouds?


8

OLR-Spectrum with Cirrus

Single scattering calculation.

Ice water content 0.01 g/m3 (contrail-cirrus), altitude 6-7 km.

Cloud reduces OLR.

Not the whole story: Clouds are active in the shortwave and in the longwave.

(Calculation: Claudia Emde)


9

The Role of Cirrus Clouds: Shortwave

Cirrus clouds reflect sunlight and thus increase the planetary albedo.

(AVHRR, Channel 1, 580-680nm, 25.1.2002, 13:30 UTC, Data Source: Met Office / Dundee Receiving Station)


10

The Role of Cirrus Clouds: Longwave

Cirrus clouds are radiatively cold and thus reduce the OLR.

Attention: grayscale is normally reversed for IR images so that clouds look white.

(AVHRR, Channel 4, 10.3-11.3μm, 25.1.2002, 13:30 UTC, Data source: Met Office / Dundee Receiving Station)


11

The Net Effect of Cirrus Clouds

For high and optically thin clouds the longwave warming effect dominates.

For lower and optically thicker clouds the shortwave cooling effect dominates.

Global net effect of all clouds is cooling. Magnitude: 4 times double CO2 (Ramanathan et al., Science, 243, 1989).

How will the net effect change for a changing surface temperature?

No good answer at the moment.


12

Cirrus Particle Sizes and Shapes

(Miloshevich et al., J. Atmos. Oceanic. Tech., 2001)

Many different particle types

For cirrus clouds the net effect depends on the size (and shape) of the ice particles.

Feedback direction unclear. (Stephens et al., J. Atmos. Sci., 47(14), 1742-1754, 1990).


13

Ice Clouds in Weather Prediction Models

In models: Ice Water Content (IWC)

(Met Office, UK, mesoscale model, Image: Sreerekha T.R.)


14

Ice Clouds in Climate Models

Climatology of zonal, annual mean IWP from various models in the IPCC AR4 data archive shows difference up to an order of magnitude.

Delta-IWP after a CO2 doubling shows also vast differences.

IWP observations are needed to resolve model differences.

(Figure by Brian Soden, University of Miami)


15

Overview



CIWSIR mission idea

Summary



16

Aircraft Campaign Locations

(Heymsfield and McFarquhar [2002].)


17

Existing Satellite Observations

Cloud emission (IR radiometry):Retrieval of ice water path (IWP) and size (D) only for thin (semitransparent) ice clouds(ATSR-2, HIRS, Meteosat, ...)

Solar reflectance (UV/Vis):Retrieval of D and gross habit classification for particles near cloud top(POLDER, Meteosat, ...)

Cloud transmission (mm-wave):Retrieval of IWP only for thick (deep convective) ice clouds(AMSU-B, SSM-T2, ...)


18

Overview



CIWSIR mission idea

Summary



19

CIWSIR Mission ProposalCIWSIR = Cloud Ice Water Sub-millimeter Imaging Radiometer

Proposal community:

Uni BremenUni BonnUni MainzUni MünchenUni KielDLR OberpfaffenhofenDWDMPI (Mainz, Hamburg)Alfred Wegener Institut

Met Office, UKECMWFUni Wisconsin, USUni Rome, ITUni Florence, ITUni Edinburgh, UKLMD, FRChalmers, SEUni Colorado, USNCAR/NESDIS, USUni Bern, CHRTH Zürich, CHUni Paris, FRUni Miami, US


20

Cirrus Measurement with Microwave Sensors

Ice cloud reduces the brightness temperature, as a part of the upwelling radiation is scattered away.

Compared to the IR, the measurement „sees“ the inside of the cloud, not just the top.

Sensitivity is strongly frequency dependent.

(Buehler et al., CIWSIR Mission Proposal, 2005, Figure by Oliver Lemke)


21

Cirrus Measurement with Microwave Sensors

ARTS Simulation

(CIWSIR Mission Proposal)

(Buehler et al., CIWSIR Mission Proposal, 2005,

simulation by Sreerekha Ravi)


22

Frequency Dependence of Ice Signal

(Figure: Sreerekha T. R., IWP = 80 g/m2, randomly oriented cylindrical ice particles, aspect ratio 4, r = 100 µm)


23

Influence of Cirrus Clouds on AMSU-B

Strong ice clouds are detectable at AMSU frequencies (183±7 GHz)

(25.1.2002, 1330 UTC

Figure: Sreerekha Ravi)


24


25

190 GHz 664 GHz

(ARTS Simulation: Sreerekha T.R.)


26

CIWSIR Channels

(Buehler et al., CIWSIR Mission Proposal, 2005, figure by Viju O. John)


27

Different Particle Sizes

Different frequencies sample different parts of the size distribution

IR sees only smallest particles, radar only largest particles

(Buehler et al., CIWSIR Mission Proposal, 2005, simulation by Claudia Emde)


28

The CIWSIR Instrument

(Antenna diameter: 30 cmPicture: Mark Jarrett)


29

satellite orbit

swath width

flight direction

SSP

footprints

fore-view

aft-view

45°flightdirection

fore-view

aft-view

view from the top

The CIWSIR InstrumentMission proposal to ESA for current explorer call.

Conical scanner.

Goal: Ice water path and effective ice particle size with 10-20 km horizontal resolution and 20-25% accuracy.


31

Performance Estimate

IWP and D median errors mostly below 25 %

IR radiances complement sub-mm channels

Requirement for CIWSIR to fly tandem with Metop (AVHRR/3, IASI)

Co-registration facilitated by high AVHRR spatial resolution

(Buehler et al., CIWSIR Mission Proposal, 2005, simulation by Frank Evans)


32

ESA Earth Explorers

CIWSIR

current

call(Adapted from R. Münzenmayer, EADS Astrium GmbH)

Humidity

Clouds


33

EarthCARECloud Profiling Radar (CPR)at 94 GHz (similar to the CPR on CLOUDSAT)

Lidar (ATLID) at 355 nm (UV)

+ other instruments

Spots of < 1 km diameter

High vertical resolution (CPR < 400 m, ATLID < 100 m)

aerosol and cloud profiles plus radiation fluxes

Point samples along flight track

IWC from CPR to factor of 2 with assumptions on size distribution


34

Figure by Andy Heymsfield

For strong clouds the lidar covers only a part of the IWP.


35

Overview



CIWSIR mission idea

Summary



36

Summary

Cirrus clouds play a crucial role in the earths climate due to their strong interaction with shortwave and longwave radiation.

Climate models and NWP models include cloud ice water content (IWC). There are large variations between models.

Global IWC or ice water path (IWP) data are urgently needed for validation.

IWP can not be directly measured with existing satellite sensors.

CIWSIR can measure IWP directly.

Low scientific and technological risk, moderate cost.

Documents

Sofia, May 17, 2006 S. A. Buehler Institute of Environmental Physics University of Bremen CIWSIR, a Mission to Study Cirrus Clouds