Column and surface measurements of H\(_2\)O and HDO at Darwin, Australia
The added value of stable isotopes to interpret atmospheric hydrology has been of recent interest following the introduction remote sensing products, in-situ measurements and increasingly sophisticated models. In-situ measurements provide point measurements with good temporal coverage and resolution, but due to small scale of some of the processes influencing stable isotopes, it can be difficult to evaluate regional and global climate models. Satellite based remote sensing products have the potential to provide datasets with appropriate spatial scales but with poor temporal resolution, and although there have been recent attempts to assess the accuracy of these techniques only a couple use well calibrated in-situ measurements. To date only a couple of assessments of dataset accuracy have been provided using well calibrated in-situ measurements from aircraft platforms (Herman 2014, Schneider 2015). Beyond these studies assessment dataset accuracy has focused on using ground based remote sensing Fourier Transform Spectrometers (FTS), however, these have been shown to suffer from large measurement biases (Schneider 2015). There is a need to both provide validation studies of these ground based FTS measurements and to evaluate a broader range of products used to study water isotopes in the hydrological cycle.
The ground based remote sensing of water isotopes has been performed through the Total Column Carbon Observing Network (TCCON; Wunch et al., 2011) and the MUlti-platform remote Sensing of Isotopologues for investigating the Cycle of Atmospheric water (MUSICA) project using the Network for the Detection Atmospheric composition Change (NDACC) Fourier Transform Spectrometers (Schneider 2012). Schneider et al. (2015) presented an evaluation of the accuracy of the MUSICA sites was provided through comparison against in-situ measurements of water vapor isotopes from an aircraft platform. They showed large positive biases for FTS retrievals. No similar comparison with in-situ measurements has been conducted for the TCCON network, although the TCCON stations have been used to determine the accuracy of a number of satellite products (Frankenberg 2013, Scheepmaker 2015, Boesch 2013).
Large positive biases consistent for all comparisons with in-situ observations (Worden 2011, Herman 2014, Schneider 2015). Comparison between satellite SWVI products and the TCCON network have shown the satellite illustrate a negative bias (Risi 2012, Boesch 2012), whether this is caused by inaccuracy in satellite or TCCON retrievals remains uncertain. Risi et al. (2012) showed that whilst the TCCON column retrievals were enriched relative to \(\delta\)\(^2\)H from the SCIAMACHY instrument, the latitudinal gradient was preserved. Boesch et al. (2012) compared the TCCON isotope retrievals with the GOSAT satellite and showed the bias between the 2 datasets varied with season and location. They showed the GOSAT retrievals were very sensitive to surface characteristics and the shape of the H\(_2\)O and HDO a priori column. Interestingly the largest biases were observed in Darwin, Australia where complex water vapor and isotope profiles would be expected due to the strong vertical mixing in this tropical region. To better assess the errors associated with satellite based retrievals determined from these studies, the TCCON measurements would greatly benefit from comparisons against accurate in-situ observations.