Abstract
Abstract. HadISDH is a near-global land surface specific humidity monitoring product providing monthly means from 1973 onwards over large-scale grids. Presented herein to 2012, annual updates are anticipated. HadISDH is an update to the land component of HadCRUH, utilising the global high-resolution land surface station product HadISD as a basis. HadISD, in turn, uses an updated version of NOAA's Integrated Surface Database. Intensive automated quality control has been undertaken at the individual observation level, as part of HadISD processing. The data have been subsequently run through the pairwise homogenisation algorithm developed for NCDC's US Historical Climatology Network monthly temperature product. For the first time, uncertainty estimates are provided at the grid-box spatial scale and monthly timescale. HadISDH is in good agreement with existing land surface humidity products in periods of overlap, and with both land air and sea surface temperature estimates. Widespread moistening is shown over the 1973–2012 period. The largest moistening signals are over the tropics with drying over the subtropics, supporting other evidence of an intensified hydrological cycle over recent years. Moistening is detectable with high (95%) confidence over large-scale averages for the globe, Northern Hemisphere and tropics, with trends of 0.089 (0.080 to 0.098) g kg−1 per decade, 0.086 (0.075 to 0.097) g kg−1 per decade and 0.133 (0.119 to 0.148) g kg−1 per decade, respectively. These changes are outside the uncertainty range for the large-scale average which is dominated by the spatial coverage component; station and grid-box sampling uncertainty is essentially negligible on large scales. A very small moistening (0.013 (−0.005 to 0.031) g kg−1 per decade) is found in the Southern Hemisphere, but it is not significantly different from zero and uncertainty is large. When globally averaged, 1998 is the moistest year since monitoring began in 1973, closely followed by 2010, two strong El Niño years. The period in between is relatively flat, concurring with previous findings of decreasing relative humidity over land.
Highlights
Specific humidity at the surface is, on a physical basis, expected to increase commensurate with rising surface temperatures, where the presence of liquid water is not a limiting factor (Held and Soden, 2000)
We describe the creation of the land surface specific humidity component of an envisaged generation HadCRUH product: HadISDH (Met Office Hadley Centre (in collaboration with the National Oceanic and Atmospheric Administration’s (NOAA) National Climate Data Center (NCDC), the National Physical Laboratory and the Climatic Research Unit) Integrated Surface Database (ISD) humidity product). This builds upon the new hourly land surface dataset HadISD (Dunn et al, 2012), which is a quality-controlled database of global synoptic data since 1973
It is first necessary to assess the likely quality of HadISDH before we can use it with any confidence
Summary
Specific humidity at the surface is, on a physical basis, expected to increase commensurate with rising surface temperatures, where the presence of liquid water is not a limiting factor (Held and Soden, 2000). We describe the creation of the land surface specific humidity component of an envisaged generation HadCRUH product: HadISDH (Met Office Hadley Centre (in collaboration with the National Oceanic and Atmospheric Administration’s (NOAA) National Climate Data Center (NCDC), the National Physical Laboratory and the Climatic Research Unit) Integrated Surface Database (ISD) humidity product) This builds upon the new hourly land surface dataset HadISD (Dunn et al, 2012), which is a quality-controlled database of global synoptic data since 1973. We intend to include relative humidity and other related variables into HadISDH at a later date This will involve the development of measurement uncertainty estimates specific to each variable and ensuring consistency across all variables after application of homogenisation procedures. Given that both of these are novel ventures, it was felt that they could be dealt with more thoroughly in a separate paper
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