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NASA NTRS1993

Low-power millimeter-wave radar observations of the atmosphere

Historically, cloud structures, dynamics, and precipitation processes have been observed and measured with sensors from two different spatial resolutions. Laser-based sensors have volume resolutions on the order of 10(exp -4) to 10(exp -2) cubic meters for a 1-s sample. Radar systems operating at wavelengths between 1 and 10 cm have resolutions on the order of 10(exp 4) to 10(exp 7) cubic meters. The resolutions of micro-wave systems depend primarily on the system RF bandwidth and antenna bandwidth. Both resolution regimes were useful in the study of cloud structures and processes - the former for determining resolution on the individual cloud particle scale and the latter for studying the coarse characteristics of cloud dynamics and structure. There are, however, cloud processes and structures that occur on scales that lie between these two regimes: the process of entrainment, where outside air is brought within the cloud boundaries; the mixing of in-cloud particles with different histories; cloud particle coalescence; and ice formation. The use of mm-wave (30- to 300-GHz) radars offers an opportunity to observe cloud processes at these scales and determine their influence on precipitation development, cloud albedos, cloud lifetimes, chemical cycling of tract substances, aircraft icing, and other meteorological phenomena. To determine the usefulness of a 35-GHz radar for observing these precipitation and cloud processes, a research program was initiated. The objectives of this program are to develop a 35-GHz radar, to measure scattering from precipitation and clouds, and to develop a model to compute scattering from clouds using the finite-difference time-domain (FDTD) technique.

Ronnau, James F.↗

DOE OSTI2020

Multiple-platform and multiple-Doppler radar observations of a supercell thunderstorm in South America during RELAMPAGO

On 10 November 2018, during the RELAMPAGO field campaign in Argentina, South America, a thunderstorm with supercell characteristics was observed by an array of mobile observing instruments, including three Doppler on Wheels (DOW) radars. This is believed to be the first such storm ever sampled by multiple Doppler radars in South America, and one of perhaps two such sets of observations in the Southern Hemisphere. The observational strategies employed within the Córdoba Province on 10 November are described, as also is the evolution of the initial deep convective clouds through their organization into a supercell. In contrast to the archetypal supercell, the updraft rotation in this storm was rather short lived (~20 min), as also was the occurrence of damaging hail, causing us to question whether this indeed was indeed a supercell. However, additional data collected within the Córdoba Province provide us with evidence of other storms with this behavior, which appears to be attributable in part to effects of the local terrain. Thus, the structure and evolution of this storm documented during RELAMPAGO may be typical in this region.

Trapp, Robert↗

NASA NTRS1985

Kyoto Meteor Radar Observation in November, 1981 and May, 1982

The Kyoto meteor radar a pulsed Doppler radar and operates at 31.57 MHz ith peak power of 10 kW. An interferometer is adopted for arrival angle measurement. Height resolution is approximately 1 to 3 km depending on both azimuth and elevation angles. Data is analyzed according to an algorithm approved by GROVES. All data in height range 84 to 104 km are separated into five layers, whose altitude width is 4 km. In each layer, least square fitting is performed for ten parameters, which are amplitudes of northward/eastward mean wind and sin/cos component of diurnal/semidiurnal tide. Reliable range for each parameter is also calculated.

Tsuda, T.↗

NASA NTRS1988

Radar observations of basaltic lava flows, Craters of the Moon, Idaho

Radar images of Craters of the Moon, Idaho were used to study the backscatter characteristics of basaltic lava flows of predominantly pahoehoe textures and to determine the ability to detct fissure vents. Four images were obtained: X-band HH, X-band HV, L-band HH, and L-band HV. Hummocky pahoehoe flows were found to have strong backscatter in all four of these images. Aa lava flows showed the greatest variation in backscatter intensities, due to an increase in multiple scattering at the L-band scale. Eruptive fissures are detectable in the radar images by virtue of associated parallel spatter ramparts which have diagnostic, strong backscatter in the X-band images that are in contrast to the weak backscatter of the surrounding shelly pahoehoe lava. The importance of look direction in the use of radar images to characterize terrains is emphasized.

Greeley, Ronald↗

NASA NTRS1989

Radar observations of asteroid 1986 JK

The asteroid 1986 JK was observed with a 3.5 cm-wavelength radar in May and June, 1986, at less than 0.029 AU; its radar echo power circular polarization ratio indicates single backscattering from smooth surface elements. A working model constructed for the asteroid in light of these radar data postulates a 1-2 km object whose shape has little elongation and some polar flattening. Orbital and physical characteristics are rather cometlike. The radar astrometric data obtained are noted to be extremely powerful for orbit-improvement, so that a search ephemeris whose uncertainty is an order-of-magnitude smaller than that based on relevant optical data alone can be prepared by combining optical and radar data.

