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Peter S. Ray
,
David P. Jorgensen
, and
Sue-Lee Wang

Abstract

Airborne Doppler radar can collect data on target storms that are quite widely dispersed. However, the relatively long time required to sample an individual storm in detail, particularly with a single aircraft, and the amplification of the statistical uncertainty in the radial velocity estimates when Cartesian wind components are derived, suggests that errors in wind fields derived from airborne Doppler radar measurements would exceed those from a ground based radar network which was better located to observe the same storm. Error distributions for two analysis methods (termed Overdetermined and Direct methods) are given and discussed for various flight configurations. Both methods are applied to data collected on a sea breeze induced storm that occurred in western Florida on 28 July 1982. Application of the direct solution, which does not use the continuity equation, and the overdetermined dual-Doppler method, which requires the use of the continuity equation, resulted in similar fields. Since the magnitude of all errors are unknown and the response of each method to errors is different, it is difficult to assess overall which analysis performs better; indeed each might be expected to perform best in different parts of the analysis domain. A flexible collection strategy can be followed with different analysis methods to optimize the quality of resulting synthesized wind fields.

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J. J. Stephens
,
Peter S. Ray
, and
R. J. Kurzeja

Abstract

Approximations to the far-field, backscattering response for an electromagnetic impulse are shown for two water sphere sizes. For small electrical sizes, the scattering is described by an electric dipole; for large electrical sizes, a combination of reflection from the front interface, creeping waves, and surface currents excited as the impulse moves across the sphere is used.

It is shown that transient effects are confined effectively to an equivalent space period of less than six diameters and can be neglected in all operational applications of pulse radar to rain detection.

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Brenda C. Johnson
,
Judith Stokes
, and
Peter S. Ray

Abstract

Optimum design of a Doppler radar system for operation in a severe storm environment will depend on the maximum unambiguous velocity. Radial velocities of severe storms are examined from four Doppler radars over several hours on 20 May 1977. The probability of a radial velocity occurrence for a given pulse repetition frequency-wavelength combination is presented.

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Peter S. Ray
,
J. J. Stephens
, and
K. W. Johnson

Abstract

An objective procedure for determining optimum radar positions in a multiple-Doppler radar network is given for the objective criteria of maximizing the integrated reliability. It is demonstrated for arrays of three to seven radars, including cases where some positions are fixed. The optimum locations for a three-Doppler radar network are at the vertices of an equilateral triangle. Subject to the chosen objective function the best deployment does not result from simply adding additional radars one-by-one to an existing radar network.

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Paul E. Bieringer
,
Peter S. Ray
, and
Andrew J. Annunzio

Abstract

A study by Bieringer et al., which is Part I of this two-part study, demonstrated analytically using the shallow-water equations and numerically in controlled experiments that the presence of terrain can result in an enhancement of sensitivities to initial condition adjustments. The increased impact of adjustments to initial conditions corresponds with gradients in the flow field induced by the presence of the terrain obstacle. In cross-barrier flow situations the impact of the initial condition adjustments on the final forecast increases linearly as the height of the terrain obstacle increases. While this property associated with initial condition perturbations may be present in an analytic and controlled numerical environment, it is often difficult to realize these benefits in a more operationally realistic setting. This study extends the prior work to a situation with actual terrain, Doppler radar wind observations over the terrain, and observations from a surface mesonet for model verification. The results indicate that the downstream surface wind forecast was improved more when the initial conditions adjusted through the assimilation of Doppler radar data originated from areas with terrain gradients than from regions where the terrain was relatively flat. This result is consistent with the findings presented in Part I and suggests that when varying terrain elevation is present upstream of a target forecast area, a greater benefit (in terms of forecast accuracy) can be made by targeting additional observations in the regions containing variable terrain than regions where the terrain is relatively flat.

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Cathy J. Kessinger
,
Peter S. Ray
, and
Carl E. Hane

Abstract

On 19 May 1977, a severe squall line formed and moved through the National Severe Storms Laboratory observing network in Oklahoma, producing heavy rain, hail, strong winds, and tornadoes. The squall line is examined at two times: 1434 and 1502 CST. Doppler analysis of part of the squall line reveals four convective cells in the line, developing cells ahead of the line, a trailing precipitation region, and a convective rainband at the western edge of the system. The updrafts within the convective cells on the leading edge tilt westward in the lower levels and eastward near the tropopause. Convective updrafts and downdrafts are fed by low-level air entering the squall line from the front. Surface network analysis and gust front penetration by an instrumented aircraft indicated strong convergence along the leading edge of one of the stronger cells in the line. Horizontal, line-relative flow perpendicular to the squall line and within the trailing precipitation area is from east to west (front to back) at all levels, weakening with height. An exception to this is an area of weak (≤3 m s−1) rear inflow into the stratiform precipitation region in the midlevels. Flow parallel to the squall line is stronger, in general, than the perpendicular flow. A composite rawinsonde analysis shows ascending motion within the troposphere over most of the squall line region. A conceptual model is developed for 19 May 1977 and is compared to conceptual models of tropical squall lines and of the 22 May 1976 Oklahoma squall line.

