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R. M. Endlich
and
R. M. Rados

Abstract

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R. L. MANCUSO
and
R. M. ENDLICH

Abstract

Pilot reports from special turbulence-reporting periods were used to investigate methods of analyzing and forecasting clear-air turbulence over the United States. Meteorological analyses for the special reporting periods were made objectively by computer using only standard upper air rawinsonde measurements. The wind analyses were built upward from the 400-mb level to the 200-mb level using thermal wind shears to compensate for missing wind data in high-speed portions of the flow. The best meteorological indicators of turbulence were found to be the vertical vector wind shear and the product of wind shear and horizontal deformation. To a somewhat lesser extent, large gradients of relative humidity (at the 400-mb level) and large magnitudes of divergence also tended to be associated with turbulent regions. Turbulence analyses based on both meteorological relationships and pilot reports were made. These analyses are in terms of the probability of encountering significant turbulence (moderate or severe) within a 100-n.mi. flight sector. Advection forecasts of the turbulence probabilities were made, and these showed reasonable skill for periods out to 12 hr in advance.

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R. M. Endlich
and
J. R. Clark

Abstract

This paper describes research in formulating objective weather analysis and forecasting aids for calculation by an electronic computer. Methods based upon single-station techniques for analysis of rawinsoundings are presented for computing wind shear, lapse rate, thermal advection and a parcel stability index. These quantities are useful in identifying phenomena such as fronts, troughs, the tropopause and the level of maximum wind. Average values of height, temperature, humidity and wind components are computed in six arbitrarily defined layers for purposes of representing the gross atmospheric structure. A simple computer technique for calculating kinematical values of horizontal divergence, vorticity, deformation, vertical motion and geostrophic departures from layer-averaged wind and height data at three stations is developed and demonstrated for an area comprised of the midwestern United States. For this region, the computations require only 20 min on a computer of modest speed; therefore, the magnitude of the calculations is no longer an obstacle to their routine application. Limitations of the methods are described. Our experience indicates that these computer routines are adaptable to several operational and research problems even though the multiple-station computations are subject to considerable noise. Further developments of objective techniques and possible means of suppressing noise are described.

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R. M. ENDLICH
and
R. L. MANCUSO

Abstract

Regions of clear air turbulence in the upper troposphere and lower stratosphere are classified into four groups, and the characteristics of each group are summarized. An empirical turbulence index is defined that describes meteorological conditions associated with a type of turbulent region that is relatively large and that sometimes contains severe turbulence. The turbulence index, Richardson's number, and other meteorological quantities are described in relation to a clearly defined case of turbulence observed by a research aircraft. For the period March 12–24, 1902, vertical shear, lapse rate, Richardson's number, and the turbulence index were calculated by electronic computer from rawinsonde data over the United States. These computed quantities are compared with pilot reports of turbulence. Individual maps are presented that illustrate substantial agreement between computed quantities and turbulence reports. Standard statistical tests show that both Richardson's number and the turbulence index have definite skill in turbulence analysis. Suggestions are given for further improvement of criteria for analyzing turbulence. To enhance turbulence research and operational analysis, the writers believe that a turbulence sensor operated as an integral part of the radiosonde system is very desirable.

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R. M. ENDLICH
and
R. L. MANCUSO

Abstract

This study describes objective analysis of the atmospheric conditions that precede or accompany severe thunderstorms and tornadoes. The data used are standard rawinsonde observations and hourly surface reports as they are transmitted over teletypewriter. In analyzing upper air data, spherical coordinates are used with grid points 2½° of lat. and long. apart. Hourly observations are analyzed on a 1¼° grid. The vertical structure of the atmosphere is represented by seven layers between the surface and 100 mb. Observational data are averaged for these layers using all points in the soundings. By use of a nondimensional pressure term as the vertical coordinate, the three layers below 500 mb. slope with the terrain, and the lowest layer contains most boundary processes.

The objective analysis procedure fits a first degree polynomial to at least five observations that are nearest to a grid point. A distance weighting factor and upstream-downstream enhancement are used. The analysis method smooths the observations lightly, but has a resolution and accuracy that appear approximately equivalent to those of hand methods. Analyzed quantities include wind components, height, temperature, and moisture. From these a number of kinematic quantities not normally available to forecasters are computed and compared with storm developments. In general, certain quantities that depend on the field of motion appear to be more directly related to storm formation than do synoptic or thermodynamic factors. Objective severe storm indicators that combine different synoptic or kinematic factors are formulated at grid points, and their patterns match areas of storm development reasonably well. The results support the belief that the forecaster's accuracy and efficiency can be increased through greater reliance on computer methods of data processing and analysis.

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R. L. MANCUSO
and
R. M. ENDLICH

Abstract

The probability is determined that an aircraft will encounter moderate or severe high-level turbulence during a 100-mi. flight segment when particular values of certain meteorological quantities exist in that locality. The turbulence data used are pilot reports collected by the U.S. Weather Bureau Clear Air Turbulence Project, for March 12–24, 1962 and February 4–9, 1963. The meteorological quantities which were computed from standard data include vertical vector wind shear, vertical wind direction shear, temperature lapse rate, horizontal wind shear, vorticity, and resultant deformation. A correlation of 0.45 was found between turbulence frequency and the product of vertical vector wind shear and deformation. This value is the highest correlation found so far with data of this type. The product of wind shear and deformation is an important factor in the development of fronts. One might expect that a tendency for frontogenesis would correlate better with turbulence than would frontolysis. The data however, indicate that both processes are equally important. Temperature lapse rate appeared to have little influence on the turbulence frequency except for a few occasions when conditions were nearly dry adiabatic. The regression equations between turbulence frequency and meteorological quantities that have been derived appear useful in estimating the risk of encountering turbulence in a given locality. Such turbulence estimates can be made at a particular time from the concurrent upper-air data, or on a climatological basis from the climatology of the pertinent meteorological factors.

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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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