4. DEFINITION OF REQUIREMENTS

Dr Josef Cihlar presented a list of requirements as defined by the GCOS/GTOS TOPC. The specifics of these requirements are contained in Version 2.0 of the GCOS/GTOS Plan for Terrestrial Climate-related Observations^[5]. It is recognised that the climate-related requirements are only a part of the ultimate GTOS needs. He explained that the philosophy used in identifying the key variables was to examine the climate-related objectives of the global observing systems and then identify variables that must be observed to meet these objectives. This was supplemented by assessing the current use of each potential variable (directly, through numerical models, etc.) and also its potential uses in the future. If a variable met one of these needs it was included in the list of key variables. The desired specifications for each variable in terms of accuracy, spatial and temporal resolution were then established. The key variables are described in detail in the above report and are also available on the World Wide Web (http://www.wmo.ch/web/gcos/pub/top_vr_2.html). Following the recent Global Observing Systems Space Panel meeting these specifications are being refined by the TOPC so as to be more meaningful to both satellite and in situ observations. It is expected that the revised requirements will be completed by September, 1997