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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)?