Publications

The estimation of air and particle motions in storms from multiple Doppler radar measurement is a long standing problem in radar meteorology. Our research interest in understanding the relationship of electrical change generation processes above the freezing level to thunderstorm life cycle, and in the detailed quantification of the eventual low altitude divergent outflow produced by the storm, demands an accurate retrieval of air and particle motions at essentially all altitudes within the storm. We found that existing approaches had deficiencies for our needs, and have developed an improved "hybrid" approach which attempts to provide high quality estimates throughout the storm volume.

The limitations to ultra-low sidelobe performance are explored using a 32-element linear array, operating at L-band, contianing transmit/receive (T/R) modules with 12-bit phase shifters. With conventional far-field calibrations, the average sidelobe level of the array was about-40dB. In theory, considerably lower sidelobe performance is expected from such an array. Initially, sidelobe performance was thought to be limited by inadequate calibrations. An examination of individual array element patterns showed a mirror-symmetric ripple which could be attributed to edge effects in a small array. Simulations indicated that more precise calibrations would not compensate for these element-pattern differences. An adaptive calibration technique was developed which iteratively adjusted the attenuator and phaser commands to create nulls in the antenna pattern in the direction of the nulls of a theoretical antenna pattern. With adaptive calibrations, the average sidelobe level can be lower to 60dB. The technique can be used for interference suppression by implementing antenna patterns with deep nulls in specified directions.

The purpose of this report is to present detailed data on microburst outflows recorded by the TDWR testbed radar (FL-2) in Huntsville, Alabama (1986) and Denver, Colorado (1987-88). Whenever possible, a microburst detected within 10 km of the radar was scanned in a vertical direction (RHI) at 1 to 2 degree azimuthal intervals about the center of divergence. The vertical profile of the outflow is pertinent to the detection capability and siting strategy of a single Doppler radar observing the microburst from a horizontal viewing angle. Additionally, outflow features are important in assessing the hazard associated with microbursts as well as the capability of other wind shear detection (LLWAS or ASR). Of particular interest is the variability of outflows depths from case to case and site to site. If the depth across the maximum velocity differential is shallow, an outflow might go undetected or underestimated by a radar, the beam ot which was not viewing the axis of peak divergence. Previous research projects in Denver reported the highest winds in a microburst typically occur near the surface with an average outflow depth (1/2 peak velocity) ranging between 500 and 600 meters: however, the vertical resolution of these data was fairly crude due to the scan strategies utilized. This report provides detailed high resolution microburst outflow vertical profile data pertinent to TDWR system studies based on RHI and closely spaced PPI scans. The median observed outflow depth in Huntsville was 200 meters shallower than in Denver while the median height of the maximum velocity varied from 100 meters AGL in Huntsville to 200 meters AGL in Denver. For those Denver events presented here, we recommend that the TDWR microburst detection scan extend to at least 200 meters AGL and 100 meters if there is adequate clutter suppression.

The FAA is deploying over 100 next generation airport surveillance radars (ASR-9) at selected major airports across the country. Like previous ASRs, the ASR-9 utilizes dual broad elevation fan beams Figure 1) along with a rapid scan rate (12.5 RPM to exercise its primary function of detecting aircraft over a 60 nmi radius. In addition, the ASR-9 has a separate dedicated weather reflectivity channel which allows air traffic controllers to display quantitative precipitation intensity reports corresponding to the NWS six-level intensity scale on their PPI display. The 30 second update rate of the weather channel coupled with the large sample volume swept by the ASR-9 fan-beam combine to provide timely and useful indications of precipitation intensity within the terminal airspace. The PPI display of precipitation intensity which is presented to the air traffic controller is essentially a 2-D representation of the 3-D reflectivity field sampled by the fan-shaped beam of the ASR-9. Since the antenna gain varies with elevation angle (Figure 1), the parameter reopned by the ASR-9 weather channel represents a beam-weighted, vertically averaged estimate of storm intensity. Previous research has shown that the vertically integrated reflectivity automatically reported by fan-beam radars such as the ASR-9 correlates well with estimates of vertically integrated liquid water content (VIL), a useful meteorological parameter which is a measure of overall storm intensity. Dobson found a linear relationship between W and fan-beam reflectivity from 30 to 60 dBZ assuming the beam is filled with precipitation (see discussion in Section 4). If the beam is non-uniformly or only partially filled with precipitation, then the inherent vertical integration introduced by the fan-beam may cause an underestimation of the storm intensity. This beam filling loss is most acute at long range, where the vertical extent of the beam intercepts more than 10 km of altitude. The magnitude of this error depends on the complex interaction between the vertical reflectivity structure of the storm and its interception by the fan-shaped beam. If the shape and altitude extent of the vertical reflectivity profile (such as could be provided by a pencil-beam radar) are known, then a suitable adjustment can be calculated and applied to the fan-beam reflectivity estimate in order to produce the desired reflectivity report. The six-level weather thresholds are stored in processor memory for each range sate as functions of receive beam (high or low). The thresholds can be adjusted to compensate for beam filling losses. The adjustments initially implemented in the ASR-9 were derived using a reflectivity profile model which assumes the maximum reflectivity of the storm is distributed constantly from the surface up to 4 km, and then falls off at 3 dBZ per km above 4 km. The success of the reflectivity correction depends on how well the model profile matches actual storm profiles. If regional variations in general storm morphology are significant, then different beam filling loss correction models may need to be developed for specific regions. Understanding the significance of these regional variations in storm vertical reflectivity structure and their impact on ASR-9 weather report accuracy provided the motivation for this study.