Ostro, S. J.↗

NASA NTRS1998

Icarus Revisted: Three Decades of Radar Observations of Asteroids

Three decades after the close encounter of 1566 Icarus, which marked the beginning of asteroid radar astronomy, we had a chance to observe the asteroid afresh by using a more developed radar system, and here we present the results of the study.

Icarus asteroid radar astronomy radar cross sectio↗

NASA NTRS2014

Assimilation of Dual-Polarimetric Radar Observations with WRF GSI

Dual-polarimetric (dual-pol) radar typically transmits both horizontally and vertically polarized radio wave pulses. From the two different reflected power returns, more accurate estimate of liquid and solid cloud and precipitation can be provided. The upgrade of the traditional NWS WSR-88D radar to include dual-pol capabilities will soon be completed for the entire NEXRAD network. Therefore, the use of dual-pol radar network will have a broad impact in both research and operational communities. The assimilation of dual-pol radar data is especially challenging as few guidelines have been provided by previous research. It is our goal to examine how to best use dual-pol radar data to improve forecast of severe storm and forecast initialization. In recent years, the Development Testbed Center (DTC) has released the community Gridpoint Statistical Interpolation (GSI) DA system for the Weather Research and Forecasting (WRF) model. The community GSI system runs in independently environment, yet works functionally equivalent to operational centers. With collaboration with the NASA Short-term Prediction Research and Transition (SPoRT) Center, this study explores regional assimilation of the dual-pol radar variables from the WSR-88D radars for real case storms. Our presentation will highlight our recent effort on incorporating the horizontal reflectivity (ZH), differential reflectivity (ZDR), specific differential phase (KDP), and radial velocity (VR) data for initializing convective storms, with a significant focus being on an improved representation of hydrometeor fields. In addition, discussion will be provided on the development of enhanced assimilation procedures in the GSI system with respect to dual-pol variables. Beyond the dual-pol variable assimilation procedure developing within a GSI framework, highresolution (≤1 km) WRF model simulations and storm scale data assimilation experiments will be examined, emphasizing both model initialization and short-term forecast of precipitation fields and processes. Further details of the methodology of data assimilation, the impact of different dual-pol variables, the influence on precipitation forecast will be presented at the conference.

Li, Xuanli↗

NASA NTRS2014

Assimilation of Dual-Polarimetric Radar Observations with WRF GSI

Li, Xuanli↗

NASA NTRS2021

An Evaluation of Size-Resolved Cloud Microphysics Scheme Numerics for use with Radar Observations Part II: Condensation and Evaporation

Accurate numerical modeling of clouds and precipitation is essential for weather forecasting and climate change research. While size-resolved (bin) cloud microphysics models predict particle size distributions without imposing shapes, results are subject to artificial size distribution broadening owing to numerical diffusion associated with various processes. Whereas Part 1 addressed collision-coalescence, here we investigate numerical diffusion that occurs in solving condensation and evaporation. In a parcel model framework, all of the numerical schemes examined converge to one solution of condensation and evaporation as the mass grid is refined, and the advection-based schemes are recommended over the reassigning schemes. Including Eulerian vertical advection in a column limits the convergence to some extent, but that limitation occurs at a sufficiently fine mass grid, and the number of iterations in solving vertical advection should be minimized to reduce numerical diffusion. Insubstantial numerical diffusion in solving condensation can be amplified if collision-coalescence is also active, which in turn can be substantially diminished if turbulence effects on collision are incorporated. Large-eddy simulations of a drizzling stratocumulus field reveal that changes in moments of Doppler spectra obtained using different mass grids are consistent with those obtained from the simplified framework, and that spectral moments obtained using a mass grid designed to effectively reduce numerical diffusion are generally closer to observations. Notable differences between the simulations and observations still exist, and our results suggest a need to consider whether factors other than numerical diffusion in the fundamental process schemes employed can cause such differences.

