Stephen M. Sekelsky

Particle Size Estimation in Ice-Phase Clouds Using Multifrequency Radar Reflectivity Measurements at 95, 33, and 2.8 GHz

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...

Retrieval of atmospheric attenuation using combined ground-based and airborne 95-GHz cloud radar measurements

Cloud measurements at millimeter-wave frequencies are affected by attenuation due to atmospheric gases, clouds, and precipitation. Estimation of the true equivalent radar reflectivity, Ze, is complicated because extinction mechanisms are not well characterized at these short wavelengths. This paper discusses cloud radar calibration and intercomparison of airborne and ground-based radar measurements and presents a unique algorithm for attenuation retrieval. This algorithm is based on dual 95-GHz radar measurements of the same cloud and precipitation volumes collected from opposing viewing angles. True radar reflectivity is retrieved by combining upward-looking and downward-looking radar profiles. This method reduces the uncertainty in radar reflectivity and attenuation estimates, since it does not require a priori knowledge of hydrometeors’ microphysical properties. Results from this technique are compared with results retrieved from the Hitschfeld and Bordan algorithm, which uses single-radar measurements with path-integrated attenuation as a constraint. Further analysis is planned to employ this dual-radar algorithm in order to refine single-radar attenuation retrieval techniques, which will be used by operational sensors such as the CloudSat radar.

Classification of particles in stratiform clouds using the 33 and 95 GHz polarimetric cloud profiling radar system (CPRS)

This paper describes the identification of regions of ice, cloud droplets, rain, mixed-phase hydrometers, and insects in stratiform clouds using 33 and 95 GHz radar measurements of reflectivity, linear-depolarization ratio (LDR), dual-wavelength ratio, and velocity from a single-antenna radar system. First, the radar system, experiment, and data products are described. Then, regions are classified using a rule-based classifier derived primarily from LDR, velocity, and altitude. Next, a region-dependent attenuation-correction algorithm is developed to remove attenuation biases in the reflectivity estimate, and histograms of the corrected data are presented for each data product and class. The labeled regions and attenuation-corrected data are then used to train a neural net and maximum likelihood classifier. These agree with the rule-based classifier 96% and 94% of the time, respectively. Finally, the paper evaluates the importance of measuring dual-frequency parameters, velocity, and depolarization ratio.

Application of Dual-Frequency Millimeter-Wave Doppler Spectra for the Retrieval of Drop Size Distributions and Vertical Air Motion in Rain

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 ...

Near-Field Reflectivity and Antenna Boresight Gain Corrections for Millimeter-Wave Atmospheric Radars

Abstract Millimeter-wavelength (MMW) cloud radars operating at Ka band (35 GHz) and W band (95 GHz) are popular atmospheric research tools because they are compact, have low prime power requirements, and are highly sensitive to small hydrometeors. In order to maximize sensitivity, ground-based systems use large diameter high-gain antennas. However, these antennas have substantial far-field distances as large as several kilometers. The far-field distance is defined as rf = 2D2/λ, where D is the antenna diameter and λ is the radar wavelength. In the Fresnel region, where 0.62D3/λ < r < rf, the antenna gain and pattern shape vary significantly with distance. Processing radar measurements obtained in the Fresnel region using the conventional radar equations gives erroneous results because these relationships assume far-field antenna characteristics. Correction factors are needed to account for the radar response to targets that lie in the near field. This paper provides correction factors for responses to tar...

Radiative Impacts of Anvil Cloud during the Maritime Continent Thunderstorm Experiment

Abstract In this study the radiative impact of three separate cirrus anvil systems that occurred during the Maritime Continent Thunderstorm Experiment is investigated. Retrievals of microphysical cloud properties and an independent radar measurement are used to develop an appropriate set of radar reflectivity factor (Z)–ice water content (IWC) parameterizations. This set of parameterizations is then applied to the reflectivity field of a scanning 5.2-cm radar. The three-dimensional ice water structure is used as input to a two-stream radiative transfer model using an independent pixel approximation for several different stages in the life cycle of the cloud system. Peak radiative heating/cooling occurs at many different levels from just below the tropopause down to the freezing level. This behavior is attributed to spatial variability of the anvil cloud–top height. There is a distinct difference between the average radiative heating profile in the presence of island-based convection as compared with ocean...

