2013-05-27: TES tropospheric O3 validated with sonde data: spatial patterns and temporal stability in the bias

Satellite measurements provide a relatively new perspective on global tropospheric ozone (O3) distributions and their changes in time. The Tropospheric Emission Spectrometer, TES, is operational since mid 2004 and is a suitable sensor to investigate multi-annual changes in tropospheric O3 from space.
Figure 1 shows the spatial distribution of 2005-2010 summer average tropospheric O3 as observed by TES.

In our study TES V004 nadir O3 profiles are validated with more than 4400 coinciding ozonesonde measurements taken across the world from the World Ozone and Ultraviolet Radiation Data Centre (WOUDC) during the period 2005-2010. An important quality indicator of the TES O3 data is whether any biases are constant in time or reflect some degree of instrument degradation.

Generally, TES V004 O3 retrievals are biased high by 2-7 ppbv (7-15%) in the troposphere. Focusing on the 464 hPa retrieval level, broadly representative for free tropospheric O3, we find differences in the TES biases for the Tropics (+3 ppbv, +7%), sub-tropics (+5 ppbv, +11%), and northern (+7 ppbv, +13%) and southern mid-latitudes (+4 ppbv, +10%) as shown in Figure 2.

The relatively long-term record (6 years) of TES-ozonesonde comparisons, allowed us to quantify temporal variations in TES biases at 464 hPa. We find that there are no discernable biases in each of these latitudinal bands; temporal variations in the bias are typically within the uncertainty of the difference between TES and ozone-sondes. Establishing these bias patterns is important in order to make meaningful use of TES O3 data in applications such as model evaluation, trend analysis, or data assimilation.

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Seasonal average (June-July-August) of tropospheric O3 concentrations at 464 hPa as observed by TES between 2005 and 2010 averaged on a 3° × 2° (longitude × latitude) grid. The effect of the variable MOZART a priori has been removed by reprocessing the TES retrievals with one global and annual mean (MOZART) a priori O3 profile following the method by Zhang et al. (2006). TES retrievals with residual clouds and errors have been excluded following the standard TES quality flags, leaving typically 50 data points in each grid cell.
Mean 2005-2010 TES bias in O3 at 464 hPa per latitude zone (black symbols) and their standard errors (including data pairs numbers and standard errors). Grey symbols indicate the seasonal mean biases and their standard errors. NMLT: Northern mid-latitudes; NStropics: Northern sub-tropics; SLMT: Southern lower and mid-latitudes.