A multi-instrument comparison of integrated water vapour measurements at a high latitude site
We compare measurements of integrated water vapour (IWV) over a subarctic\nsite (Kiruna, Northern Sweden) from five different sensors and retrieval\nmethods: Radiosondes, Global Positioning System (GPS), ground-based\nFourier-transform infrared (FTIR) spectrometer, ground-based microwave\nradiometer, and satellite-based microwave radiometer (AMSU-B). Additionally,\nwe compare also to ERA-Interim model reanalysis data. GPS-based IWV\ndata have the highest temporal coverage and resolution and are chosen\nas reference data set. All datasets agree reasonably well, but the\nground-based microwave instrument only if the data are cloud-filtered.\nWe also address two issues that are general for such intercomparison\nstudies, the impact of different lower altitude limits for the IWV\nintegration, and the impact of representativeness error. We develop\nmethods for correcting for the former, and estimating the random\nerror contribution of the latter. A literature survey reveals that\nreported systematic differences between different techniques are\nstudy-dependent and show no overall consistent pattern. Further improving\nthe absolute accuracy of IWV measurements and providing climate-quality\ntime series therefore remain challenging problems.