Search Results

You are looking at 11 - 20 of 26 items for

  • Author or Editor: J. A. Weinman x
  • Refine by Access: All Content x
Clear All Modify Search
J. A. Weinman
and
P. N. Swarztrauber

Abstract

The equation of radiative transfer was approximated by the method of Giovanelli to provide the albedo of an externally illuminated, plane-parallel striated medium. The medium was assumed to consist of isotropic scatterers with a scattering coefficient per unit depth of
01lx
that depended periodically on one horizontal coordinate. The dependence of the albedo on the angle of incidence of the solar radiance, the mean optical thickness, and the wavelength and amplitude of the striations is presented for media with κ1/&kappa0 = 0.1 and 0.9.
Full access
K. E. Kunkel
and
J. A. Weinman

Abstract

Monte Carlo techniques are utilized to compute monostatic lidar returns from turbid atmospheres. Examples are evaluated for thick hazes, clouds, fogs and rain. The effects of multiple scattering are significant in the cases considered. Results are compared with those obtained by Eloranta (1972) to describe doubly scattered lidar returns and the agreement is satisfactory, provided that higher orders of multiple scattering are negligible.

Full access
T. L. Barber
and
J. A. Weinman

Abstract

A rugged portable laser-ranging pilot balloon tracking system was developed to measure wind profiles in the boundary layer. The portable laser-ranging system provides digital output and can be operated by one relatively unskilled person. Additional personnel may be needed to launch the pilot balloons. Wind speeds were measured at 50-m intervals with an accuracy of 0.25 m s−1. Under clear sky conditions, the nominal range of the system is 3000 m.

Full access
J. A. Weinman
,
R. Meneghini
, and
K. Nakamura

Abstract

This study compares precipitation rate profiles derived from a single frequency radar and radiometer with such profiles derived from a dual-frequency radar.

Measurements obtained during the 1985–86 CRL/NASA rain measuring experiment from airborne X- and Ka-band radars and an X-band passive microwave radiometer were used to derive rainfall rate profiles over the Atlantic Ocean. The rainfall retrieval employs the classical Hitschfeld-Bordan radar equation constrained by a measurement of the path integrated extinction derived from passive radiometry.

The path-integrated extinction obtained from the radiometric measurements was compared with that obtained from coincident dual-frequency radar reflection measurements from the ocean surface. The mean rainfall rate derived from the path-integrated extinction retrieved from the measured microwave radiances agreed within 25% with the mean rainfall rate obtained from the reflected radar signals.

An analysis of the errors in the retrieval algorithm showed that errors in the path-integrated extinction significantly affect the retrieved rainfall profiles near the surface. A least squares linear extrapolation of the profile in the lowest kilometer was used to revise the boundary condition in the retrieval. The profiles were solved iteratively until the rainfall rate at the surface was within the range of scatter about the linear profile at higher altitudes.

An optimization analysis was applied to the derivation of rainfall rate profiles retrieved from a dual-frequency radar data. The results of the retrieval were compared to those obtained from the radar-radiometer retrievers.

The availability of only an X-band radiometer limited the retrieval of rainfall rate profiles to maritime cases. It appears that it will be possible to measure rainfall under most conditions when radiometers operating at several higher frequencies become available on future airborne radar experiments.

Full access
J. H. Joseph
,
W. J. Wiscombe
, and
J. A. Weinman

Abstract

This paper presents a rapid yet accurate method, the “delta-Eddington” approximation, for calculating monochromatic radiative fluxes in an absorbing-scattering atmosphere. By combining a Dirac delta function and a two-term approximation, it overcomes the poor accuracy of the Eddington approximation for highly asymmetric phase functions. The fraction of scattering into the truncated forward peak is taken proportional to the square of the phase function asymmetry factor, which distinguishes the delta-Eddington approximation from others of similar nature. Comparisons of delta-Eddington albedos, transnmissivities and absorptivities with more exact calculations reveal typical differences of 0–0.022 and maximum differences of 0.15 over wide ranges of optical depth, sun angle, surface albedo, single-scattering albedo and phase function asymmetry. Delta-Eddington fluxes are in error, on the average, by no more than 0.5%0, and at the maximum by no more than 2% of the incident flux. This computationally fast and accurate approximation is potentially of utility in applications such as general circulation and climate modelling.

