Confusion with metpy water vapor functions

Question

I'm trying to solve for surface turbulent latent heat flux and I need surface saturation humidity (qs) and surface air humidity (qa)

I currently have the 2m dew point temperature and 2m temperature from the ERA5 dataset.

If what I'm thinking is correct, surface saturation humidity (qs) is just the specific humidity at the dew point. I therefore used metpy's specific_humidity_from_dewpoint function.

But isn't the specific humidity at the dew point also just the saturation mixing ratio at the dew point? I used metpy's saturation_mixing_ratio function but it gave me a ever so slightly different result from the first one.

I'm also trying to solve for the surface air humidity. I thought of doing relative_humidity_from_dewpoint, mixing_ratio_from_relative_humidity, and then finally specific_humidity_from_mixing_ratio

Am I doing this correctly?

Answer 1

• Specific humidity is the ratio of the mass of water vapor to the mass of all air: $q = m_v / (m_v + m_d)$)
• Water vapor mixing ratio is the ratio of the mass of water vapor to the mass of dry air: $w = m_v / m_d$

See here for more information. So no, the specific humidity at the dewpoint is not the saturation mixing ratio at the dew point.

I'm rusty on my heat flux calculations, but I think you need to stick with the specific humidity functions for your needs.