and boundary layer fluxes cannot be determined directly from current in situ observations over large enough domains. Even those quantities that are observed, such as precipitation or snowcover, are not measured very well, and comparisons between analyses and measurements can yield new insights.
GCIP's focus on assimilation of land surface variables should result in significant improvements in forecast accuracy and range. Snow water equivalent is beginning to be assimilated, but soil moisture remains a problem. We do not know how to estimate soil moisture from observations on a continental scale, much less how to specify its initial value for numerical weather prediction. Also unknown is how to assimilate subsurface temperature. Streamflow and precipitation measurements could provide additional information about how to update snow and soil moisture as well as the atmospheric dynamical fields that are implicitly dependent upon their values.
GCIP's focus on precipitation should also result in significant improvements in precipitation and streamflow forecasts and climatologies. Prior to GCIP, precipitation was reported only from individual stations. With the advent of GEWEX, the Global Precipitation Climatology Project (GPCP) began, and we now have global archives for a number of years (Xie and Arkin, 1996). GCIP is playing a major role in extending these coarse-scale global climate products (2.5°) to operational high-resolution (4-km) gridded precipitation for the continental United States (Baldwin and Mitchell, 1996). This analysis effort will further help to combine the disparate scales of atmospheric weather and climate with surface hydrologic scales.
A number of model analyses, including the NCEP and the ECMWF global data assimilation systems, have been available since the late 1970s. NCEP's reanalysis (Kalnay et al., 1996) will further extend these analyses back to the 1950s. These analysis products are available at NCAR. Many scientists have used these and similar analyses to describe the global hydrologic cycle as well as the continental hydrologic cycle over the continental United States (e.g., Roads et al., 1994; Higgins et al., 1996).
Regional analysis systems (such as the NCEP nested grid model analysis system) were introduced in the 1980s to make better use of the high-resolution data available in certain regions (e.g., over the United States). However, regional analyses were archived only at NCEP and other numerical weather prediction centers and were not readily available to the community. GCIP has changed this; a number of analysis centers are participating in the GCIP project and are archiving their products on common horizontal grids (40 km on a Lambert-conformal projection). As discussed in Chapter 2, the NCEP regional Eta Model Data Assimilation System