Water vapor tomography with low cost GPS receivers [presentation]

We propose to apply low cost single frequency GPS receivers, deployed in a dense array, to tomographic estimation of small scale three-dimensional (3-D) atmospheric water vapor fields. The proposed proof-of-concept experiment will be conducted during the Atmospheric Radiation Measurement (ARM) program’s Intensive Observation Period (IOP) of July 1999 near the Lamont Cloud and Radiation Testbed (CART). The system will utilize 25-30 GPS receivers deployed at 1 to 3 km spacing in a 10 x 10 km or larger area. Carrier phase data from this array will be analyzed to determine line-of-sight tropospheric delays caused by atmospheric water vapor. These line-ofsight delays shall be inverted, using tomographic techniques, and optionally first guess fields based on other atmospheric data available at the ARM CART site, to estimate 3-D atmospheric water vapor fields at 30 min or smaller time intervals. An alternative approach will be to assimilate the slant measurements into a high resolution numerical weather model like MM5. The purpose of the proposed effort is to develop and demonstrate a new atmospheric sensing technique to measure small scale atmospheric water vapor fields, which are important to provide the initial and boundary conditions for the Single Column Modeling (SCM) efforts conducted under the ARM program. Background Improved knowledge of the water vapor field is needed for a variety of atmospheric research applications and for improved weather forecasting. Within the ARM program an all weather, continuously operating system to monitor water vapor fields is needed to provide the initial conditions for the SCM efforts, aimed at improving the treatment of atmospheric radiation in Global Climate Models. Recently developed methods for sensing precipitable water vapor (PWV) using the Global Positioning System (GPS) promise to provide needed water vapor information for weather and climate models [ Bevis et al., 1992;Rocken et al., 1993;Rocken et al., 1995a;Businger et al., 1996;Duan et al., 1996]. For example, the GPS Research Group at UCAR/UNAVCO is currently analyzing GPS data from the NOAA Forecast Systems Laboratory (FSL) GPS network [ Gutman et al., 1994] in near real time, as shown in Figure 1. Scientists at FSL and at the National Center for Atmospheric Research (NCAR) are working on assimilating these observations into weather models to determine their effect on short term forecasting. Figure 1: PWV estimates of the NOAA/FSL GPS network. These results are taken from the near real time analysis of the GPS Research Group at UCAR. These two figures show a significant gradient of water vapor through the center portion of the network, where the proposed tomographic network will be located. PWV measurements are derived by analyzing delays on the GPS signal caused by the neutral atmosphere. Traditionally this zenith delay is estimated as a parameter that maps as the cosecant of the satellite elevation angle [ Niell, 1996]. The parameter is defined as: