Validation of ACE-FTS satellite data in the upper troposphere/lower stratosphere (UTLS) using non-coincident measurements

Abstract

Abstract. CO, O 3 , and H 2 O data in the upper troposphere/lower stratosphere (UTLS) measured by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) on Canada's SCISAT-1 satellite are validated using aircraft and ozonesonde measurements. In the UTLS, validation of chemical trace gas measurements is a challenging task due to small-scale variability in the tracer fields, strong gradients of the tracers across the tropopause, and scarcity of measurements suitable for validation purposes. Validation based on coincidences therefore suffers from geophysical noise. Two alternative methods for the validation of satellite data are introduced, which avoid the usual need for coincident measurements: tracer-tracer correlations, and vertical tracer profiles relative to tropopause height. Both are increasingly being used for model validation as they strongly suppress geophysical variability and thereby provide an "instantaneous climatology". This allows comparison of measurements between non-coincident data sets which yields information about the precision and a statistically meaningful error-assessment of the ACE-FTS satellite data in the UTLS. By defining a trade-off factor, we show that the measurement errors can be reduced by including more measurements obtained over a wider longitude range into the comparison, despite the increased geophysical variability. Applying the methods then yields the following upper bounds to the relative differences in the mean found beCorrespondence to: M. I. Hegglin (michaela@atmosp.physics.utoronto.ca) tween the ACE-FTS and SPURT aircraft measurements in the upper troposphere (UT) and lower stratosphere (LS), respectively: for CO ±9% and ±12%, for H 2 O ±30% and ±18%, and for O 3 ±25% and ±19%. The relative differences for O 3 can be narrowed down by using a larger dataset obtained from ozonesondes, yielding a high bias in the ACE-FTS measurements of 18% in the UT and relative differences of ±8% for measurements in the LS. When taking into account the smearing effect of the vertically limited spacing between measurements of the ACE-FTS instrument, the relative differences decrease by 5-15% around the tropopause, suggesting a vertical resolution of the ACE-FTS in the UTLS of around 1 km. The ACE-FTS hence offers unprecedented precision and vertical resolution for a satellite instrument, which will allow a new global perspective on UTLS tracer distributions.

Assessment of upper tropospheric and stratospheric water vapor and ozone in reanalyses as part of S-RIP

“…All reanalyses contain high biases relative to the SDI MIM at pressures greater than 100 hPa (see also Jiang et al, 2015), although this may in part be explained by the increase in measurement uncertainty of satellite limb sounders with decreasing altitude in the upper troposphere . Several studies have shown that Aura MLS contains a dry bias in the upper troposphere/lower stratosphere around 200 hPa (e.g., Davis et al, 2016;Vömel et al, 2007), and similarly a dry bias has been found in the upper troposphere for ACE-FTS (Hegglin et al, 2008). The comparisons in Fig.…”

Jet characterization in the upper troposphere/lower stratosphere (UTLS): applications to climatology and transport studies

“…However, atmospheric features may also be captured by only one of the two instruments not only because of different sampling in the horizontal, but also because of vertical sampling and resolution differences. ACE-FTS has been shown to often achieve an effective vertical resolution near 1 km in the UTLS (Hegglin et al, 2008); while the ability of MLS to resolve smallvertical-scale features has yet to be explored in detail, Fig. 9 and the accompanying discussion suggest that it does capture variability on scales that might not be obvious given the nominal resolution.…”

Sensitivity analysis of the potential impact of discrepancies in stratosphere–troposphere exchange on inferred sources and sinks of CO<sub>2</sub>

“…We use version 2.2 of the ACE-FTS ozone data. Hegglin et al (2008) found that the version 2.2 ACE-FTS profiles have an effective vertical resolution of 1 km in the UTLS. Validation of the ACE-FTS O 3 data suggested that the relative mean difference between ACE-FTS O 3 data and independent measurements is less than 8 % between 16-44 km.…”

“…Validation of the ACE-FTS O 3 data suggested that the relative mean difference between ACE-FTS O 3 data and independent measurements is less than 8 % between 16-44 km. Hegglin et al (2008) evaluated the data in the UTLS and reported mean differences relative to aircraft and ozonesonde data of about 8 % in the lower stratosphere and a high bias of 18 % in the upper troposphere. We restrict our use of the ACE-FTS data to the lower stratosphere.…”