Brad W. Orr

Cloud physics studies with 8 mm wavelength radar

Abstract Results from recent cloud experiments with an 8 mm wavelength Doppler radar demonstrate that millimeter wavelength radar can provide important new information about nonprecipitating and lightly precipitating clouds. Millimeter wave radar can be used to document small-scale spatial structure of cirrus and marine stratus clouds. Its data can be used to estimate profiles of ice content, particle size and concentration in cirrus clouds and profiles of liquid water content and turbulence in marine stratus clouds. New results with mm wave radar suggest that plate-like crystals may be distinguishable from aggregates with polarization techniques. Quantitative information about cirrus cloud ice crystal fallspeeds, and therefore ice mass flux, can also be produced with newly developed techniques that exploit the sensitivity and velocity precision of such radars. The good sensitivity to cloud particles, the immunity from ground clutter contamination, and the good spatial resolution of millimeter wavelength radar make it an excellent instrument for documenting quantitative microphysical and dynamical properties of non-precipitating and lightly precipitating clouds.

Nocturnal Low-Level Jet in a Mountain Basin Complex. Part I: Evolution and Effects on Local Flows

A Doppler lidar deployed to the center of the Great Salt Lake (GSL) basin during the Vertical Transport and Mixing (VTMX) field campaign in October 2000 found a diurnal cycle of the along-basin winds with northerly up-basin flow during the day and a southerly down-basin low-level jet at night. The emphasis of VTMX was on stable atmospheric processes in the cold-air pool that formed in the basin at night. During the night the jet was fully formed as it entered the GSL basin from the south. Thus, it was a feature of the complex string of basins draining toward the Great Salt Lake, which included at least the Utah Lake basin to the south. The timing of the evening reversal to down-basin flow was sensitive to the larger-scale north‐south pressure gradient imposed on the basin complex. On nights when the pressure gradient was not too strong, local drainage flow (slope flows and canyon outflow) was well developed along the Wasatch Range to the east and coexisted with the basin jet. The coexistence of these two types of flow generated localized regions of convergence and divergence, in which regions of vertical motion and transport were focused. Mesoscale numerical simulations captured these features and indicated that updrafts on the order of 5 cm s 21 could persist in these localized convergence zones, contributing to vertical displacement of air masses within the basin cold pool.

A Method for Estimating Particle Fall Velocities from Vertically Pointing Doppler Radar

Abstract A method is presented that estimates particle fall velocities from Doppler velocity and reflectivity measurements taken with a vertically pointing Doppler radar. The method is applicable to uniform, stratified clouds and is applied here to cirrus clouds. A unique aspect of the technique consists of partitioning the Doppler velocities into discrete cloud height and cloud reflectivity bins prior to temporal averaging. The first step of the method is to temporally average the partitioned Doppler velocities over an hour or two to remove the effects of small-scale vertical air motions. This establishes relationships between particle fall velocity and radar reflectivity at various levels within the cloud. Comparisons with aircraft in situ observations from other experiments show consistency with the remote-sensing observations. These results suggest that particle fall speeds can be determined to within 5–10 cm s−1 by means of this technique.

The Lake Ontario Winter Storms (LOWS) Project

Abstract Snowstorms generated over the Great Lakes bring localized heavy precipitation, blizzard conditions, and whiteouts to downwind shores. Hazardous freezing rain often affects the same region in winter. Conventional observations and numerical models generally are resolved too coarsely to allow detection or accurate prediction of these mesoscale severe weather phenomena. The Lake Ontario Winter Storms (LOWS project was conducted to demonstrate and evaluate the potential for real-time mesoscale monitoring and location-specific prediction of lake-effect storms and freezing rain, using the newest available technologies. LOWS employed an array of specialized atmospheric remote sensors (a dual-polarization short wavelength radar, microwave radiometer, radio acoustic sounding system, and three wind profilers) with supporting observing systems and mesoscale numerical models. An overview of LOWS and its initial accomplishments is presented.

Doppler Radar Observations of the Development of a Boundary-Layer Nocturnal Jet

Abstract A single Doppler radar obtained detailed clear-air measurements of the development of a strong boundary-layer nocturnal jet in North Dakota during the summer of 1989. The evolution of the jet was monitored by the radar with a high degree of vertical and temporal resolution using a repetitive sequence of four different elevation scans. A new variation of the velocity-azimuth display (VAD) analysis technique provided vertical profiles of the mean wind components and several turbulence terms. Boundary-layer wind speeds began to increase in the late afternoon, well before sunset, as surface cooling began. Wind speeds accelerated faster after sunset and eventually produced a jet that exceeded 23 m s−1 at about 0.5 km AGL. The wind veered with height and time and followed the expected inertial oscillation pattern. Measured shear stresses, vertical fluxes of momentum, and velocity variances, which were initially large, decreased sharply after the surface began to cool. The directly measured vertical vel...

