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David W. Martin and Anthony J. Schreiner

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This article describes the size, intensity, trajectory, lifetime and distribution of the GATE cloud clusters of West Africa and the eastern Atlantic Ocean and relates their distribution to the summer climate of the region. SMS-1 infrared and visible 3 h pictures for 85 days of GATE, starting 27 June 1974, were used. It was found that over 500 clusters occurred. Size averaged 2 × 105 km2; lifetime, one day. Although both were highly variable, in general, lifetime increased with maximum size. The clusters occurred in a band oriented west-southwest to east-northeast over the ocean and eastward over land. Nodes were observed at intervals of 5–7° along the axis of maximum frequency of occurrence. Clusters at all latitudes moved generally westward, having straighter tracks and faster speeds over land. From July to September the axis of the cluster band shifted northward 100–300 km, and tended to split over the ocean. Clusters on the northern flank of the band were associated with African easterly waves, especially during Phase III; however, most of the clusters of GATE occurred to the south of the surface pressure trough and surface confluence, apparently in association with convergence within the mean low-level monsoon circulation.

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Jun Li, W. Paul Menzel, and Anthony J. Schreiner

Abstract

The optimal nonlinear inversion or one-dimensional variational (1DVAR) method was used to retrieve the cloud-top height and effective cloud amount from Geostationary Operational Environmental Satellite (GOES) sounder longwave spectral-band cloudy radiance measurements. The cloud-top pressure and effective cloud amount derived from the carbon dioxide (CO2)–slicing technique served as the background or first guess in the 1DVAR retrieval process. The atmospheric temperature profile, moisture profile, and surface skin temperature from the forecast analysis were used for the radiative transfer calculation in both the CO2-slicing method and the 1DVAR retrieval processing. Simulation studies were made to investigate the accuracy (the retrievals were compared with truth) of the cloud-top pressures and the effective cloud amounts derived from both the CO2-slicing and 1DVAR algorithms. Significant improvement of 1DVAR over CO2-slicing cloud properties was found in the simulation studies; an improvement of 10–50 hPa for root-mean-square error was obtained in 1DVAR over the CO2-slicing-derived cloud-top pressures, depending on the cloud height (high, mid, or low). This improvement came largely from the reduction of the bias in the 1DVAR retrievals over the CO2-slicing cloud-top pressures. The 1DVAR approach was applied to process the GOES-8 sounder cloudy radiance measurements; consistent with the simulation results, CO2 slicing assigned high and low clouds to lower levels than 1DVAR did.

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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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Anthony J. Schreiner, David A. Unger, W. Paul Menzel, Gary P. Ellrod, Kathy I. Strabala, and Jackson L. Pellet

A processing scheme that determines cloud height and amount based on radiances from the Visible Infrared Spin Scan Radiometer Atmospheric Sounder (VAS) using a CO2 absorption technique has been installed on the National Environmental Satellite Data and Information Service VAS Data Utilization Center computer system in Washington, D.C. The processed data will complement the Automated Surface Observing System (ASOS). ASOS uses automated ground equipment that provides near-continuous observations of surface weather data that are currently manually obtained. Geostationary multispectral infrared measurements are available every hour with information on clouds above the ASOS laser ceilometer viewing limit of 12 000 ft. The combined ASOS/satellite system will be able to depict cloud conditions at all levels up to 50 000 ft. The error rate of combined ASOS and satellite observations is less than 4% of the total sample in a comparison test with manual observations performed by National Weather Service personnel during March and April 1992. An attempt to distinguish thin from opaque clouds, by using a satellite-determined effective cloud amount, resulted in a substantial reduction in the discrepancies.

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W. Paul Menzel, Frances C. Holt, Timothy J. Schmit, Robert M. Aune, Anthony J. Schreiner, Gary S. Wade, and Donald G. Gray

Since April 1994 a new generation of geostationary sounders has been measuring atmospheric radiances in 18 infrared spectral bands and thus providing the capability for investigating oceanographic and meteorological phenomena that far exceed those available from the previous generation of Geostationary Operational Environmental Satellites (GOES). Menzel and Purdom foreshadowed many of the anticipated improvements from the GOES-8/9 sounders. This article presents some of the realizations; it details the in-flight performance of the sounder, presents both validated operational as well as routinely available experimental products, and shows the impact on nowcasting and forecasting activities.

For the first time operational hourly sounding products over North America and adjacent oceans are now possible with the GOES-8/9 sounders. The GOES-8/9 sounders are making significant contributions by depicting moisture changes for numerical weather prediction models over the continental United States, monitoring winds over oceans, and supplementing the National Weather Service's Automated Surface Observing System with upper-level cloud information. Validation of many sounding products has been accomplished by comparison with radiosondes and aircraft measurements. Considerable progress has been made toward assimilation of soundings from clear skies and cloud properties in cloudy regions in operational as well as research forecast models; GOES-8/9 moisture soundings are now being used in the operational Eta regional forecast model.

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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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