David Atlas

Bio: David Atlas is an academic researcher from Goddard Space Flight Center. The author has contributed to research in topics: Radar & Convective storm detection. The author has an hindex of 44, co-authored 124 publications receiving 6399 citations. Previous affiliations of David Atlas include Jet Propulsion Laboratory & University of Chicago.

Doppler radar characteristics of precipitation at vertical incidence

Abstract: A comprehensive review and extension of the theoretical bases for the measurement of the characteristics of rain and snow with vertically pointing Doppler radar are presented. The drop size distribution in rain can be computed from the Doppler spectrum, provided that the updraft can be estimated, but difficulties are involved in the case of snow. Doppler spectra and their moments are computed for rain by using various power law relations of fall speed υ versus particle diameter D and an exponential fit to the actual fall speed data. In the former case, there is no sharp upper bound to the spectra and all the spectral moments increase with rainfall rate R without limit; in the latter case, there is a sharp upper bound of the spectra corresponding to the limiting terminal velocity of raindrops, and the spectral moments approach an asymptote. Accordingly, the power laws are useful approximations over only limited ranges of precipitation rate. A comparison of theoretical and experimental mean Doppler velocity 〈υ〉 as a function of radar reflectivity factor Z shows that the empirical relation 〈υ〉 = 2.6Z0.107 of J. Joss and A. Waldvogel seems to be the only practical relation; even so, the scatter in 〈υ〉 is about ±1 m sec−1. This is also the kind of error to be expected in measuring updraft speeds by present methods. Such updraft errors result in unacceptably large errors in the drop number concentration estimated from Doppler spectra. In the absence of updrafts the mean Doppler velocity 〈υ〉 is uniquely related to Λ, the slope of the exponential drop size distribution. Simultaneous measurements of Z and 〈υ〉 can then be used to estimate N0, Λ, D0, M, and R, where N0 is the intercept of the exponential drop size distribution at D = 0, D0 is the median volume diameter, and M is the liquid-water content.

Path- and Area-Integrated Rainfall Measurement by Microwave Attenuation in the 1–3 cm Band

Abstract: At a wavelength of about 0.9 cm, microwave attenuation is demonstrated to be linearly related to rainfall rate and independent of drop size distribution and temperature. In addition, practical methods for measuring path- and area-averaged rainfall rate are reviewed. A compromise between maximum path-averaged rainfall rate sensitivity and minimum sensing errors may be achieved by the use of one-way methods between the transmitter and the receiver, with a wavelength of 1.5 to 2.0 cm. Corrections for nonspherical drops and for multiple scattering are also discussed.

General Probability-matched Relations between Radar Reflectivity and Rain Rate

Abstract: An improved method for transforming radar-observed reflectivities Ze into rain rate R is presented. The method is based on a formulation of a Ze-R function constrained such that (1) the radar-retrieved pdf of R and all of its moments are identical to those determined from the gauges over a sufficiently large domain, and (2) the fraction of the time that it is raining above a low but still has an accurately measurable rain intensity is identical for both the radar and for simultaneous measurements of collocated gauges on average. Data measured by a 1.65-deg beamwidth C-band radar and 22 gauges located in the vicinity of Darwin, Australia, are used. The resultant Ze-R functions show a strong range dependence, especially for the rain regimes characterized by strong reflectivity gradients and substantial attenuation. The application of these novel Ze-R functions to the radar data produces excellent matches to the gauge measurements without any systematic bias.

Advances in Radar Meteorology

Abstract: Publisher Summary In this chapter there were two main objectives: To summarize significant developments in radar meteorology since 1958 and to integrate them properly within the framework of knowledge existing prior to that time. The emphasis in this chapter is on the basic principles and concepts of radar meteorology. In order to relate the measured reflectivity to the size and concentration of the particles in the scattering volume, and thus to the conventional meteorological parameters, such as liquid water content and precipitation rate, this chapter is first to determine the radar cross section of an individual scatterer as a function of its refractive index, size, shape, and orientation. Because of the importance of precipitation rate in a variety of meteorological problems, ranging from flood warning to research on the water budget of a storm, extensive consideration is given to its measurement by radar. Radar measurements of storm structure and dimensions suffer from beam width and side lobe effects. In particular, the problem of estimating true storm height is discussed in some detail in the next section, and it is shown how the heights of intense storms may be greatly overestimated by the effects of side lobes, while the visible tops of some weaker storms may be underestimated. The use of Doppler or coherent radar techniques permits the measurement of the velocities of the scatterers, and so, provides a vital new dimension in radar probing of the atmosphere. The basic Doppler theory and the relation of the Doppler spectrum of the scatterers to the fluctuation spectrum of echo intensity on incoherent (conventional) pulse radar are reviewed in some detail. The final section is devoted to a comprehensive review of the long-elusive phenomenon of “angel” echoes.

