Abstract

Millimeter-wave Doppler spectra obtained from the dual-frequency Cloud Profiling Radar System (CPRS) are used to retrieve both the drop size distribution and the vertical air motion in rain. CPRS obtains collocated spectra at W and Ka bands through a single 1-m-diameter lens antenna. The vertical air motion is determined primarily from the 95-GHz Mie scattering from rain, whereas turbulence effects are minimized by correlating the drop size distributions measured at both the 95- and 33-GHz frequencies. The authors describe an iterative procedure that estimates the drop sizes and vertical motions with range and horizontal resolution of 60 m and temporal resolution of 2 s. Model drop size distributions are used to initiate the procedure, but the retrieved distributions and vertical air motions are seen to be independent of the particular model used.

Data were gathered to test the procedure during the Ground-Based Remote Sensing Intensive Observation Period (GBRS IOP) sponsored by the Department of Energy Atmospheric Radiation Measurement (DOE ARM) program. The measurements represent the first simultaneous Doppler spectra of rain at these frequencies. The experiment took place in April 1995 at the Cloud and Radiation Testbed (CART) site in Lamont, Oklahoma. Radiosonde and surface measurements of temperature and pressure were used in the retrieval algorithm. Rain events from stratiform and transition region (i.e., decaying from the convective region toward the stratiform region of a storm) clouds were observed and are analyzed in this paper. The rain rate for the stratiform rain case was relatively uniform with small amounts of vertical air motion. Variations of the vertical winds for the transition region case, however, were larger and more frequent and were accompanied by short intense downbursts. The algorithm’s results are best for rain rates higher than 1 mm h⁻¹.

Abstract

Multifrequency radar measurements collected at 2.8 (S band), 33.12 (Ka band), and 94.92 GHz (W band) are processed using a neural network to estimate median particle size and peak number concentration in ice-phase clouds composed of dry crystals or aggregates. The model data used to train the neural network assume a gamma particle size distribution function and a size–density relationship having decreasing density with size. Results for the available frequency combinations show sensitivity to particle size for distributions with median volume diameters greater than approximately 0.2 mm.

Measurements are presented from the Maritime Continent Thunderstorm Experiment, which was held near Darwin, Australia, during November and December 1995. The University of Massachusetts—Amherst 33.12/94.92-GHz Cloud Profiling Radar System, the NOAA 2.8-GHz profiler, and other sensors were clustered near the village of Garden Point, Melville Island, where numerous convective storms were observed. Attenuation losses by the NOAA radar signal are small over the pathlengths considered so the cloud-top reflectivity values at 2.8 GHz are used to remove propagation path losses from the higher-frequency measurements. The 2.8-GHz measurements also permit estimation of larger particle diameters than is possible using only 33.12 and 94.92 GHz. The results suggest that the median particle size tends to decrease with height for stratiform cloud cases. However, this trend is not observed for convective cloud cases where measurements indicate that large particles can exist even near the cloud top.

Abstract

Observations were made of unbroken marine stratus off the coast of Oregon using the combined capabilities of in situ probes and a 95-GHz radar mounted on an aircraft. Reflectivity and Doppler velocity measurements were obtained in vertical and horizontal planes that extend from the flight lines. Data from three consecutive days were used to examine echo structure and microphysics characteristics. The clouds appeared horizontally homogeneous and light drizzle reached the surface in all three cases.

Radar reflectivity is dominated by drizzle drops over the lower two-thirds to four-fifths of the clouds and by cloud droplets above that. Cells with above-average drizzle concentrations exist in all cases and exhibit a large range of sizes. The cells have irregular horizontal cross sections but occur with a dominant spacing that is roughly 1.2–1.5 times the depth of the cloud layer. Doppler velocities in the vertical are downward in all but a very small fraction of the cloud volumes. The cross correlation between reflectivity and vertical Doppler velocity changes sign at or below the midpoint of the cloud, indicating that in the upper parts of the clouds above-average reflectivities are associated with smaller downward velocities. This correlation and related observations are interpreted as the combined results of upward transport of drizzle drops and of downward motion of regions diluted by entrainment. The in situ measurements support these conclusions.

Abstract

Scatterometer model functions that directly estimate friction velocity have been developed and are being tested with radar and in situ data acquired during the Surface Wave Dynamics Experiment (SWADE) of 1991. K_u-band and C-band scatterometers were operated simultaneously for extensive intervals for each of 10 days during SWADE. The model function developed previously from the FASINEX experiment converts the K_u-band normalized radar cross-section (NRCS) measurements into friction velocity estimates. These are compared to in situ estimates of surface wind stress and direction across a wide area both on and off the Gulf Stream (for hourly intervals), which were determined from buoy and meteorological measurements during February and March 1991. This involved the combination of a local, specially derived wind field, with an ocean wave model coupled through a sea-state-dependent drag coefficient. The K_u-band estimates u∗ magnitude are in excellent agreement with the in situ values. The C-band scatterometer measurements were coincident with the K_u-band NRCSs, whose u∗ estimates are then used to calibrate the C band. The results show the C-band NRCS dependence at 20°, 30°, 40°, and 50° to be less sensitive to friction velocity than the corresponding cases for K_u band. The goal is to develop the capability of making friction velocity estimates (and surface stress) from radar cross-sectional data acquired by satellite scatterometers.

