Water vapor retrieval from OMI visible spectra

Abstract

Abstract. There are distinct spectral features of water vapor in the wavelength range covered by the Ozone Monitoring Instrument (OMI) visible channel. Although these features are much weaker than those at longer wavelengths, they can be exploited to retrieve useful information about water vapor. They have an advantage in that their small optical depth leads to fairly simple interpretation as measurements of the total water vapor column density. We have used the Smithsonian Astrophysical Observatory (SAO) OMI operational retrieval algorithm to derive the slant column density (SCD) of water vapor using the 430-480 nm spectral region after extensive optimization. We convert from SCD to vertical column density (VCD) using the air mass factor (AMF), which is calculated using look-up tables of scattering weights and assimilated water vapor profiles. Our Level 2 product includes not only water vapor VCD but also the associated scattering weights and AMF. In the tropics, our standard water vapor product has a median SCD of 1.3 × 10 23 molecules cm −2 and a median relative uncertainty of about 11 %, about a factor of 2 better than that from a similar OMI algorithm that uses a narrower retrieval window. The corresponding median VCD is about 1.2 × 10 23 molecules cm −2 . We have examined the sensitivities of SCD and AMF to various parameters and compared our results with those from the GlobVapour product, the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Aerosol Robotic NETwork (AERONET).

Validation and update of OMI Total Column Water Vapor product

“…These features are several orders of magnitude weaker than those at longer wavelengths. However, they can still be exploited to retrieve TCWV (Wagner et al, 2013;Wang et al, 2014). Since water vapor is a weak absorber in the blue spectral range, the retrieval is free from the complication of nonlinearity due to saturation.…”

“…First, the slant column density (SCD) is derived from a spectral fitting algorithm that considers water vapor, O 3 , NO 2 , O 2 -O 2 , liquid water, C 2 H 2 O 2 , the Ring effect, the water Ring effect, third-order closure polynomials, wavelength shift, under-sampling, and common mode. The median SCD fitting uncertainty is about 11 % (Wang et al, 2014). Then, the vertical column density (VCD) is obtained by dividing the SCD with an air mass factor (AMF) that is based on a radiative transfer calculation.…”

“…Water vapor derived from the blue spectral range can greatly enhance the scientific return of satellites, especially for those with instruments that lack spectral coverage at longer wavelengths (e.g., OMI). Wang et al (2014) retrieved TCWV from OMI spectra using the 430-480 nm retrieval window. The retrieval method consists of two steps.…”

“…Satellite remote sensing of water vapor has led to products retrieved from the visible (e.g., GOME, Wagner et al, 2003;Lang et al, 2007;SCIAMACHY, Noël et al, 2005; GOME-2, Grossi et al, 2015; Ozone Monitoring Instrument (OMI), Wang et al, 2014), near-infrared (e.g., SCIAMACHY, Schrijver et al, 2009;MODIS, Diedrich et al, 2015;MERIS, Lindstrot et al, 2012), infrared (e.g., MODIS, Seemann et al, 2003;AIRS, Bedka et al, 2010;IASI, Pougatchev et al, 2009), microwave (e.g., SSM/I, Schlüssel and Emery, 1990;Wentz, 1997), and GPS radio signals (e.g., Wang et al, 2007;Kishore et al, 2011). These data sets offer the unique opportunity to study water vapor distribution on a global scale.…”

“…Visible measure-ments are available for both land and ocean but are limited to nearly cloud-free daytime conditions. Wang et al (2014) derived total column water vapor (TCWV, also known as the total precipitable water) using the spectra measured by OMI. The Level 2 data for 2005-2009 generated using the Wang et al (2014) algorithm (version 1.0) have been archived at the Aura Validation Data Center (AVDC, http://avdc.gsfc.nasa.gov/index.php?…”