Engineering topics

Jameson, A. R.

Publications and source records attributed to Jameson, A. R..

17 records

NASA NTRS1996

A Possible Origin of Linear Depolarization Observed at Vertical Incidence in Rain

Recent observations by two different nadir-pointing airborne radars with some polarization capabilities have detected surprisingly large linear depolarization ratios at times in convective tropical rain. This depolarization can be explained if the rain is considered to be a mixture of a group of apparent spheres and another group of drops that are distorted in the horizontal plane perpendicular to the direction of propagation of the incident wave. If confirmed in future observations, this suggests that at times the larger raindrops are oscillating, in part, because of collisions with smaller drops. Since many of the interpretations of radar polarization measurements in rain by ground-based radars presume that the raindrop shapes correspond to those of the well-known "equilibrium" drops, the present observations may require adjustments to some radar polarization algorithms for estimating rainfall rate, for example, if the shape perturbations observed at nadir also apply to measurements along other axes as well.

Jameson, A. R.↗

NASA NTRS1996

SIR-C/X-SAR Observations of Rain Storms

Teh SIR-C/X-sar radar observations of rain storms are the first multi-polarization and multi-frequency observations of percipitation from space. In addition to numerous, often dramatic images of severe weather systems obtained by forming a synthetic aperture in the usual side-looking attitude, several data takes were performed while the radar antennas were parallel to the ground and the radar beams were pointing at nadir. These oppoprtunities coincided with the passage of the Shuttle over Tropical Cyclone Odille in the southern Indian Ocean during the first flight, and over Typhoon Seth in the Western Pacific during the second flight. The resulting observations, or more apropriately, the resulting measurements, demonstrate for the first time the capability of a spaceborne multi-frequency multi-polarization microwave radar system to quantify precipitation rates, to detect hydrometer phase, and to classify rain type.

SIR-C↗

NASA NTRS1995

The use of optimal polarizations for studying the microphysics of precipitation: Nonattenuating wavelengths

The objective of this work is to explore relationships between the microphysical properties of precipitation and optimal polarizations. The dependence of three optimal polarization parameters (asymmetry ratio A, optimal tilt tau(sub op), and optimal ellipticity epsilon(sub op) on the reflectivity-weighted mean drop shape, mean canting angle, and standard deviation of a Gaussian canting angle distribution is studied. This is accomplished by using computer simulations that provide the rms scattering matrix for an ensemble of canted drops with a prescribed two-parameter canting angle distribution. Also examined are the effects of propagation on the polarization parameters for nonattenuating wavelengths. The asymmetry ratio A is simply the ratio of the maximal to minimal total backscattered energy (ratio of the largest and smallest eigenvalue of the Graves power matrix G is identically equal to S dagger S). Similar to Z(sub DR), this ratio decreases with increasing mean axial ratio, but unlike Z(sub DR), it is not affected by canting (for a single drop). The dependence of A on the reflectivity-weighted mean drop shape is examined, and a power-law relationship similar to that which exists for Z(sub DR) is established. The asymmetry ratio A can be regarded as a generalization of Z(sub DR) because it requires only a measurement of linear depolarization ratio (in addition to Z(sub DR)), is independent of the propagation phase, and is less sensitive to canting. In a similar manner, the dependence of optimal ellipticity and tilt on the microphysical parameters is studied. In particular, it appears that the rms tilt of the optimal polarization ellipse is proportional to the variance of the canting angle distribution. Several other promising relationships between optimal polarizations and the microphysical variables of an ensemble of hydrometeors are also discussed.

