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R. M. Endlich
and
G. S. McLean

Abstract

Due to a lack of observational data, little information exists in the literature concerning quantitative relationships between turbulent gust intensity in the free atmosphere and commonly measured meteorological quantities. Measurements of turbulence, wind, and temperature made by a B-47 research aircraft are used to investigate such relationships in order to satisfy several practical needs. It is found that a quantity that is the product of wind speed and turning of the wind with height is more closely related to turbulent gust intensity than vertical wind shear or Richardson's number. Variations in the frequency of occurrence of turbulence are determined as the quantities mentioned above increase in magnitude. Moderate or severe turbulence has a frequency of occurrence greater than 50 per cent when the product of wind speed and vertical change of direction exceeds 0.8 deg sec−1, when vertical shear exceeds 0.027 sec−1, or when Richardson's number is less than 0.4 (for calculations made over 2000-ft layers). The writers believe that the results are representative of winter condition in mid-latitudes.

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R. M. Endlich
and
G. S. McLean

Abstract

No Abstract Available.

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R. M. Endlich
and
G. S. McLean

Abstract

Wind measurements made by aircraft of Project Jet Stream during forty-eight flights are compared with geostrophic and gradient winds (computed on upper-air charts) in order to determine geostrophic and gradient departures. Due to random errors in radiosonde data and to a lesser extent in the aircraft winds, individual departures are not reliable. Therefore, the jet stream area, as represented on a vertical cross-section perpendicular to the wind flow, is arbitrarily divided into nine sectors. Average departures are computed for each sector and for certain combinations of sectors.

In cyclonic jet streams (i.e., in the vicinity of upper troughs), these calculations gave the following results: observed wind speeds were, on the average, 27.5 kn (or 18.4 per cent) less than geostrophic speeds but were in excellent agreement with gradient speeds. Although the average gradient departure was approximately zero, these departures tended to be negative (observed winds less than gradient winds) on the south side of the jet core and slightly positive on the north side. The standard deviation of this population of true geostrophic departures was 20 kn, while the standard deviation of true gradient departures was 6 kn. Gradient winds were therefore considerably superior to geostrophic winds as an approximation to observed winds in cyclonically curved jet streams.

In straight jet streams, observed winds were 2.4 kn (2.8 per cent) sub-geostrophic (and sub-gradient) on the average; however, this departure should be considered as essentially zero. Sub-geostrophic flow was most pronounced in the layer immediately above the jet core. Sufficient data were not available for determining the departures in anticyclonic jet streams.

The statistical significance and theoretical consequences of the departures are discussed.

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R. M. Endlich
and
G. S. McLean

Abstract

Typical jet stream characteristics are illustrated by discussions of an average, a strong and a weak jet stream. The basic data were obtained by aircraft of Project Jet Stream in flights carried out mainly over the southeastern United States. An empirical model, based on observations obtained during some fifty flights, illustrates the average wind and temperature fields in the vicinity of the jet stream core and the structure of the “jet stream front.” It is suggested that this model may be used to define jet stream flow in mid-latitudes. Average distributions of turbulence, clouds and contrails around the jet stream are presented.

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R. M. Endlich
and
G. S. McLean

Abstract

Aircraft observations over the central United States are used to construct empirical models of jet streams that agree well with models based on data from other regions of the mid-latitudes. Individual jet streams conform to speeds of the model within certain limits. These limits are represented by a percentage variability that is larger to the north of the jet core than to the south. On the other hand, variations in the structure of the average speed field with season and with position in the upper flow appear to be rather small so that a single model describes the jet stream adequately. The field of average transverse wind component indicates appreciable motion toward high pressure in a layer beneath the jet-stream core. Average mesoscale vertical motion as large as −0.4 m sec−1 occurs below and slightly north of the core. Turbulence has a minimum frequency of occurrence at the maximum wind level immediately south of the jet core. Vertical wind shear has appreciably larger magnitudes than thermal wind shear in two shallow layers separated by the maximum wind level. The vorticity field corresponding to the wind and temperature fields is described. Several unexplained features of jet streams are mentioned briefly.

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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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F. Vitart
,
C. Ardilouze
,
A. Bonet
,
A. Brookshaw
,
M. Chen
,
C. Codorean
,
M. Déqué
,
L. Ferranti
,
E. Fucile
,
M. Fuentes
,
H. Hendon
,
J. Hodgson
,
H.-S. Kang
,
A. Kumar
,
H. Lin
,
G. Liu
,
X. Liu
,
P. Malguzzi
,
I. Mallas
,
M. Manoussakis
,
D. Mastrangelo
,
C. MacLachlan
,
P. McLean
,
A. Minami
,
R. Mladek
,
T. Nakazawa
,
S. Najm
,
Y. Nie
,
M. Rixen
,
A. W. Robertson
,
P. Ruti
,
C. Sun
,
Y. Takaya
,
M. Tolstykh
,
F. Venuti
,
D. Waliser
,
S. Woolnough
,
T. Wu
,
D.-J. Won
,
H. Xiao
,
R. Zaripov
, and
L. Zhang

Abstract

Demands are growing rapidly in the operational prediction and applications communities for forecasts that fill the gap between medium-range weather and long-range or seasonal forecasts. Based on the potential for improved forecast skill at the subseasonal to seasonal time range, the Subseasonal to Seasonal (S2S) Prediction research project has been established by the World Weather Research Programme/World Climate Research Programme. A main deliverable of this project is the establishment of an extensive database containing subseasonal (up to 60 days) forecasts, 3 weeks behind real time, and reforecasts from 11 operational centers, modeled in part on the The Observing System Research and Predictability Experiment (THORPEX) Interactive Grand Global Ensemble (TIGGE) database for medium-range forecasts (up to 15 days).

The S2S database, available to the research community since May 2015, represents an important tool to advance our understanding of the subseasonal to seasonal time range that has been considered for a long time as a “desert of predictability.” In particular, this database will help identify common successes and shortcomings in the model simulation and prediction of sources of subseasonal to seasonal predictability. For instance, a preliminary study suggests that the S2S models significantly underestimate the amplitude of the Madden–Julian oscillation (MJO) teleconnections over the Euro-Atlantic sector. The S2S database also represents an important tool for case studies of extreme events. For instance, a multimodel combination of S2S models displays higher probability of a landfall over the islands of Vanuatu 2–3 weeks before Tropical Cyclone Pam devastated the islands in March 2015.

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