# How to calculate the specific humidity based on the saturated specific humidity?

I am using ERA5 data to drive the land surface model. One of the forcing required by LSM is specific humidity. We want to use the 2-m specific humidity. So how can I calculate the specific humidity based on dew point at 2 meters, surface pressure at 2 meters and temperature at 2 meters? I saw in ERA5 documentation we can calculate the 'saturated specific humidity' by some equations (https://confluence.ecmwf.int/display/CKB/ERA5%3A+data+documentation#ERA5:datadocumentation-Computationofnear-surfacehumidityandsnowcover). But how can I get the specific humidity?

Thanks and hope for your early reply!

## 2 Answers

The answer is easy. Replace $$T$$ with $$T_d$$, where $$T_d$$ is the dewpoint temperature.

I will say I am not all that familiar with those equations, but that is besides the point.

Just in case it will save a great deal of time for others in the future, ill recommend the metpy package.

https://unidata.github.io/MetPy/latest/api/generated/metpy.calc.specific_humidity_from_dewpoint.html

• Please expand the answer to detail the specifics of the package Jun 5 at 12:20
• The specifics are detailed in the link. In summary, it's a python function that requires the surface pressure and the dew point temperature and computes the specific humidity from these two variables. Jun 5 at 13:43