Final Remarks

A number of new data sets promise to significantly enhance the capability to quantify the relative strength of mechanisms in two-way land-atmosphere interaction. Much of these observations have been collected during several field experiments that specifically focused on multi-sensor ( in situ subsurface, meteorological tower, aircraft- and satellite-based passive and active radiation remote sensing) data collection. The primary goals of these field experiments are to investigate micrometeorologic processes, boundary layer dynamics and the spatial scaling of surface flux. The FIFE [First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment] experiment was conducted on a 1515 kilometers area near Manhattan, Kansas [ Sellers et al., 1992]. The two intensive periods of data collection covered several weeks in the summers of 1987 and 1989. Results of these experiments are mostly reported in the November 1992 special issue of Journal of Geophysical Research. The HAPEX (Hydrologic Atmospheric Pilot Experiment) focused on the semiarid Sahel environment. The intense data collection period for this experiment occurred during August and September 1991. The Monsoon 90 field experiment in Arizona focused on the use of remotely sensed data to estimate surface flux. Results of this experiment are collected in the May 1994 issue of Water Resources Research. Kustas [1994, this issue] provides a review of these experiments and develops a summary of their principal findings.

There are also a number of larger scale experiments planned to study variability in the hydrologic cycle and climate. These principally represent coordinated efforts to monitor and centrally archive the heat and moisture state of the land and atmosphere over the continents. The GEWEX (Global Energy and Water Cycle Experiment) Continental-Scale International Project (GCIP) is focused on the Mississippi basin. Its primary purpose is:

``To improve climate models by bridging the gap between those scales significant for modeling discrete processes of the hydrologic and energy cycles over land and those that are practical for modeling the global climate system and predicting the impacts of climate change on continental regions'' [ WCRP, 1993b].

A number of data assimilation products are also now available. For example Schubert et al. (1993) introduce a comprehensive and consistent gridded data set of atmospheric variables based on rawinsonde, satellite retrieval, aircraft, ship, surface and rocketsonde reports. Similarly, Kalnay and Jenne [1991] at the National Meteorological Center are performing a re-analysis of the available observations to provide consistent model-assimilated fields of relevant climate and hydrologic cycle variables. Giorgi et al. [1993] introduce an accurate multiyear climatology of western United States. They use a regional atmospheric model to disaggregate assimilated data over complex topography.

For surface variables, Mintz and Walker [1993] introduce estimated fields of global monthly soil moisture and evapotranspiration based on precipitation and temperature observations. Vinnikov and Yeserkepova [1991] introduce a set of soil moisture (zero to one meter depth) observations across the former Soviet Union territory. This data set covers the 1972-1985 period.

Lettenmaier et al. [1994] and Wallis et al. [1991] perform extensive data quality checks on a hydroclimatologic data base for the continental United States. They provide multi-decadal time-series of high-quality daily precipitation, stream discharge and air temperature for a large number of points within this region. Such time-series may be extended using proxy data. Meko et al. [1993] use tree rings to develop a United States drought data set extending back to the year 1705.

Satellite remote sensing provides an opportunity to develop high spatial resolution data sets for the study of land-atmosphere interaction. Although the duration of data coverage is limited, the areal extent is large and it covers unpopulated regions that are deficient in observing stations. For example, Choudhury [1993] shows that multi-spectral observations may be used to effectively quantify the spatial extent and interannual fluctuations in desertification induced by land surface processes. Gutman [1990] focuses on the Great Plains and shows that remotely sensed thermal data may be used to monitor drought for this region.

The agenda for research in land-atmosphere interaction is defined in the National Research Councils (NRC) 1991 report Opportunities in the Hydrologic Sciences. The notion that the atmospheric forcing of surface hydrologic processes is itself dependent on surface conditions, and hence analyses of the fluctuations in hydrologic variables require the representation of two-way land-atmosphere coupling is firmly established in this report. A number of important hypotheses and open questions regarding land-atmosphere interaction and variability in the hydrologic cycle and hydroclimatic system are listed in NRC [1991]. The increased availability of high quality observation data sets as mentioned above and the science questions posed in NRC [1991] provide the bases for substantial advances in understanding and characterizing land-atmosphere interaction. Significant contributions to both hydrologic and atmospheric science are now possible.