Full access
E. W. Eloranta
,
J. M. King
, and
J. A. Weinman

Abstract

Vertical profiles of the horizontal radial wind component in the lowest kilometer of the atmosphere have been measured remotely with lidar. Wind speed determinations were made by observing the motion of naturally occurring aerosol density inhomogeneities. Lidar wind measurements compare favorably with simultaneous pilot balloon observations of the wind.

Full access
G. David Alexander
,
James A. Weinman
, and
J. L. Schols

Abstract

A technique is described in which forecasts of the locations of features associated with marine cyclones may be improved through the use of microwave integrated water vapor (IWV) imagery and image warping of forecast mesoscale model fields. Here, image warping is used to optimally match mesoscale model output to observations of IWV measured by microwave sensors. In the mesoscale model simulations presented here (one of the March 1993 “superstorm,” one of a rapidly deepening cyclone observed in the North Atlantic in February 1992, and one of the ERICA IOP 4 cyclone), the Pennsylvania State University–National Center for Atmospheric Research MM5 model is initialized from the standard National Meteorological Center (recently renamed the National Centers for Environmental Prediction) operational analysis. The simulations are then run until a time at which a Special Sensor Microwave/Imager (SSM/I) overpass occurs. For each simulation, the forecast pattern of IWV is then compared to the field shown in the SSM/I image. In all three cases, the MM5 moves the cyclones too slowly, and therefore places distinguishing features in the forecast IWV fields significantly upstream of their locations as revealed in the microwave imagery. To rectify these errors, the grid on which the source image (forecast field) is defined is then warped to match the target image (remotely observed IWV field) by choosing pairs of tie points corresponding to similar features in the two images. The values of all model moisture variables at all vertical levels are then carried to the new warped grid points and interpolated back to the original model grid. Model integration then proceeds with the new model fields. The model results at a subsequent time after the warping is applied are then compared with simultaneous model results in simulations in which no warping was applied as well as with model simulations in which a standard nudging technique is applied. Warping results in improved forecasts of cyclone minimum sea level pressure, tracks, and IWV fields over both the control simulations and the nudged simulations.

Full access
S. T. Shipley
,
E. W. Eloranta
, and
J. A. Weinman

Abstract

Monostatic lidar is explored as a means for determining the rainfall rate over an extended atmospheric path with a spatial resolution comparable to that of rain gages. An empirical relationship is established between the optical extinction coefficient of rain β r (km−1) and the rainfall rate R (mm hr−1). Correlation of lidar-derived rainfall extinction and gage rainfall rates at Madison gives
β r R0.74
.

The β r -R relations obtained from the work of other authors compare well with this relationship.

A lidar equation which accounts for the multiple scattering of light in rain is presented. A numerical procedure which derives estimates of β r as a function of range from lidar returns is developed. Examples of lidar-derived rainfall rate range profiles in spatially inhomogeneous thunderstorms are given.

Full access
K. E. Kunkel
,
E. W. Eloranta
, and
J. A. Weinman

Abstract

Procedures are described for the analysis of lidar data to remotely measure 1) spectra of aerosol density fluctuations, 2) radial and transverse components of the mean wind and turbulent fluctuations of the transverse component of the wind velocity in the convective boundary layer, and 3) the kinetic energy dissipation rate. Results were compared with independent data obtained with a bivane anemometer installed at the 70 m level on a tower within the scanning sector of the lidar. Good agreement was obtained whenever the lidar data had adequate signal-to-noise characteristics (i.e., S/ greater than unity).

Full access
J. A. Weinman
,
J. T. Twitty
,
S. R. Browning
, and
B. M. Herman

Abstract

The intensity of sunlight multiply scattered in model atmospheres is derived from the equation of radiative transfer by an analytical small-angle approximation. The approximate analytical solutions are compared to rigorous numerical solutions of the same problem. Results obtained from an aerosol-laden model atmosphere are presented. Agreement between the rigorous and the approximate solutions is found to be within a few percent.

The analytical solution to the problem which considers an aerosol-laden atmosphere is then inverted to yield a phase function which describes a single scattering event at small angles. The effect of noisy data on the derived phase function is discussed.

Full access