David M. Babb

An Evaluation of a 94-GHz Radar for Remote Sensing of Cloud Properties

Abstract The performance of a 94-GHz radar is evaluated for a variety of cloud conditions. Descriptions of the radar hardware, signal processing, and calibration provide an overview of the radars capabilities. An important component of the signal processing is the application of two cloud-mask schemes to the data to provide objective estimates of cloud boundaries and to detect significant returns that would otherwise be discarded if a simple threshold method for delectability was applied to the return power. Realistic profiles of atmospheric pressure, temperature, and water vapor are used in a radiative transfer model to address clear-sky attenuation. A physically relevant study of beam extinction and backscattering by clouds is attempted by modeling cloud drop size distributions with a gamma distribution over a range of number concentrations, particle mean diameters, and distribution shape factors; cloud liquid water contents and mean drop size diameters reported in the literature are analyzed in this c...

Drizzle Suppression in Ship Tracks

Abstract Although drizzle was a relatively infrequent occurrence during the Monterey Area Ship Track study, diverse measurements from several sources produced data signals consistent with a reduction in drizzle drops in stratus clouds affected by ship effluents. Concurrent increases in liquid water in the cloud droplet size range, due to redistribution from the drizzle mode, were not always observed, possibly because of the relatively small and often negligible amounts of water in the drizzle mode. Significant changes in cloud droplet size distribution, as well as reductions in drizzle flux and concentrations of drops >50-μm radius, were observed in ship tracks when drizzle was more uniformly present in the ambient cloud. Radiometric measurements showed that increased droplet concentrations in ship tracks, which resulted in reduced droplet sizes, can significantly alter the liquid water path. Radar observations indicated that the reduced reflectivities of ship tracks compared with ambient clouds may be du...

The Appearance and Disappearance of Ship Tracks on Large Spatial Scales

The 1-km advanced very high resolution radiometer observations from the morning, NOAA-12,and afternoon, NOAA-11, satellite passes over the coast of California during June 1994 are used to determine the altitudes, visible optical depths, and cloud droplet effective radii for low-level clouds. Comparisons are made between the properties of clouds within 50 km of ship tracks and those farther than 200 km from the tracks in order to deduce the conditions that are conducive to the appearance of ship tracks in satellite images. The results indicate that the low-level clouds must be sufficiently close to the surface for ship tracks to form. Ship tracks rarely appear in low-level clouds having altitudes greater than 1 km. The distributions of visible optical depths and cloud droplet effective radii for ambient clouds in which ship tracks are embedded are the same as those for clouds without ship tracks. Cloud droplet sizes and liquid water paths for low-level clouds do not constrain the appearance of ship tracks in the imagery. The sensitivity of ship tracks to cloud altitude appears to explain why the majority of ship tracks observed from satellites off the coast of California are found south of 35 8N. A small rise in the height of low-level clouds appears to explain why numerous ship tracks appeared on one day in a particular region but disappeared on the next, even though the altitudes of the low-level clouds were generally less than 1 km and the cloud cover was the same for both days. In addition, ship tracks are frequent when lowlevel clouds at altitudes below 1 km are extensive and completely cover large areas. The frequency of imagery pixels overcast by clouds with altitudes below 1 km is greater in the morning than in the afternoon and explains why more ship tracks are observed in the morning than in the afternoon. If the occurrence of ship tracks in satellite imagery data depends on the coupling of the clouds to the underlying boundary layer, then cloud-top altitude and the area of complete cloud cover by low-level clouds may be useful indices for this coupling.

Retrieval of Cloud Microphysical Parameters from 94-GHz Radar Doppler Power Spectra

Abstract A technique is presented that uses Doppler spectra collected from a vertically pointing 94-GHz radar to reconstruct cloud and precipitation drop size distributions. A conceptual model describing the broadening of Doppler spectra by turbulence was adapted from earlier works presented in the literature. This model was then used as a basis for an algorithm that solves for parameters describing the turbulence and drop distribution. Numerically simulated Doppler spectra, calculated from known drop distributions, were first used to test the accuracy of the retrieval algorithm. The tests indicate that the retrieval algorithm can accurately retrieve the turbulence parameter and characteristic diameter but is less able to correctly determine the shape parameter. The technique was then applied to actual Doppler spectra collected from a liquid-phase stratus cloud. Vertical profiles of cloud properties such as liquid water content (LWC), effective radius, total number concentration, and mean vertical wind we...

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.

The Removal of Turbulent Broadening in Radar Doppler Spectra Using Linear Inversion with Double-Sided Constraints

Abstract Remote sensing instruments have the ability to collect data over extensive temporal periods and spatial regions. A common thread between all these sensors is the need to relate the measured quantity to a meaningful observation of a system property. If the relationship between each measurement and the set of atmospheric quantities that influence that measurement is known, the problem can be reduced to a set of linear equations. Solving for the unknown atmospheric quantities then becomes a linear algebra problem, where the solution vector is equal to the inverse of the kernel matrix multiplied by the set of independent measurements. However, in most remote sensing applications, inversion of the kernel matrix is unstable, resulting in the amplification of measurement and computational uncertainties. Techniques to circumvent this error amplification have focused on methods of constraining the solution. In this paper, the authors adapt an existing technique to do such an inversion. Noise reduction is ...

Processing Millimeter Wave Profiler Radar Spectra

Abstract Spectral processing algorithms employed in millimeter-wave profiling radars typically obtain good signal-to-noise ratios from weakly scattering clouds by incoherently averaging many spectra. Radar operating characteristics dictate sampling times on the order of a few seconds. Presented here are analyses showing that changes in the vertical wind during the sampling period can be a major contributor to the measured spectrum width. Such broadened spectra violate the assumptions made in spectral inversion techniques, and may lead to incorrect interpretations of the turbulent and microphysical characteristics of the radar volume. Moreover, it is shown that there are several factors involved in determining the measured spectral shape: the averaging time window and horizontal advection velocity of the cloud, as well as horizontal inhomogeneities in cloud vertical velocity and microphysical fields. Current processing algorithms do not allow for distinction between these effects, leading to potential for ...

Ground-Based Remote Sensing of Cloud Properties Using Millimeter-Wave Radar

The elements of radar signal processing are described in the context of climate applications of radar. To this end descriptions of radar hardware are minimized and emphasis is placed on describing the transformation of the radar signal as it propagates from the transmitter up into the atmosphere and then back to the radar receiver after a scattering event. For the final radar observables to be of use to the climate community, they must be turned into quantitative measures of the location and properties of clouds. Descriptions of recent algorithm development efforts are described to illustrate approaches and techniques that are being developed to accomplish the goal of producing quantitative descriptions of cloud properties from the radar observables.

Measurements of atmospheric extinction using combined airborne and ground-based 95 GHz radar observations

During the boreal summer of 1998 three 95-GHz (W-band) radars were deployed by the University of Massachusetts (UMass), the Pennsylvania State University (Penn State) and NASAs Jet Propulsion Laboratory (JPL). Ground-based systems from UMass and Penn State were located at the the U.S. Department of Energys Atmospheric Radiation Measurement (ARM) program field site in northern Oklahoma. (The UMass system was later deployed in New Iberia, Louisiana.) The third 95-GHz radar, jointly developed by UMass and NASA/JPL, was mounted in a nadir-pointing orientation on the NASA DC-8 research aircraft. The experiment serves several purposes. First, it provides an opportunity to intercompare the calibrations of active research radars that are adding to the scientific archives of W-band radar observations. Second, simultaneous airborne and ground-based measurements are combined to measure precipitation attenuation.