Ground-Based Remote Sensing of Cloud Particle Sizes during the 26 November 1991 FIRE II Cirrus Case: Comparisons with In Situ Data

Abstract The paper presents the results of retrieving characteristic particle sizes for the November 26 1991 FIRE II case using two methods that utilize ground-based remotes sensors. The size information for the complete vertical depth of the cloud was obtained for a 3-hour period from 1830 to 2130 UTC using combined Doppler radar and IR radiometer measurements and for two shorter periods using radar reflectivity and CO2 lidar backscatter measurements. The results obtained with both remote sensing techniques are compared for these two periods. Possible retrieval uncertainties are discussed. Comparisons yielded an agreement with a relative standard deviation of 15%-20% between the two methods. Particle sizes retrieved by both methods were compared with 2D particle probe data sampled during 10 time intervals when a research aircraft was crossing the hub area. The relative standard deviation of particle sizes retrieved with the radar-radiometer method from those obtained from 2D probes is about 30% for nine ...

Recent radar measurements of turbulence and microphysical parameters in marine boundary layer clouds

We present a collection of research related to radar measurements of turbulence and microphysical properties in clouds. The radars used in these studies operate at frequencies ranging from 404 MHz to 34.6 GHz. We discuss the relative contributions made by the two primary radar scattering mechanisms to the measured values of radar reflectivity at the different frequencies. The desired turbulence and microphysical information is obtained from the radar reflectivity and other elements of the Doppler velocity spectra. Methods and examples are given with emphasis on liquid water clouds associated with the marine boundary layer.

Multiwavelength Observations of a Developing Cloud System: The FIRE II 26 November 1991 Case Study

Simultaneous multiwavelength measurements of a developing cloud system were obtained by NOAA Doppler lidar, Doppler radar, Fourier transform infrared interferometer, and microwave and infrared radiometers on 26 November 1991. The evolution of the cloud system is described in terms of lidar backscatter, radar reflectivity and velocity, interferometer atmospheric spectra, and radiometer brightness temperature, integrated liquid water, and water vapor paths. Utilizing the difference in wavelength between the radar and lidar, and therefore their independent sensitivity to different regions of the same cloud, the cloud top, base, depth, and multiple layer heights can be determined with better accuracy than with either instrument alone. Combining the radar, lidar, and radiometer measurements using two different techniques allows an estimation of the vertical profile of cloud microphysical properties such as particle sizes. Enhancement of lidar backscatter near zenith revealed when highly oriented ice crystals were present. The authors demonstrate that no single instrument is sufficient to accurately describe cirrus clouds and that measurements in combination can provide important details on their geometric, radiative, and microphysical properties.

The 1995 Arizona Program: Toward a Better Understanding of Winter Storm Precipitation Development in Mountainous Terrain

The 1995 Arizona Program was a field experiment aimed at advancing the understanding of winter storm development in a mountainous region of central Arizona. From 15 January through 15 March 1995, a wide variety of instrumentation was operated in and around the Verde Valley southwest of Flagstaff, Arizona. These instruments included two Doppler dual-polarization radars, an instrumented airplane, a lidar, microwave and infrared radiometers, an acoustic sounder, and other surface-based facilities. Twenty-nine scientists from eight institutions took part in the program. Of special interest was the interaction of topographically induced, storm-embedded gravity waves with ambient upslope flow. It is hypothesized that these waves serve to augment the upslope-forced precipitation that falls on the mountain ridges. A major thrust of the program was to compare the observations of these winter storms to those predicted with the Clark-NCAR 3D, nonhydrostatic numerical model.

Detection of Weakly Precipitating Winter Clouds by a NOAA 404-MHz Wind Profiler

Abstract Recent studies have demonstrated that the 404-MHz wind profilers of the National Oceanic and Atmospheric Administration WPDN (Wind Profiler Demonstration Network) can detect precipitation under most circumstances. Their ability to detect nonprecipitating and weakly precipitating clouds, however, has remained the subject of debate. To address this question, a 35-GHz Ka-band cloud-sensing radar was operated side by side, with a WPDN profiler in Colorado during the winter of 1993. The short wavelength (0.87 cm), finescale resolution, and excellent sensitivity of the Ka-band system to small hydrometeors make it very well suited for detailed measurements of clouds and weak precipitation. Comparisons of data from the two instruments show that in addition to detecting precipitation, the profiler did indeed detect nonprecipitating ice clouds under some circumstances that can be approximately delineated by profiler reflectivity and vertical velocity thresholds. These thresholds are a function of height fo...