Effect of Atmospheric Water Vapor

1. Effect of Atmospheric Water Vapor Rafael Rosolem 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 Arizona 2 University of Wisconsin COSMOS 3rd Workshop December 10, 2012

2. Simulating Different “Atmospheres” in MCNPx PW (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

3. 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

4. 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)

5. 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)

6. 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)

7. The Effect of Atmospheric Water Vapor on Fast Neutrons How do we correct for the signal to remove the effect of water vapor variations?

8. Water Vapor Correction/Scaling Factor NCORR = NMEAS . CWV Maximum change on the order of ~12% of neutron signal

9. Park Falls (WI) WLEF TV Tower Wisconsin 461 m

10. Park Falls: Vertical Profile versus Surface Meteorology or

11. Santa Rita (AZ): Comparison with TDT Network

12. 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

13. 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 Temp RH

14. Water Vapor Correction to be Implemented Online COSMOS Level 2 data NCORR = NMEAS . CWV Placeholder for this correction already available in our database

15. Where should CWV matter most? NCEP Reanalysis: Monthly climatology (1948-2011) Water vapor correction factor relative to fully dry atmosphere

16. 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 sensor • Questions • 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)?