Directionally dependent Lambertian-equivalent reflectivity (DLER) of the Earth's surface measured by the GOME-2 satellite instruments

Lieuwe G Tilstra,Piet Stammes,Ping Wang,Olaf N E Tuinder

doi:10.5194/amt-14-4219-2021

Abstract

Abstract. In this paper we introduce the new concept of directionally dependent Lambertian-equivalent reflectivity (DLER) of the Earth's surface retrieved from satellite observations. This surface DLER describes Lambertian (isotropic) surface reflection which is extended with a dependence on the satellite viewing geometry. We apply this concept to data of the GOME-2 satellite instruments to create a global database of the reflectivity of the Earth's surface, providing surface DLER for 26 wavelength bands between 328 and 772 nm as a function of the satellite viewing angle via a second-degree polynomial parameterisation. The resolution of the database grid is 0.25∘ by 0.25∘, but the real, intrinsic spatial resolution varies over the grid from 1.0∘ by 1.0∘ to 0.5∘ by 0.5∘ down to 0.25∘ by 0.25∘ by applying dynamic gridding techniques. The database is based on more than 10 years (2007–2018) of GOME-2 data from the MetOp-A and MetOp-B satellites. The relation between DLER and bi-directional reflectance distribution function (BRDF) surface reflectance is studied using radiative transfer simulations. For the shorter wavelengths (λ<500 nm), there are significant differences between the two. For instance, at 463 nm the difference can go up to 6 % at 30∘ solar zenith angle. The study also shows that, although DLER and BRDF surface reflectances have different properties, they are comparable for the longer wavelengths (λ>500 nm). Based on this outcome, the GOME-2 surface DLER is compared with MODIS surface BRDF data from MODIS band 1 (centred around 645 nm) using both case studies and global comparisons. The conclusion of this validation is that the GOME-2 DLER compares well to MODIS BRDF data and that it does so much better than the non-directional LER database. The DLER approach for describing surface reflectivity is therefore an important improvement over the standard isotropic (non-directional) LER approaches used in the past. The GOME-2 surface DLER database can be used for the retrieval of atmospheric properties from GOME-2 and from previous satellite instruments like GOME and SCIAMACHY. It will also be used to support retrievals from the future Sentinel-5 UVNS (ultraviolet, visible, near-infrared, and short-wave infrared) satellite instrument.

Highlights

Most satellite retrievals of atmospheric composition require accurate information about the reflectivity of the Earth’s surface to achieve accurate retrieval results
The parameter presented in the right column is the surface anisotropy parameter, defined here as the difference between the Global Ozone Monitoring Experiment (GOME)-2 surface dependent Lambertian-equivalent reflectivity (DLER) at viewing angles θv of +45◦ and −45◦
The choice for MODIS band 1 is based on the fact that (i) it is close enough to one of the DLER wavelength bands, and (ii) based on the results presented in Sect. 3 we may expect only small differences between DLER and bi-directional reflectance distribution function (BRDF) for wavelengths longer than 600 nm

Summary

Introduction

Most satellite retrievals of atmospheric composition require accurate information about the reflectivity of the Earth’s surface to achieve accurate retrieval results. This includes the retrieval of trace gases, such as ozone (O3), nitrogen dioxide (NO2), bromine oxide (BrO), formaldehyde (CH2O), water vapour (H2O), carbon dioxide (CO2), carbon monoxide (CO), and methane (CH4), and of cloud and aerosol information. Many of these retrievals use Lambertian surface reflection in the radiative transfer calculations and, adopt the use of Lambertian (isotropic) surface albedo climatologies. Relying on Lambertian reflection is common practice, using a bi-directional reflectance distribution function (BRDF) (Nicodemus et al, 1992; SchaepmanStrub et al, 2006) to describe the surface reflectivity would. According to a recent study by Lorente et al (2018), the simplification of using Lambertian surface reflection can lead to errors of a factor of 2 in the surface reflection for vegetated surfaces

Results

Discussion

Conclusion