A multi-instrument comparison of integrated water vapour measurements at a high latitude site

Abstract

Abstract. We compare measurements of integrated water vapour (IWV) over a subarctic site (Kiruna, Northern Sweden) from five different sensors and retrieval methods: Radiosondes, Global Positioning System (GPS), ground-based Fourier-transform infrared (FTIR) spectrometer, groundbased microwave radiometer, and satellite-based microwave radiometer (AMSU-B). Additionally, we compare also to ERA-Interim model reanalysis data. GPS-based IWV data have the highest temporal coverage and resolution and are chosen as reference data set. All datasets agree reasonably well, but the ground-based microwave instrument only if the data are cloud-filtered. We also address two issues that are general for such intercomparison studies, the impact of different lower altitude limits for the IWV integration, and the impact of representativeness error. We develop methods for correcting for the former, and estimating the random error contribution of the latter. A literature survey reveals that reported systematic differences between different techniques are study-dependent and show no overall consistent pattern. Further improving the absolute accuracy of IWV measurements and providing climate-quality time series therefore remain challenging problems.

A multi-site intercomparison of integrated water vapour observations for climate change analysis

“…Each of these techniques has their strengths and weaknesses, and need to be inter-compared carefully under different conditions if progress is to be made on understanding the water vapour distribution and its time variability. Buehler et al (2012) concluded that a literature survey reveals that reported systematic differences between different techniques are study-dependent and show no overall consistent pattern. Further improving the absolute accuracy of IWV measurements and providing climate-quality time series therefore remain challenging problems.…”

“…Although an IWV multiinstrument intercomparison has been the subject of at least a dozen of papers in the literature (see Buehler et al, 2012, for a number of these, and Hocke and Martine, 2013, for tables of intercomparison studies), the added value of our analysis lies in focusing on homogeneous data sets or data sets taking part in a homogenisation procedure. Moreover, several studies in the literature concentrate on a multi-sensor intercomparison at a single site (e.g.…”

“…Moreover, several studies in the literature concentrate on a multi-sensor intercomparison at a single site (e.g. Pałm et al, 2010;Schneider et al, 2010;Buehler et al, 2012). In this paper, we will first focus on the mid-latitude site Brussels (Belgium), as a case study (see Sect.…”

“…We nevertheless incorporate these RS measurements in our intercomparison, as they have been widely used in such analyses in the literature so far (see e.g. Buehler et al, 2012) and they provide the longest record (up to 60 years) of upper-air temperatures and humidity. Moreover, they have near-global coverage and high vertical resolution, are launched under all weather conditions and these daily daytime and nighttime observations take place at a large number of sites (see also Table 1).…”

“…For RS stations located above IGS sites, RS surface data are extrapolated hydrostatically with an assumed temperature lapse rate of −6.5 K km −1 , while q is extrapolated by assuming that the dewpoint temperature depression (temperature minus dewpoint temperature) is constant with height, as explained by Deblonde et al (2005). An alternative approach was proposed by Buehler et al (2012) for the Arctic station Esrange, who found a relative bias IWV / IWV of −3.5 % per 100 m altitude difference. However, the actual scaling factors seem to depend on location…”