Kwiatkowski, John M.↗

NASA NTRS1995

Improved Coupled Z-R and k-R Relations and the Resulting Ambiguities in the Determination of the Vertical Distribution of Rain from the Radar Backscatter and the Integrated Attenuation

Several algorithms to calculate a rain-rate profile from a single-frequency air-or spaceborne radar backscatter profile and a given path-integrated attenuation have been proposed. The accuracy of any such algorithm is limited by the ambiguities between the (multiple) exact solutions, which depend on the variability of the parameters in the Z-R and k-R relations used. In this study, coupled Z-R and k-R relations are derived based on the drop size distribution. It is then shown that, because of the coupling, the relative difference between the multiple mutually ambiguous rain-rate profiles solving the problem must remain acceptably low, provided the available path-integrated attenuation value is known to within 0.5 dB.

Haddad, Z. S.↗

NASA NTRS1994

The meteorological parameterization of specific attenuation in rain viewed at Nadir

Polynomial expressions are presented for the parameterization of the specific attenuation in rain from 9 to 38 GHz that are applicable to a wide range of naturally occurring drop size distributions and for viewing angles close to nadir. Because the temperature T affects the specific attenuation at some frequencies, expressions for the polynomial coefficients as functions of T are also provided for -10C less than or equal to T less than or equal to 30C. The advantage of this parameterization is that even without a detailed specification of the drop size distribution, useful estimates of the specific attenuation are often possible given only two out of three parameters, namely, the rainfall rate R, the rainwater content W, or D(sub m) (the mass-weighted mean drop diameter), particularly if the temperature is also specified.

Jameson, A. R.↗

NASA NTRS1994

Measuring rainwater content by radar using propagation differential phase shift

While radars measure several quantities closely coupled to the rainfall rate, for frequencies less than 15 GHz, estimates of the rainwater content W are traditionally computed from the radar reflectivity factor Z or the rate of attenuation A--quantities only weakly related to W. Consequently, instantaneous point estimates of W using Z and A are often erroneous. A more natural, alternative parameter for estimating W at these frequencies is the specific polarization propagation differential phase shift phi(sub DP), which is a measure of the change in the difference between phases of vertically (V) and horizontally (H) polarized waves with increasing distance from a radar. It is now well known that W is nearly linearly related to phi(sub DP) divided by (1 - reversed R), where reversed R is the mass-weighted mean axis ratio of the raindrops. Unfortunately, such relations are not widely used in part because measurements of phi(sub DP) are scarce but also because one must determine reversed R. In this work it is shown that this parameter can be estimated using the differential reflectivity (Z(sub H)/Z(sub V) at 3 GHz. An alternative technique is suggested for higher frequencies when the differential reflectivity becomes degraded by attenuation. While theory indicates that it should be possible using phi(sub DP) to estimate W quite accurately, measurement errors increase the uncertainty to +/- 18%-35% depending on reversed R. While far from ideal, it appears that these estimates are likely to be considerably more accurate than those deduced using currently available methods.

Jameson, A. R.↗

NASA NTRS1994

A new approach to estimating rainwater content by radar using propagation differential phase shift

As microwaves propagate through rain, the rate of phase change with increasing distance is different depending upon whether the transmissions are polarized horizontally or vertically. This rate of change is the so-called specific propagation differential phase shift phi(sub DP). This paper demonstrates that at several frequencies and over a wide domain the ratio of phi(sub DP) to the rainwater content W is nearly linearly related to D(sub m), the mass-weighted mean drop size. An investigation of errors indicates that this new approach is likely to yield more accurate estimates of W than the other classical reflectivity factor Z, attenuation, or polarization techniques. The most accurate estimates of W are most likely at the highest frequency considered, 13.80 GHz. In lieu of such high-frequency measurements, these somewhat esoteric results are made more concrete through an analysis of 3-GHz radar measurements collected during the Convection and Precipitation Experiment in a tropical rainstorm in Florida. Among the principal advantages of using phi(sub DP) to measure rain are that an absolute calibration of the radar is no longer required and the estimates are decoupled from measurements of the radar reflectivity factor. Consequently, temporal and spatial structures of rain estimates do not simply mimic those of the reflectivity factor, as happens for classical estimation techniques using Z.

