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S. Businger, R. Johnson, and R. Talbot

This paper provides an overview of the trials and successes in the development of an autonomous balloon instrument platform (smart balloon) and reviews scientific insights gained through its employment as a marker in a Lagrangian strategy during recent field experiments. The smart balloons are designed and constructed at the National Oceanic and Atmospheric Administration Air Resources Laboratory Field Research Division in collaboration with the University of Hawaii. In a 2004 field deployment a smart balloon carrying a miniature ozone sensor successfully crossed the Atlantic Ocean from Long Island, New York, to the African coast of Morocco. Significant progress has been made through field experiments such as this in our understanding of the relationships between the evolution of marine boundary layers and the chemistry of aerosol and gaseous constituents in clean and polluted air masses. Innovation in design and advances in instrument and communication technology have opened a dramatic new range of applications for the smart balloon in atmospheric research, including, for example, the interesting prospect of making observations very near the ocean surface in hurricanes and typhoons, which are not possible with research aircraft.

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S. Businger, R. McLaren, R. Ogasawara, D. Simons, and R. J. Wainscoat

The summit of Mauna Kea is arguably the best site on Earth for astronomical observations and the capital investment in telescopes on Mauna Kea has exceeded $600 million. The success of astronomical observations on Mauna Kea is strongly influenced by weather conditions. During prevailing clear periods, astronomical observing quality varies substantially due to changes in the vertical profiles of temperature, wind, moisture, and turbulence. Cloud and storm systems occasionally cause adverse or even hazardous conditions. To facilitate the best possible use of good atmospheric conditions and to support operation safety on the 4200-m mountain summit, a new interdisciplinary research program has been initiated that provides custom weather forecasts/nowcasts and meteorological data to the Mauna Kea Observatories. An operational mesoscale numerical modeling effort provides crucial forecast guidance for astronomical image quality, or seeing, during prevalent fair weather and for adverse weather. Of the existing telescopes on Mauna Kea, several commonly have multiple instruments or detectors mounted simultaneously, increasing the observational choices and thereby also increasing the utility of the custom weather forecast products provided by the newly established Mauna Kea Weather Center. Summit temperature forecasts allow mirrors to be set to the ambient temperature to reduce image degradation. Precipitable water forecasts allow infrared and submillimeter observations to be prioritized according to atmospheric opacity. Forecasts of adverse weather protect the safety of personnel, mitigate the hazard to telescope facilities, and allow for scheduling of maintenance when observing is impaired by cloud. This paper provides an overview of the unique forecast requirements and challenges faced by Mauna Kea weather forecasters. Progress toward meeting these challenges and opportunities for future research are discussed.

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J. A. Businger, W. F. Dabberdt, A. C. Delany, T. W. Horst, C. L. Martin, S. P. Oncley, and S. R. Semmer

The Atmosphere-Surface Turbulent Exchange Research (ASTER) facility developed at the National Center for Atmospheric Research (NCAR) will support observational research on the structure of the atmospheric surface layer. ASTER will provide state-of-the-art measurements of surface fluxes of momentum, sensible heat, and water vapor, and support measurements of surface fluxes of trace chemical species. The facility will be available to the scientific community in the spring of 1990. The motivation for the development of ASTER and the elements that constitute this new national facility are briefly discussed.

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Steven Businger, Steven R. Chiswell, Michael Bevis, Jingping Duan, Richard A. Anthes, Christian Rocken, Randolph H. Ware, Michael Exner, T. VanHove, and Fredrick S. Solheim

This paper provides an overview of applications of the Global Positioning System (GPS) for active measurement of the Earth's atmosphere. Microwave radio signals transmitted by GPS satellites are delayed (refracted) by the atmosphere as they propagate to Earth-based GPS receivers or GPS receivers carried on low Earth orbit satellites.

