The primary purpose of this algorithm is to calculate water-vapor mixing ratio at the 25- and 60-m levels of the tower at the Southern Great Plains (SGP) Central Facility. Since there are no barometric pressure sensors at those levels on the tower, the hypsometric equation is used along with pressure values from either the surface meteorological observation system (SMOS) or the temperature, humidity, wind, and pressure system (THWAPS) to derive barometric pressures at those altitudes. After this is done, water vapor mixing ratio can be calculated directly.
At the same time, this code serves as a “best-estimate” temperature/relative humidity/pressure product for the surface and the 25- and 60-meter levels. The primary input at the surface is the SMOS, while the primary inputs at the 25- and 60-m levels come from the sensors in the southeast (SE) elevator. The alternate observations come from the THWAPS and the west (W) elevator for the surface and 25- to 60-m levels, respectively. This primary and alternates can be switched manually with a command line switch at runtime.
For more details, see the Tower Water-Vapor Mixing Ratio Value-Added Product technical report.