Global IWV trends and variability in atmospheric reanalyses and GPS observations

Abstract

Abstract. This study investigates the means, variability, and trends in integrated water vapour (IWV) from two modern reanalyses (ERA-Interim and MERRA-2) from 1980 to 2016 and ground-based GPS data from 1995 to 2010. It is found that the mean distributions and inter-annual variability in IWV in the reanalyses and GPS are consistent, even in regions of strong gradients. ERA-Interim is shown to exhibit a slight moist bias in the extra-tropics and a slight dry bias in the tropics (both on the order of 0.5 to 1 kg m−2) compared to GPS. ERA-Interim is also generally drier than MERRA-2 over the ocean and within the tropics. Differences in variability and trends are pointed out at a few GPS sites. These differences can be due to representativeness errors (for sites located in coastal regions and regions of complex topography), gaps and inhomogeneities in the GPS series (due to equipment changes), or potential inhomogeneities in the reanalyses (due to changes in the observing system). Trends in IWV and surface temperature in ERA-Interim and MERRA-2 are shown to be consistent, with positive IWV trends generally correlated with surface warming, but MERRA-2 presents a more general global moistening trend compared to ERA-Interim. Inconsistent trends are found between the two reanalyses over Antarctica and most of the Southern Hemisphere, and over central and northern Africa. The uncertainty in current reanalyses remains quite high in these regions, where few in situ observations are available, and the spread between models is generally important. Inter-annual and decadal variations in IWV are also shown to be strongly linked with variations in the atmospheric circulation, especially in arid regions, such as northern Africa and Western Australia, which add uncertainty in the trend estimates, especially over the shorter period. In these regions, the Clausius–Clapeyron scaling ratio is found not to be a good humidity proxy for inter-annual variability and decadal trends.

Consistency and representativeness of integrated water vapour from ground-based GPS observations and ERA-Interim reanalysis

“…due to inhomogeneity and/or changes in the quality in either of the datasets). The changes over time are small in magnitude (Parracho et al 2018) and have negligible impact on the average statistics computed here. After establishing the contribution of representativeness errors, we address the following specific questions: 1) by which means is it possible to mitigate the representativeness errors?…”

“…Vertical correction is especially important for variables such as IWV because the water vapour mixing ratio is the largest in the atmospheric boundary layer. Variation of biases/differences between GPS and models is also observed as a function of latitude and season (Roman et al, 2012;Ning et al, 2013;Parracho et al, 2018). Absolute differences have a tendency to be larger in moister and warmer regions/periods while relative differences tend to be larger in colder and drier regions/period, globally.…”

“…The Zenith Tropospheric Delay (ZTD) estimates, which are available with a time sampling of 5 minutes, are first screened for outliers as described in Parracho et al, 2018, and averaged in hourly bins centred on the round hours (00 UTC, 01 UTC…). Next, the hourly ZTDs are converted to IWV using 6-hourly surface pressure, Ps, and weighted mean temperature, Tm, computed from ERA-Interim pressure level data (see Appendix B in Parracho et al, 2018). No temporal interpolation is applied here so that only the 1-hourly ZTD estimates matching the times of the reanalysis (00 UTC, 06 UTC…) are converted.…”

“…Indeed, model values are representative of spatial averages. Large biases are thus often observed in coastal and mountainous regions (Hagemann et al, 2003;Bock et al, 2005;Parracho et al, 2018). In coastal areas, model grid cells can contain a fraction of IWV over sea not consistent with the GPS observations over land.…”

Diurnal cycle of short-term fluctuations of integrated water vapour above Switzerland

“…Our study is focused on the IWV observations of the TROpospheric WAter RAdiometer (TROWARA). TROWARA is a dual-channel microwave radiometer, and its design and construction were described by Peter and Kämpfer (1992) and Morland (2002). Two ferrite circulator switches at each frequency channel of the radiometer perform the change from the antenna to the noise diodes.…”