Search Results

You are looking at 11 - 20 of 42 items for

  • Author or Editor: R. Atlas x
  • Refine by Access: All Content x
Clear All Modify Search
David Atlas
,
Kenneth R. Hardy
, and
Keikichi Naito

Abstract

A general analysis is made of the turbulent refractivity spectrum in and beyond the limiting microscale and a relation derived for its scattering reflectivity in either the back or bistatic directions. Radar reflectivity is computed as a function of wavelength for regions of CAT. The results are compared to the minimum detectable reflectivity of airborne radars having optimum state of the art characteristics at each wavelength. It is shown that the best radars now feasible can barely detect the most reflective CAT at 10 n mi (i.e., 1 minute warning). A 20-db improvement in sensitivity is required for detection of most CAT, which appears to be just attainable by pre-detection integration. The optimum wavelength to implement is 5–6 cm. The best radar at this wavelength will also detect circus clouds reliably. Whether detecting clouds or chaff a measure of the echo fluctuation (or Doppler) spectrum is required to identify the intensity of CAT. However, in the case of high altitude clear air echoes, there is an indication that the reflectivity in excess of some minimum threshold value is a sign of some degree of mechanical turbulence. It is also demonstrated that a ground-based forward-scatter link holds great promise for reliable CAT detection. A tentative quantitative classification of CAT severity is also proposed.

Full access
J. M. Vergin
,
D. R. Johnson
, and
R. Atlas

Abstract

The results of a quasi-Lagrangian diagnostic study of two 72 h Goddard Laboratory for Atmospheric Sciences (GLAS) model cyclone predictions from 0000 GMT 19 February 1976 are presented and compared with observed results. One model forecast (SAT) was generated from initial conditions which included satellite sounding data, and the other model forecast (NOSAT) was generated from initial conditions that excluded satellite sounding data. Examination of the mass and angular momentum budget statistics for the SAT and NOSAT forecasts reveals substantial differences. The improvement in the SAT forecast is established from the more realistic SAT budget statistics, and results from the modifications of initial atmospheric structure due to satellite information.

The assimilation of satellite data caused modifications of the horizontal mass and eddy angular momentum transports at the zero hour. The assimilation of satellite data resulted in colder temperatures and weaker stabilities in the lower layers of the northwest quadrant of the budget volume, and thus an improved structure of the cold polar air mass over a relatively warm ocean surface. In the southwest quadrant of the budget volume, the SAT assimilation produced an increase in the stability of the middle and lower layers and an increase in temperatures throughout much of the troposphere. These modifications in the temperature structure were the primary reasons for the improved mass and eddy angular momentum transports which contributed to the better SAT forecast for the cyclone event.

Full access
G. R. Gray
,
R. J. Serafin
,
D. Atlas
,
R. E. Rinehart
, and
J. J. Boyajian

Color enhancement of images has become a powerful tool in rapid evaluation of grey-scale information. Recent advances in semiconductor technology have made possible the construction of an inexpensive digital real-time color-enhanced (or false-color) display for meteorological radar information such as reflectivity and Doppler velocities. Variable magnification allows detailed analysis of selected areas of the radar coverage.

The display was interfaced to a Doppler/reflectivity processor on the NHRE S-band radar at Grover, Colorado, during the 1974 hail season. A preliminary meteorological analysis of the Doppler color displays of the storm of 7 August 1974 demonstrates a large variety of significant features which may be observed either in real-time or subsequently. These include the regions of convergence and vorticity, major inflow and outflow regions, and turbulence. Most importantly, it is shown that the updraft cores can be identified with the easterly-momentum air which has been transported upward with the drafts from the lower levels. In view of the slow eastward motion of the storm system, the very large Doppler components found at the leading edge of the higher-level echo pattern also indicate rapid evaporation of the particles as they move out into the clear, dry environmental air. It is the resulting evaporative cooling which is responsible for the downdrafts in this vicinity. Among the many real-time applications of the color Doppler display, perhaps the most important in the artificial modification of convective storms is the location of the major inflow and updraft regions. These determine where seeding should be focused. The use of the color display also permits the ready discrimination of storm echoes from ground clutter in which they are frequently obscured. Its applicability to the detection of tornado cyclones and hurricane velocity mapping is also self-evident.

