The Aura Microwave Limb Sounder (MLS) team has announced the release of a new version 6 (v6) dataset. The most significant changes between v6 and the previous v5 data record are in the nitrous oxide (N2O) and temperature products. Although changes for other products are minimal, the MLS team recommends employing v6 in all new scientific studies using MLS data.
The v6 N2O product shows significantly larger abundances (as much as 20%) in the lower stratosphere (100-10 hectopascal(hPa)) than v5, especially in the tropics, improving agreement with expectations and other observations. The v6 MLS temperature product has significantly better precision (up to 20% improvement) and vertical resolution (up to 30% improvement) in the upper stratosphere and lower mesosphere (10-0.1 hPa) compared to v5 and prior versions.
The entire MLS data record, since the July 2004 launch of the Aura satellite to date, has been reprocessed using the v6 data processing software. The v6 software is also being used for forward-processing of incoming observations. To avoid disrupting scientific studies already well underway, forward-processing of incoming data using the previous v5 software will continue for the foreseeable future. In keeping with NASA ESDIS policy, with the release of MLS v6, the MLS v4 data record will no longer be available from the Goddard Earth Sciences Data and Information Services Center (GES DISC).
A full assessment of MLS v6 data quality, along with guidelines for appropriate use of the data, is provided in the data quality document. Importantly, this document describes updates to data screening rules for multiple products in v6.
Of particular note is the limited remaining life of the MLS 190-GHz subsystem, which measures H2O, N2O, HCN, and upper stratospheric HNO3. Since May 2024, the 190-GHz measurements are only made for a few days each calendar month. The spectral regions MLS uses to measure several other species (notably CH3Cl, CH3CN, ClO, HCl, and cloud ice) are also affected by emissions from water vapor.
During periods when the 190-GHz subsystem is deactivated, in the absence of MLS H2O measurements, retrievals of those other species must assume climatological water vapor abundances. This introduces biases in those species such that — although they are still useful from a purely morphological or qualitative perspective — observations on affected days cannot be quantitatively compared to measurements taken on surrounding days or in previous years when the 190-GHz radiometer was operational. This issue applies equally to the MLS v5 data record (for observations starting in May 2024) and users of v5 data are advised to consult the v6 document for more information on this issue.
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Questions or comments? Email the NASA GES DISC Help Desk or reach out on the Earthdata Forum.