G2.23   Possible SZA dependence in the FTIR CH4 retrievals during polar vortex overpasses

Gap detailed description

Possible SZA (solar zenith angle) dependence in the retrieval during of CH4 measured by polar vortex overpasses may influence CH4 retrievals. During polar vortex overpasses, stratospheric profiles of CH4 are expected to differ from those measured outside the polar vortex.  This may influence some measurements at high latitudes in winter. Applying more accurate winter time a priori profiles would reduce residuals in the retrieval. Currently there is a lack of accurate CH4 profile measurements under wintertime conditions.

Activities within GAIA-CLIM related to this gap             

We propose to investigate the effect of profile shape changes on CH4 retrievals measured with FTIR instruments. New measurement techniques have become available recently, such as AirCore (Karion et al., 2010; Paul et al., 2016) which provide the potential for analysis of samples collected in-situ. For example, in Sodankylä it is possible to sample stratospheric air using AirCore at the site of TCCON FTIR measurements (although there will be some spatial mismatch owing to balloon drift). Thus the effect of vortex variability can be estimated based on both FTIR and AirCore measurements.

Gap remedy(s)

Remedy #1

Specific remedy proposed

Use AirCore measurements to understand SZA dependence effects.  Currently there is limited availability of these AirCore data. However, new measurements will become available during the project. For example, Lindenberg has initiated a series of AirCore launches. There are also ongoing activities to increase the usability and decrease the costs of AirCore measurements, thus allowing more sites to participate in future.

Measurable outcome of success

The measurements will contribute to the analysis, development and verification of the next version of the FTIR retrieval.

Achievable outcomes

Technological / organizational viability: medium / low.  The development is ongoing. The transfer to network management provides some organizational challenges.

Indicative cost estimate: medium (>1million)/ low (<1 million).


The remedy will contribute to improve the network wide retrieval method. This will also help to address G2.24, by providing more information on accurate profiles in both, the stratosphere and troposphere.  


2 years.

Gap risks to non-resolution

Identified future risk / impact

Probability of occurrence if gap not remedied

Downstream impacts on ability to deliver high quality services to science / industry / society

Quality of some FTIR products varying throughout the network.

Medium /low

Lack of network-harmonised products will lead to ambiguity in interpretation and to a reduced value in applications including satellite characterisation.


Work package: