STARTWAVE

STudies in Atmospheric Radiative Transfer
and Water VApour Effects

Validation of STARTWAVE water vapour measurements

It is difficult to measure Integrated Water Vapour. The only direct method is to calculate IWV from the observations made during a radiosounding. However, the humidity and temperature sensors on radiosondes are themselves affected by measurement errors. An alternative method is to use ground-based remote sensing. These methods rely on detecting the absorption of infrared or microwave radiation by the water vapour in the earth's atmosphere. It is important to compare measurements made by the different instruments in order to validate the measurements. The plot below compares all the available measurements made at Bern and Davos for the month of January 2003. In general, there is good agreement between the different methods. Over a period of several years, the mean differences in the IWV measured by the different instruments are within +/- 1mm.

GPS refers to the Global Positioning System receiver, SPM to the sun photometer, PFR to the precision filter radiometer (similar to the sun photometer) and Sonde to IWV from the Payerne radiosounding calculated from the height of the station. TROWARA and ASMUWARA refer to the TROWARA and ASMUWARA microwave radiometers.

This comparison is described in: J. Morland, B. Deuber, D. G. Feist, L . Martin, S. Nyeki, N. Kämpfer, C. Mätzler, P. Jeannet, L. Vuilleumier, The STARTWAVE atmospheric water database, ACPD Atmos. Chem. Phys. Discuss., 5, 10839 10879, 2005 www.atmos-chem-phys.org/acpd/5/10839/ SRef-ID: 1680-7375/acpd/2005-5-10839 Atmospheric Chemistry