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Invert Ambaum (2020) in dewpoint, consistent with saturation_vapor_pressure - #4153
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…essure dewpoint still inverted the Bolton (1980) fit while saturation_vapor_pressure uses Ambaum (2020), so the two were no longer inverses (up to -0.2 K at 50 C). The liquid-water Ambaum formula has an exact solution on the -1 branch of the Lambert W function, which dewpoint now uses. Expected values that depend on dewpoint are updated. Closes Unidata#3976
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The mixing ratio lines are computed with dewpoint and move by a fraction of a pixel. Images taken from the CI results of the previous commit. Signed-off-by: Guillaume Marty <279286709+LemonCondor@users.noreply.github.com>
Signed-off-by: Guillaume Marty <279286709+LemonCondor@users.noreply.github.com>
Signed-off-by: Guillaume Marty <279286709+LemonCondor@users.noreply.github.com>
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Description Of Changes
dewpointstill inverted the Bolton (1980) fit whilesaturation_vapor_pressureuses Ambaum (2020) since #3726, so the two are no longer inverses:dewpoint(saturation_vapor_pressure(T)) - Tis about -0.02 K at 20 °C and -0.2 K at 50 °C, andrelative_humidity_from_dewpoint/dewpoint_from_relative_humiditydo not round-trip (#3976).With
x = T0 / T, the liquid-water Ambaum formula used by_saturation_vapor_pressure_liquid(latent heat varying linearly with temperature) reduces toa ln x - b x = ln(e / e_s0) - bwitha = (Cp_l - Cp_v) / Rvandb = Lv / (Rv T0) + a. This has an exact solution on the -1 branch of the Lambert W function (already used in this module for the Romps LCL), sodewpointnow inverts it in closed form, without iteration and without adding a parameter.dewpoint(saturation_vapor_pressure(T))matchesTto ~1e-13 K from -80 °C to 60 °C.dewpoint(0 hPa)gives 0 K (Bolton gave -243.5 °C).test_ccl_with_mltwo nearly tangent intersections (784.1 and 782.7 hPa) merge into one.Checklist
test_dewpoint_saturation_vapor_pressure_inverse,test_dewpoint_relative_humidity_roundtrip; both fail on main)