Abstract

Counterrotating 500-m-scale vortices in the boundary layer are documented in the right-moving member of a splitting supercell thunderstorm in northeastern Oklahoma on 17 May 1995 during the Verification of the Origins of Rotation in Tornadoes Experiment. A description is given of these vortices based upon data collected at close range by a mobile, 3-mm wavelength (95 GHz), pulsed Doppler radar. The vortices are related to a storm-scale, pseudo-dual-Doppler analysis of airborne data collected by the Electra Doppler radar (ELDORA) using the fore–aft scanning technique and to a boresighted video of the cloud features with which the vortices were associated. The behavior of the storm is also documented from an analysis of WSR-88D Doppler radar data.

The counterrotating vortices, which were associated with nearly mirror image hook echoes in reflectivity, were separated by 1 km. The cyclonic member was associated with a cyclonically swirling cloud base. The vortices were located along the edge of a rear-flank downdraft gust front, southeast of a kink in the gust front boundary, a location previously found to be a secondary region for tornado formation. The kink was coincident with a notch in the radar echo reflectivity. A gust front located north of the kink, along the edge of the forward-flank downdraft, was characterized mainly by convergence and density current–like flow, while the rear-flank downdraft boundary was characterized mainly by cyclonic vorticity.

Previously documented vortices along gust fronts have had the same sense of rotation as the others in the group and are thought to have been associated with shearing instabilities. The symmetry of the two vortices suggests that they may have been formed through the tilting of ambient horizontal vorticity. Although the vortices did not develop into tornadoes, it is speculated that similar vortices could be the seeds from which some tornadoes form.

Abstract

A continental stratus cloud layer was studied by advanced ground-based remote sensing instruments and aircraft probes on 30 April 1994 from the Cloud and Radiation Testbed site in north-central Oklahoma. The boundary layer structure clearly resembled that of a cloud-topped mixed layer, and the cloud content is shown to be near adiabatic up to the cloud-top entrainment zone. A cloud retrieval algorithm using the radar reflectivity and cloud droplet concentration (either measured in situ or deduced using dual-channel microwave radiometer data) is applied to construct uniquely high-resolution cross sections of liquid water content and mean droplet radius. The combined evidence indicates that the 350–600 m deep, slightly supercooled (2.0° to −2.0°C) cloud, which failed to produce any detectable ice or drizzle particles, contained an average droplet concentration of 347 cm⁻³, and a maximum liquid water content of 0.8 g m⁻³ and mean droplet radius of 9 μm near cloud top. Lidar data indicate that the K_a-band radar usually detected the cloud-base height to within ∼50 m, such that the radar insensitivity to small cloud droplets had a small impact on the findings. Radar-derived liquid water paths ranged from 71 to 259 g m⁻² as the stratus deck varied, which is in excellent agreement with dual-channel microwave radiometer data, but ∼20% higher than that measured in situ. This difference appears to be due to the undersampling of the few largest cloud droplets by the aircraft probes. This combination of approaches yields a unique image of the content of a continental stratus cloud, as well as illustrating the utility of modern remote sensing systems for probing nonprecipitating water clouds.

Abstract

The local structure and evolution of the convective boundary layer (CBL) are studied through measurements obtained with a volume-imaging radar, the turbulent eddy profiler (TEP). TEP has the unique ability to image the temporal and spatial evolution of both the velocity field and the local refractive index structure-function parameter, C̃ ² _n . Volumetric images consisting of several thousand pixels are typically formed in as little as 1 s. Spatial resolutions are approximately 30 m by 30 m by 30 m.

CBL data obtained during an August 1996 deployment at Rocks Springs, Pennsylvania, are presented. Measurements of the vertical C̃ ² _n profile are shown, exhibiting the well-known bright band near the capping inversion at z _i , as well as intermittent plumes of high C̃ ² _n . Horizontal profiles show coherent 100-m-scale C̃ ² _n and vertical velocity (w) structures that correspond to converging horizontal velocity vectors. To quantify the scales of structures, the vertical and streamwise horizontal correlation distances are calculated within the TEP field of view.

To study the statistics and scales of larger structures, effective volumes larger than the TEP field of view are constructed through Taylor’s hypothesis. Statistics of C̃ ² _n and w time series are compared to an appropriately scaled large eddy simulation (LES). While w time series comparisons agree very well, the LES C̃ ² _n predictions agree only with some of the measured data. Finally, the scales of C̃ ² _n structures in the TEP time series measurements are calculated and compared to the scales in the LES spatial domain. Good agreement is found only near the capping inversion layer, the area of largest structures. This study highlights the unique capabilities of the TEP instrument, and shows what are believed to be the first statistical comparisons of measured C̃ ² _n data with LES derived results.

Abstract

This paper describes the turbulent eddy profiler (TEP), a volume-imaging, UHF radar wind profiler designed for clear-air measurements in the atmospheric boundary layer on scales comparable to grid cell sizes of large eddy simulation models. TEP employs a large array of antennas—each feeding an independent receiver—to simultaneously generate multiple beams within a 28° conical volume illuminated by the transmitter. Range gating provides 30-m spatial resolution in the vertical dimension. Each volume image is updated every 2–10 s, and long datasets can be gathered to study the evolution of turbulent structure over several hours. A summary of the principles of operation and the design of TEP is provided, including examples of clear-air reflectivity and velocity images.

Abstract

The Polarization Diversity Pulse-Pair (PDPP) technique can extend simultaneously the maximum unambiguous range and the maximum unambiguous velocity of a Doppler weather radar. This technique has been applied using a high-resolution 95-GHz radar to study the reflectivity and velocity structure in severe thunderstorms. This paper documents the technique, presents an analysis of the first two moments of the estimated mean velocity, and provides a comparison of the results with experimental data, including PDPP images of high-vorticity regions in supercell storms.