Field test of spectral line intensity parameters for tropospheric water vapor

Abstract

[1] We report the results of a field experiment designed to study atmospheric water vapor absorption in the visible and near-infrared spectral regions between 550 and 1000 nm. We carried out spectroscopic ground measurements of direct solar radiation under clear-sky conditions in Boulder, Colorado. The data with a spectral resolution of approximately 1 nm were analyzed using the differential optical absorption spectroscopy (DOAS) technique in five different absorption bands of water vapor. We show that this technique can reveal the broadband effects of errors and inconsistencies in absorption spectra information for the water molecule. Retrievals of column tropospheric water vapor from the field spectra were compared to simultaneous independent estimates from Global Positioning System (GPS) data and radiosonde soundings. The data set is used to critically assess the widely used High-Resolution Transmission Molecular Absorption Database (HITRAN) [Rothman et al., 1998]. The results indicate that line intensities in the 3n + d polyad centered at 820 nm are underestimated by 21% with respect to the strong 3n polyad centered at 940 nm, while the 4n polyad at 720 nm shows agreement within the measurement accuracy of 3%. Two weaker bands centered at 650 and 590 nm were found to be overestimated by about 8-10%. The effect of the proposed corrections on the absorption of incoming solar flux for a clear-sky atmosphere is estimated to be 0.6 W/m 2 for an overhead Sun.

Field measurements of water vapor continuum absorption in the visible and near‐infrared

“…We carried out a field experiment in Boulder, Colorado, over a 1 month period in summer 2001 with the goal of studying various aspects of atmospheric absorption by tropospheric water vapor in the Vis and NIR spectral regions. The experiment has been briefly described in a paper investigating the accuracy of spectral line parameters from molecular databases [ Sierk et al , 2003]. Here we will describe how the data set from the same field experiment was used to study water vapor continuum absorption.…”

“…For the DOAS analysis we form the ratio of the foreground and background spectrum T d measured by our instruments, as in good approximation this eliminates I 0 (λ) in . (The error introduced by the assumption, that the application of Beer‐Lambert's law and the convolution with the instrument function are interchangeable, the so called I 0 effect, was shown to be negligible for our experiment [ Sierk et al , 2003]. ) In the RTM we simulate the measurement T d by computing the two spectra for the corresponding conditions at high spectral resolution (0.01 cm −1 ) and forming the ratio …”

“…Each GPS site was additionally equipped with meteorological ground sensors for pressure and temperature, which enabled the separation of the wet and the dry component of the refraction delay. Further details on the processing of the GPS data are given by Sierk et al [2003]. The GPS technique has been proven to provide estimates of zenith column water with an absolute accuracy of 1 kg/m 2 [e.g., Kruse et al , 1999].…”

First retrieval of global water vapour column amounts from SCIAMACHY measurements

“…The radiative transfer code SCIATRAN (Buchwitz et al, 2000b;Rozanov et al, 2002) has been used instead of its predecessor GOMETRAN (Rozanov et al, 1997), including the latest update of the HI-TRAN data base (Rothman et al, 2003;Coheur et al, 2003). Recent field tests have shown that especially the 4ν water vapour band around 720 nm used in the present study is well described by HITRAN (Sierk et al, 2003). Furthermore, the grid of solar zenith angles for radiative transfer calculations has been extended to 88 • .…”

Assessment of the consistency of near‐infrared water vapor line intensities using high‐spectral‐resolution ground‐based Fourier transform measurements of solar radiation

“…In addition, the data that make up the spectral line databases [e.g., Rothman et al , 2003] are also derived in a piecemeal way, with different bands measured by different groups at different times. Sierk et al [2003] have quantified inconsistencies in the HITRAN 2001 [ Rothman et al , 2003] database between five water vapor bands around the visible (0.59 to 0.94 μm) from surface based observations; earlier versions of the database been assessed using surface based observations in the same spectral region by Smith and Newnham [2001], Veihelmann et al [2002] and Albert et al [2004]. In our paper, we use high‐spectral‐resolution measurements of near‐infrared radiation reaching the surface over a large spectral range to provide a consistent assessment of water vapor absorption across the whole near‐infrared region.…”