Full access
L. Cucurull
,
R. Atlas
,
R. Li
,
M. J. Mueller
, and
R. N. Hoffman

Abstract

Experiments with a global observing system simulation experiment (OSSE) system based on the recent 7-km-resolution NASA nature run (G5NR) were conducted to determine the potential value of proposed Global Navigation Satellite System (GNSS) radio occultation (RO) constellations in current operational numerical weather prediction systems. The RO observations were simulated with the geographic sampling expected from the original planned Constellation Observing System for Meteorology, Ionosphere, and Climate-2 (COSMIC-2) system, with six equatorial (total of ~6000 soundings per day) and six polar (total of ~6000 soundings per day) receiver satellites. The experiments also accounted for the expected improved vertical coverage provided by the Jet Propulsion Laboratory RO receivers on board COSMIC-2. Except that RO observations were simulated and assimilated as refractivities, the 2015 version of the NCEP’s operational data assimilation system was used to run the OSSEs. The OSSEs quantified the impact of RO observations on global weather analyses and forecasts and the impact of adding explicit errors to the simulation of perfect RO profiles. The inclusion or exclusion of explicit errors had small, statistically insignificant impacts on results. The impact of RO observations was found to increase the length of the useful forecasts. In experiments with explicit errors, these increases were found to be 0.6 h in the Northern Hemisphere extratropics (a 0.4% improvement), 5.9 h in the Southern Hemisphere extratropics (a significant 4.0% improvement), and 12.1 h in the tropics (a very substantial 28.4% improvement).

Full access
Allen J. Lenzen
,
Donald R. Johnson
, and
Robert Atlas

Abstract

Quasi-Lagrangian diagnostics of mass, angular momentum, water vapor, and kinetic energy are evaluated for four different Goddard Laboratory for Atmospheres model simulations of the Queen Elizabeth II storm of 9–11 September 1978 to study the impact of Seasat-A satellite Scatterometer (SASS) winds and horizontal resolution in numerical prediction. In a four-way comparison, the diagnostics investigate the impact of including dealiased SASS winds in the initial conditions of the model and doubling the horizontal resolution on 36 h simulations of the QE II storm. The largest impact on the simulation stemmed from doubling the model's horizontal resolution from 4° × 5° to 2° × 2.5°. The increased resolution resulted in a storm track much closer to that observed, a much deeper surface development, a stronger mass circulation, stronger heating, and stronger increase of angular momentum. The inclusion of SASS data resulted in an approximately 2–3-mb-deeper surface cyclone for both the 2° × 2.5° and 4° × 5° resolution simulations. The inclusion also led to substantial increases in the horizontal mass circulation and heating for the 2° × 2.5° simulation. During the early explosive deepening phase of the cyclone, the inward lateral transport of water vapor in lower layers was larger in the 2° × 2.5° SASS than in the 2° × 2.5° NOSASS (exclusion of SASS surface winds) simulation. During the period of most rapid development, the results from the SASS simulation revealed a larger generation of kinetic energy throughout the troposphere and increased outward transport of kinetic energy in upper layers.

Full access
W. E. Baker
,
R. Atlas
,
M. Halem
, and
J. Susskind

Abstract

In this study we examine the sensitivity of forecast to individual components of the First GARP (Global Atmospheric Research Programme) Global Experiment database as well as to some modifications in the data analysis techniques. Several short assimilation experiments (0000 GMT 18 January 1979 through 0000 21 January) are performed in order to test the effects of each database or analysis change. Forecasts are then generated from the initial conditions provided by these experiments. The 0000 21 January case is chosen for a detailed investigation because or the poor forecast skill obtained earlier over North America for that particular case. Specifically, we conduct experiments to test the sensitivity of forecast skill to: 1) the addition of individual satellite observing system components; 2) temperature data obtained with different satellite retrieval methods; and 3) the method of vertical interpolation between the mandatory pressure analysis levels and the model sigma levels.

For the single case examined, TIROS-N infrared land retrievals produced operationally are found to degrade the forecast, while the use of TIROS-N retrievals produced with a physical inversion method as part of an analysis/forecast cycle results in an improved forecast. The use of oceanic VTPR (Vertical Temperature Profile Radiometer) satellite retrievals also results in an improved forecast over North America. The forecast is also found to be sensitive to the method of vertical interpolation between the mandatory pressure analysis levels and the model sigma levels.

