The need for quality assessments of Essential Climate Variables (ECVs) Climate Data Records (CDRs) is part of the GCOS guidelines for the generation of data products. The assessement process gives an overview of available data records and enables users to judge the quality and fitness for purpose of CDRs by informing them about the strengths and weaknesses of existing and readily available information. This is achieved by inter-comparison and evaluation, and, if possible, also providing the reasons behind differences and limitations. Assessments of data records related to the global energy and water cycles became an integral part of GEWEX activities over the last decades.

In this context, the GEWEX Data and Assessment Panel (GDAP) has initiated the GEWEX Water Vapor Assessment (G-VAP) in 2011. The main objectives of G-VAP are:

• Quantifying the state of the art in water vapour products being constructed for climate applications, and, by this:
• Supporting GDAP’s selection process of water vapour products for its generation of globally consistent water and energy cycle products.

All three GCOS Essential Climate Variables (ECV) on water vapour will be considered:

• Total column water vapour (TCWV),
• Upper tropospheric humidity (UTH),
• Water vapour and related temperature profiles (WVP).

While the requirements on resolution are similar between GCOS-200 and the GEWEX needs, G-VAP will consider the GCOS requirements on accuracy and stability as baseline guidance. The assessment of atmospheric profiles (specific humidity preferred) is of highest interest to GDAP as such profiles are the input to the GEWEX products (see http://www.gewex.org for an overview). The usage of the products within GDAP activities essentially implies to study long-term data records.