Abstract

Abstract. Of all satellite measurements of ozone, only two instruments have coincident, spatially overlapping measurements to allow direct comparison of tropospheric column ozone (TCO): the Ozone Monitoring Instrument (OMI) and the Tropospheric Emission Spectrometer (TES) on the NASA Aura spacecraft. For two years (2005–2006), we collect all observations between 60° S and 60° N from nadir (~65 000 from OMI and TES) and cross-track swaths (~30 000 000 from OMI) and compare with a chemistry-transport model (CTM) simulating each observation with corresponding spatial and temporal coincidence. High-frequency TCO variations are indicative of stratospheric intrusions of ozone-rich air, and the individual, level 2 data provide access to these short-lived phenomena. Although we can identify some seasonal and large-scale biases in the model, the CTM as a transfer standard identifies weaknesses in the observations and further helps quantify the measurement noise of individual profiles. The relatively noise-free CTM bridges these two satellite measurements and improves their cross-validation to better precision than a simple direct comparison. Previous validation studies of TES TCO versus ozonesondes found a bias of about +4 Dobson Units (DU) for large regions. The three-way comparison and the CTM transfer method that use a far greater number of coincidences, indicate that monthly zonal mean OMI-TES TCO biases fall within 5–10%, and thus quantifies the zonal mean OMI TCO bias at a few DU. For small regions (i.e., 5 × 5°), however, the monthly mean OMI-TES differences can exceed ±10 DU at many places (e.g., tropics for the direct OMI-TES comparison) due to different tropospheric sensitivities of the two instruments at these locations. Partly removing the influence of different sensitivities by applying the CTM as the transfer standard, the OMI-TES differences generally decrease, especially over the tropics. In addition, the CTM-TES comparison split into day versus night observations shows no apparent bias in TES at very low levels, ±1 DU. These OMI-TES-CTM comparisons highlight the importance of the a priori ozone profiles that went into each satellite retrieval, including a false agreement due to CTM-a priori similarity, and the importance of including the vertical information (i.e., averaging kernel) in the retrieval products. This study also highlights the advantages of overlapping measurements in terms of cross-validation and the application of a model as the transfer standard.

Highlights

  • Among the four Earth Observing System (EOS) Aura instruments – High Resolution Dynamics Limb Sounder (HIRDLS), Microwave Limb Sounder (MLS), Ozone Monitoring Instrument (OMI), and Tropospheric Emission Spectrometer (TES) – only OMI and TES have coincident ozone (O3) measurements through the troposphere, providing the information content and the opportunity to compare tropospheric column ozone (TCO) with these two datasets directly on a single, overlapping measurement basis as is presented here

  • Considering that the differences generally become smaller in the Northern Hemisphere (NH) middle latitudes and no systematic bias is found for the model in the Middle East, this large (∼10 Dobson Units (DU)) OMI-TES bias spot is likely real and it is masked by the noise in the direct comparison

  • In this paper we present a detailed analysis of the NASA Aura level 2 (L2) tropospheric column ozone (TCO) data from OMI and TES

Read more

Summary

Introduction

Among the four Earth Observing System (EOS) Aura instruments – High Resolution Dynamics Limb Sounder (HIRDLS), Microwave Limb Sounder (MLS), Ozone Monitoring Instrument (OMI), and Tropospheric Emission Spectrometer (TES) – only OMI and TES have coincident ozone (O3) measurements through the troposphere, providing the information content and the opportunity to compare tropospheric column ozone (TCO) with these two datasets directly on a single, overlapping measurement basis as is presented here. Level 2 (L2) orbit/swath data are used in this study: the OMI OMO3PR V003 data (de Haan and Veefkind, 2009) and the TES version 4 (V004, F05 07) nadir global survey product Both datasets report ozone profiles, they contain only about 1–2 degrees of freedom for the signal (DOFS) in the troposphere (Nassar et al, 2008; Zhang et al, 2010), and provide limited profile resolution but reasonable TCO information. Equation (10) is introduced by Zhang et al (2010) as the “CTM method” to compare two satellite observations, which shows advantages over the direct comparison method (Eq 6), as the modeled profile is generally closer to the true values than the a priori and adding less noise to the real OMI-TES difference bOMI − bTES. We note that Zhang et al (2010) adjusts both OMI and TES retrieval to a fixed a priori, which probably enhances the difference between the original a priori and the true state

Comparisons of monthly 2-D TCO PDFs for NH middle latitudes and tropics
Comparisons of monthly mean TCO latitude-by-longitude patterns
Findings
Discussion and conclusions
Full Text
Published version (Free)

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call