Jet Streams of the Atmosphere By Herbert Riehl Department of Atmospheric Science Colorado State University Fort Collins, Colorado May 1962 CER62HR35 Jet Streams of the Atmosphere by Herbert Riehl Technical Report No. 32 Department of Atmospheric Science Colorado State University Fort Collins. Colorado May 1962 The price for this report is $1.50. Copies may be obtained from the Department of Atmospheric Science Colorado State University Fort Collins, Colorado JET STREAMS OF THE ATMOSPHERE by Herbert Riehl Colorado State University With a chapter on The Jet Stream and Aircraft Operations by Cdr. J. W. Hinkelman, Jr., USN Technical Paper Number 32 Department of Atmospheric Science Colorado State University Fort Collins. Colorado May 1962 CER62HR35 FOREWORD The presence of jet streams in the atmosphere is one of the most important and exciting facts in the field of Atmospheric Science. The practical impact created by the narrow bands of high wind speed goes far beyond air route planning, downstream with the current, upstream at a judicious distance from the core. Missile flight planning must take account of this current. The cloud systems that bring rain and snow to the ground often are shaped by the jet stream and in their turn may react on it. Many features of surface cli­ mate are closely related to jet stream incidence. In the field of theory, the observed concentration of kinetic energy into narrow cores is a highly challenging problem. No theory of the general circulation and of the daily weather disturbances will be satisfactory without an explanation of the wind concentration and its role in the mechanics and energy transformations of the atmosphere. Closely allied are problems of smaller scale; for example the turbulence sometimes experienced by subsonic jets and other aircraft at the height of the jet­ stream core. Almost a decade has passed since preparation of the first comprehensive monograph entitled "The Jet Stream" by H. Riehl and Collaborators. It was published by the U. S. Navy (NAVAER 50-1R-249) and sub­ sequently, in expanded form, by the American Meteorological Society as Monograph #7. Since that time, the large increase in the rawin network over the globe has furnished a new wealth of data on jet stream occurrence and on the gross structure of the current. Special research missions have con­ tributed to our knowledge of finer structural details, as have satellite observations to the description of the associated cloud systems. Jet-stream like velocity concentrations have been generated in geophysical model experiments. With so much additional information it appears appropriate to give a new state-of-the-art resume con­ cerning this very challenging area of modern meteorology, which should be welcome to meteorologists as reference and text material. Moreover, Dr. Riehl has managed to present the subject with minimum use of mathematics. This volume, therefore, should be a valuable guide to aviators and others concerned with aero­ space problems and use who require a succinct presentation without the burden of cumbersome derivations of formulae. Special attention is directed to Chapter X which contains an outline of procedures for numerical jet stream analysis. This may be considered as a large step forward toward objective forecasting -- a subject neglected in much of the research effort. Commander J. W. Hinkelman USN, currently detailed to the Fed­ eral Aviation Agency, is the author of Chapter XI. There, he has undertaken one of the very rare, yet much needed, analyses that connect meteorology proper with the requirements of one of its chief consumers. Cdr. Hinkelman's clear statement of the present position and his ideas about the future should prove to become important guides in the development of meteorological aids to aviation. As in case of the first monograph, the writing of this volume was supported by the U. S. Navy Weather Research Facility, NAS, Norfolk, Va. We may hope that the present edition will enjoy as wide a circulation as its predecessor for the benefit of increased understanding and application of the intriguing, yet by no means fully explained, phenomenon of the jet stream. Elmar R. Reiter Associate Professor of Atmospheric Science Fort Collins, Colorado May 1962 ACKNOWLEDGEMENTS Thanks of the author are due to the U. S. Navy Weather Research Facility, NAS, Norfolk, Va., and especially to Capt. D. F. Rex, Cdr. C. A. Palmer and Mr. A. L. Morris, for encouraging and supporting the writing of this monograph. Thanks are due also to Mrs. P. Eggleston for editing the text and for styling the volume, and to Mrs. H. Sargent for preparing the manuscript. Publication of the monograph was facilitated by the Administration of Colorado State University. H. RIEHL T ABLE OF CONTENTS Page Foreword .... I. Introduction II. Wind Structure of the Jet Stream 3 Data Sources and Errors . 3 Highest Wind Speeds in Jet Streams 3 Vertical Wind Structure. 4 Lateral Wind Profiles. 5 Horizontal View . 