ISSI Publication on Monitoring Water Vapor

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.

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.

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