# Humidity

## Regional Models

Regional models will set atmospheric relative humidity to an annually average constant value of 90 %.

## Global Models

Global AOMIP models will use the monthly NCEP humidity climatology available here (not yet but soon!). It is proposed that this data set will be blended with the AOMIP humidity forcing to produce an appropriate global forcing data set. The blending would occur over the latitude band 60-65 degrees North. The blended data set would have the same monthly-climatological temporal resolution as the AOMIP humidity.

## Humidity Conversion

The following formulas (Gill, 1982, Appendix 4) are used to convert relative humidity (i.e., percent saturation) to specific humidity (i.e., in g/kg units) for direct use in calculation of evaporation rates and latent heat fluxes.

- Compute saturation vapor pressure (Pa) over ocean surface as

Actually, the saturation vapor pressure over a salt solution is less than over fresh water, For sea water the reduction is about 2 %. We ignore this discrepancy in the above formula.

- Compute saturation vapor pressure (Pa) over ice/snow surface as

- Compute vapor pressure (Pa) in air as

The vapor pressure formulae are **only** valid in the range -40.0 to +40.0 degrees Celsius. From them, we derive the specific humidity.

- Compute specific humidity (kg/kg) over ocean surface as

- Compute specific humidity (kg/kg) over ice/snow surface as

- Compute specific humidity (kg/kg) in air as