7 Vertical Cross Sections of Wind 12 III. Thermal Structure of the Jet Stream . 14 Thermal Wind. 14 Stratospheric Temperature Field 16 Vertical Cross Sections. 17 Temperature Charts . 18 Dewpoint Cross Section ..... 19 IV. A Jet Stream Example - 6 March 1958 24 The Jet Stream from Balloon Soundings 24 The Jet Stream from Aircraft Reconnaissance 27 V. The Subtropical Jet Stream . 30 The Subtropical Jet Stream of Winter . 30 The Subtropical Jet Stream in Summer 36 VI. The Polar Night Jet Stream .. 42 VII. Fluctuations of the Jet Stream 48 Individual Currents . 48 Hemispheric Fluctuations 48 VIII. The Jet Stream in Relation to Weather and Cyclones 53 Jet Streams and Cyclones ...... 53 Clouds and Precipitation in Jet Streams 57 Clear-Air Turbulence 63 IX. Climatic Aspects of the Jet Stream ..... 65 Mean Circulation . 65 Mean Cross Sections for Various Longitudes 68 X. High-Level Wind Analysis 74 Evaluation of Balloon Soundings 74 Secondary Wind Fluctuations . 75 LMW Parameters . 78 Analysis of Jet Stream Charts . 80 XI. The Jet Stream and Aircraft Operations 85 Introduction . 85 Aircraft Operational Aspects 85 Navigational Aspects . 85 Meteorological Aspects 86 Flight Planning . 87 Air Operations and Air Traffic Management of the Future 91 Table of Contents - Cont'd. Page XII. On the Formation and Maintenance of Jet Streams 93 The Work of C. -G. Rossby ... 93 General Circulation Calculations 96 On the Energy of Jet Streams 97 Transverse Circulation 102 Potential Vorticity Field 104 Model Experiments 107 Bibliography . 113 I. INTRODUCTION Flight missions in the altitude range from winter. During that season we encounter three dis­ :30,000 to 40,000 feet have become commonplace in tinct jet stream systems: the military services; since the 1950's, regular commercial service has begun. Sounding balloons 1) The 'subtropical' jet stream, which marks penetrate to the tropopause and beyond at many the poleward limit of the trade wind cell of the gen­ points around the globe. At least once daily they eral circulation; provide information on wind, temperature and heights of isobaric surfaces to great altitudes. New 2) the 'polar front' jet stream, which is as­ types of measuring equipment, such as transosond.e sociated with the principal frontal zones and cyclones balloons, furnish data. All of these sources of in­ of middle and subpolar latitudes; and formation document amply that the air currents of the tropopause region in the temperate zone and in other climatic belts pos sess narrow, high­ velocity cores, bordered on both sides by broad -20 -10 expanses of more sluggish motion. -5 The leadership in the early phases of jet stream 200 research was carried by C .-G. Rossby. He looked for evidence of high -velocity cores from the late 1930's, following his work on the Gulf Stream 400 (Rossby 1936), a narrow ocean current with speeds of meters per second embedded in an environment with speeds of centimeters per second (fig. 1. 1). At first Rossby met discouragement because winds 600 at 10,000 feet-a standard level for upper-air anal­ ysis for many years - varied only gradually with ~ latitude. Then, in the early 1940' s, reports began Q) to arrive that spoke of extraordinary wind speeds W 800 encountered by high-altitude bombing planes. Near 6 I 30, 000 feet these winds on occasion attained the --- -100 t­ 0 10 speed of the planes themselves. o.. 4 --_ ~IOOO -" \ With the rise in aircraft ceilings, the frequency \ of radiosonde ascents reaching the tropopause also 5 increased. Beginning in 1945 maps of the tropo­ pause region and vertical cross sections to 300 mb 1200 and higher could be drawn in several parts of the \ northern hemisphere. With use of geostrophic or r\ gradient wind formulae, wind speeds were calcu­ o 40 80 120 1400 I ".1. I. I "II .1 lated on these charts and sections. In startling Nautical Miles similarity to the Gulf Stream picture, these calcu­ "- lations revealed that the flow configuration aloft, " "--- especially in the middle latitudes in winter, is 1600 --- suggestive of a broad stream of air meandering eastward around the hemisphere in wave-like pat­ terns, with the energy of motion concentrated in o narrow bands of high speed. 1800 In analogy with fluid dynamics, Rossby (Rossby 1947 a, Staff Members, University of Chicago, 1947) Fig. 1.1 Vertical cross section through the Gulf called the newly discovered current 'jet stream' . Stream off Chesapeake Bay, looking This name has been widely accepted, though its downstream. Solid lines are lines of merits may be disputed. During the initial phases equal velocity of ocean current (cm/ sec), of research, it seemed likely that there was a depicting narrow high -speed core. Dash­ single jet stream around each hemisphere. Follow­ ed lines are isotherms (OC) portraying ing extension of the sounding networks in arctic and abrupt temperature drop from right to tropical regions, plus the advent of rawins, this left across the Gulf Stream (adapted simple picture had to be amended, especially in from Iselin, 1936).