<FONT COLOR="#00009c" FACE="georgia","arial">Precipitable Water

A major objective of NASA's Earth Science Enterprise program is the description of the global hydrologic cycle. Since water vapor is an important link connecting the various components of the hydrological cycle, an understanding of the role of water vapor in the hydrological climate system and its variability on all scales will be necessary to meet this objective. A full understanding of its role in regulating climate will require a knowledge of its distribution, horizontal and vertical transport, and interactive processes. An understanding of each of these aspects will require measurements of water vapor on scales ranging from local observational campaigns for improving climate model parameterizations to global scales for the development of a global climatology of water vapor. To adequately analyze global and regional climate and hydrologic systems, observations will need to employ a vertical resolution compatible with the inherent vertical scales of atmospheric moisture structure and will require global measurements on a comparable scale. Present satellite observations are inadequate in this respect. Even radiosonde observations produced by the operational international network are often inadequate in terms of vertical resolution. In addition, radiosonde observations provide poor coverage of large portions of the planet. Regional (national) differences in humidity sensor characteristics and performance is another serious problem. Further refinements in present retrieval techniques, especially those using combinations of data from various and planned observing systems are expected to yield significant improvements.

An important water vapor parameter currently being obtained from satellite and radiosonde measurements is precipitable water. Precipitable water is the total atmospheric water vapor contained in a vertical column of unit cross-sectional area extending from the surface to the top of the atmosphere. Precipitable water is commonly expressed in terms of the height to which that water substance would stand if completely condensed and collected in a vessel of the same unit cross section. Climatologies of PW are currently being compiled using measurements from the Defense Meteorological Satellite Program (DMSP), the National Oceanic and Atmospheric Administration (NOAA) polar orbiting satellites and the operational network of radiosondes. These data sets are the first step in providing information on the horizontal and temporal variability of water vapor. Though these data sets do not provide the vertical distribution of water vapor that will be needed for a complete understanding of water vapor processes in the climate system, they do provide important information. For example, climate models have shown significant increases in water vapor in response to global warming. PW climatologies may, therefore, be indicators of such climate warming. PW climatologies would also provide verification of climate model performance over multiyear periods.

One area of research that the Infrared Measurements Group at GHCC is involved in is the determination of PW from GOES. Current research activities include retrieval algorithm development and evaluation, and PW data set development. Even though GOES measurements are not global in extent, its high spatial and temporal resolution can provide important applications. For example, in order to improve the ability to predict climate variability, better parameterizations of water vapor processes such as surface evaporation, convection, and condensation and their interactions with the circulation of the atmosphere will be needed in climate models. Improvements in parameterizations of these processes in climate models will depend on observations that quantify these processes at various scales. PW data sets from GOES can provide the temporal and spatial resolution to study these processes. An example of a very important atmospheric forcing involving the above processes is the diurnal cycle of PW. The ability of climate models to capture this forcing is important. Analysis has shown that when there is diurnal forcing present in the models, land surfaces are cooler with more evaporation and less sensible heat flux than when no forcing is present. Also precipitation rate decreases over land particularly in the monsoon regions. Determining the diurnal cycle of PW would be useful in model analysis studies and parameterizations of this type. GOES is the only satellite capable of high temporal resolution which can provide the needed observations to characterize the diurnal variability of PW.

An important aspect of the Earth Science Enterprise program is its regional field measurement campaigns. One objective of these campaigns is to provide qualitative evaluations of water vapor measurements deduced from various satellite and radiosonde observations. The results should help provide the basis or verification of particular measuring system techniques or algorithms that best meet observational requirements. GOES PW retrievals with its temporal spatial resolution can provide an additional verification for these global satellite measurement systems. Finally, GOES PW data sets can provide additional data to PW climatologies especially over land areas where measurements from satellite microwave instruments are not presently successful. Though the GOES PW data would not be global it would provide useful climatologies of high interest regions such as those containing the Amazon deforestation and the El Nino/ Southern Oscillation (ENSO).

Technical Information

• Retrieval Algorithm Development and Evaluation
  • GOES-8 Evaluation Activities
• Data Set Development and Analysis
  • Precipitable Water Variability Studies

Last updated on: November 2, 1999