Jameson, A. R.↗

NASA NTRS1994

An alternative approach to estimating rainfall rate by radar using propagation differential phase shift

In this work it is shown that for frequencies from 3 to 13 GHz, the ratio of the specific propagation differential phase shift phi(sub DP) to the rainfall rate can be specified essentially independently of the form of the drop size distribution by a function only of the mass-weighted mean drop size D(sub m). This significantly reduces one source of substantial bias errors common to most other techniques for measuring rain by radar. For frequencies 9 GHz and greater, the coefficient can be well estimated from the ratio of the specific differential attenuation to phi(sub DP), while at nonattenuating frequencies such as 3 GHz, the coefficient can be well estimated using the differential reflectivity. In practice it appears that this approach yields better estimates of the rainfall rate than any other current technique. The best results are most likely at 13.80 GHz, followed by those at 2.80 GHz. An optimum radar system for measuring rain should probably include components at a both frequencies so that when signals at 13.8 GHz are lost because of attenuation, good measurements are still possible at the lower frequency.

Jameson, A. R.↗

NASA NTRS1993

Spaceborne radar sensing of precipitation above an ocean surface - Polarization contrast study

The potential benefits of polarization adjustment for spaceborne radar sensing of precipitation are explored, with emphasis on the role of the wave polarization in separating or 'distinguishing' ocean surface return from the hydrometeor echoes of a 'chirped' signal. Experimental as well as computational data for the polarization scattering matrices of hydrometeors and ocean surfaces are obtained and used to calculate ocean and precipitation 'response' to the transmitted pulse for various rain rates and incidence angles. Polarization, which provides the best contrast between rain and ocean returns, is found to vary from almost circular near nadir to elliptical at large off-nadir look angles of incidence. Calculations show an order of magnitude improvement in the ratio of the returns when compared with the traditional choice of horizontal transmit and receive polarization.

Kostinski, Alexander B.↗

NASA NTRS1992

The effect of temperature on attenuation-correction schemes in rain using polarization propagation differential phase shift

The study elucidates and quantifies differences in the response of the rate of change of polarization propagation differential phase shift Phi, the rate of attenuation for a horizontally/vertically polarized wave A(H,V), and the rate of polarization differential attenuation A(H-V) to temperature. It is shown that if the effects of temperature when estimating A(H) and A(H-V) from Phi are neglected, the average fractional standard error increases only slightly at 9 GHz but significantly at 5 and 3 GHz. Errors at 5 and 3 GHz are about two to three times those at 9 GHz. The performance of Phi-based schemes of attenuation correction at these lower frequencies is much more significantly degraded by temperature uncertainty than at 9 GHz. It is concluded that it is best to use Phi to correct for attenuation at the least-attenuating frequencies.

Jameson, A. R.↗

NASA NTRS1992

A dual-frequency microwave technique for measuring rainwater content

This study shows that the dual-frequency (38, 25 GHz) differential attenuation (A(38-25)) coefficient can provide accurate estimates of rainwater content W potentially over a wide range of rainwater contents. While measurements along a microwave link are fairly easily implemented, radar estimates of A(38-25) can become clouded by differences between the radar reflectivity factors at the two frequencies (Z38, Z25). Root-mean-square deviations (epsilon) of the estimated W from the actual W are calculated for a wide variety of drop-size distributions and rainwater contents. The computed epsilon include the effects of standard measurement errors and differences between Z38 and Z25. Accurate estimates appear possible using a 38-25-GHz radar when W is not less than 1.5-2 g/cu m, depending upon the desired spatial resolution, and along a microwave link when W is not less than 0.5 g/cu m.

