OBS 15: New Satellite Sensors to study Cirrus Clouds in the sub-mm Spectral Range

COST 723 UTLS Summerschool

Cargese, Corsica, Oct. 3-15, 2005

Stefan A. Buehler

Institute of Environmental Physics

University of Bremen

www.sat.uni-bremen.de

OBS 15: New Satellite Sensors to study Cirrus Clouds

in the sub-mm Spectral Range

Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005

2

Overview

Ice clouds in the earths radiation balance

Existing ice cloud observations

Observing ice clouds in the sub-mm spectral range

Summary and outlook

(Picture by Claudia Emde)


3

Overview




Summary and outlook



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.


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But what about Clouds?


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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)


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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)


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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 (compare lecture by Klaus Gierens)

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)(compare lecture by Francois Bouttier)

(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




Summary and outlook



16

Aircraft Campaign Locations

(Heymsfield and McFarquhar [2002].)


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Particle Size

Larger size for warmer temperature, but

Large natural variability

Overall:

Large instrumentation Inhomogeneity

Need global satellite data to validate GCMs

(Heymsfield and McFarquhar [2002].)


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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, ...)


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ESA Earth Explorers

CIWSIR

current

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

Humidity

Clouds


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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


21

Overview




Summary and outlook



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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)


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Cirrus Measurement with Microwave Sensors

ARTS Simulation

(CIWSIR Mission Proposal)

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

simulation by Sreerekha Ravi)


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Frequency Dependence of Ice Signal

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


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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)


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190 GHz 664 GHz

(ARTS Simulation: Sreerekha T.R.)


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New Cirrus Sensors

Strong Cirrus clouds have an important influence on AMSU-B measurements near 183 GHz, but

to determine the ice water content of weaker clouds (and ice particle size/shape) we need more channels at higher frequencies.

NASA proposal SIRICE (PI Steve Ackerman).

ESA Opportunity Mission Proposal CIWSIR (Cloud Ice Water Sub-millimeter Imaging Radiometer).


29

Different Particle Sizes

Different frequencies sample different parts of the size distribution

IR sees only smallest particles, radar only largest particles (compare lecture by Geraint Vaughan)

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


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CIWSIR Channels

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


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The CIWSIR Instrument

(Antenna diameter: 30 cmPicture: Mark Jarrett)


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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 15 km horizontal resolution and 25% accuracy.

Preparation: Aircraft campaigns with sub-mm receivers and simultaneous in-situ measurements.


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Retrieval by Bayesian Interpolation

Compare lecture by Francois Bouttier.

Create training dataset that covers the atmospheric variability (atmospheric states plus simulated radiances).

Bayes’ theorem quantifies the notion that if the measurement looks similar then the underlying state is likely to be also similar.

Retrieved IWP is the mean of all IWP in the training dataset, weighted with the “closeness” of the simulated radiances to the measured radiances.

For Gaussian statistics same result as 1D-Var.


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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)


35

Odin Data

There are no meteorological sub-mm sensors (yet)

Odin SMR is a stratospheric chemistry instrument with a band at 502 GHz

Choose measurements with low tangent altitude

Ice clouds clearly visible(Tangent altitude 6 km, arbitrary simulation, IWP not fitted,Source: Patrick Eriksson)


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Ice Clouds in Odin sub-mm Data

(Ice column above 10.5 km. Top: Odin-SMR Retrieval fall 2002. Bottom: ECHAM autumn average. Source: Patrick Eriksson)


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Simulated Polarization Signal in MLS Data

R2(V): 200.5 GHz

R3(H): 230 GHz

Similar absorption in both channels

Observe difference in cloud signal between channels

(Emde, 2005)


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EOS-MLS Data

Simulation MLS Measurement

Davis et al. [2005] found effective aspect ratio of 1.2±0.2

Not to be confused with the large aspect ratios of individual particles!


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Overview




Summary and outlook



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Summary and OutlookCirrus 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.

IWC can not be directly measured with existing satellite sensors.

CLOUDSAT and EarthCARE will be a big steps towards a better understanding of cloud processes and cirrus climatology, but will not provide daily accurate global IWP data.

A sensor in the sub-mm, combined with existing IR data, can measure IWP directly.

SIRICE (NASA) and CIWSIR (ESA) proposals.

Aircraft campaigns are needed to validate retrieval algorithms.


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Thanks for your attention.Questions?

...

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OBS 15: New Satellite Sensors to study Cirrus Clouds in the sub-mm Spectral Range

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