P2.65
Strong absorption of solar NIR by precipitating clouds
W.F.J. Evans, North West Research Associates, Bellevue, WA
Measurements of transmission spectra indicate certain clouds strongly absorb NIR radiation; radiation codes such as MODTRAN4 do not model this strong absorption well. The absorption fingerprint in cloud spectra matches liquid water. A garden hose spray shows liquid water absorption. The same spectrum observed in transmission of drizzling fog, indicating it has an association with drizzle in clouds. The spectral signature of liquid water absorption was observed from aircraft on the AIRS project in drizzle clouds. Precipitating clouds including Virga have a bimodal droplet distribution with water droplets> 200 mm radius. This liquid water absorption is not explained by Mie theory for cloud droplets of size from 10 to 20 microns using MODTRAN or GCM radiation codes. A modified MODTRAN4 simulation with added drizzle droplets > .5 mm only partially simulated the observed NIR absorption. Daily measurements with 2 filtered pyranometers used to investigate the statistics of this effect. Under clear conditions, the ratio of NIR to Total short wave is about 40 %. Under cloudy conditions this ratio is reduced from 40% to 15 %. The absorbed flux is in the 50 to 150 W/m2 range . On a regional basis this effect corresponds to 20 W/m2 with estimated ground temperature impacts 2 to 10 K. The effect is 2.5 W/m2 or 0.5 K on a global basis. Absorption of NIR short wave by drizzling clouds is a missing factor in GCMs, regional climate models and forecast models. This NIR absorption is missing in current radiation schemes; large droplets need to be included as bi-modal distributions. Simulations needed to test the impact of this error in models .
Poster Session 2, Cloud Physics Poster Session II
Wednesday, 12 July 2006, 5:00 PM-7:00 PM, Grand Terrace
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