Abstract:
This CM SAF climate data record provides daily estimates of Upper Tropospheric Humidity (UTH) derived from passive microwave sounders.
UTH contributes significantly to the atmospheric greenhouse effect by having a strong influence on the outgoing longwave radiation, despite the smaller concentration by mass in comparison to lower troposphere. Long-term observations of UTH can therefore be used to monitor changes in upper tropospheric water vapour loading and subsequently to infer changes in the radiative effect of water vapour in the atmosphere.
The CM SAF UTH v1.0 data set is based on observations from the Advanced Microwave Humidity Sounding Unit B (AMSU-B, onboard NOAA-15, NOAA-16, NOAA-17) and the Microwave Humidity Sounder (MHS, onboard NOAA-18, MetOp-A and MetOp-B), and covers the period 1 January 1999 to 31 December 2015. The UTH product is a mere transformation of brightness temperature, measured by the 183.31±1.00 GHz channel to a more intuitive humidity unit. The relative humidity Jacobian, which is used to define the UTH, moves vertically depending on the water vapour load in the atmosphere. It is generally assumed that the Jacobian covers a broad atmospheric layer between 500 and 200 hPa. However, in a drier atmosphere, the Jacobian will have significant contributions from altitudes below the 500 hPa level and in a wetter atmosphere there can be contributions from altitudes above the 200 hPa level.
The UTH retrieval is generally not valid outside ±60° latitudes because of the very low water vapour loading here in the upper troposphere. However, there will be some valid UTH retrievals in these regions, thus a global data record is provided to enable maximum exploitation of the data.
Note that observations affected by deep convective or precipitating clouds, together with those sensing the surface due to dry atmosphere or high orography, have been removed from the data set. The CM SAF UTH product has been evaluated against an equivalent UTH derived from ERA-Interim renanalysis. The validation activity suggests that the data record fulfils the GCOS requirements of 5% measurement accuracy and 0.3% decadal stability within ±60° latitude. This is a Thematic Climate Data Record (TCDR).
