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S. M. Chiswell

Abstract

Pressure sensors used in CTDs (conductivity temperature depth) respond to transients in temperature. It is often assumed that these transients have a negligible effect on pressure. However, in a Sea-Bird CTD used in Hawaiian waters, these transients lead to pressure errors as high as 8 db.

We describe how we correct these errors using linear system theory by computing the response function of the pressure sensor to temperature transients. The CTD housing insulates the pressure sensor from the water to some extent, so that the effective response function is a combination of the intrinsic response of the pressure transducer convolved with a response function due to transfer of heat through the housing. Using this method, pressure is corrected to within 1 db.

The impulse response functions for two similar pressure tranducers are quite different, probably due to small manufacturing variations. Thermal insulation of pressure sensors also varies from CTD to CTD. The net effect is that the response functions vary considerably from CTD to CTD.

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Allen J. Riordan, J. Thomas Anderson, and S. Chiswell

Abstract

The analysis of the rainband structure and wind fields associated with a coastal front along the North Carolina shoreline is described. Dual-Doppler radar and the augmented GALE (Genesis of Atlantic Lows Experiment) ensemble of in situ stations depict shallow, convective rainbands that overtake the front from the warm-air sector and intensify at the surface front location. Clockwise band rotation is shown to be caused by the difference in alignment between the echo motion and the rainband axes and by new development ahead of the front.

Radar measurements depict the circulation systems associated with a portion of one rainband in the cold air ahead of the front. Here shallow precipitation cores are vertically tilted due to the frontal wind shear. Circulation cells and most precipitation cores are centered just above the frontal inversion, as inferred by the wind shift line aloft. This feature is nearly horizontal in the cross-frontal direction but slopes downward in a direction roughly parallel to the front.

Ahead of the front, main updrafts in and above the cold air are found near the upwind portion of precipitation cores and along two well-defined lines aligned roughly perpendicular to the front. These lines propagate northward and affect several nearby surface sites prior to frontal passage. The speed of northward propagation is consistent with gravity wave theory, while on the larger scale the front appears to behave as the leading edge of a density current. The major features found in this case are compared and contrasted with those of a synoptic-scale warm front.

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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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