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In this paper, an investigation of the potential of rapid refractivity retrieval is presented. The retrieval technique utilizes radar phase measurements of ground clutter to derive near-surface refractivity, which has been commonly used as a proxy for humidity, given its close relation to vapor pressure. Surface humidity is an important meteorological parameter and has been known to play an important role in convective initiation. In this paper, the refractivity retrieval technique is exploited by using smaller numbers of samples for phase calculation, which is a fundamental process in refractivity retrieval. The impetus for this paper is to explore the possibility of rapid refractivity retrieval by exploiting the rapid beam-steering capability of a phased-array radar. Using the National Weather Radar Testbed in Norman, OK, a 64-pulse per radial raw-data set was collected for conventional refractivity processing. Then, subsets of the 64 samples were extracted to emulate shorter dwell periods and the corresponding more rapid experiments. The test cases that were considered are 2, 4, 8, 16, and 32 samples. Refractivity fields retrieved using smaller numbers of samples are compared against the reference field, which was obtained using the entire 64-sample data set. It will be shown that, statistically, significant refractivity fields can be obtained from as short as a two-sample dwell. Index Terms--Array signal processing, meteorological radar, phased array radar, rapid refractivity retrieval, surface refractivity.

Mammatus clouds are an intriguing enigma of atmospheric fluid dynamics and cloud physics. Most commonly observed on the underside of cumulonimbus anvils, mammatus also occur on the underside of cirrus, cirrocumulus, altocumulus, altostratus, and stratocumulus, as well as in contrails from jet aircraft and pyrocumulus ash clouds from volcanic eruptions. Despite their aesthetic appearance, mammatus have been the subject of few quantitative research studies. Observations of mammatus have been obtained largely through serendipitous opportunities with a single observing system (e.g., aircraft penetrations, visual observations, lidar, radar) or tangential observations from field programs with other objectives. Theories describing mammatus remain untested, as adequate measurements for validation do not exist because of the small distance scales and short time scales of mammatus. Modeling studies of mammatus are virtually nonexistent. As a result, relatively little is known about the environment, formation mechanisms, properties, microphysics, and dynamics of mammatus. This paper presents a review of mammatus clouds that addresses these mysteries. Previous observations of mammatus and proposed formation mechanisms are discussed. These hypothesized mechanisms are anvil subsidence, subcloud evaporation/sublimation, melting, hydrometeor fallout, cloud-base detrainment instability, radiative effects, gravity waves, Kelvin--Helmholtz instability, Rayleigh--Taylor instability, and Rayleigh-Benard-like convection. Other issues addressed in this paper include whether mammatus are composed of ice or liquid water hydrometeors, why mammatus are smooth, what controls the temporal and spatial scales and organization of individual mammatus lobes, and what are the properties of volcanic ash clouds that produce mammatus? The similarities and differences between mammatus, virga, stalactites, and reticular clouds are also discussed. Finally, because much still remains to be learned, research opportunities are described for using mammatus as a window into the microphysical, turbulent, and dynamical processes occurring on the underside of clouds.

The 1998 International Workshop on Remote Sensing in Geophysics Using Doppler Techniques was held to discuss various applications of Doppler remote sensing from sea surface sensing to radar astronomy and from wind profiling to atmospheric boundary layer sensing. Some of the topics discussed in the workshop include use of Doppler spectrum to study atmospheric turbulence, techniques in seismic prospecting, radar astronomy and meteorological radar simulation.

We present observations of biological scatterers with a polarimetric weather radar. The radar has a pencil beam, a high power, and a wavelength of 10 cm. It transmits horizontally and vertically polarized waves alternately. The available polarimetric variables are differential reflectivity, differential phase, and correlation coefficient between orthogonally polarized returns. Two types of biological scatterers, insects and birds, are contrasted. Observed polarimetric signatures of these are compared and explained with a simple scattering model. Finally, we discuss the implications of recognizing the biological scatterers' type and the size of birds. Index Terms - Radar polarimetry, scattering, weather radar.

The authors contrast rainfall in two Oklahoma squall lines: one with deep convection occurred in the spring and the other with shallower convection in the winter. Both passed over a micronetwork of densely spaced rain gauges and were observed with the National Severe Storm Laboratory's polarimetric weather radar. Polarimetric measurements reveal differences in storm structure that in turn imply that microphysical processes caused the drop size distributions to be quite distinct for the two events. In the winter squall line the conventional R(Z) algorithm for estimating rainfall fails badly, whereas in the summer squall line it performs well. The method based on specific differential phase measurements, however, yields a very good match between radar-derived areal precipitation amount and rain depth obtained from the micronetwork of densely located rain gauges for both events.

We present observations of the copolar correlation coefficient between horizontally and vertically polarized echoes [vertical bar][[Rho].sub.hv](0)[vertical bar]. These were made with ground-based and airborne weather radars at nearly vertical incidence. A sharp decrease of [vertical bar][[Rho].sub.hv](0)[vertical bar] occurs at the bright band bottom, and is attributed to a varying mixture of hydrometeors with diverse shape, size, and thermodynamic phase. The largest contribution to decorrelation seems to come from wet aggregates; this is substantiated by consideration of two simple models. One consists of randomly oriented wet prolate spheroids, and the other considers an ensemble of distorted spheres. Prolates with axis ratios of 3 or distorted spheres with rms roughness equal to 15% of the diameter decrease the correlation to 0.8 at S band. At Ku band and for the size range encountered in the bright band, the decrease is a function of equivalent diameter because scattering is in the Mie regime. [vertical bar][[Rho].sub.hv](0)[vertical bar] measurement at 13.8 GHz and from the aircraft are the first ever. Also, differential phase and differential reflectivity at a 10 [degrees] off nadir are the first of its kind. These last two variables showed a distinct signature in the bright band. This is significant because it might lead to applications on airborne or spaceborne platforms.

Linear-prediction-based methods are applied to mean Doppler frequency estimation of weather radar signals in the presence of ground clutter when only a small number of samples is available for processing. We show the ability of these methods to resolve weather signals from ground clutter in unfavorable conditions, i.e., when the clutter is stronger and its spectrum is narrower than that of the signal. Given a priori information about the ground clutter, Prony's method can be applied and simplified to derive a convenient formula for mean Doppler frequency estimation. This estimator may be considered as a generalization of the pulse-pair (PP) estimator and it can give satisfactory results in resolving weather signals from ground clutter for signal-to-noise ratios (SNRs) above 20 dB. The eigendecomposition-based minimum-norm (MN) method is applied to lower the SNR threshold to 10 dB. The performance of these estimators is compared with the signal-only lower bound and with a processor consisting of various ground clutter filters and the PP estimator.

Investigations reveal a possible integrated approach to the Tufts-Kumaresan (TK) technique, the minimum-norm (MN) technique and the total least squares (TLS) technique for frequency estimation in the presence of noise. The MN and TLS techniques are marginally better than the TK technique. The MN technique is an altered form of the TK technique, which, when altered, can provide the MN and TLS solution vectors.

The paper proposes a technique for calibrating a large polarimetric phased array weather radar The focus is on the use of computational electromagnetics. It validates the accuracy of the computations on a scaled-down antenna model and derives calibration parameters for the same model. The proposed technique is demonstrated using the WIPL-D software. The calibration is illustrated with all mathematical concepts required to carry the calibration out. The concept is adapted to use the results of numerical simulations which is part of the paper's novelty. Additionally, a roadmap of polarimetric phased array calibration is proposed and illustrated. [ABSTRACT FROM AUTHOR]