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Captain Leo S. Bielinski

In cases of strong winds aloft, the upper-air wind reports are often erratic and unreliable and frequently do not extend to the level of the jet-stream core. The jet maxima shown at standard levels on conventional constant-pressure charts are more likely to be reflections of the “true” jet maximum which is located between the standard levels. Since the temperature reports of upper-air observations are reliable and available in most cases, a method is introduced which utilizes the temperature soundings to determine the location of the jet maxima. The “true” jet maxima can then be identified by barometric pressure height, temperature, thermal wind shear, and velocity.

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Project Jet Stream

The Observation and Analysis of the Detailed Structure of the Atmosphere near the Tropopause

R. M. Endlich, Patrick Harney, G. S. McLean, Robert M. Rados, Maj. O. J. Tibbets, and W. K. Widger Jr.

The importance of “jet streams” and other near-tropopause phenomena to aircraft operations led to the establishment by the Air Force of a project for probing these regions. Two specially instrumented aircraft, a B-47 jet bomber and a B-29, are the data gathering vehicles. Their instrumentation is discussed and examples of the data gathered thus far are given. These data show that significant maxima and minima of wind speed can occur between reporting stations and indicate that at jet stream levels, the gradient wind is a much better approximation to observed space-averaged winds than the geostrophic.

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Jean T. Lee and Joseph G. Galway

In a previous paper [5] the authors have indicated the observed relationship between the jet at 200 mb, the −60C isotherm at 200 mb and tornado occurrences. The location of this jet has become important in severe local storm forecasting procedures. Often, the lack of a band of strong winds at 500 mb, especially during the summer months, made this level ineffective as an aid in locating the jet some height above. The 20-mb data are not transmitted until more than six hours after time of observation; thus, the formulation of a chart which is not a constant-pressure chart and called by SELS “The JET CHART” was conceived to remedy this situation. The preparation, analysis, and use of this chart in severe local weather forecasting are set forth here.

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Julie Haggerty, Eric Defer, Adrianus De Laat, Kristopher Bedka, Jean-Marc Moisselin, Rodney Potts, Julien Delanoë, Frédéric Parol, Alice Grandin, and Stephanie Divito

Ingestion of large amounts of ice crystals by jet engines, known as the ice crystal icing (ICI) hazard, appears to be the culprit in more than 150 jet engine power-loss and damage events during the past two decades ( Fig. 1 ). Typically occurring near tropical convective systems, ICI events may also impact heated inlets used by an aircraft’s air data system. Although the heat within an engine or inlet would presumably prevent any ice buildup, analyses of engine power-loss events attributed to

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Robert R. Dickson

Data, acquired by specially instrumented aircraft, are presented for two levels through a northwesterly jet stream. Wind shear on the cyclonic side of this jet stream is roughly twice that on the anticyclonic side. Stronger areas of clear air turbulence appear closely related to strong vertical wind shear. An area of uniform absolute vorticity exists for about 160 nautical miles north of the jet stream. Measured microvariations of the temperature along a pressure surface—up to 3.1 C° in 8.5 nautical miles—give indirect evidence of jet stream “fingers” of high velocity.

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George S. McLean

Cloud observations made by crew members during flights of Project Jet Stream are used to determine the frequency of occurrence of various types of clouds in the vicinity of jet streams. These cloud distributions may be useful in upper-air analysis and forecasting. Furthermore, a knowledge of cloud distributions may be used as an in-flight aid for identifying and locating jet streams.

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Joseph Vederman

A model of the life cycle of the jet stream corresponding to the Bjerknes life cycle of extratropical cyclones is proposed. Diagrams are given showing various stages of development of the jet stream. The model is compared with analyzed charts of jet streams and surface cyclonic frontal systems.

