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Richard J. Reed

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Lance F. Bosart

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Juanzhen Sun and James W. Wilson

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Assimilation of radar data is one of the key scientific challenges for numerical weather prediction of convective systems. Considerable progress has been made in recent years including retrieval of boundary layer winds from single-Doppler observations, assimilation of radar observations into convective-scale numerical models for explicit thunderstorm prediction, and assimilation of radar estimates of rainfall and wind into mesoscale models. However, the assimilation of radar data for weather prediction remains an important scientific area that demands further investigation. In this paper, the techniques that are currently being used and have demonstrated potential in radar data assimilation are presented. The progress on the research and applications is described and the future directions and challenges are outlined.

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Daniel Rosenfeld and Carlton W. Ulbrich

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The question of the connections between raindrop-size distributions (RDSDS) and radar reflectivity–rainfall rate ZR relationships is explored from the combined approach of rain-forming physical processes that shape the RDSD, and a formulation of the RDSD into the simplest free parameters of the rain intensity R, rainwater content W, and median volume drop diameter D 0. This is accomplished through examination of integral parameters deduced from the RDSD associated with the host of ZR relations found in the literature. These latter integral parameters are deduced from the coefficient and exponent of empirical ZR relations using a gamma RDSD. A physically based classification of the RDSDs shows remarkable ordering of the D 0W relations, which provides insight into the fundamental causes of the systematic differences in ZR relations.

The major processes forming the RDSD are examined with respect to a mature equilibrium RDSD, which is taken as the eventual distribution. Emphasis is placed on cloud microstructure (with the two end members being “continental” and “maritime”) and cloud dynamics (with end members “convective” and “stratiform”). The influence of orography is also considered. The ZR classification scheme can explain large systematic variations in ZR relations, where R for a given Z is greater by a factor of more than 3 for rainfall from maritime compared to extremely continental clouds, a factor of 1.5–2 greater R for stratiform compared to maritime convective clouds, and up to a factor of 10 greater R for the same Z in orographic precipitation. The scheme reveals the potential for significant improvements in radar rainfall estimates by application of a dynamic ZR relation, based on the microphysical, dynamical, and topographical context of the rain clouds.

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David P. Jorgensen and Tammy M. Weckwerth

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From its initial deployment as a research tool following the second World War, radar has played a fundamental role in revealing the forces that initiate and organize severe storms and larger mesoscale convective systems composed of a conglomeration of convective storm cells. Early radar observations were primarily descriptive and showed the tremendous variety of precipitating moist convection types and sizes. Examples include single convective storms, longer-lived multicellular storms, fast-moving squall lines, slower-moving linear and nonlinear convective systems, and long-lived supercell storms. Certain modes or types of convective systems were shown to possess a variety of hazardous weather that includes very heavy rain, large hail, straight-line damaging winds, tornadoes, and lightning. It was soon recognized that the type of convective system was strongly dependent on the environment in which it was embedded. Researchers determined that two variables were particularly important in describing convective behavior: the vertical profile of the horizontal wind and potential instability of the air feeding the system [convective available potential energy (CAPE)]. The types of convective systems are discussed here according to their typical shear and CAPE values. In addition to the knowledge gained from observational radar studies, considerable advancement in understanding of convective system dynamics has resulted from high-resolution numerical simulations.

In addition to being a critical factor in determining the particular structure and organization that convective systems assume once convection is initiated, radar (particularly in clear air mode) has been a leading tool in identifying forcing mechanisms for convective initiation. In particular, the role of “boundary layer forcing” in initiating convection has received much attention in recent years. Boundary layer circulations, which are sometimes precursors to deep convective development, are clearly observed by radar as reflectivity fine lines and/or discontinuities in Doppler velocity. Some of these mesoscale boundary layer mechanisms for producing upward motion include horizontal convective roles, sea-breeze circulations, drylines, gust fronts, orographic circulations (e.g., mountain–valley), and circulations resulting from horizontal inhomogeneities in surface character. Convection initiation sometimes does not occur continuously along boundaries but only at preferred along-boundary locations. Location preferences can sometimes be identified with boundary intersections, such as colliding gust fronts, sea-breeze fronts and rolls, and drylines and rolls. It is not always clear, however, why convection forms at certain locations along boundaries and not others. It is possible that low-level waves, bores, and other features, which may not always be apparent in radar data, may also play an important role in convection initiation processes.