Full access
E. Stratmann
,
D. Atlas
,
J. H. Richter
, and
D. R. Jensen

Abstract

A method of calibrating a fixed vertically pointing radar is presented. The technique involves the firing of B-B shot of known radar cross section through the beam while making successive trajectory corrections until the absolute maximum signal is attained. The results agree closely with an independent calibration of antenna gain. The approach is particularly suited to an FM-CW radar with high range resolution because the pellets reach heights well in excess of the minimum range and errors in range are negligible. Corrections are presented for the reduction in maximum two-way gain resulting from intersecting beams whose full gain is attained only at the point of intersection. It is also shown that Probert-Jones’ k 2 factor is significantly smaller for this system, and possibly for others, than the generally accepted value of unity. The method can be readily extended to any sufficiently sensitive pulsed radar by using small elevation angles and direct measurements of range rather than those obtained from the echoes.

Full access
R. Meneghini
,
K. Nakamura
,
C. W. Ulbrich
, and
D. Atlas

Abstract

For a spaceborne meteorological radar, the use of frequencies above 10 GHz may be necessary to attain sufficient spatial resolution. As the frequency increases, however, attenuation by rain becomes significant. To extend the range of rain rates that can be accurately estimated, methods other than the conventional Z-R, or backscattering method, are needed. In this paper, tests are made of two attenuation-based methods using data from a dual-wavelength airborne radar operating at 3 cm and 0.87 cm. For the conventional dual-wavelength method, the differential attenuation is estimated from the relative decrease in the signal level with range. For the surface reference method, the attenuation is determined from the difference of surface return powers measured in the absence and the presence of rain. For purposes of comparison, and as an indication of the relative accuracies of the techniques, the backscattering, (Z-R), method, as applied to the 3 cm data, is employed. As the primary sources of error for the Z-R, dual-wavelength, and surface reference methods are nearly independent, some confidence in the results is warranted when thew methods yield similar rain rates. Cases of good agreement occur most often in stratiform rain for rain rates between a few mm h−1 to about 15 mm h−1; that is, where attenuation at the shorter wavelength is significant but not so severe as to result in a loss of signal. When the estimates disagree, it is sometimes possible to identify the likely error source by an examination of the return power profiles and a knowledge of the error sources.

Full access
David Atlas
,
R. C. Srivastava
, and
W. S. Marker Jr.

Abstract

Specular reflection from a stratum of sharp, mean, vertical refractivity gradient frequently accompanies the scatter from the turbulent perturbations in refractivity which tend to be maximized close to the gradient. As a result, the signal intensity falls more rapidly, and the magnitude of the mean Doppler shift increases less rapidly with beam offset angle from the great circle than is the case for pure turbulent scatter. Also, in transmission via the great circle path, the Doppler spread, signal fading rate, and multi-path spread may be greatly reduced from that expected for turbulent scatter alone. Because transmission along the great circle may be greatly influenced by specular reflections, the strength of which is a function of the form and sharpness of the mean refractivity gradient, past experiments relating signal-wavelength dependence or signal-scatter angle dependence to the form of the turbulent refractivity spectrum are suspect.

Full access
David Atlas
,
Carlton W. Ulbrich
, and
Christopher R. Williams

Abstract

A unique set of Doppler and polarimetric radar observations were made of a microburst-producing storm in Amazonia during the Tropical Rainfall Measuring Mission (TRMM) Large-Scale Biosphere–Atmosphere (LBA) field experiment. The key features are high reflectivity (50 dBZ) and modest size hail (up to 0.8 mm) in high liquid water concentrations (>4 g m−3) at the 5-km 0°C level, melting near the 3-km level as evidenced by the Doppler spectrum width on the profiler radar (PR), by differential polarization on the S-band dual-polarized radar (S-POL), and a sharp downward acceleration from 2.8 to 1.6 km to a peak downdraft of 11 m s−1, followed by a weak microburst of 15 m s−1 at the surface. The latter features closely match the initial conditions and results of the Srivastava numerical model of a microburst produced by melting hail. It is suggested that only modest size hail in large concentrations that melt aloft can produce wet microbursts. The narrower the distribution of hail particle sizes, the more confined will be the layer of melting and negative buoyancy, and the more intense the microburst. It is hypothesized that the timing of the conditions leading to the microburst is determined by the occurrence of an updraft of proper magnitude in the layer in which supercooled water accounts for the growth of hail or graupel.

Full access