Retrieval of daytime total columnar water vapour from MODIS measurements over land surfaces

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Abstract

A retrieval of total column water vapour (TCWV) from MODIS (Moderate-resolution Imaging Spectroradiometer) measurements is presented. The algorithm is adapted from a retrieval for MERIS (Medium Resolution Imaging Spectrometer) from Lindstrot et al. (2012). It obtains the TCWV for cloud-free scenes above land at a spatial resolution of 1 km × 1 km and provides uncertainties on a pixel-by-pixel basis. The algorithm has been extended by introducing empirical correction coefficients for the transmittance calculation within the forward operator. With that, a wet bias of the MODIS algorithm against ARM microwave radiometer data has been eliminated. The validation against other ground-based measurements (GNSS water vapour stations, GUAN radiosondes, and AERONET sun photometers) on a global scale reveals a bias between -0.8 and -1.6 mm and root mean square deviations between 0.9 and 2 mm. This is an improvement in comparison to the operational TCWV Level 2 product (bias between -1.9 and -3.2 mm and root mean square deviations between 1.9 and 3.4 mm). The comparison to MERIS TCWV for an example overpass exposes a systematic dry bias.

Figures

  • Figure 1. Normalized frequencies of occurrence for the comparison of the MODIS L2 TCWV product (MOD05) to ground-based microwave radiometers (3 sites: ARM SGP, TWP, NSA; left upper panel) and global ground-based GNSS data (right upper panel), both for the period 2003–2011, and AERONOT sun photometer measurements (lower panel) for the period 2003–2014. Figure 9 shows the geographical distribution of the validation data. See text for detailed information.
  • Table 1. Position and widths of five NIR bands (original MODIS design specification) used for the TCWV retrieval (Xiong and Barnes, 2006).
  • Figure 2. Simulated total atmospheric transmittance due to water vapour in the NIR (absorption coefficients from HITRAN database Rothman et al., 2010, black curve) and actual relative response function of the five used MODIS Aqua bands (blue curves).
  • Table 2. Regression coefficients between for the determination of the first guess from the transmittance of band 17 (Eq. 7).
  • Figure 3. Normalized frequencies of occurrence for comparisons of TCWV retrieved from the MODIS retrieval using just one absorption band to MWR TCWV data in mm; see text for detailed description.
  • Figure 4. Simulated versus measured atmospheric transmittances due to water vapour for each MODIS band using the same data as in Fig. 3; see Sect. 4 for detailed description of the compared variables.
  • Figure 5. Assessment of the radiance–central wavelength dependency for MODIS Aqua: bias (upper row) and RMSD (lower row) between simulated and measured radiances as a function of the assumed central wavelength for each absorption band. Each curve represents 1 of the 10 detectors. The vertical dashed lines indicate the position of the original nominal wavelength of the band. The vertical dotted lines indicate the position of zero bias and minimum RMSD. See text for further discussion (Sect. 4).
  • Figure 6. Assessment of the radiance–central wavelength dependency for MODIS Terra: bias (upper row) and RMSD (lower row) between simulated and measured radiances as a function of the assumed central wavelength for each absorption band. Each curve represents 1 of the 10 detectors. The vertical dashed lines indicate the position of the original nominal wavelength of the band. The vertical dotted lines indicate the position of zero bias and minimum RMSD. See text for further discussion (Sect. 4).

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APA

Diedrich, H., Preusker, R., Lindstrot, R., & Fischer, J. (2015). Retrieval of daytime total columnar water vapour from MODIS measurements over land surfaces. Atmospheric Measurement Techniques, 8(2), 823–836. https://doi.org/10.5194/amt-8-823-2015

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