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V. Chandrasekar, R. Meneghini, and I. Zawadzki

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Radars have played an important role in the observation of precipitation and will continue to do so in the future. With the recent introduction of space-based radar for measuring precipitation on the Tropical Rainfall Measurement Mission (TRMM) satellite, weather radar applications now range from local to global scales. The radar basis for characterizing precipitation lies in the scattering and propagation properties of electromagnetic waves through precipitation, and is summarized in this paper. The methodologies for converting the backscattering and propagation measurements such as radar reflectivity, differential reflectivity, differential propagation phase, and attenuation to precipitation estimates are provided for both ground-based and space-based radars. Quantitative precipitation estimation has been a challenging problem for over four decades. This challenge has inspired extensive progress in the area of precipitation microphysics, remote sensing techniques, and in situ observations. Another major advance in quantitative precipitation estimation is the understanding of the critical role player by practical engineering considerations. Techniques for developing precipitation algorithms from space and ground observations as well as strategies for validating the estimates are also presented. Following a summary of the various challenges, the discussion focuses on those areas with potential for significant future progress for the estimation of both local and global precipitation.

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Frédéric Fabry and R. Jeffrey Keeler

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The design and implementation of signal-processing algorithms are specialized trades of radar meteorology practiced by a small group of experts and poorly understood by most other radar data users. Yet signal processing is the essential first step of radar data processing, and the skill with which it is done determines the type and quality of data that will be available to radar meteorologists. Like many other facets of radar meteorology, it is undergoing a rapid evolution as computing capabilities expand exponentially. In this chapter, an overview of the current state and evolution of signal processing for the nonspecialist is provided. To achieve this, the nature and the properties of the radar signal itself is first described, as it determines the type and quality of the information that can be obtained. After these foundations are laid, the current state of signal processing on operational radars and then some of the latest developments that may shape the future are described.

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Keith A. Browning

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Extratropical cyclones are responsible for significant weather in the form of heavy precipitation and strong winds. The capability of numerical weather prediction models to predict the synoptic-scale structure of such cyclones has improved greatly over recent years but much of the significant weather itself is associated with small and mesoscale processes not properly represented even in today's relatively high-resolution models. As a result, the detailed prediction of significant weather, even for the period 1–12 h ahead, still falls far short of requirements. In order to find out what improvements are needed by way of increased model resolution, better parameterizations, and/or improved observations/assimilation, it is first necessary to learn more about the structure, mechanism, and interaction of the small-scale and mesoscale processes. This is the subject of this review.

The review focuses on the structure and organization of slantwise and upright convection within extratropical cyclones, particularly cold-season maritime cyclones. These subsynoptic-scale features are set within a broader context using the conveyor-belt and frontal-fracture paradigms. It is shown that there is a common tendency for slantwise convection to occur in the form of vertically stacked multiple circulations, sometimes associated with lines of upright convection that are themselves broken into chains of line elements.

The review also examines the nature and significance of evaporation/sublimation and shearing instability within frontal zones. These processes play opposing roles in, respectively, sharpening and diffusing the individual slantwise convective circulations. Although shearing instability occurs mostly at very small scales, individual events can lead to breakdown of laminar flow over layers as much as 1 km deep. Such events may be attributed to potential shearing instability, which, like its counterpart in convective instability, can suddenly be released where a layer of air is lifted to saturation.

The studies of the above processes described in this review make extensive use of observations from many different types of radar. Although it is generally necessary to interpret radar observations within the context of other information, the pivotal role of radar in these studies is clear. The writer owes a debt of gratitude of Dave Atlas who from an early stage inspired him to attempt to use the full potential of radar.

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Howard B. Bluestein and Roger M. Wakimoto

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Most severe convective storms are too remote, occur too infrequently, or translate too rapidly to be resolved at high enough spatial and temporal scales by fixed-site, ground-based radars. Fortunately, the recent introduction of mobile radar platforms into the field has had a major impact on advancing our understanding of the internal structure of severe convection. These systems can be broadly divided into airborne, spaceborne, and ground-based mobile platforms. The National Oceanic and Atmospheric Administration (NOAA) P-3, Electra Doppler Radar (ELDORA) ER-2 Doppler Radar (EDOP) are examples of aircraft equipped with radars that have successfully collected data on supercell storms, tornadoes, microbursts, and intense squall lines. Spaceborne platforms might be considered of limited use for studying severe convective storms owing to their high altitude, poor temporal resolution over a particular geographic region, and narrow swath with respect to the earth. However, an example of a synthetic-aperture radar detecting microbursts over the ocean and the ability of the Tropical Rainfall Measuring Mission (TRMM) radar to provide the first global data of severe convective storms are discussed.