Rainfall Microphysics and Radar Properties: Analysis Methods for Drop Size Spectra

Abstract: Analyses are performed of experimental drop size spectra to explore the relationships among integral parameters for rain. The data used in this work were acquired with an airborne optical 2D precipitation probe in TOGA COARE during a 4-month period in 1992–93. It is assumed that the experimental size spectra can be described by a gamma drop size distribution (DSD) of the form N(D) = N0Dμ exp(−ΛD) involving three parameters (N0, μ, Λ), which are determined using a new method of truncated moments. The method allows for truncation of the DSD at the large-diameter end of the spectrum due in part to instrumental effects and also in part to the trajectory of the aircraft through a rain streamer that has been sorted by wind shear. An effect analogous to truncation can occur at the small-diameter end of the size spectrum due to evaporation. However, truncation of the spectrum at the small-diameter end is not considered in this work. It is found that spectra with small space and timescales display conside...

The Changing Character of Precipitation

Abstract: From a societal, weather, and climate perspective, precipitation intensity, duration, frequency, and phase are as much of concern as total amounts, as these factors determine the disposition of precipitation once it hits the ground and how much runs off. At the extremes of precipitation incidence are the events that give rise to floods and droughts, whose changes in occurrence and severity have an enormous impact on the environment and society. Hence, advancing understanding and the ability to model and predict the character of precipitation is vital but requires new approaches to examining data and models. Various mechanisms, storms and so forth, exist to bring about precipitation. Because the rate of precipitation, conditional on when it falls, greatly exceeds the rate of replenishment of moisture by surface evaporation, most precipitation comes from moisture already in the atmosphere at the time the storm begins, and transport of moisture by the storm-scale circulation into the storm is vital....

Survey on Free Space Optical Communication: A Communication Theory Perspective

Abstract: Optical wireless communication (OWC) refers to transmission in unguided propagation media through the use of optical carriers, i.e., visible, infrared (IR), and ultraviolet (UV) bands. In this survey, we focus on outdoor terrestrial OWC links which operate in near IR band. These are widely referred to as free space optical (FSO) communication in the literature. FSO systems are used for high rate communication between two fixed points over distances up to several kilometers. In comparison to radio-frequency (RF) counterparts, FSO links have a very high optical bandwidth available, allowing much higher data rates. They are appealing for a wide range of applications such as metropolitan area network (MAN) extension, local area network (LAN)-to-LAN connectivity, fiber back-up, backhaul for wireless cellular networks, disaster recovery, high definition TV and medical image/video transmission, wireless video surveillance/monitoring, and quantum key distribution among others. Despite the major advantages of FSO technology and variety of its application areas, its widespread use has been hampered by its rather disappointing link reliability particularly in long ranges due to atmospheric turbulence-induced fading and sensitivity to weather conditions. In the last five years or so, there has been a surge of interest in FSO research to address these major technical challenges. Several innovative physical layer concepts, originally introduced in the context of RF systems, such as multiple-input multiple-output communication, cooperative diversity, and adaptive transmission have been recently explored for the design of next generation FSO systems. In this paper, we present an up-to-date survey on FSO communication systems. The first part describes FSO channel models and transmitter/receiver structures. In the second part, we provide details on information theoretical limits of FSO channels and algorithmic-level system design research activities to approach these limits. Specific topics include advances in modulation, channel coding, spatial/cooperative diversity techniques, adaptive transmission, and hybrid RF/FSO systems.

Impact of Cloud Microphysics on the Development of Trailing Stratiform Precipitation in a Simulated Squall Line: Comparison of One- and Two-Moment Schemes

Abstract: A new two-moment cloud microphysics scheme predicting the mixing ratios and number concentrations of five species (i.e., cloud droplets, cloud ice, snow, rain, and graupel) has been implemented into the Weather Research and Forecasting model (WRF). This scheme is used to investigate the formation and evolution of trailing stratiform precipitation in an idealized two-dimensional squall line. Results are compared to those using a one-moment version of the scheme that predicts only the mixing ratios of the species, and diagnoses the number concentrations from the specified size distribution intercept parameter and predicted mixing ratio. The overall structure of the storm is similar using either the one- or two-moment schemes, although there are notable differences. The two-moment (2-M) scheme produces a widespread region of trailing stratiform precipitation within several hours of the storm formation. In contrast, there is negligible trailing stratiform precipitation using the one-moment (1-M) scheme. The primary reason for this difference are reduced rain evaporation rates in 2-M compared to 1-M in the trailing stratiform region, leading directly to greater rain mixing ratios and surface rainfall rates. Second, increased rain evaporation rates in 2-M compared to 1-M in the convective region at midlevels result in weaker convective updraft cells and increased midlevel detrainment and flux of positively buoyant air from the convective into the stratiform region. This flux is in turn associated with a stronger mesoscale updraft in the stratiform region and enhanced ice growth rates. The reduced (increased) rates of rain evaporation in the stratiform (convective) regions in 2-M are associated with differences in the predicted rain size distribution intercept parameter (which was specified as a constant in 1-M) between the two regions. This variability is consistent with surface disdrometer measurements in previous studies that show a rapid decrease of the rain intercept parameter during the transition from convective to stratiform rainfall.

On the distribution and continuity of water substance in atmospheric circulations

Abstract: The conservation and distribution of water substance in atmospheric circulations is considered within a frame of continuity principles, model air flows, and models of microphysical processes. The simplest considerations of precipitation involve its vertical distribution in an updraft column, where condensate appears immediately as precipitation with uniform terminal fallspeed. The study also treats steady two-dimensional air circulations in which time-dependent distributions of water vapor, cloud and precipitation respond to model microphysical processes.