Abstract. The Global Positioning System/Meteorology (GPS/MET) Program was established in 1993 by the University Corporation for Atmospheric Research (UCAR) to demonstrate active limb sounding of the Earth's atmosphere using the radio occultation technique. The demonstration system observes occulted GPS satellite signals received by a low Earth orbiting (LEO) satellite, MicroLab-1, launched April 3, 1995. The system can profile ionospheric electron density and neutral atmospheric properties. Neutral atmospheric refractivity, density, pressure, and temperature are derived at altitudes where the amount of water vapor is low. At lower altitudes, vertical profiles of density, pressure, and water vapor pressure can be derived from the GPS/MET refractivity profiles if temperature data from an independent source are available. This paper describes the GPS/MET data analysis procedures and validates GPS/MET data with statistics and illustrative case studies. We compare more than 1200 GPS/MET neutral atmosphere soundings to correlative data from operational global weather analyses, radiosondes, and the GOES, TOVS, UARS/MLS and HALOE orbiting atmospheric sensors. Even though many GPS/MET soundings currently fail to penetrate the lowest 5 km of the troposphere in the presence of significant water vapor, our results demonstrate iøC mean temperature agreement with the best correlative data sets between 1 and 40 km. This and the fact that GPS/MET observations are all-weather and self-calibrating suggests that radio occultation technology has the potential to make a strong contribution to a global observing system supporting weather prediction and weather and climate research.

Abstract. The launch of the proof-of-concept mission GPS/MET (Global Positioning System/Meteorology) in 1995 began a revolution in profiling Earth's atmosphere through radio occultation (RO). GPS/MET; subsequent single-satellite missions CHAMP (CHAllenging Minisatellite Payload), SAC-C (Satellite de Aplicaciones Cientificas-C), GRACE (Gravity Recovery and Climate Experiment), METOP-A, and TerraSAR-X (Beyerle et al., 2010); and the six-satellite constellation, FORMOSAT-3/COSMIC (Formosa Satellite mission #3/Constellation Observing System for Meteorology, Ionosphere, and Climate) have proven the theoretical capabilities of RO to provide accurate and precise profiles of electron density in the ionosphere and refractivity, containing information on temperature and water vapor, in the stratosphere and troposphere. This paper summarizes results from these RO missions and the applications of RO observations to atmospheric research and operational weather analysis and prediction.

Thermodynamic states in clouds are closely related to physical processes such as phase changes of water and longwave and shortwave radiation. Global Positioning System (GPS) radio occultation (RO) data are not affected by clouds and have high vertical resolution, making them ideally suited to cloud profiling on a global basis. By comparing the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) RO refractivity data with those of the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis and ECMWF analysis for soundings in clouds and clear air separately, a systematic bias of opposite sign was found between large-scale global analyses and the GPS RO observations under cloudy and clear-sky conditions. As a modification to the standard GPS RO wet temperature retrieval that does not distinguish between cloudy-and clear-sky conditions, a new cloudy retrieval algorithm is proposed to incorporate the knowledge that in-cloud specific humidity (which affects the GPS refractivities) should be close to saturation. To implement this new algorithm, a linear regression model for a sounding-dependent relative humidity parameter a is first developed based on a high correlation between relative humidity and ice water content. In the absence of ice water content information, a takes an empirical value of 85%. The in-cloud temperature profile is then retrieved from GPS RO data modeled by a weighted sum of refractivities with and without the assumption of saturation. Compared to the standard wet retrieval, the cloudy temperature retrieval is consistently warmer within clouds by ;2 K and slightly colder near the cloud top (;1 K) and cloud base (1.5 K), leading to a more rapid increase of the lapse rate with height in the upper half of the cloud, from a nearly constant moist lapse rate below and at the cloud middle (;68C km 21 ) to a value of 7.78C km 21 , which must be closer to the dry lapse rate than the standard wet retrieval.

A tmospheric water vapor is the dominant greenhouse gas in the earth's atmosphere, and so quantifying the feedback of water vapor in global warming is therefore of paramount importance. This feedback is also crucial for other feedback processes, such as snow-sea ice and clouds, that also play a significant role in climate. The lack of detailed knowledge of the hydrological cycle thus is a major limiting factor toward a better understanding of the earth's climate system. The inaccuracy is substantial and concerns practically all aspects of the hydrological cycle. At present, even such a basic quantity as the global annual precipitation rate is probably only possible to be determined to an accuracy of some 10% (Adler et al. 2001). The main reason for this appears to be the high spatial and temporal variability of the water cycle, for which the sampling properties of current observing systems are wholly inadequate. To find new and cost-efficient ways to improve the global observing system is a central objective for the weather and climate prediction communities.Global Positioning System (GPS)-based measurements offer here new and promising possibilities. One of these is the capability to provide data at similar More systematic, integrated use of GPS techniques could lead to better information about the highly variable features of the hydrologic cycle and more. AFFILIATIONS:

A Global Positioning System Meteorology (GPS/MET) proof-of-concept experiment became a reality on 3 April 1995. A small satellite carrying a modified GPS receiver was launched into earth orbit to demonstrate the feasibility of active limb sounding of the earth's neutral atmosphere and ionosphere using the radio occultation method. On 22 October 1995, a GPS/MET occultation took place over northeastern China where a dense network of radiosonde observations was available within an hour of the occultation. The GPS/MET refractivity profile shows an inflection, and the corresponding temperature retrieval displays a sharp temperature inversion around 310 mb. Subjective analyses based on radiosonde observations indicate that the GPS/MET occultation went through a strong upper-level front. In this paper, the GPS/MET sounding is compared with nearby radiosonde observations to assess its accuracy and ability to resolve a strong mesoscale feature. The inflection in the refractivity profile and the sharp frontal inversion seen in the GPS/MET sounding were verified closely by a radiosonde located about 150 km to the east of the GPS/MET occultation site. A similar frontal structure was also found in other nearby radiosonde observations. These results showed that high-quality GPS/MET radio occultation data can be obtained even when the occultation goes through a sharp temperature gradient associated with an upper-level front. 1. Introduction A small satellite (MicroLab 1) carrying a global positioning system (GPS) receiver, modified to sound the atmosphere, was launched on 3 April 1995. This instrument is the centerpiece of a proof-of-concept experiment to demonstrate the feasibility of sensing the terrestrial atmosphere by the GPS limb-sounding method (Fjeldbo et al. 1971; Melbourne et al. 1994). Ware et al. (1996) reviewed the theory and history of this experiment and presented preliminary results obtained from this GPS meteorology (GPS/MET) experiment. They compared 11 representative temperature