Wet Bulb Globe Temperature

[dopplershift]

NWS will be adding Wet Bulb Globe Temperature (WBGT) to the NDFD grids. It'd be great to have an implementation in MetPy. There's documentation of the formula here.

@SPiltz01 seeing your name on that paper 😉 anything we should know?

[SPiltz01]

It really needs more work, but it was a good start.

[dopplershift]

@SPiltz01 a couple of questions if you don't mind:

1. The paper never specifies values for the f_db and f_dif (direct beam and diffuse radiation from the sun). Were the values of 0.67 and 0.33 from Hunter and Maynard (1999) used?
2. The replacement of 0.115 with 0.315 is a little confusing, since right above that part of the paper it says something about setting the coefficient to 0.437 gives better results.

[dopplershift]

@mcallic2 I also found this version of the paper. It may or may not be helpful.

[deeplycloudy]

As part of work on a class project, @icastorm and @mhamelttu have made a draft implementation of WBGT. They implemented the WFO Tulsa method as well as one commonly used in Australia. Please see the notebook at their repository for a comparison of the two methods, and a discussion of the uncertainties they uncovered, which echo points made up-thread. There is still a need to include units support, but there’s a good start on an implementation here.

[deeplycloudy]

Just came across the paper below by some colleagues. It is meant for when an actual globe temperature measurement is used, which seems a bit orthogonal to this issue, but is likely good reference for a deep dive on harmonizing various approaches in thermal comfort measurements. https://link.springer.com/article/10.1007/s00484-021-02131-y

[akrherz]

See ecmwf-projects/thermofeel for a WBGT implementation.

[deeplycloudy]

Here are a couple more references that are focused on using regular meteorological measurements:

Patel et al. (2013), "Comparison of methods for estimating Wet-Bulb Globe Temperature index from standard meteorological measurements"

Lemkie and Kjellstrom (2012), "Calculating Workplace WBGT from Meteorological Data: A Tool for Climate Change Assessment", which compares various estimation methods.

To the questions by @dopplershift above, I have it on good authority that in an early (~2020) implementation of the NDFD, the direct and diffuse radiation fractions were were taken directly from the proportion of sky covered, which might not be the right approach - diffuse can refer to blue sky too, I think? The early NDFD implementation also used 0.315 for the heat transfer coefficient. I have an outstanding message trying to get some insight as to the final state of the production NDFD code.

[deeplycloudy]

Another new paper Guyer et al. 2021, addressing WBGT differences over different surfaces: "Identifying the need for locally-observed wet bulb globe temperature across outdoor athletic venues for current and future climates in a desert environment."

I now have access to the production NDFD code, which is likely to have one more change by next summer. I think we should implement what's already there and then open a new issue tracking the need for one more sync with the NOAA/NWS approach.

[deeplycloudy]

@SPiltz01 a couple of questions if you don't mind:

1. The paper never specifies values for the f_db and f_dif (direct beam and diffuse radiation from the sun). Were the values of 0.67 and 0.33 from Hunter and Maynard (1999) used?
2. The replacement of 0.115 with 0.315 is a little confusing, since right above that part of the paper it says something about setting the coefficient to 0.437 gives better results.

The current NDFD implementation, which resulted from some interagency code review uses:

1. f_db=0.75 and f_dif=0.25 , based on Oke (1978)[1].
2. The heat coefficient currently used is h=0 at night, and h=0.228 during the day.

[1] Oke, T.R., 1978: Boundary Layer Climates, 339- 390, Methuen & Co., New York, New York.

[akrherz]

I am lazily wondering if anybody watching this issue has an update on if there is an "official" WBGT equation from WMO/NWS/AMS etc?

[SPiltz01]

The NWS has an equation that is used for NDFD. I will see if I can find it.

…

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