Ground-based radars have been used to map the wind field near and within severe convective features close to the ground at very high update cycles. Doppler spectra in tornadoes suggesting F5 wind speeds were collected by a continuous wave radar developed by the Los Alamos National Laboratory. The University of Massachusetts—Amherst built a W-band radar that was mounted in a van and later a truck. This radar was designed with a beamwidth of 0.18°, allowing for ultrahigh spatial resolution. Moreover, a polarization diversity pulse-pair technique was implemented so that the maximum unambiguous Doppler velocity was large enough to be useful in determining maximum wind speeds in tornadoes. The University of Massachusetts radar and an X-band system, developed jointly by the University of Oklahoma, the National Severe Storms Laboratory, and the National Center for Atmospheric Research and also mounted on a truck, known as the Doppler on Wheels (DOW), have collected unprecedented data on the finescale structure of tornadoes. “Eyes” and spiral bands in the radar reflectivity fields were shown to be ubiquitous. For the first time, data suggesting the existence of multiple vortices within tornadoes have been collected.

There has been a burgeoning growth of mobile radar systems that continues to this day. Two C-band systems have been built and are in the early stages of being tested. These two systems known as the Shared Mobile Research and Teaching Radars (SMART-Rs) and the Seminole hurricane hunter are both equipped with polarization diversity. These radars will be able to provide more details of the precipitation physics within severe storms and the range and velocity ambiguities will be reduced. A DOW radar capable of rapid scanning is under development by the University of Oklahoma and an X-band phased array is being converted for meteorological use by the University of Massachusetts. Other examples are provided.

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Kenneth S. Gage and Earl E. Gossard

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This review begins with a brief look at the early perspectives on turbulence and the role of Dave Atlas in the unfolding of mysteries concerning waves and turbulence as seen by powerful radars. The remainder of the review is concerned with recent developments that have resulted in part from several decades of radar and Doppler radar profiler research that have been built upon the earlier foundation.

A substantial part of this review is concerned with evaluating the intensity of atmospheric turbulence. The refractivity turbulence structure-function parameter C 2 n , where n is radio refractive index, is a common metric for evaluating the intensity of refractivity turbulence and progress has been made in evaluating its climatology. The eddy dissipation rate is a common measure of the intensity of turbulence and a key parameter in the Kolmogorov theory for locally homogeneous isotropic turbulence. Much progress has been made in the measurement of the eddy dissipation rate under a variety of meteorological conditions including within clouds and in the presence of precipitation. Recently, a new approach using dual frequencies has been utilized with improved results.

It has long been recognized that atmospheric turbulence especially under hydrostatically stable conditions is nonhomogeneous and layered. The layering means that the eddy dissipation and eddy diffusivity is highly variable especially in the vertical. There is ample observational evidence that layered fine structure is responsible for the aspect sensitive echoes observed by vertically directed very high frequency VHF profilers. In situ observations by several groups have verified that coherent submeter-scale structure is present in the refractivity field sufficient to account for the “clear air” radar echoes. However, despite some progress there is still no consensus on how these coherent structures are produced and maintained.

Advances in numerical modeling have led to new insights by simulating the structures observed by radars. This has been done utilizing direct numerical simulation (DNS) and large eddy simulation (LES). While DNS is especially powerful for examining the breaking of internal waves and the transition to turbulence, LES had been especially valuable in modeling the atmospheric boundary layer.

Internal gravity waves occupy the band of intrinsic frequencies bounded above by the Brunt–Väisälä frequency and below by the inertial frequency. These waves have many sources and several studies in the past decade have improved our understanding of their origin. Observational studies have shown that the amplitude of the mesoscale spectrum of motions is greater over mountainous regions than over flat terrain or oceans. Thus, it would appear that flow over nonuniform terrain is an important source for waves. Several numerical studies have successfully simulated the generation of internal waves from convection. Most of these are believed to result from deep convection with substantial wave motion extending into the upper troposphere, stratosphere, and mesosphere. Gravity waves known as convection waves are often seen in the stable free atmosphere that overlay convective boundary layers.

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