The delay in GPS signals reaching Earth-based receivers due to the presence of water vapor is nearly proportional to the quantity of water vapor integrated along the signal path. Measurement of atmospheric water vapor by Earth-based GPS receivers was demonstrated during the GPS/STORM field project to be comparable and in some respects superior to measurements by ground-based water vapor radiometers. Increased spatial and temporal resolution of the water vapor distribution provided by the GPS/STORM network proved useful in monitoring the moisture-flux convergence along a dryline and the decrease in integrated water vapor associated with the passage of a midtropospheric cold front, both of which triggered severe weather over the area during the course of the experiment.

Given the rapid growth in regional networks of continuously operating Earth-based GPS receivers currently being implemented, an opportunity exists to observe the distribution of water vapor with increased spatial and temporal coverage, which could prove valuable in a range of operational and research applications in the atmospheric sciences.

The first space-based GPS receiver designed for sensing the Earth's atmosphere was launched in April 1995. Phase measurements of GPS signals as they are occluded by the atmosphere provide refractivity profiles (see the companion article by Ware et al. on page 19 of this issue). Water vapor limits the accuracy of temperature recovery below the tropopause because of uncertainty in the water vapor distribution. The sensitivity of atmospheric refractivity to water vapor pressure, however, means that refractivity profiles can in principle yield information on the atmospheric humidity distribution given independent information on the temperature and pressure distribution from NWP models or independent observational data.

A discussion is provided of some of the research opportunities that exist to capitalize on the complementary nature of the methods of active atmospheric monitoring by GPS and other observation systems for use in weather and climate studies and in numerical weather prediction models.

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R. Ware, M. Exner, D. Feng, M. Gorbunov, K. Hardy, B. Herman, Y. Kuo, T. Meehan, W. Melbourne, C. Rocken, W. Schreiner, S. Sokolovskiy, F. Solheim, X. Zou, R. Anthes, S. Businger, and K. Trenberth

This paper provides an overview of the methodology of and describes preliminary results from an experiment called GPS/MET (Global Positioning System/Meteorology), in which temperature soundings are obtained from a low Earth-orbiting satellite using the radio occultation technique. Launched into a circular orbit of about 750-km altitude and 70° inclination on 3 April 1995, a small research satellite, MicroLab 1, carried a laptop-sized radio receiver. Each time this receiver rises and sets relative to the 24 operational GPS satellites, the GPS radio waves transect successive layers of the atmosphere and are bent (refracted) by the atmosphere before they reach the receiver, causing a delay in the dual-frequency carrier phase observations sensed by the receiver. During this occultation, GPS limb sounding measurements are obtained from which vertical profiles of atmospheric refractivity can be computed. The refractivity is a function of pressure, temperature, and water vapor and thus provides information on these variables that has the potential to be useful in weather prediction and weather and climate research.

Because of the dependence of refractivity on both temperature and water vapor, it is generally impossible to compute both variables from a refractivity sounding. However, if either temperature or water vapor is known from independent measurements or from model predictions, the other variable may be calculated. In portions of the atmosphere where moisture effects are negligible (typically above 5–7 km), temperature may be estimated directly from refractivity.

This paper compares a representative sample of 11 temperature profiles derived from GPS/MET soundings (assuming a dry atmosphere) with nearby radiosonde and high-resolution balloon soundings and the operational gridded analysis of the National Centers for Environmental Prediction (formerly the National Meteorological Center). One GPS/MET profile was obtained at a location where a temperature profile from the Halogen Occultation Experiment was available for comparison. These comparisons show that accurate vertical temperature profiles may be obtained using the GPS limb sounding technique from approximately 40 km to about 5–7 km in altitude where moisture effects are negligible. Temperatures in this region usually agree within 2°C with the independent sources of data. The GPS/MET temperature profiles show vertical resolution of about 1 km and resolve the location and minimum temperature of the tropopause very well. Theoretical temperature accuracy is better than 0.5°C at the tropopause, degrading to about 1°C at 40-km altitude.

Above 40 km and below 5 km, these preliminary temperature retrievals show difficulties. In the upper atmosphere, the errors result from initial temperature and pressure assumptions in this region and initial ionospheric refraction assumptions. In the lower troposphere, the errors appear to be associated with multipath effects caused by large gradients in refractivity primarily due to water vapor distribution.

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