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S. Sokolovskiy, W. Schreiner, C. Rocken, and D. Hunt

Abstract

GPS radio occultation remote sensing of the neutral atmosphere requires ionospheric correction of L1 and L2 signals. The ionosphere-corrected variables derived from radio occultation signals—such as the phase, Doppler, and bending angle—are affected by small-scale ionospheric effects that are not completely eliminated by the ionospheric correction. They are also affected by noise from mainly the L2 signal. This paper introduces a simple method for optimal filtering of the L4 = L1 − L2 signal used to correct the L1 signal, which minimizes the combined effects of both the small-scale ionospheric residual effects and L2 noise on the ionosphere-corrected variables. Statistical comparisons to high-resolution numerical weather models from the European Centre for Medium-Range Weather Forecasts (ECMWF) validate that this increases the accuracy of radio occultation inversions in the stratosphere.

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Y-R. Guo, Y-H. Kuo, J. Dudhia, D. Parsons, and C. Rocken

Abstract

On 19 September 1996, a squall line stretching from Nebraska to Texas with intense embedded convection moved eastward across the Kansas–Oklahoma area, where special observations were taken as part of a Water Vapor Intensive Observing Period sponsored by the Atmospheric Radiation Measurement program. This provided a unique opportunity to test mesoscale data assimilation strategies for a strong convective event. In this study, a series of real-data assimilation experiments is performed using the MM5 four-dimensional variational data assimilation (4DVAR) system with a full physics adjoint. With a grid size of 20 km and 15 vertical layers, the MM5-4DVAR system successfully assimilated wind profiler, hourly rainfall, surface dewpoint, and ground-based GPS precipitable water vapor data. The MM5-4DVAR system was able to reproduce the observed rainfall in terms of precipitation pattern and amount, and substantially reduced the model errors when verified against independent observations.

Additional data assimilation experiments were conducted to assess the relative importance of different types of mesoscale observations on the results of assimilation. In terms of the assimilation model’s ability to recover the vertical structure of moisture and in reproducing the rainfall pattern and amount, the wind profiler data have the maximum impact. The ground-based GPS data have a significant impact on the rainfall prediction, but have relatively small influence on the recovery of moisture structure. On the contrary, the surface dewpoint data are very useful for the recovery of the moisture structure, but have relatively small impact on rainfall prediction. The assimilation of rainfall data is very important in preserving the precipitation structure of the squall line. All the data are found to be useful in this mesoscale data assimilation experiment.

Issues related to the assimilation time window, weighting of different types of observations, and the use of accurate observation operator are also discussed.

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Y.-H. Kuo, X. Zou, S. J. Chen, W. Huang, Y.-R. Guo, R. A. Anthes, M. Exner, D. Hunt, C. Rocken, and S. Sokolovskiy

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.

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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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R. A Anthes, P. A Bernhardt, Y. Chen, L. Cucurull, K. F. Dymond, D. Ector, S. B. Healy, S.-P. Ho, D. C Hunt, Y.-H. Kuo, H. Liu, K. Manning, C. McCormick, T. K. Meehan, W J. Randel, C. Rocken, W S. Schreiner, S. V. Sokolovskiy, S. Syndergaard, D. C. Thompson, K. E. Trenberth, T.-K. Wee, N. L. Yen, and Z Zeng

The radio occultation (RO) technique, which makes use of radio signals transmitted by the global positioning system (GPS) satellites, has emerged as a powerful and relatively inexpensive approach for sounding the global atmosphere with high precision, accuracy, and vertical resolution in all weather and over both land and ocean. On 15 April 2006, the joint Taiwan-U.S. Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC)/Formosa Satellite Mission 3 (COSMIC/FORMOSAT-3, hereafter COSMIC) mission, a constellation of six microsatellites, was launched into a 512-km orbit. After launch the satellites were gradually deployed to their final orbits at 800 km, a process that took about 17 months. During the early weeks of the deployment, the satellites were spaced closely, offering a unique opportunity to verify the high precision of RO measurements. As of September 2007, COSMIC is providing about 2000 RO soundings per day to support the research and operational communities. COSMIC RO data are of better quality than those from the previous missions and penetrate much farther down into the troposphere; 70%–90% of the soundings reach to within 1 km of the surface on a global basis. The data are having a positive impact on operational global weather forecast models.

With the ability to penetrate deep into the lower troposphere using an advanced open-loop tracking technique, the COSMIC RO instruments can observe the structure of the tropical atmospheric boundary layer. The value of RO for climate monitoring and research is demonstrated by the precise and consistent observations between different instruments, platforms, and missions. COSMIC observations are capable of intercalibrating microwave measurements from the Advanced Microwave Sounding Unit (AMSU) on different satellites. Finally, unique and useful observations of the ionosphere are being obtained using the RO receiver and two other instruments on the COSMIC satellites, the tiny ionosphere photometer (TIP) and the tri-band beacon.

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