Robert A. Kropfli

An Unattended Cloud-Profiling Radar for Use in Climate Research

A new millimeter-wave cloud radar (MMCR) has been designed to provide detailed, long-term observations of nonprecipitating and weakly precipitating clouds at Cloud and Radiation Testbed (CART) sites of the Department of Energys Atmospheric Radiation Measurement (ARM) program. Scientific requirements included excellent sensitivity and vertical resolution to detect weak and thin multiple layers of ice and liquid water clouds over the sites and long-term, unattended operations in remote locales. In response to these requirements, the innovative radar design features a vertically pointing, single-polarization, Doppler system operating at 35 GHz (Ka band). It uses a low-peak-power transmitter for long-term reliability and high-gain antenna and pulse-compressed waveforms to maximize sensitivity and resolution. The radar uses the same kind of signal processor as that used in commercial wind profilers. The first MMCR began operations at the CART in northern Oklahoma in late 1996 and has operated continuously the...

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.

Estimation of ice cloud parameters from ground‐based infrared radiometer and radar measurements

A technique is presented to estimate ice cloud particle characteristic sizes and concentrations as well as the integrated ice water path from simultaneous ground-based radar and infrared radiometer measurements. The approach is based on the theoretical consideration of infrared thermal radiative transfer within a cloud and can be applied to clouds that are semitransparent in the infrared “window” and horizontally extensive. The suggested technique is applied to radar and infrared radiometer data collected during the Cloud Lidar and Radar Exploratory Test (CLARET-I) experiment. Retrieved values of ice cloud microphysical parameters are in general agreement with results obtained by other methods.

Prospects for Measuring Rainfall Using Propagation Differential Phase in X- and Ka-Radar Bands

Abstract Model calculations and measurements of the specific propagation and backscatter differential phase shifts (KDP and δo, respectively) in rain are discussed for X- (λ ∼ 3 cm) and Ka-band (λ ∼ 0.8 cm) radar wavelengths. The details of the drop size distribution have only a small effect on the relationships between KDP and rainfall rate R. These relationships, however, are subject to significant variations due to the assumed model of the drop aspect ratio as a function of their size. The backscatter differential phase shift at X band for rain rates of less than about 15 mm h−1 is generally small and should not pose a serious problem when estimating KDP from the total phase difference at range intervals of several kilometers. The main advantage of using X-band wavelengths compared to S-band (λ ∼ 10–11 cm) wavelengths is an increase in KDP by a factor of about 3 for the same rainfall rate. The relative contribution of the backscatter differential phase to the total phase difference at Ka band is signif...

Estimation of Ice Hydrometeor Types and Shapes from Radar Polarization Measurements

Abstract An approach to distinguish between various types of ice hydrometeors and to estimate their shapes using radar polarization measurements is discussed. It is shown that elevation angle dependencies of radar depolarization ratios can be used to distinguish between planar crystals, columnar crystals, and aggregates in reasonably homogeneous stratiform clouds. Absolute values of these ratios depend on the reflectivity-weighted mean particle aspect ratio in the polarization plane. Circular depolarization ratios depend on this ratio, and linear depolarization ratios depend on this ratio and particle orientation in the polarization plane. The use of nearly circular elliptical polarization provides a means of measuring depolarization for low reflectivity scatterers when the circular polarization fails due to low signal level in one of the receiving channels. Modeling of radar backscattering was applied to the elliptical depolarization ratios as measured by the Ka-band radar developed at the NOAA Environme...

On the Use of Radar Depolarization Ratios for Estimating Shapes of Ice Hydrometeors in Winter Clouds

Abstract An approach is suggested to relate measurements of radar depolarization ratios and aspect ratios of predominant hydrometeors in nonprecipitating and weakly precipitating layers of winter clouds. The trends of elevation angle dependencies of depolarization ratios are first used to distinguish between columnar-type and plate-type particles. For the established particle type, values of depolarization ratios observed at certain elevation angles, for which the influence of particle orientation is minimal, are then used to estimate aspect ratios when information on particle effective bulk density is assumed or inferred from other measurements. The use of different polarizations, including circular, slant-45° linear, and two elliptical polarizations, is discussed. These two elliptical polarizations are quasi-circular and quasi-linear slant-45° linear, and both are currently achievable with the National Oceanic and Atmospheric Administration Environmental Technology Laboratory’s Ka-band radar. In compari...

The Potential of 8-mm Radars for Remotely Sensing Cloud Drop Size Distributions

Abstract This paper describes the use of a vertically pointing 8.6-mm-wavelength Doppler radar for measuring drop size spectra in clouds. The data used were collected in the Atlantic Stratocumulus Transition Experiment in 1992. This paper uses the full Doppler velocity spectrum from the time series of Doppler radial velocities to extract information farther into the small-drop regime than previously attempted. The amount of liquid residing in the cloud regime is compared with that found in the precipitation regime where drop fall velocities are resolvable. Total liquid is compared with that measured with a collocated three-channel microwave radiometer. Examples of number density spectra, liquid water spectra, and flux spectra are shown and compared with what is known of these quantities from various in situ measurements by aircraft in similar clouds. Error estimates and uncertainties are discussed. It is concluded that 8-mm Doppler radars have the potential for broader use in cloud and precipitation studi...

Evaluation of a 45° slant quasi-linear radar polarization state for distinguishing drizzle droplets, pristine ice crystals, and less regular ice particles

Abstract A remote sensing capability is needed to detect clouds of supercooled, drizzle-sized droplets, which are a major aircraft icing hazard. Discrimination among clouds of differing ice particle types is also important because both the presence and type of ice influence the survival of liquid in a cloud and the chances for occurrence of these large, most hazardous droplets. This work shows how millimeter-wavelength dual-polarization radar can be used to identify these differing hydrometeors. It also shows that by measuring the depolarization ratio (DR), the estimation of the hydrometeor type can be accomplished deterministically for drizzle droplets; ice particles of regular shapes; and to a considerable extent, the more irregular ice particles, and that discrimination is strongly influenced by the polarization state of the transmitted microwave radiation. Thus, appropriate selection of the polarization state is emphasized. The selection of an optimal polarization state involves trade-offs in competin...

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.