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NDACC News and Highlights
This section highlights significant items of interest within NDACC, with updates at least annually following the NDACC Steering Committee meeting.
October 23, 2012 marks the official opening of the the Southern Tropical high altitude NDACC Maido Observatory in Reunion Island. After more than 15 years effort from CNRS-INSU, the Region and the University of Reunion Island, the construction of the Observatoire de Physique de l'Atmosphere (OPAR) was completed in July 2012. The lidars and FTIR instruments are being installed for the long term monitoring of the composition of the atmosphere.
The NDACC Working Group on Water Vapor announces the publication: ISSI Scientific Report 10: Monitoring Atmospheric Water Vapour edited by Niklaus Kampfer. The aim of the book is to assess in detail in situ and remote sensing techniques presently used to monitor on a regular basis the distribution of atmospheric water vapor. The basics of the different measurement methodologies is reviewed on a level that is considered helpful for the non expert to obtain insight and sufficient knowledge to make the most out of available data in a sense that also limitations, caveats and problem areas of the corresponding technique are mentioned. The authors felt it extremely helpful also to point out weaknesses in order to understand what realistically can be expected from a certain type of sensor. A special weight was also put on the retrieval aspects of the sensor types as most sensors do not directly measure humidity itself but a signature of humidity such as emission or absorption spectra, backscattered signals etc. In many cases the signal of interest can be retrieved only after complex mathematical treatment and by using additional information that might falsify in the worst case the data product or that in minimum must be considered when properly interpreting data.
The book is structured in three sections plus appendices. A first section is devoted to in situ methods as normally used on balloons or on aircraft. Thin film capacitive sensors and frost point hygrometry is covered as well as fluorescence methods. Section two is devoted to remote sensing techniques operated from the ground and covers passive microwave radiometry, infrared Fourier transform spectrometry using the sun as a background source and lidar where the two approaches, Raman lidar and differential absorption lidar are covered. The third section approaches the topic from the point of view of networks and measurements on more global frame from satellites. The value of networks to probe the atmosphere with a special weight on water vapor is highlighted. A detailed overview of the different concepts used from satellite is given. A special chapter is devoted to the difficulties encountered when combining data from different platforms with different viewing geometries, different altitude resolutions and different sampling schemes. These aspects too often are neglected when using data from different sources and resources. A last chapter finally is devoted to the aspect of validation and inter-comparison of different techniques. The chapter essentially covers all the different techniques and the multitude of performed inter-comparisons. Most use of this chapter can however be made in the interactive pdf-version available at http//www.watervapor.org.
The book ends with several appendices that the authors consider to provide useful information in a concise way. Appendix A is built up of so called fact sheets about the different techniques. They summarize the basic facts about the corresponding technique such as accuracy, resolution etc. but also problem areas and what has to be kept in mind when using data from the respective methodology. The same appendix also provides fact sheets of individual instruments such as used in the frame of NDACC. Appendix B covers an overview of many available equations for water vapor saturation pressure over liquid water and over ice.
More information can be found at http://www.springer.com/earth+sciences+and+geography/remote+sensing/book/978-1-4614-3908-0.
In 1996, in an attempt to resolve an inconsistency between ozone profile trends obtained from satellite vs. ground-based measurements, the Stratospheric Processes and their Role in Climate (SPARC) project of the World Climate Research Programme (WCRP) initiated a collaboration with the International Ozone Commission (IO3C) to carefully re-evaluate the ground-based and satellite ozone data. This study, headed by the SPARC Panel on Understanding Ozone Trends, did not simply review the published literature but conducted a critical re-analysis and interpretation of ozone vertical profiles. A significant focus of the study included validation of the data quality and quantification of the errors to determine if such aspects limited trends determinations as a function of altitude or latitude. The resulting SPARC/IO3C/GAW Assessment of Trends in the Vertical Distribution of Ozone was published in 1998 (SPARC Report No. 1) and was used extensively in the 1998 WMO/UNEP Ozone Assessment. This complete report can be downloaded from the SPARC web site at http://www.sparc-climate.org/publications/sparc-reports/sparc-report-no1/.
Since this 1998 trends assessment, the end of certain satellite records (the SAGE instruments in particular) has limited our observations of global changes in the vertical distribution of ozone. More specifically, while a number of new satellite instruments have been launched since 2000, no thorough assessment of how well these new measurements agree with each other or with the SAGE record has been conducted. In addition, ozone profile measurements from various ground-based networks have matured through an additional 15 years of operations and through improved geographical coverage. Thus, in an effort to improve our knowledge and understanding of the past changes in the vertical distribution of ozone, a new SPARC/IO3C/IGACO/NDACC (SI2N) initiative has been organized (see http://igaco-o3.fmi.fi/VDO/index.html and recent SPARC Newsletter articles (Harris et al., 2011 and Harris et al., 2012 appearing in SPARC Newsletters 37 and 39 respectively, which can be accessed at http://www.sparc-climate.org/publications/newsletter/).
Under this initiative satellite, ground-based, and sonde measurements of ozone are being critically analyzed, as are methods of preparing combined data sets in an effort to provide input to the next WMO/UNEP Scientific Assessment of Ozone Depletion anticipated for 2014. Within the SI2N initiative, an essential aim is to better understand all instrumental records and to improve the methods for combining them. Finally the SI2N initiative highlights the need for, and challenges associated with, keeping high quality profiling operational for at least the next two decades or so. It should be noted, however, that while the SI2N initiative will hopefully make broad gains in our abilities to utilize and combine data from diverse measurement suites for ozone profiles, temperature profiles, and even trace gas abundances and profiles, it cannot fill all the gaps created by measurement cessation.
Such an SI2N focus on the quality assurance of long-term data records, verification of instrument performance, and intercomparisons of data sets from a broad spectrum of instruments has been a corner stone of the NDACC since its inception as the NDSC. Three NDACC instrument working groups (led by S. Godin-Beekman, T. Leblanc, and W. Steinbrecht; N. Kampfer and G. Nedoluha; and J. Hannigan and M. De Maziere) for lidar, microwave and FTIR respectively are coordinating the NDACC contributions by these instruments to the SI2N initiative. NDACC scientists are also contributing to the efforts of two additional SI2N working groups (Ozonesondes led by S. Oltmans and H. Smit, and Umkehr led by T. McElroy and I. Petropavlovskikh). A broad range of SI2N-focused activities is now underway within the NDACC and the associated publications are in preparation. These include studies dealing specifically with lidar, microwave, or FTIR measurements as well as those that combine data from multiple instrument types including both ozonesondes and Umkehr. The full details of these activities are summarized in the latest NDACC Newsletter to be released in early 2013.