Cloud microphysics↗

NASA NTRS1981

Spaceborne radar observation of the earth surface

Seasat SAR images are being analyzed to determine the potential of spaceborne radars for earth resources and ocean surface observation. Examples are presented for a variety of applications in structural mapping, lithological classification, soil moisture detection, polar ice motion monitoring and ocean features observation. These examples are briefly discussed with emphasis on the future research needed to further the capability of radar sensors, by themselves or in combination with other sensors. A brief discussion is then given on the spaceborne sensors which are required and planned to meet these needs.

Elachi, C.↗

NASA NTRS2013

Polarimetric and Multi-Doppler Radar Observations of Electrified and Unelectrified Wildfire Smoke Plumes

Pyrocumulus clouds above three Colorado wildfires (Hewlett Gulch, High Park, and Waldo Canyon; all occurred during summer 2012) electrified and produced small intracloud discharges whenever the smoke plumes grew to high altitudes (over 10 km above mean sea level, or MSL). This occurred during periods of rapid wildfire growth, as indicated by the shortwave infrared channel on a geostationary satellite, as well as by incident reports. In the Hewlett Gulch case, the fire growth led to increased updrafts within the plume, as inferred by multiple‐ Doppler radar syntheses, which led to the vertical development and subsequent electrification ‐ a life cycle as short as 30 minutes. The lightning, detected by a threedimensional lightning mapping network, was favored in high‐altitude regions (~10 km MSL) containing modest reflectivities (25 dBZ and lower), ~0 dB differential reflectivity, and reduced correlation coefficient (~0.6‐0.7). This indicated the likely presence of ice particles (crystals and aggregates, possibly rimed) mixed with ash. Though neither multiple‐Doppler nor polarimetric observations were available during the electrification of the High Park and Waldo Canyon plumes, their NEXRAD observations showed reflectivity structures consistent with Hewlett Gulch. In addition, polarimetric and multiple‐Doppler scanning of unelectrified High Park plumes indicated only irregularly shaped ash, and not ice, was present (i.e., reflectivities < 25 dBZ, differential reflectivity > 5 dB, correlation < 0.4), and there was no broaching of the 10 km altitude. Based on these results, the electrification likely was caused by ice‐based processes that did not involve significant amounts of graupel. The results demonstrate the scientific value of multiple‐Doppler and polarimetric radar observations of wildfire smoke plumes ‐ including the ability to distinguish between regions of pure hydrometeors, regions of pure ash, and mixtures of both ‐ and also suggest a possible new application for lightning data in monitoring wildfires.

Lang, Timothy J.↗

NASA NTRS1988

Surface contour radar observations of the directional wave spectrum during Fasinex

The surface control radar (SCR), a 36-GHz computer-controlled airborne radar which generates a false-color coded elevation map of the sea surface below the aircraft in real time, is described. The SCR turned out to be ideal for documenting the wave spectra during Fasinex (the Frontal Air-Sea Interaction Experiment) due to its high spatial resolution and rapid mapping capability over extensive areas. Synoptic weather maps for February 15-18, 1986 are presented.

Walsh, E. J.↗

NASA NTRS1992

Experimental measurement and theoretical modeling of microwave scattering and the structure of the sea surface influencing radar observations from space

The electromagnetic bias is an error present in radar altimetry of the ocean due to the non-uniform reflection from wave troughs and crests. A study of the electromagnetic bias became necessary to permit error reduction in mean sea level measurements of satellite radar altimeters. Satellite radar altimeters have been used to find the upper and lower bounds for the electromagnetic bias. This report will present a theory using physical optics scattering and an empirical model of the short wave modulation to predict the electromagnetic bias. The predicted electromagnetic bias will be compared to measurements at C and Ku bands.

Arnold, David↗

NASA NTRS1992

On the application of multifrequency polarimetric radar observations for sea-ice classification

The use of multifrequency polarimetric radar imagery to enhance the ability to separate different sea-ice types using single-frequency, single-polarization synthetic aperture radar (SAR) data is investigated. Backscatter characteristics of six radiometrically and polarimetrically distinct sea-ice types are selected in an unsupervised range-dependent analysis of multifrequency polarimetric SAR data using the maximum a posteriori (MAP) polarimetric classifier. Maximum ice discrimination is achieved with combined C- and L-band full polarimetry, and collocated passive microwave imagery suggests greater than 90 percent classification accuracy. C-band VV-pol alone achieves only 68 percent relative accuracy because it confuses multiyear and rough compressed first year ice. L-band, relative classification accuracy is 75 percent, 83 percent, and 85 percent, using HH-pol, HH- and VV-combined, or the full polarimetry, respectively. P-band is less accurate. Combinations of two frequencies at a single polarization show the greatest improvement over a single channel.

