atmospheric radiation, with a particular focus on the influence of clouds and the role of the cloud radiative feedback (Stokes and Schwarz, 1994). Measurements of water vapor play a major role in the integrated radiative flux experiments and single-column model experiments. The ARM program is sponsoring extensive observational facilities for a period of 10 years at 3 sites: the southern great plains (centered at Lamont, OK), the tropical western Pacific Ocean (centered at Nauru), and the north slope of Alaska (centered at Barrow). GPS receivers have been installed at the southern great plains site.

In the context of understanding the role of water vapor in climate, precipitable water data from GPS (and eventually some information on water vapor profiles) will be useful mainly in those regions of globe where radiosondes are unavailable and/or where SSM/I and TOVS retrievals have large errors.

GVaP is relying on passive microwave observations of column water amount over oceans. This poses some problems in the tropics. SSM/I has a local coverage of twice per day in the tropics, and SSM/I precipitable water retrievals are unavailable under precipitating conditions. Sheu and Liu (1996) performed an intercomparison of 6 different SSM/I precipitable water algorithms against precipitable water derived from ship-borne radiosondes during TOGA COARE. The algorithms showed 10-15% errors when compared with the radiosondes. It is not clear at present whether SSM/I precipitable water retrievals in this environment can be improved to a more acceptable level of 5%. GPS may provide improved accuracy in this environment.

The importance of the diurnal cycle in tropical airsea interaction is being increasingly appreciated (e.g. Webster et al., 1996). It has been hypothesized that the diurnal cycle may influence lower frequency variations. GPS may provide the only method to observe the diurnal cycle in precipitable water.

Numerous islands in the equatorial oceans could in principle be used as locations for GPS receivers. Additionally, use of the TAO buoy array in the equatorial Pacific Ocean could be explored as potential platforms for GPS receivers.

Another oceanic region where current satellite observations of precipitable water are inadequate is theArctic Ocean. Simulation of clouds by climate and numerical weather prediction models is poor in the Arctic because of poor data bases and lack of understanding of physical processes that determine water vapor amount. Water vapor in the Arctic is of particular importance because of the hypothesized “hyper” water vapor feedback in the Arctic (Curry et al., 1995). At present, there are no radiosondes in the Arctic Ocean, and passive microwave techniques to retrieve precipitable water do not work over ice. TOVS retrievals of water vapor at high latitudes are also problematic. Since precipitable water values are commonly less than 1 g cm⁻² in the Arctic, a considerable challenge is posed to any observing system.

Improved measurements of atmospheric water vapor would be useful for a variety of climate applications. The WCRP GEWEX GVaP program is the major national/international forum for addressing water vapor issues. The DOE ARM program is cooperating with GVaP to evaluate and improve techniques to determine water vapor amount. GPS technology has made some minor inroads into these programs, but there is some skepticism of the contribution to be made from GPS, beyond the observing network that is already in place.

GPS has the greatest potential for contributing to the global water vapor data base in remote land areas where there are no radiosondes, in the moist equatorial oceanic regions where passive microwave algorithms do not presently perform very well, and in the very dry polar regions.

Curry, J.A., J.L. Schramm, MC. Serreze, and E.E. Ebert, 1995: Water vapor feedback over the Arctic Ocean. J. Geophys. Res., 100, 14,223-14,229.

Sheu, R.-S. and G. Liu, 1995: Atmospheric humidity variations associated with westerly wind bursts during TOGA COARE. J. Geophys. Res., in press.

Stokes and Schwarz, 1994: The Atmospheric Radiation Measurement Program. Bull. Amer. Meteorol. Soc., 1201-1221.

Webster, P.J., C.A. Clayson, and J.A. Curry, 1995: Clouds, radiation, and the diurnal cycle of sea surface temperature in the tropical western Pacific. J. Clim., in press

Front Matter (R1-R6)

Contents (R7-R10)

I Workshop Summary (1-2)

Executive Summary (3-5)

1 Introduction (6-9)

2 Networks and Data Sources (10-13)

3 Remote Sensing of the Atmosphere (14-17)

4 Dynamic Positioning and Navigation (18-20)

5 Static Positioning (21-22)

II Workshop Proceedings (23-24)

6 Civilian GPS Planning and Policy Making in the Federal Government (25-39)

7 Networks and Data Sources (40-109)

8 Remote Sensing of the Atmosphere (110-163)

9 Dynamic Positioning and Navigation (164-200)

10 Static Positioning (201-226)

Appendices (227-227)

A Statement of Task (228-228)

B Biographical Sketches of Steering Committee Members (229-230)

C Workshop Attendees (231-233)

D Working Group Participants (234-235)

E Workshop Agenda (236-239)