solar forcing effects of shallow cumuli at acrf sgp...
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Solar Forcing Effects of Shallow Cumuli at ACRF SGP Site
David MillsUniversity of South Carolina
GCEP End-of-Summer WorkshopPacific Northwest National Laboratory
Shallow Cumulus are clouds primarily with flat bottoms and round tops formed by:
Moisture Lifting Mechanism Cloud Condensation Nuclei
Shallow Cumulus occur over a large portion of continental and tropical regions.
Cumulus Cloud Development
http://www.atmos.ucla.edu/as3/scrns/clouddev/Note11.htmhttp://www.atmos.ucla.edu/as3/scrns/clouddev/Note11.htm
Shallow Cumulus Clouds
These clouds are smaller than the grid spacing used in most numerical models of the atmosphere
Because of their sub-grid sizes, their radiative effects in climate models have large uncertainties
http://www.crh.noaa.gov/images/iwx/program_areas/wxpics/clouds_sun/faircu1.jpg
http://images.arm.gov/armimages.nsf/by+id/TENG-5EWU4M
ACRF SGP Site(ARM Climate Research Facility, Souther Great Plains site)
http://images.arm.gov/armimages.nsf/by+id/TENG-734R4B
http://images.arm.gov/armimages.nsf/by+id/TENG-6EGS9Q
ARSCL VAP(Actively Remote Sensed Clouds Location, Value-added Product)
Compilation of various measurement instruments:Microwave RadiometerMillimeter-Wavelength Cloud RadarVaisala CeilometerMicropulse Lidar
Used to provide the best estimate of:Cloud Base HeightCloud Top HeightCloud Thickness
Total Sky Imager (TSI)
Provides a visual of Hemispheric sky conditions during daylight hours.
Our criteria for Identification of shallow cumulus clouds: Cloud Base Height between 0.3 - 3 kmCloud Top Height between 1.5 - 3 kmCloud Thickness ~1 km Verify using Total Sky Imager
Eliminated days with:Clouds caused by large scale circulation (Cirrus) Agricultural burningPrecipitating cloudsCumulus clouds transitioning to Stratocumulus
Cloud Identification
Shortwave Flux Analysis Value Added Product
The Shortwave Flux Analysis Value Added Product uses methods provided by Charles Long and Thomas Ackerman (2000) which uses various methods and tests to measure and predict the shortwave irradiance.
This analysis provides an empirical fit, to estimate the clear sky irradiance based on clear conditions near the day of interest.
The hemispheric Cloud Fraction (CF) defines the percentage of the sky covered with clouds.
1000
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Forc
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W/m
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233.0232.5232.0231.5231.0230.5Time
1.00.80.60.40.20.0
Clo
ud F
ract
ion
Shortwave Irradiance Fitted Irradiance Cloud Fraction
Clear Few Clouds Cloudy
Shortwave Flux Analysis Value added Product
Data was collected from the Summers (May-August) of 2000 - 2007
Cloud forcing was computed for each 15-minute IntervalComputed by subtracting net clear sky empirical fit from the actual recorded shortwave irradiance.
Integrated forcing over the daylight period and analyzed by summer, month, and period.
Computed arithmetic mean for each period to compare to previous studies containing all clouds.
Analysis
I↓,Obs − I↑,Obs( )− I↓,Fit − I↑,Fit( )
Impact of Cloud Cover and Duration
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W/m
2
20072006200520042003200220012000
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CC
Hou
rs
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Num
Days
0.42
0.40
0.38
0.36
0.34
0.32
0.30
0.28
Avg C
F0.42
0.40
0.38
0.36
0.34
0.32
0.30
0.28
Avg
CF
CC_Hours # Days Avg_CF
Summer Forcing(123 days) Avg_CF
There were 206 days with Shallow Cumulus out of the 8 summers (984 days).
The summer shortwave forcing effect of Shallow Cumulus for the 8 summers was -6 W m-2 .
The extrapolated annual shortwave forcing effect of Shallow Cumulus for the entire 8 year period was -2 W m-2.
Assumed there were no shallow cumulus outside of the summers.
Results
The -2 W m-2 forcing is larger than the forcing associated with various greenhouse gases reported by the IPCC.
Results
Summary
The effects of the Shallow Cumulus are very significant in accurately modeling climate. The SGP site is representative of many continental regions as well as Tropical regions.
The extrapolated annual shortwave forcing of shallow cumulus for the entire 8 year period (2922 days) was -2 W m-2.This value is comparable to, and larger than most of the forcingassociated with various greenhouse gases as reported by the IPCC (IPCC 2007), which are 1-2 W m-2.
Thanks
Pacific Northwest National LabLarry K. BergEvgueni KassianovWilliam Shaw
Department of Energy
Global Change Education ProgramJeff GaffneyNancy MarleyMilton Constantin
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