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Paul E. Bieringer
,
Peter S. Ray
, and
Andrew J. Annunzio

Abstract

The concept of improving the accuracy of numerical weather forecasts by targeting additional meteorological observations in areas where the initial condition error is suspected to grow rapidly has been the topic of numerous studies and field programs. The challenge faced by this approach is that it typically requires a costly observation system that can be quickly adapted to place instrumentation where needed. The present study examines whether the underlying terrain in a mesoscale model influences model initial condition sensitivity and if knowledge of the terrain and corresponding predominant flow patterns for a region can be used to direct the placement of instrumentation. This follows the same concept on which earlier targeted observation approaches were based, but eliminates the need for an observation system that needs to be continually reconfigured. Simulations from the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) and its adjoint are used to characterize the locations, variables, and magnitudes of initial condition perturbations that have the most significant impact on the surface wind forecast. This study examines a relatively simple case where an idealized mountain surrounded by a flat plain is located upwind of the forecast verification region. The results suggest that, when elevated terrain is present upstream of the target forecast area, the largest forecast impact (defined as the difference between the simulation with perturbed initial conditions and a control simulation where the initial condition was not perturbed) occurs when the initial analysis perturbations are made in regions with complex terrain.

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Conrad L. Ziegler
,
Peter S. Ray
, and
Donald R. MacGorman

Abstract

This paper addresses aspects of the airflow, microphysics, and electrification in a mountain thunderstorm which occurred on 7 August 1979 over the Langmuir Laboratory new Socorro, New Mexico, site of the Thunderstorm Research International Program (TRIP). Single Doppler observations are used to form a conceptual model of the essentially one-dimensional storm updraft which is expressed in simple analytical form. A one-dimensional kinematic numerical cloud model is employed with the analytic updraft profile to diagnose the evolution of temperature, war substance, radar reflectivity, space charge density and axial electric field in the main updraft region. Retrieved thermal, microphysical, and electrical variables are verified with in situ aircraft and balloon observations and measured radar reflectivity. The calculated rate of noninductive charge transfer accompanying collision and separation of ice crystals and riming graupel particles is in direct proportion to cloud and precipitation content, and attains a peak value of about 10 C km−3 min−1 between −30° and −40°C. Agreement between calculations and balloon measurements of space charge density and vertical electric field imply that the noninductive graupel-ice charge separation mechanism accounts for a substantial portion of the storm's total separated charge. The peak noninductive charging rate appears to balance the discharge rate implied by the observed flash rate.

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Karen L. Sangren
,
Peter S. Ray
, and
Gene B. Walker

Abstract

Doppler velocity spectra collected at vertical incidence contain information on vertical air motions and drop-size distributions with high spatial and temporal resolution. In the past, the computational interdependence between vertical air velocities and drop-size distributions has severely limited the accuracy with which they could be estimated. A dual-wavelength technique is applied in which vertical air motion is determined independently of the drop-size distribution. The Rogers reflectivity method and an extended version of the Hauser-Amayenc method are also applied. The latter technique fits Doppler spectra in a nonlinear least-squares sense using two exponential drop-size distribution models. Results of applying each method to Oklahoma squall line data are compared and the strengths and weaknesses of the three techniques are assessed. For the methods tested, there is a trade-off between potential accuracy and potential for successful application. For example, the dual wavelength method is theoretically quite accurate but is extremely sensitive to poor data quality.

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Donald W. Burgess
,
Larry D. Hennington
,
Richard J. Doviak
, and
Peter S. Ray

Abstract

A single-Doppler data display has been developed which allows simultaneous presentation of the principal moments of the Doppler spectrum while retaining ease of viewing and interpretation. Display severe storm signatures are described and an example of data collected in a tornadic storm is presented. A tornado signature is identified, its evolution is followed, and comparisons between the signature and the tornado damage track are made.

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