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Estimation equation implemented in function svapor may be swaped for improved estimation #43

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dschlaep opened this issue Sep 4, 2017 · 1 comment
Closed

Estimation equation implemented in function svapor may be swaped for improved estimation #43

dschlaep opened this issue Sep 4, 2017 · 1 comment
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Appendix improvem...

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dschlaep commented Sep 4, 2017

Errors resulting from the current equation are up to -60% at -40˚C, close to 0% near 0˚C, and -10% at 50˚C when compared against tabulated values by Haynes 2017 (see attached figure ). It appears that the large errors at temperatures away from 0˚C arise because \Delta H_{vap}^{H_{2}O} is not incorporated as a function of temperature but instead used at a fixed temperature of 0˚C. I see two approaches to improve on this:

• Calculate the latent heat of water vaporization as a function of temperature.

• Replace e^{CC} and e^{ARM} with empirically derived equations by Alduchov & Eskridge 1996 (with deviations of < 0.38% between -40 and +50˚C) or by Buck 1981/2012
(with deviations < 0.01% within the temperature range of -80 to 50˚C).

Additionally: We could add the 'enhancement factor' to the calculation of e for humid air instead of some ideal air (i.e, gas mixture that corresponds to the ideal gas law).

Alduchov, O. A., and R. E. Eskridge. 1996. Improved Magnus Form Approximation of Saturation Vapor Pressure. Journal of Applied Meteorology 35:601–609.
Buck, A. L. 1981. New Equations for Computing Vapor Pressure and Enhancement Factor. Journal of Applied Meteorology 20:1527–1532.
Buck, A. L. 2012. MODEL CR-1A hygrometer with autofill: operating manual. Page 26. Buck Research Instruments L.L.C, Boulder, CO.

fig_saturation-vapor-pressure-equation_comparison
@dschlaep dschlaep added this to the Appendix improvements milestone Sep 4, 2017
dschlaep referenced this issue Jul 3, 2020
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Consistent and improved set of equations for saturation vapor pressur…
79ce146 
…e and derivative

- Previously, we had two functions `svapor` based on Hess 1959 that estimated saturation vapor pressure over water in [mmHg] and `slope_svp_to_t` that estimated the derivative based on Allen et al. 2005, but not of the implemented equation. The equation by Hess 1959 was increasingly (but slightly) underestimating saturation vapor pressure at temperatures > 20 C compared to Haynes 2017.

--> Replace functions with `svp` that calculates saturation vapor pressure over water and ice based on equations by Huang 2018 and estimates the gradient by taking the derivation of these equations.

- Updated unit tests

Haynes, W. M., editor. 2017. Vapor pressure and other saturation properties of water. Page 6.5-6.6 CRC Handbook of Chemistry and Physics. 97th Edition (Internet Version). CRC Press/Taylor & Francis, Boca Raton, FL.
Huang, J. 2018. A Simple Accurate Formula for Calculating Saturation Vapor Pressure of Water and Ice. Journal of Applied Meteorology and Climatology 57:1265–1272.
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dschlaep commented Sep 3, 2021

closed by 79ce146

@dschlaep dschlaep closed this as completed Sep 3, 2021
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