Jameson, A. R.↗

NASA NTRS1991

Polarization radar measurements in rain at 5 and 9 GHz

Potential techniques for measuring rainfall rate R and rainwater content W at 9 and 5 GHz are explored. An investigation is made of a previous technique to use the polarization propagation differential phase shift with increasing distance from the radar Phi(H-V) to estimate and remove the effects of specific and polarization differential attenuation from Z(H) and Z(DR), respectively. It is shown that in rain this technique is sensitive to variations in the drop-size distribution. It is confirmed that Phi(H-V) can be used to extend the distance over which useful measurements of Z(H) and Z(DR) can be obtained. While residual errors introduced by attenuation and the 'noise' from the correction scheme itself eclipse the potential of many possible techniques for quantitative rainfall measurements at these frequencies, the attenuation recovery scheme is argued to be adequate for obtaining useful polarization microphysical measurements, particularly above the melting level, even after encounters with rain.

Jameson, A. R.↗

NASA NTRS1991

The effect of drop-size distribution variability on radiometric estimates of rainfall rates for frequencies from 3 to 10 GHz

An attempt is made to quantify and document the dependence of the strength and the linearity of the relation between the absorption coefficient of the rain k sub a and rainfall rates R for frequencies from 3 to 10 GHz. It is shown that the physical link between R and k sub a varies depending upon microwave frequency. The weaker the relation the more sensitive k sub a and R to variations in the drop-size distribution. The scatter in k sub a and R is found to be greatest at 8 GHz and smallest at 3 GHz in response to variations to drop-size distribution.

Jameson, A. R.↗

NASA NTRS1991

A comparison of microwave techniques for measuring rainfall

The purpose of this paper is to evaluate within a common framework a large number of techniques for nearly instantaneous microwave measurements of rainfall and to determine the range of rainfall rates best suited to the various techniques and estimators. The physical basis of a technique as transformed by measurement imperfections of real instruments determines the ultimate performance capability of any microwave rain estimator. While many of the defects of the measurement process apply to all techniques, the physics behind each estimator differs. A method is presented for objectively evaluating the physical bases of the techniques and for quantifying estimator performance for perfect instruments. These results are then tempered by standard measurement errors to yield more realistic results. Analysis demonstrates that, in general, the minimization of rainfall estimate errors over a wide range of rainfall rates requires the simultaneous application of more than one microwave rainfall measurement technique.

Jameson, A. R.↗

NASA NTRS1990

The functional correlation between rainfall rate and extinction coefficient for frequencies from 3 to 10 GHz

The relationship between the rainfall rate (R) obtained from radiometric brightness temperatures and the extinction coefficient (k sub e) is investigated by computing the values of k sub e over a wide range of rainfall rates, for frequencies from 3 to 25 GHz. The results show that the strength of the relation between the R and the k sub e values exhibits considerable variation for frequencies at this range. Practical suggestions are made concerning the selection of particular frequencies for rain measurements to minimize the error in R determinations.

Jameson, A. R.↗

NASA NTRS1989

Theoretical analysis and meteorological interpretation of the role of raindrop shape on microwave attenuation and propagation phase shifts - Implications for the radar measurement of rain

The effects of raindrop shape on the attenuation and propagation phase shift at vertical and horizontal linear polarizations is examined, focusing on the implications of these effects on rain measurements by radars. Analytic expressions for the imaginary and real components of the forward scattering amplitude function are dervied for a wavelength of 2.2 cm. It is shown that the summation attenuation rate and the rate of summation propagation phase shift are both insensitive to the shapes of raindrops.

Jameson, A. R.↗

NASA NTRS1989

On the calibration of climatological satellite rainfall measurements using a transportable ground-based polarization radar

The use of a ground-based polarization radar is proposed for the calibration of climatological satellite rainfall measurements. A distribution matching method for improving the correlations between radar reflectivity and rainfall brightness temperatures is discussed. The importance of collecting measurements of all observables at the same spatial resolution is demonstrated using GATE rainrate data (Patterson et al., 1979). The use of the matched distribution method to calibrate techniques such as HART (Atlas et al., 1988; Rosenfeld et al., 1988) is examined. Also, consideration is given to the application of polarization radar data to the interpretation of brightness temperatures.

Jameson, A. R.↗