The FAA is deploying over 100 new ariport surveillance radars (ASR-9) across the country. In contrast to earlier ASRs, the ASR-9 utilized a separate digital weather processing channel to provide air traffic controllers with timely, calibrated displays of precipitation intensity. The ASR-9 utilizes dual selectable fan-shaped elevation beams designed to track aircraft over a large volume. As a consequence, weather echoes received from these fan-shaped beams represent vertically-averaged quantities. If the precipitation only partially or nonuniformly fills the beam, then the vertically integrated reflectivity may underestimate the actual intensity of the storm. The ASR-9 weather channel corrects for this by adjusting the range-dependent six-level relfectivity thresholds. The appropriateness of the currently implemented correction has not been carefully examined and may require modification to take into account regional and morphological variability in storm structure. This report discusses the method used to derive new beam filling loss adjustments. An extensive database of volumetric pencil-beam radar data were used in conjunction with our ASR-9 simulation facility to derive adjustments aimed at calibrating the precipitation intensity reports to the maximum perceived hazard. The new corrections yield substantially improved results over the current corrections in producing these reflectivity reports.

Techniques for the suppression of ground and storm clutter to permit the detection of low altitude windshear by pulse Doppler radars are described. Novel features of the system include the use of clutter residue and range aliased weather echo editing maps which edit out the range-azimuth cells on a "data adaptive" basis.

The Lincoln Laboratory Terminal Doppler Weather Radar (TDWR) testbed operated in Denver, CO in 1987-88. This radar is a prototype of the wind shear detection radars scheduled to be installed by the FAA to provide warnings of possibly hazardous wind shear conditions in airport terminal areas. To obtain the required coverage at low altitudes (down to 100-200 meters above ground level), the antenna beam is required to scan at or very near the earth's surface. Strong ground clutter returns at these low elevation angles present a major problem in the detection of low reflectivity wind shear signals and pose a significant challenge to the mission of these radars. To address this problem, steps along several fronts are taken to mitigate the effects of clutter contamination. These include the use of narrow pencil-beam antennas to minimize ground illumination, suppression by high-pass clutter filters, and the use of clutter residue map editing. This report deals with the latter step, and focuses on the clutter environment experienced at the testbed site during April-October 1988 and its effect on clutter residue map usage. Since the clutter environment is subject to change over time -- due either to man-made or natural causes -- the residue maps require periodic updates to reflect the changing nature of the clutter. This is particularly important for radar systems such as these which rely on automated algorithms to detect subtle patterns and features in the radar returns. To study the frequency with which residue maps required replacement in Denver, clutter measurements recorded during this period were analyzed and are presented in this report as a series of clutter residue maps. The maps are compared and the short and long term changes analyzed. It is concluded that the overall changes during this time were relatively small and gradual, and that map updates at one to two month intervals were sufficient. The generation of the residue maps is described and the importance of collecting clutter data on clear, weather-free days, without the presence of anomalous propagation conditions is addressed. This report also describes the use of median estimation in the construction of the maps as an effective method of eliminating the occasional strong returns from moving reflectors, such as aircraft and vehicles, which would otherwise distort the maps.

A very low grazing angle (

An adaptive procedure for selecting radar pulse repetition frequency (PRF) has been developed as the primary means of minimizing the occurence of range aliased echoes within operationaly significant coverage aread (e.g., airport runways) of the Terminal Doppler Weather Radar (TDWR) system. This procedure underwent extensive testing at the S-Band TDWR testbed while located in Denver, CO, where it was judged to be highly successful at preserving the integrity of data collected within the vicinity of the Stapleton International Airport runways. The actual TDWR system will operate at a C-Band frequency, and an increase in potential range obscuration is expected over that experience by the S-band testbed. This report discusses the anticipated performance of the PRF selection procedure in the C-Band environment by extrapolating results obtained using S-Band testbed data. The results conclusively demonstrate the efficacy of adaptive PRF selection as a method by which to reduce potential range obscuration. A worst-case scenario, for example, indicates that over 20% of the TDWR radar collected about the airport runways has the potential for being contaminated with range aliased echoes at any given time during TDWR surveillance operations. With adaptive PRF selection, however, the expected obscuration is reduced to only 3%. (The corresponding figures for the S-Band testbed are shown to be 14& rather than 20% and 1% rather than 3%). While adaptive PRF selection can substantially reduce range obscuration, it cannot totally eliminate the problem. An enhancement to the PRF selection strategy, which further reduces the potential range obscuration, is introduced and recommended. Additionally, the complementary use of pulse-to-pulse phase modulation to exact valid velocity measurements in the presence of range contamination is discussed and preliminary experimental results presented.

The ASR-9, the next generation airport surveillance radar, will be deployed by the FAA at over 100 locations throughout the United States. The system includes a weather channel designed to provide ATC personnel with timely and accurate weather reflectivity information as a supplement to normal aircraft information. This report presents results of an assessment of the ASR-9 weather channel performance. Two issues addressed are: (1) whether the ASR-9 weather channel performs according to FAA specifications, and (2) whether the ASR-9 weather channel adequately represents weather reflectivity for ATC purposes. These measurement results are intended to support the FAA in developing the operational use of ASR-9 weather information. Comparisons between data from an ASR-9 in Huntsville, Alabama, recorded during design qualification and testing, and data from two other "reference" radars were used as the basis for the assessment. Several storm cases were analyzed, comprised of stratiform rain, isolated convective storms, squall lines, and cold fronts containing multiple simultaneous convective storms. Results suggest that, with the exception of an apparent 3 dB discrepancy between the weather products of the ASR-9 and the "reference" radars, the ASR-9 weather channel seems to perform according to FAA specifications. Although the ASR-9 products give a reasonable representation of the extent and severity of potentially hazardous weather in Huntsville, the results suggest that the static storm model used to determine beamfill corrections for the ASR-9 should be optimized for the particular climatic region in which an ASR-9 will be operated.