implement Tdew to RH conversions

[mikofski]

Is your feature request related to a problem? Please describe.
In case weather file doesn't have RH or Pwat, I would like to be able to use Tdew to calculate spectral mismatch

Describe the solution you'd like
This capability is already implemented in MetPy relative_humidity_from_dewpoint

Describe alternatives you've considered
reimplement in pvlib? import metpy and call it? ignore b/c most weather files already include RH or Pwat, or uncertainty in chain of conversions too much?

Additional context
I thought this topic came up somewhere else but I couldn't find it in google group, discussions, or issues/pr's

[adriesse]

I would prefer to duplicate very minor models/functions in pvlib rather than expand the list of dependencies.

[AdamRJensen]

I find using metpy annoying due to the requirement of specifying units; thus I agree that it would be nice to have these simple functions in pvlib.

[wholmgren]

I agree with Anton that these models and implementations are simple enough that it's better to just copy (with attribution/license) than to introduce a non-trivial dependency. I also agree with Adam that the metpy units requirement is annoying.

The dewpoint to relative humidity calculation is essentially exact for this purpose so I don't have any concerns about introducing additional uncertainty.

If we agree that we should add this function then I might have someone that would be interested in contributing a PR.

[adriesse]

I suggest looking up the equation in a textbook and writing a new function referencing that. I did this years ago using the meteonorm documentation as reference.

[mikofski]

A coworker shared this reference: http://www.bom.gov.au/climate/averages/climatology/relhum/calc-rh.pdf, which is a calculation of the ratio of vapor to saturation vapor pressures based on ambient and dew point temperatures

[kurt-rhee]

Another reference that may be useful for saturation vapor pressures. PlantPredict uses the AEKR coefficients over water for the magnus form equations.

https://www.osti.gov/servlets/purl/548871-PjpxAP/webviewable/

[uakkoseo]

I'm currently working on this. There are three different options. MetPy uses Bolton1980, @kurt-rhee mentions PlantPredict uses the AEKR, @mikofski pointed us to the document that uses Abbot 1985. It's unclear to me whether it is preferable to choose one approach over another. Any thoughts?

[wholmgren]

Unidata/MetPy#508 also relevant. I suspect all of the above is fine for PV modeling applications.

[AdamRJensen]

I'm currently working on this. There are three different options. MetPy uses Bolton1980, @kurt-rhee mentions PlantPredict uses the AEKR, @mikofski pointed us to the document that uses Abbot 1985. It's unclear to me whether it is preferable to choose one approach over another. Any thoughts?

Personally I'd love to see all three of them. If they have similar input arguments they could all be bundled in one function that has a model argument, like this:

temp_dew_from_relative_humidity(relative_humidity, model='aekr')

[adriesse]

I would prefer to see just one good one rather than a range of options since this is peripheral to PV modelling.

[kurt-rhee]

@uakkoseo

I don't have a strong preference for either implementation, I believe that the model accuracy in either case must be quite rough. If it helps, here is the C# implementation of the equation mentioned above:

/* 
 * These magnus coefficients below are the AEKR coefficients which reduce overall errors the most between -40 and 50 degrees C according to research by NOAA:
 * https://www.osti.gov/servlets/purl/548871-PjpxAP/webviewable/
 * 
 * magnusA variable is not explicity needed since the value is cancelled out by a division operation, but the variable is kept here
 * for the sake of keeping the complete equation.
 * 
 */
double magnusA = 6.1094;
double magnusB = 17.625;
double magnusC = 243.04;

// e:  amount of water vapor in the air
var e = magnusA * Math.Exp((magnusB * Temperature) / (magnusC + Temperature));

// es:  amount of water vapr in the air if the air were entirely saturated at the same temperature and pressure
var es = magnusA * Math.Exp((magnusB * dewpoint) / (magnusC + dewpoint));

return 100 * (es / e);

[uakkoseo]

Thanks @kurt-rhee ! I think it is easy enough to bundle them all in one function and give the option to choose the model, unless one is sufficient or deemed better than others. From what I understand, all three of these models are used often, the uncertainties seem to be similar for different temperature ranges. The AEKR model has the largest range for temperature out of the three. If used in the stated temperature range Bolton and Abbot seem to be as applicable. (I understand as the issue @wholmgren linked also points to this as well)