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GEWEX/ESA DUE GlobVapour workshop on long term water vapour data sets and their quality assessment

8-10 March 2011, ESA/ESRIN, Via Galieo Galilei, Frascati, Italy

 

Water vapour is a key climate variable. In the lower troposphere, condensation of water vapour into precipitation provides latent heating which dominates the structure of tropospheric diabatic heating. Water vapour is also the most important gaseous source of infrared opacity in the atmosphere, accounting for about 60% of the natural greenhouse effect for clear skies, and provides the largest positive feedback in model projections of climate change.

The network of radiosonde measurements provides the longest record of water vapour measurements in the atmosphere, dating back to the mid-1940s. However, early radiosonde sensors suffered from significant measurement biases, particularly for the upper troposphere, and changes in instrumentation with time often lead to artificial discontinuities in the data record (e.g., see Elliott et al., 2002). Consequently, most of the analysis of radiosonde humidity has focused on trends for altitudes below 500 hPa and is restricted to those stations and periods for which stable instrumentation and reliable moisture soundings are available.

Satellite observations relevant for water vapour climatology were mostly build for weather forecasting purposes and their utilisation for building climate data records is rather complex and involves the creation of high quality radiance records, so called Fundamental Climate Data Records (FCDR) and subsequently so called ECV products[1] that are water vapour parameters. Currently, water vapour climatology is obtained from satellite observations either using retrieval schemes or through assimilation into a Numerical Weather Prediction Model forming model-based reanalysis products.

GEWEX and in particular its Radiation Panel (www.gewex.org/projects-GRP.htm) is generating climate data records concerning the Earth Energy and Water Cycle by means of dedicated reprocessing activities. Currently, a missing element is a water vapour product that can be used in a consistent way with the other products. This workshop shall help to answer questions on how such a water vapour climatology should be designed to be most useful in the context of GEWEX.

Global organisations as CEOS, CGMS and WMO help to organise the creation of FCDRs and ECV products leading to the fulfilment of GCOS requirements. As a consequence space agencies have started several programs and projects to create both types of data records.

 

Motivation

 

There is a clear need of a global time series of temperature and humidity profiles for studies of the 3D T, q distribution, e.g., for water vapour transport, and the use within the derivation of cloud property data sets and components of the radiation budget climatology. Such a data set should cover a period of approximately 30 years starting in the late 1970ties. Taking the ISCCP data set as an example the use of T-, q-profile data from satellite data real time processing systems can easily corrupt the quality of a data set for long term climate variability analysis. Currently, no single retrieved data set or NWP model-based reanalysis is capable of completely fulfilling the needs in terms of temporal coverage, homogeneity and accuracy. There are manifold reasons for this:
  • The work on high quality radiance records for relevant satellite instruments has only started in the last decade. Many problems as for instance the diurnal drift of satellites and the consequences for a derived data set have not been studied to great extent.
  • The instrumentation for deriving atmospheric profiles before the advent of IASI and AIRS suffers from the inherent problem that the information content on the profile is rather low. This forces the use of a priori information where the impact of uncertainties in the a priori on the final data sets has been barely investigated.
  • Most existing water vapour products focus on the use of one particular instrument or instrument type. As a result many data sets only cover parts of the needed time series. Additionally, all problems related to specific instrument capabilities or sampling properties remain in the final data set, e.g., an SSM/I total column water vapour can only be derived over oceans.
  • Merging of different products is a very difficult task as to reach a very high quality the error characteristics of individual products needs to be understood. Most of the existing data sets do not have measures of uncertainty within the products. Early attempts of merging data did not reveal data sets that improved much on real time data processing.
  • Early NWP based reanalysis data sets suffered from inadequate data assimilation techniques that prohibited the full use of satellite observations, e.g., most of the observations were only used over oceans and under clear sky conditions.


[1] The term ECV product was introduced by GCOS. Other documents also use the term Thematic Climate Data Record (Climate Data Records from Environmental Satellites, National Research Council, 2004) or integrated climate product (GCOS Implementation Plan).


 

Workshop Concept Paper