The Absorption of NIR Solar Radiation by Precipitation from Clouds

W.F.J. Evans,Trent University,Peterborough,Ontario K9J7B8

Email: wevans@trentu.ca

ACKNOWLEDGEMENTSE. Puckrin,co-investigatorS. Cober for droplet distributions.H. Barker and CSA for field support.D. Travers for data analysisMSC and G. Isaac for providing AIRS aircraft flights and Mirabel siteEnbridge Consumers Gas for funding support. NSERC for matching IOR grant .

FTS BOMEM DA-8 at Mirabel

AIRS Convair 580 Aircraft

FTS Magna 550 on Aircraft

Ground Cloud Observations with Magna FTS

NIR Cloud Transmission Spectra

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N ear-infrared spectra of the clear sky (curve A), of light overcast c loud (curve B) and of very heavy overcast c loud (curve C) obta ined on April 11 and 12, 1995 at Peterborough, O ntario.

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Cloud Spectrum Matches Transmission of 2 mm of Liquid Water

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C lo u d tra n sm iss io n sp e c tra fo r lig h t o ve rca s t c lo u d (cu rve A ) a n d ve ry h e a vy o ve rca s t c lo u d (cu rve B ). C u rve C re p re se n ts th e la b o ra to ry tra n sm iss io n sp e c tru m a ttr ib u te d to a 2 -m m p a th o f liq u id w a te r.

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Transmission of a Droplet Spray from a Hose Showing Liquid Water Absorption 5600 to 6600 cm-1

Le ft H and S ide C u rve from D e tec tion o f L iqu id W a te r20 o S ca tte ring /D irec t M easu rem en t

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Transmission of a Droplet Spray from a Hose Showing Liquid Water Absorption below 8000 cm-1

H o s e T ra n s m is s io n C o m p a re d W ith 0 .5 m m W a te r C e ll

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Rain at 85o/ 65o

Rain at 85o/ 65o

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Precipitating Cloud vs Rain Radiance SpectraR

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Transmission Spectrum of Drizzling Fog showing Liquid Absorption Compared to .5 mm Water Cell

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Sequence Showing Liquid Absorption at 9:45

W ater Sequence for Dec 7 1999

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NIR Spectrum Showing Vapour,Liquid Water and Ice Absorptions

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liq u id w a te r tra n sm iss io nice tra n s m iss io nm e a s u re d c lo u d tra n sm iss io n (h e a vy c lo u d /lig h t c lo u d )

F ig u re 8 .4 : T h e tra n s m is s io n sp e c tru m o f h e a vy c lo u d m e a s u re d in th e n e a r in fra re d re g io n . A b so rp tio n b a n d s o f w a te r va p o u r, liq u id w a te r a n d ic e a re c le a rly e v id e n t in th e sp e c tru m , w h ic h w a s m e a su re d a t a re so lu tio n o f 1 6 cm -1.

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Comparison of Aircraft with Ground Spectra Showing Ice and Liquid Water Absorption

MODTRAN4 Simulation of Strong Absorption

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Measurement overcast cloud

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Total and NIR pyranometers measuring the ratio of NIR/ Visible shortwave

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Change in Ratio of NIR/Visible Short Wave Radiation due to Absorption by Cloud

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Droplet size distribution in rain

Estimate of path length of rain drops from rainfall rate

• Assume rainfall rate is 6 mm/hour• and there is enough liquid water to rain for 10 minutes • (from fall velocity of drops at 1 m/sec and 600 m cloud)• then there is about 1mm thickness of water • in large drops available for strong NIR absorption• multiply by path length factor of secant 60 0• gives 2 mm path length of liquid water

Estimate of absorption by 2 mm of liquid water in rain

• Rain rate gave 2mm path length of liquid water = l• Optical Depth = kx l = 10/cm x 0.2 cm = 2.0 OD• Transmission = e- OD = .135• Absorption = 1.0 - Transmission = .864• Energy abs = Absn x flux =.864 x 100W/m2 = 86 W/m2• Since NIR flux available for absorption = 100W/m2• (under thick cloud)

Combined satellite radar image of precipitating clouds

Abstraction of Absorbed Flux

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W avenumber (cm-1)A comparison of the energy absorbed primarily by liquid water in very heavy overcast cloud (curve A) and the modelled result of a liquid water in a 1 km thick stratus cloud at a resolution of 50 cm-1 (curve B). The spectrally integrated liquid water absorption was determ ined to be 80.0 W /m2 for the observed and 95.5 W /m2 for the modelled results [Davies et al., 1984].

Heavy overcast cloud

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CONCLUSIONS a• Measurements of NIR spectra of cloudy skies indicate precipitation

strongly absorbs NIR radiation. This effect is not included in models.• Precipitation spectra matches absorption fingerprint of liquid water.• A measurement of garden hose spray shows droplets absorb like bulk

liquid water.• The measured radiance spectrum of rain shows strong NIR absorption• The absorption spectrum observed in transmission of drizzling fog ,

indicating absorption by drizzle in clouds.• The spectral signature of liquid water absorption observed in drizzle clouds

on the AIRS project.• Rain droplet distribution measurements indicate that precipitation contains

over 1 mm of liquid water. • Physics shows that 1 mm can absorb over 100W/m2 of NIR radiation. • Precipitation from clouds frequently includes Virga. • Absorption of NIR short wave by precipitation is a missing factor in GCMs,

regional climate models and forecast models. This strong NIR absorption is missing in current radiation schemes.

• Simulations are needed to test the impact of this error in models .