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The JET2000 Project: Aircraft Observations of the African Easterly Jet and African Easterly Waves

Aircraft Observations of the African Easterly Jet and African Easterly Waves

C. D. Thorncroft, D. J. Parker, R. R. Burton, M. Diop, J. H. Ayers, H. Barjat, S. Devereau, A. Diongue, R. Dumelow, D. R. Kindred, N. M. Price, M. Saloum, C. M. Tayor, and A. M. Tompkins

Scientific background and motivation for the JET2000 aircraft observing campaign that took place in West Africa during the last week of August 2000 are presented. The Met Research Flight CI30 aircraft made two flights along the African easterly jet (AEJ) between Sal, Cape Verde, and Niamey, Niger, and two “box” flights that twice crossed the AEJ at longitudes near Niamey. Dropsondes were released at approximately 0.5°–10° intervals. The two box flights also included low-level flights that sampled north–south variations in boundary layer properties in the baroclinic zone beneath the AEJ.

Preliminary results and analysis of the JET2000 period including some of the aircraft data are presented. The JET2000 campaign occurred during a relatively dry period in the Niamey region and, perhaps consistent with this, was also associated with less coherent easterly wave activity compared to other periods in the season. Meridional cross sections of the AEJ on 28 and 29 August (after the passage of a mesoscale system) are presented and discussed. Analysis of dropsonde data on 28 August indicates contrasting convective characteristics north and south of the AEJ with dry convection more dominant to the north and moist convection more dominant to the south. The consequences of this for the AEJ and the relationship with the boundary layer observations are briefly discussed.

Preliminary NWP results indicate little sensitivity to the inclusion of the dropsonde data on the AEJ winds in European Centre for Medium-Range Weather Forecasts (ECMWF) and Met Office analyses. It is proposed that this may be due to a good surface analysis and a realistic model response to this. Both models poorly predict the AEJ in the 5-day forecast indicating the need for more process studies in the region.

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Johanna Yepes, Germán Poveda, John F. Mejía, Leonardo Moreno, and Carolina Rueda

Scientists and students from Colombia and the United States participated in CHOCO-JEX, the first field campaign to provide upper-air observations over the far eastern Pacific and western Colombia, a region that has one of the rainiest spots on Earth. The Chocó low-level jet (ChocoJet) is a westerly low-level jet, an extension of the southwesterly cross-equatorial flow that converges over the far eastern Pacific (EPAC) and western Colombia, which exerts a strong modulation over the region

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C. Vera, J. Baez, M. Douglas, C. B. Emmanuel, J. Marengo, J. Meitin, M. Nicolini, J. Nogues-Paegle, J. Paegle, O. Penalba, P. Salio, C. Saulo, M. A. Silva Dias, P. Silva Dias, and E. Zipser

Moisture is transported in South America westward from the tropical Atlantic Ocean to the Amazon basin, and then southward toward the extratropics. A regional intensification of this circulation to the east of the Andes Mountains is called the South American low-level jet (SALLJ), with the strongest winds found over eastern Bolivia. SALLJ is present all year and channels moisture to the La Plata basin, which is analogous to the better-known Amazon basin in terms of its biological and habitat diversity, and far exceeds the latter in its economic importance to southern and central South America in terms of hydroelectricity and food production. The relatively small SALLJ spatial scale (compared with the density of the available sounding network) has a limited understanding of and modeling capability for any variations in the SALLJ intensity and structure as well as its possible relationship to downstream rainfall.

The SALLJ Experiment (SALLJEX), aimed at describing many aspects of SALLJ, was carried out between 15 November 2002 and 15 February 2003 in Bolivia, Paraguay, central and northern Argentina, western Brazil, and Peru. Scientists, collaborators, students, National Meteorological Service personnel, and local volunteers from South American countries and the United States participated in SALLJEX activities in an unprecedented way, because SALLJEX was the most extensive meteorological field activity to date in subtropical South America, and was the first World Climate Research Program/Climate Variability and Prediction Program international campaign in South America.

This paper describes the motivation for the field activity in the region, the special SALLJEX observations, and SALLJEX modeling and outreach activities. We also describe some preliminary scientific conclusions and discuss some of the remaining questions

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