Engineering topics

Vivekanandan, J.

Publications and source records attributed to Vivekanandan, J..

9 records

NASA NTRS1993

Microwave radiative transfer studies of precipitation

Since the deployment of the DMSP SSM/I microwave imagers in 1987, increased utilization of passive microwave radiometry throughout the 10 - 100 GHz spectrum has occurred for measurement of atmospheric constituents and terrestrial surfaces. Our efforts have focused on observations and analysis of the microwave radiative transfer behavior of precipitating clouds. We have focused particular attention on combining both aircraft and SSM/I radiometer imagery with ground-based multiparameter radar observations. As part of this and the past NASA contract, we have developed a multi-stream, polarized radiative transfer model which incorporates scattering. The model has the capability to be initialized with cloud model output or multiparameter radar products. This model provides the necessary 'link' between the passive microwave radiometer and active microwave radar observations. This unique arrangement has allowed the brightness temperatures (TB) to be compared against quantities such as rainfall, liquid/ice water paths, and the vertical structure of the cloud. Quantification of the amounts of ice and water in precipitating clouds is required for understanding of the global energy balance.

Bringi, V. N.↗

NASA NTRS1992

Multiparameter radar and advanced microwave precipitation radiometer observations of tropical convection

Combinations of both active and passive microwave sensors have been proposed for experiments such as the tropical rainfall measuring mission (TRMM). During the summer 1991 Convection and Precipitation/Electrification Experiment (CaPE) in central Florida, both cold- and warm-rain precipitation processes were studied with the NCAR CP-2 multiparameter radar, operating at 3 and 10 GHz. The Advanced Microwave Precipitation Radiometer (AMPR), which operates at 10.7, 19.35, 37.1, and 85.5 GHz, was flown aboard NASA's ER-2 high-altitude aircraft over storms being scanned by the CP-2 radar. Top-of-atmosphere multifrequency TB from the AMPR is presented along with coincident CP-2 radar observations. Joint analysis of both radar and radiometer data sets allows refinement of new and existing precipitation retrieval techniques which will utilize the multifrequency TB from an integrated spaceborne microwave radiometer/radar system.

Vivekanandan, J.↗

NASA NTRS1991

Ice water path estimation and characterization using passive microwave radiometry

Model computations of top-of-atmospheric microwave brightness temperatures T(B) from layers of precipitation-sized ice of variable bulk density and ice water content (IWC) are presented. It is shown that the 85-GHz T(B) depends essentially on the ice optical thickness. The results demonstrate the potential usefulness of scattering-based channels for characterizing the ice phase and suggest a top-down methodology for retrieval of cloud vertical structure and precipitation estimation from multifrequency passive microwave measurements. Attention is also given to radiative transfer model results based on the multiparameter radar data initialization from the Cooperative Huntsville Meteorological Experiment (COHMEX) in northern Alabama. It is shown that brightness temperature warming effects due to the inclusion of a cloud liquid water profile are especially significant at 85 GHz during later stages of cloud evolution.

Vivekanandan, J.↗

NASA NTRS1991

Bulk ice characterization using coupled cloud, multiparameter radar, and radiative transfer models

The study demonstrates the potential utility of employing scattering-based passive microwave channels (not less than 37 GHz) to estimate the amount of ice water path that exists above rain in precipitating clouds. This methodology can be used along with lower-frequency emission-based channels to infer the columnar melt water and ice water paths. Portions of the ice column that coexist with cloud water hide the usually steep brightness temperature cooling through this region and inhibit characterization of the total ice column.

Turk, J.↗

NASA NTRS1991

Remote sensing of precipitation structures using combined microwave radar and radiometric techniques

After reviewing published multiparameter radar observations and their interpretation for microphysical retrieval, the paper presents radiative transfer model simulations performed using radar measurements and a cloud model. The qualitative and quantitative aspects of the remote sensing of precipitation are discussed with particular attention given to the potential of estimating scattering-based ice water path.

Vivekanandan, J.↗

NASA NTRS1990

Microwave radiative transfer studies using combined multiparameter radar and radiometer measurements during Cohmex

Theoretical calculations of the upwelling microwave radiances from clouds containing layers of rain, ice, and a melting region were performed at frequencies of 18, 37, and 92 GHz. These frequencies coincide with high-resolution microwave radiometer measurements taken aboard the NASA ER-2 high-altitude aircraft during the summer 1986 Cohmex (Cooperative Huntsville Meteorological Experiment) in Alabama. For purposes of brightness temperature computations, the storms were modeled with rain, melting phase, and ice layers. The melting phase region was composed of water-coated ice spheres defined by a 'melt index' in terms of the volume fraction of water. Single scatter albedo, scattering, and extinction coefficients were computed at the above frequencies as a function of the rain rate and melt index. In addition, multiparameter radar observations of the storm were mapped into a Cartesian space and averaged over regions comparable to the radiometer footprint.

Vivekanandan, J.↗

NASA NTRS1990

Multiparameter radar and microwave radiative transfer modeling of nonspherical atmospheric ice particles

Multiparameter radar and polarized microwave radiative transfer models for arbitrarily shaped particles are developed. The ice crystals modeled are horizontally oriented hexagonal plates, columns, and needles. The ice water content is fixed at 0.1 g/cu m, and a realistic size distribution is used. The radar modeling is done for S through K-band, and the passive microwave calculations are at 37, 85, and 157 GHz. The modeling results show that particle shape is important for both multiparameter radar and passive microwave radiometry. Radar reflectivity and upwelling microwave brightness temperatures depend strongly on the individual particle volume, which in turn depends on the ice crystal shape. Radar differential reflectivity is high for the plates and lower for needles and columns. Linear depolarization calculations indicate that oblate ice crystals such as plates can be distinguished from prolate crystals such as columns. At 85 and 157 GHz, significant polarization brightness temperature differences are calculated for plates and columns. The particle bulk density strongly affects the radar and radiometer observables. Recent measurements of ice crystals are discussed to show that the model results have practical applicability.

Evans, K. Franklin↗

NASA NTRS1989

Joint analysis of multiparameter radar and radiometer measurements of convective storms

An account is given of the radiative properties of hydrometeors in order to evaluate the influences of water, ice, and melting layers on the upwelling microwave radiances of convective storms. Attention is given to storm overflights conducted by the ER-2 aircraft in coordination with the NCAR dual-polarization/dual-wavelength mode radar. Radar-radiometer comparisons are conducted for one overflight from each day of the study campaign, using the various multiparameter radar observations to delineate regions of water, ice, and melting, as well as to obtain raindrop-size distribution. Model-computed brightness temperatures are compared with the ER-2 aircraft measurements.

Vivekanandan, J.↗

NASA NTRS1989

Millimeter wave radiative transfer studies for precipitation measurements

Scattering calculations using the discrete dipole approximation and vector radiative transfer calculations were performed to model multiparameter radar return and passive microwave emission for a simple model of a winter storm. The issue of dendrite riming was addressed by computing scattering properties of thin ice disks with varying bulk density. It was shown that C-band multiparameter radar contains information about particle density and the number concentration of the ice particles. The radiative transfer modeling indicated that polarized multifrequency passive microwave emission may be used to infer some properties of ice hydrometers. Detailed radar modeling and vector radiative transfer modeling is in progress to enhance the understanding of simultaneous radar and radiometer measurements, as in the case of the proposed TRMM field program. A one-dimensional cloud model will be used to simulate the storm structure in detail and study the microphysics, such as size and density. Multifrequency polarized radiometer measurements from the SSMI satellite instrument will be analyzed in relation to dual-frequency and dual-polarization radar measurements.

Vivekanandan, J.↗