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The model I use needs the data of Air humidity and unit is 'mb' (there is no other explanations about this variable), but now I only have two datasets and there are Relative humidity (%) or Specific humidity (kg/kg). (Specific humidity: This parameter is the mass of water vapour per kilogram of moist air. The total mass of moist air is the sum of the dry air, water vapour, cloud liquid, cloud ice, rain and falling snow)
So how can I convert RH or SH to the humidity with unit 'mb'?
I guess what you need is water vapour pressure - that is one way of expressing humidity as the partial pressure of water vapour - in mb or hPa (they are equal).
According to World Meteorological Organization (WMO No.8 Guide to Instruments and Methods of Observation, Vol 1 Measurement of Meteorological Variables, ANNEX 4.B. FORMULAE FOR THE COMPUTATION OF MEASURES OF HUMIDITY), saturation vapour pressure - ews(t) - in hPa may be derived by the following formula, where t is temperature in degrees Celsius:
ews(t) = 6.112 exp [17.62 t/(243.12 + t)]
in hPa and degrees C, for liquid pure water (–45 °C to 60 °C)
This is the saturation water vapour pressure - a kind of "maximum" water vapour that can normally exist at a certain temperature t.
To know what the actual water vapour pressure - ew(t) - is, you must multiply it by the relative humidity (Rh) (and divide by 100 if Rh is in percentage):
ew(t) = ews(t) * Rh(%) / 100
So,
ew(t) = 6.112 exp [17.62 t/(243.12 + t)] * Rh(%) / 100
This is your "humidity" in mb, if you use Celsius for t.