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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 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, R. L. Mancuso, and R. E. Nagle

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R. L. Mancuso, S. M. Serebreny, and R. H. Blackmer Jr.

Abstract

A cloud simulation model that is based on a Monte Carlo method was developed for testing and designing automatic ceilometer systems. With this model, cloud plan views and profile views an generated for several different cloud layers. The statistical properties of the cloud layers are controlled by a number of parameters. The influence of these control parameters on the generated plan and profile views are described and illustrated. The model allows a wide variety of realistic cloud conditions to be simulated and has been implemented in the form of a computer program. This computer program is currently being used to investigate the performance of various designs of automatic ceilometer systems in regard to determining true cloud amounts and ceiling heights.

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R. O. Duda, R. L. Mancuso, and S. M. Serebreny

Abstract

A computer simulation program was used to investigate the sampling problems that are associated with using automated instrument systems to determine cloud amount. The simulated cloud conditions were generated in the computer by specifying different values for parameters such as cloud amount and mean cloud length. The simulated instrument responses, which indicate the presence or absence of a cloud overhead, were used to make estimates of the cloud amount. The accuracy of these estimates was determined as a function of the number of instruments, area size, cloud size, sampling rate and averaging time. Conventional vertically pointing ceilometer instruments were found to be capable of producing cloud-amount estimates more accurate than those of human observers if four instruments are used and if the data are properly averaged. The results of this study can be used as a guide for designing instrument systems and data processing procedures.

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R. M. ENDLICH, R. L. MANCUSO, H. SHIGEISHI, and R. E. NAGLE

Abstract

A method is presented for combining remotely measured temperature soundings and wind data to provide a pressure-height reference level in the upper troposphere. In an initial phase of the study, balanced heights over North America were computed solely from standard wind reports and were found to conform closely to comparable National Meteorological Center height analyses. For oceanic application, height values from processed satellite infrared spectrometer (SIRS) data were objectively analyzed to obtain a geostrophic wind field. This wind field was used as a first guess in analyzing winds reported by commercial aircraft. The various terms of the balance equation were computed from the gridpoint winds, and balanced heights were determined by relaxation. These balanced heights blend temperature profile observations and wind data. They were used as upper level reference heights, and SIRS thickness values were subtracted from them to obtain height fields in the lower troposphere. Some typical results are illustrated.

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W. Viezee, S. M. Serebreny, R. L. Mancuso, and W. E. Shenk

Abstract

Distributions of relative vorticity and balanced height are computed from the cloud velocities associated with the cloud structure of an extratropical cyclone over the continental United States during a three-day period in March 1970. Using the SRI/NASA Electronic Display System, cloud motions are obtained from the abundance of cloud elements shown in series of ATS III photographs. Because of the absence of data on the specific height of cloud elements measured, cloud motions are assigned to a “mid-level” (when subjectively classified as middle clouds), or to a “high-level” (when subjectively classified as cirrus clouds). Derived vorticity and balanced height are compared with concurrent NMC analyses and also with similar kinematic quantities obtained from rawins at three constant-pressure levels. The computations of relative vorticity using “mid-level” cloud motion vectors show encouraging results. Patterns of computed cyclonic vorticity are related to the development, location and movement of the surface cyclone. Computed absolute values of relative vorticity show moderate correlation with 500-mb NMC values but show good positive correlation with vorticity computed from concurrent rawinsonde data for the 700-mb level. The analyses suggest that the “mid-level” corresponds best to the 700-mb level.

The vorticity analysis from the “high-level” motion vectors presented difficulties resulting from the wide latitudinal range in the height of cirrus. It is concluded that, with the addition of concurrent radiometric data, kinematic computations from cloud motions will improve beyond the results found in this investigation.

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