Swiss H2O Hub: Water vapor measurements from ground to the thermosphere

High-quality water vapor measurements from the ground to space are essential for assessing Earth's radiative balance. Water vapor contributes substantially to the uncertainty of the outgoing longwave radiation, emphasizing the need for accurate and reliable measurements. The current generation of commercially available operational radiosondes exhibits rapidly degrading reliability above the tropopause, leaving a critical observational gap. The Cryogenic Frost Point Hygrometer (CFH), a widely used instrument for water vapor profile measurements relies on the R23 coolant, which was banned under the Montreal Protocol.

The Swiss H2O Hub addresses this challenge using two balloon-borne instruments and a newly developed cryogenic water vapor radiometer. These observations are complemented by the MeteoSwiss RALMO Lidar system. ALBATROSS is a compact mid-IR laser spectrometer that measures the water vapor absorption spectrum in an open-path  multipass cell, providing SI-traceable water vapor data. The second balloon-borne instrument is the Peltier Cooled Frost Point Hygrometer (PCFH) that replaces the R23 coolant with a Peltier cooler and measures the frost point temperature via a thermocouple calibrated at METAS (SI-traceable). Stratospheric and mesospheric water vapor is measured with two passive radiometers: the already established MIAWARA instrument at the Zimmerwald Observatory near Bern and a newly developed cryogenic instrument operated at the laboratory building.

  

In total, we conducted 29 balloon soundings with various payload configurations and performed detailed instrument intercomparisons for the troposphere and the lower stratosphere. Both balloon instruments achieve breakthrough requirements for the stratosphere and reach threshold requirements for the troposphere.

These combined observations enabled the detection of increased water vapor volume-mixing ratios in the lower stratosphere and mesosphere above Switzerland. Both the passive radiometers and the balloon soundings revealed enhanced water vapor levels in the years following the eruption.