Computation, accuracy and applications of trajectories—A review and bibliography

This paper reviews the status of forecasting convective precipitation for time periods less than a few hours (nowcasting). Techniques for nowcasting thunderstorm location were developed in the 1960s and 1970s by extrapolating radar echoes. The accuracy of these forecasts generally decreases very rapidly during the first 30 min because of the very short lifetime of individual convective cells. Fortunately more organized features like squall lines and supercells can be successfully extrapolated for longer time periods. Physical processes that dictate the initiation and dissipation of convective storms are not necessarily observable in the past history of a particular echo development; rather, they are often controlled by boundary layer convergence features, environmental vertical wind shear, and buoyancy. Thus, successful forecasts of storm initiation depend on accurate specification of the initial thermodynamic and kinematic fields with particular attention to convergence lines. For these reasons ...

https://opensky.ucar.edu/islandora/object/articles%3A3309/datastream/PDF/view

Nowcasting Thunderstorms: A Status Report

Discusses the problem of determining the three-dimensional model and movement of an object from a sequence of two-dimensional images A solution to this problem depends on solving a system of nonlinear equations using a modified least-squared error method Two views of six points or three views of four points are needed to provide an overdetermined set of equations when the images are noisy It is shown, however, that this numerical method is not very accurate unless the images of considerably more points are used

Determining the movement of objects from a sequence of images

In this paper extensions to an existing tracking algorithm are described. These extensions implement adaptive tracking constraints in the form of regional upper-bound displacements and an adaptive track smoothness constraint. Together, these constraints make the tracking algorithm more flexible than the original algorithm (which used fixed tracking parameters) and provide greater confidence in the tracking results. The result of applying the new algorithm to high-resolution ECMWF reanalysis data is shown as an example of its effectiveness.

/pdf/adaptive-constraints-for-feature-tracking-4sfpxl6lkk.pdf

Adaptive Constraints for Feature Tracking

In order to produce gridded fields of pollutant concentration data and surface wind data for use in an air quality model, a number of techniques for interpolating sparse data values are compared. The techniques are compared using three data sets. One is an idealized concentration distribution to which the exact solution is known, the second is a potential flow field, while the third consists of surface ozone concentrations measured in the Los Angeles Basin on a particular day. The results of the study indicate that fitting a second-degree polynomial to each subregion (triangle) in the plane with each data point weighted according to its distance from the subregion provides a good compromise between accuracy and computational cost.

/pdf/a-comparison-of-interpolation-methods-for-sparse-data-4o76lz24b2.pdf

A Comparison of Interpolation Methods for Sparse Data: Application to Wind and Concentration Fields

The structure of the atmosphere in regions of clear-air turbulence is investigated by means of aircraft observations of wind and temperature in combination with objective and subjective turbulence records. The nature of the aircraft data and the assumptions inherent in the analyses are described. The detailed fields of vertical and horizontal wind shear, stability and Richardson number are presented for turbulence encountered in three different patterns of flow, viz., a sharp trough, an anticyclonic jet stream, and an intense straight jet. Severe turbulence (equivalent in intensity to that measured by the same aircraft in a mature thunderstorm) was found in certain portions of the trough and ridge, and moderate turbulence existed in the straight jet. Comparison of these cases and of twenty other flights indicates certain similarities and certain differences of the flow conditions in the turbulent regions. The necessary and sufficient conditions for the existence of clear-air turbulence have not b...

The Mesoscale Structure of Some Regions of Clear-Air Turbulence

Grid-point values of eastward and northward wind components imply that specific fields of divergence, vorticity and deformation exist. For certain purposes, one may wish to change the winds slightly to make them non-divergent, or to make them conform to fields of vorticity or deformation somewhat different than the original values. Control over the kinematic properties of a wind field can be exercised by the procedure given in this paper. The desired wind fields are obtained by a point iterative method applied to the two simultaneous linear partial differential equations that define horizontal divergence and relative vorticity, or stretching and shearing deformation. Some possible applications of this technique in analysis and forecasting are described briefly.

An Iterative Method for Altering the Kinematic Properties of Wind Fields.

Recent geosynchronous satellites (ATS-I and ATS-III) have obtained photographs of clouds over broad regions of the earth at intervals of ∼20 min. Sequences of such photographs show the direction and speed of cloud motions. To extract the motion information in real time it is desirable to process the raw data by computer methods. One approach to this problem is to locate a limited number of centers of brightness that represent the cloud patterns in a photograph. Such centers, which am determined on the basis of area and quantified brightness, are analogous to centre of gravity in mechanics. Measurements of displacements of these centers in successive photographs provide an indication of cloud motion. Brightness centers are found by an objective computer technique called “ISODATA” that was developed in an earlier program of pattern recognition. The average distance between brightness centers is determined in part by specifying values of certain parameters in the ISODATA program. This average distan...

Use of a Pattern Recognition Technique for Determining Cloud Motions from Sequences of Satellite Photographs

Observed winds and temperature profiles can be used to generate three-dimensional, mass-conserving wind fields that reflect topographical influences. The concept of critical dividing streamlines is used to define quasi-horizontal, flow-confining two-dimensional surfaces. Adjustment toward two-dimensional nondivergence on those surfaces forces flow around obstacles under stable conditions when some flow surfaces intersect higher terrain features. Unlike most mass-conserving wind models, the approach described here includes objective evaluation of the effects of atmospheric stability. Efficiency is achieved by casting the three-dimensional problem as several two-dimensional problems and by using an iterative scheme to adjust toward nondivergence. A 20 × 20 × 5 gridpoint analysis requires approximately 2 min on an IBM-AT personal computer.

Use of Mass Conservation and Critical Dividing Streamline concepts for Efficient Objective Analysis of Winds in Complex Terrain

An improved automatic processing method for the tracking of cloud motions as revealed by satellite imagery is presented and applications of the method to GOES observations of Hurricane Eloise and Meteosat water vapor and infrared data are presented The method is shown to involve steps of picture smoothing, target selection and the calculation of cloud motion vectors by the matching of a group at a given time with its best likeness at a later time, or by a cross-correlation computation Cloud motion computations can be made in as many as four separate layers simultaneously For data of 4 and 8 km resolution in the eye of Hurricane Eloise, the automatic system is found to provide results comparable in accuracy and coverage to those obtained by NASA analysts using the Atmospheric and Oceanographic Information Processing System, with results obtained by the pattern recognition and cross correlation computations differing by only fractions of a pixel For Meteosat water vapor data from the tropics and midlatitudes, the automatic motion computations are found to be reliable only in areas where the water vapor fields contained small-scale structure, although excellent results are obtained using Meteosat IR data in the same regions The automatic method thus appears to be competitive in accuracy and coverage with motion determination by human analysts

Roy M. Endlich

Papers

The Mesoscale Structure of Some Regions of Clear-Air Turbulence

An Iterative Method for Altering the Kinematic Properties of Wind Fields.

Use of a Pattern Recognition Technique for Determining Cloud Motions from Sequences of Satellite Photographs

Use of Mass Conservation and Critical Dividing Streamline concepts for Efficient Objective Analysis of Winds in Complex Terrain

Automatic Cloud Tracking Applied to GOES and METEOSAT Observations