Rignot, Eric↗

NASA NTRS1997

Surface Properties of the Moon, Venus and Small Bodies from Radar Observations

Studies of the moon during the period of the grant revolved around the issues of the possible presence of ice at the lunar poles and the determination of the electrical properties of the maria regoliths. The search for ice at the poles was conducted using measurements of the radar backscatter cross sections and circular polarization ratios measured from 125 m resolution Arecibo radar imagery at 13 cm wavelength obtained by Nicholas Stacy. No clear indication of the presence of ice was found in areas thought to be in permanent shadow from solar radiation. Then Cornell graduate student Greg Black modeled the radar backscattering behavior of the icy Galilean satellites using three wavelength measurements of their radar backscattering properties obtained with the Arecibo and Goldstone radars. The radar scattering properties of Europa, Ganymede, and Callisto are unlike those of any other object observed with planetary radars. They are strongly backscattering with specific radar cross sections that can exceed unity. Polarization ratios are also high, approx. 1.5, indicative of multiple scattering, and the echos follow a diffuse scattering law at all incident angles with no indication of quasi-specular reflections. 3) Most of our effort on small bodies went into developing and investigating methods for long baseline radar synthesis imaging of near-earth asteroids and comets. At X-band, the width of the synthesized beam of the Very Long Baseline Array (VLBA) is approximately 15 m at 0.03AU, a typical close approach distance for near-earth asteroids. A small amount of work was done analyzing Venus data from Arecibo and the Magellan mission.

Campbell, Donald B.↗

NASA NTRS1976

Imaging radar observations of frozen Arctic lakes

A synthetic aperture imaging L-band radar flown aboard the NASA CV-990 remotely sensed a number of ice-covered lakes about 48 km northwest of Bethel, Alaska. The image obtained is a high resolution, two-dimensional representation of the surface backscatter cross section, and large differences in backscatter returns are observed: homogeneous low returns, homogeneous high returns and/or low returns near lake borders, and high returns from central areas. It is suggested that a low return indicates that the lake is frozen completely to the bottom, while a high return indicates the presence of fresh water between the ice cover and the lake bed.

Elachi, C.↗

NASA NTRS1981

Imaging radar observations of volcanic features in Medicine Lake Highland, California

Synthetic aperture L-band radar images of Medicine Lake Highland, California, as obtained from the JPL-NASA aircraft and Seasat orbital systems, are presented. Image interpretation is based on two types of information: slope and topographic effects for geomorphic information, and reflectivity or backscatter in flat terrain which is related to surface roughness and to the surface dielectric constant. Cinder cones and lava tubes are visible (geomorphic features), and three types of lava surfaces are: aa, pahoehoe, and block. In addition, to infer lava flow relative ages, overlapping flow fronts and roughness attenuation observations may be used. It is hoped that this research will improve the understanding of the radar signature of volcanic areas, and will be helpful in interpreting data obtained from spaceborne sensors over the earth and, in particular, Venus.

Farr, T. G.↗

DOE OSTI2021

Climatology of Melting Layer Heights Estimated From Cloud Radar Observations at Various Locations

A melting layer (ML) detection algorithm for cloud radar with polarimetric capability was developed and applied to the cloud radar data collected from five different sites around the world for several years. The retrieved melting layer top height (MLH) showed a very good correspondence with the ECMWF Reanalysis 5 zero degree level data for all five sites. The ML characteristics were distinctively different for different sites, revealing climatological characteristics of ML forming clouds in different regions. Generally, ML tended to occur more frequently in summer than in winter except for a maritime site, where low stratiform clouds formed frequently in summer, the top of which might be lower than the freezing level. In contrast, at two Arctic sites, ML occurred almost exclusively in summer because it was too cold to have an ML in the other seasons. The MLH also varied significantly from site to site but generally was higher during warmer seasons. Based on MLH, two new indices, bulk temperature lapse rate (BLR) and relative depth (RD) of liquid cloud below MLH (RD) were developed, which were useful to explain the environmental characteristics of the five sites. BLR generally increased with the surface temperature at all sites except at the marine site that showed an opposite trend, where a unique synoptic pattern in winter generated high BLR in this cold season. Finally, these findings confirm that studies on thermodynamic structures using cloud radars can be broadened, taking advantage of BLR and RD information, as these indices can represent environmental thermodynamic characteristics of the clouds that have ML.

54 ENVIRONMENTAL SCIENCES↗