effect of atmospheric water vapor
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Effect of Atmospheric Water Vapor
Rafael Rosolem
COSMOS 3rd WorkshopDecember 10, 2012
W. J. Shuttleworth1, M. Zreda1, T. Franz1, X. Zeng1, S. A. K. Papuga1, Z. M. S. Mejia1, A. R. Desai2, J. S. Halasz1
1 University of Arizona2 University of Wisconsin
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Simulating Different “Atmospheres” in MCNPxPW (mm)
Start with U.S. Standard Atmosphere 1976 (78% N2 and 22% O2) Any addition of H2O replaces some of N2 and O2 proportionally These changes are made only within the sensor footprint (height of
influence found to be ~400 m), but the simulated domain reaches up to ~7.5 km in the atmosphere
Precipitable Water (PW) is computed if water vapor profile extrapolated up to 300 hPa level
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How Does Atmospheric Water Vapor Affect Fast Neutrons?
Simulations were made with varying soil moisture (0 to 0.40 m3 m-3 with 0.01 m3 m-3 intervals), and varying atmospheric water vapor (0 to 22 g m-3 with 2 g m-3 intervals) total number of paired combinations = 492
The left hand side figure shows some selected cases only
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The Effect of Atmospheric Water Vapor on Fast Neutrons
Assume atmosphere on the day of calibration was dry (ρv = zero): this defines the red curve on the right hand side panel
On that day, the normalize neutron count, N, was taken to be 0.24 corresponding to θ = 0.20 m3 m-3 (point A)
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The Effect of Atmospheric Water Vapor on Fast Neutrons
Now assume the measurement, N = 0.21, was actually taken with a fully wet atmosphere
If the presence of water vapor is disregarded the reduction in neutron counts will be interpreted as an increase in soil moisture (point B)
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The Effect of Atmospheric Water Vapor on Fast Neutrons
However, if we account for the changes in water vapor, the reduction of neutron counts is solely caused by the presence of water vapor in the atmosphere, hence measured soil moisture is unchanged (point C)
The difference is that the measurement was actually taken with a different calibration curve, i.e., that associated with wet atmosphere (blue curve, right panel)
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The Effect of Atmospheric Water Vapor on Fast Neutrons
How do we correct for the signal to remove the effect of water vapor variations?
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NCORR = NMEAS . CWV
Maximum change on the order of ~12% of neutron
signal
Water Vapor Correction/Scaling Factor
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WLEF TV TowerWisconsin
461 m
Park Falls (WI)
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Park Falls: Vertical Profile versus Surface Meteorology
or
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Santa Rita (AZ): Comparison with TDT Network
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Santa Rita (AZ): Comparison with TDT Network
Δρv = ρv – ρvREF (departure from reference atmosphere: ρvREF = 2.2 g m-3, measured during sensor calibration)
Forced slope = 1 for linear fit above
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Additional Sensors
Standard meteorological measurements will now be added to the sensor:
External temperature and relative humidity
Sensor already measures pressure
A kit will be provided to currently deployed probes
TempRH
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Water Vapor Correction to be Implemented Online
Placeholder for this correction
already available in our database
COSMOS Level 2 data
NCORR = NMEAS . CWV
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Where should CWV matter most?NCEP Reanalysis: Monthly climatology (1948-2011)
Water vapor correction factor relative to fully dry atmosphere
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Summary Water vapor effects on fast neutron flux should not be
neglected, especially at sites with strong seasonality A simple correction factor has been developed which
needs standard meteorological data Reducing water vapor related observation “noise” is also
desirable for data assimilation application Additional Temperature and Relative Humidity sensors are
to be added to the cosmic-ray sensorQuestions
How best to correct past data (using measurements from co-located sites, gridded data, nearby weather station)?
Should we also add rain gauges (not directly related to water vapor correction but valuable for data assimilation)?
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