Evaluation of tropospheric ozone columns derived from assimilated GOME ozone profile observations

A T J De Laat,R J Van Der A,M Van Weele

doi:10.5194/acp-9-8105-2009

Abstract

Abstract. Tropospheric O3 column estimates are produced and evaluated from spaceborne O3 observations by the subtraction of assimilated O3 profile observations from total column observations, the so-called Tropospheric O3 ReAnalysis or TORA method. Here we apply the TORA method to six years (1996–2001) of ERS-2 GOME/TOMS total O3 and ERS-2 GOME O3 profile observations using the TM5 global chemistry-transport model with a linearized O3 photochemistry parameterization scheme. Free running TM5 simulations show good agreement with O3 sonde observations in the upper-tropospheric and lower stratospheric region (UTLS), both for short day-to-day variability as well as for monthly means. The assimilation of GOME O3 profile observations counteracts the mid-latitude stratospheric O3 drift caused by the overstrong stratospheric meridional circulation in TM5. Assimilation of GOME O3 profile observations also improves the bias and correlations in the tropical UTLS region but slightly degrades the model-to-sonde correlations and bias of extra-tropical UTLS. We suggest that this degradation is related to the large ground pixel size of the GOME O3 measurements (960×100 km) in combination with retrieval and calibration errors. The added value of the assimilation of GOME O3 profiles compared to stand-alone model simulations lays in the long term variations of stratospheric O3, not in short term synoptic variations. The evaluation of daily and monthly tropospheric O3 columns obtained from total column observations and using the TORA methodology shows that the use of GOME UV-VIS nadir O3 profiles in combination with the spatial resolution of the model does not result in satisfactory residual tropospheric ozone columns.

Highlights

Global monitoring of tropospheric ozone (O3) is a challenge because tropospheric O3 shows large spatio-temporal variability and the number and representativity of observational sites is limited for a complete assessment of long-term global tropospheric O3 trends and tropospheric O3 distribution changes (IPCC, 2007)
A baseline needs to be defined: how well does a free TM5 run – i.e. without assimilation – compare with sonde observations? Results for upper-tropospheric and lower stratospheric region (UTLS) O3 are discussed in Sect. 4.1.1 and for the total tropospheric O3 column (TTOC) in 4.1.2
This paper presents an evaluation of level-2 residual tropospheric O3 columns derived from subtracting free-run and assimilated model stratospheric O3 columns from total column observations, the so-called TORA method

Summary

Introduction

Global monitoring of tropospheric ozone (O3) is a challenge because tropospheric O3 shows large spatio-temporal variability and the number and representativity of observational sites is limited for a complete assessment of long-term global tropospheric O3 trends and tropospheric O3 distribution changes (IPCC, 2007). Observations of tropospheric O3 from satellites have been reported within the tropics and subtropics – defined as the region from 30◦ S to 30◦ N – for indirect or residual methods. Indirect methods estimate the stratospheric O3 column which is subtracted from the total O3 column. Their success is to a large extent related to the relatively small tropical stratospheric O3 variability in comparison to tropospheric O3 variability (NRC, 2008). Outside of the tropics the large and rapid tropospheric O3 variability complicates determining tropospheric O3 as it requires individual observations to be of sufficient accuracy (de Laat et al, 2005).

Objectives

Results

Conclusion