Parallax Errors and Corrections for Dual-Antenna Millimeter-Wave Cloud Radars

Abstract Dual-antenna radar designs avoid using a transmit/receive switch. In order to measure radar reflectivity accurately and to avoid a general decrease in system sensitivity, these systems require precise alignment of their high-gain/narrow-beamwidth antennas, which is difficult. Given precisely aligned antennas, a parallax correction to account for antenna beam overlap, which is range-dependent, must be used with the correct alignment information to produce accurate reflectivities. Calculations show that dual-antenna parallax errors are extremely sensitive to the alignment of the two antennas, especially for the current generation of W-band radars, which tend to use 0.91- and 1.21-m Cassegrain antennas with half-power beamwidths of typically ≤0.25°. For example, the minimum detectable reflectivity of a W-band radar system may be degraded by more than an order of magnitude for alignment errors on the order of the antenna half-power beamwidth. Moreover, parallax errors are essentially independent of r...

Cirrus Clouds Millimeter-Wave Reflectivity Comparison with In- Situ Ice Crystal Airborne Data

In an effort to evaluate scattering models for particle size distributions of ice crystals within cirrus clouds, simultaneous data was collected in March 2000 during the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Cloud Intensive operational period (Cloud IOP) at the Cloud and Radiation Testbed (CART) site in Lamont, Oklahoma. In situ measurements of ice particles were collected using the National Center for Atmospheric Research (NCAR) Video Ice Particle Sampler (VIPS), which flew on the University of North Dakota Citation research aircraft. Ground-based vertical radar profiles were collected using the University of Massachusetts (UMass) 33GHz/95GHz Cloud Profiler Radar System (CPRS). Data from both sensors was used to retrieve and compare the equivalent radar reflectivity at Ka band (33GHz). The equivalent radar reflectivity measured by the ground-based, zenith-looking, CPRS radar at Ka band and compared to the reflectivity computed from the airborne VIPS samples of particle size distribution, N(D), using Mie theory. As anticipated the equivalent reflectivity of the radar and VIPS were similar at the time the UND Citation overflew the radar.

Multi-frequency radar observations of the melting layer

A multi-frequency radar that combines millimeter-wave (MMW) and microwave signals provides additional hydrometeor microphysical information that is not available from a single frequency system. The additional information is contained in the Mie scattering and signal extinction experienced by the MMW signal. These effects can be quantified by comparing MMW data, with measurements simultaneously collected at a second, usually lower, frequency. This technique has been used previously to estimate particle size distributions in rain and in deep ice clouds. Unfortunately, entire precipitating cloud columns cannot yet be characterized using these methods because scattering and extinction from melting particles is not well understood at millimeter-wave frequencies.

Active rain-gauge concept for liquid clouds using W-band and S-band Doppler radars

The use of multi-frequency radars Doppler Spectrum to study different aspects of precipitation has demonstrated its utility as an accurate profiling rain-gauge method. Recent studies used this concept to retrieve the drop-size distribution and vertical air motion in rain using dual-frequency Cloud Profiling Radar System, which operates at 33 GHz (Ka-band) and 95 GHz (W-band). This study was performed for low to moderate rain-rates because the use of the Ka-band frequency limited the accuracy of the measurements for high rain-rates due to the attenuation this signal suffers while it passes through the cloud. In this work we use a non-attenuating frequency, 2.8 GHz, instead of the Ka-band, to obtain measurements over a wider dynamic range of rain conditions, extending the active rain-gauge concept to heavier rain-rates. The W-band signal provides accurate measurement of the vertical air motion in rain. The actual drops shapes must be corrected for heavy rain in which case large non-spherical raindrops exist. Data will be processed as suggested by Firda et al., 1999 considering the drops shape corrections. This researchs goal is to develop IDL codes to align, process, and analyze the collected data to retrieve several cloud characterization parameters, such as drop size distribution and vertical air motion that would be used to study the inner processes of rain. Rain-rate approximations and the vertical air motion retrieval will be presented.