Brooks E. Martner

Objective Determination of Cloud Heights and Radar Reflectivities Using a Combination of Active Remote Sensors at the ARM CART Sites

Abstract The U.S. Department of Energy’s Atmospheric Radiation Measurement (ARM) Program is deploying sensitive, millimeter-wave cloud radars at its Cloud and Radiation Test Bed (CART) sites in Oklahoma, Alaska, and the tropical western Pacific Ocean. The radars complement optical devices, including a Belfort or Vaisala laser ceilometer and a micropulse lidar, in providing a comprehensive source of information on the vertical distribution of hydrometeors overhead at the sites. An algorithm is described that combines data from these active remote sensors to produce an objective determination of hydrometeor height distributions and estimates of their radar reflectivities, vertical velocities, and Doppler spectral widths, which are optimized for accuracy. These data provide fundamental information for retrieving cloud microphysical properties and assessing the radiative effects of clouds on climate. The algorithm is applied to nine months of data from the CART site in Oklahoma for initial evaluation. Much of...

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...

X-Band polarimetric radar measurements of rainfall

Abstract A combined polarimetric estimator for rainfall rate (R) retrievals from polarimetric radar measurements at X band is proposed. This estimator uses the horizontal polarization radar reflectivity Ze, differential reflectivity ZDR, and specific differential phase shift KDP, and it intrinsically accounts for changes in how drop oblateness increases with size. Because this estimator uses power measurements (i.e., Ze and ZDR), a procedure for correcting these measurements for effects of partial attenuation and differential attenuation using the differential phase measurement is suggested. An altitude correction for estimates of rainfall rates is also suggested. The proposed combined polarimetric estimator that uses KDP, ZDR, and Ze, an estimator that uses KDP alone for equilibrium drop shapes, and different Ze–R relations were applied to the 15 rain events observed with the NOAA X-band transportable polarimetric radar during the eight-week field campaign at the NASA Wallops Island facility in Virginia....

The Atmospheric Radiation Measurement Program Cloud Radars: Operational Modes

Abstract During the past decade, the U.S. Department of Energy (DOE), through the Atmospheric Radiation Measurement (ARM) Program, has supported the development of several millimeter-wavelength radars for the study of clouds. This effort has culminated in the development and construction of a 35-GHz radar system by the Environmental Technology Laboratory (ETL) of the National Oceanic and Atmospheric Administration (NOAA). Radar systems based on the NOAA ETL design are now operating at the DOE ARM Southern Great Plains central facility in central Oklahoma and the DOE ARM North Slope of Alaska site near Barrow, Alaska. Operational systems are expected to come online within the next year at the DOE ARM tropical western Pacific sites located at Manus, Papua New Guinea, and Nauru. In order for these radars to detect the full range of atmospheric hydrometeors, specific modes of operation must be implemented on them that are tuned to accurately detect the reflectivities of specific types of hydrometeors. The set...

The Utility of X-Band Polarimetric Radar for Quantitative Estimates of Rainfall Parameters

Abstract The utility of X-band polarimetric radar for quantitative retrievals of rainfall parameters is analyzed using observations collected along the U.S. west coast near the mouth of the Russian River during the Hydrometeorological Testbed project conducted by NOAA’s Environmental Technology and National Severe Storms Laboratories in December 2003 through March 2004. It is demonstrated that the rain attenuation effects in measurements of reflectivity (Ze) and differential attenuation effects in measurements of differential reflectivity (ZDR) can be efficiently corrected in near–real time using differential phase shift data. A scheme for correcting gaseous attenuation effects that are important at longer ranges is introduced. The use of polarimetric rainfall estimators that utilize specific differential phase and differential reflectivity data often provides results that are superior to estimators that use fixed reflectivity-based relations, even if these relations were derived from the ensemble of drop...

Rain versus Snow in the Sierra Nevada, California: Comparing Doppler Profiling Radar and Surface Observations of Melting Level

Abstract The maritime mountain ranges of western North America span a wide range of elevations and are extremely sensitive to flooding from warm winter storms, primarily because rain falls at higher elevations and over a much greater fraction of a basin’s contributing area than during a typical storm. Accurate predictions of this rain–snow line are crucial to hydrologic forecasting. This study examines how remotely sensed atmospheric snow levels measured upstream of a mountain range (specifically, the bright band measured above radar wind profilers) can be used to accurately portray the altitude of the surface transition from snow to rain along the mountain’s windward slopes, focusing on measurements in the Sierra Nevada, California, from 2001 to 2005. Snow accumulation varies with respect to surface temperature, diurnal cycles in solar radiation, and fluctuations in the free-tropospheric melting level. At 1.5°C, 50% of precipitation events fall as rain and 50% as snow, and on average, 50% of measured pre...

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.

Extending the Dynamic Range of an S-Band Radar for Cloud and Precipitation Studies

Abstract A new S-band vertical profiler with a coupler option for extending the dynamic range of the radar’s receiver is discussed. The added dynamic range allows the profiler to record radar reflectivity measurements in moderate to heavy precipitation that otherwise would not have been possible with this system because of receiver saturation. The radar hardware, signal processor, and operating software are based on existing S-band and UHF profiler technology. Results from a side-by-side comparison with a calibrated Ka-band radar are used to determine the calibration and sensitivity of the S-band profiler. In a typical cloud profiling mode of operation, the sensitivity is −14 dBZ at 10 km. Examples taken from a recent field campaign are shown to illustrate the profiler’s ability to measure vertical velocity and radar reflectivity profiles in clouds and precipitation, with particular emphasis on the benefit provided by the coupler technology.

Raindrop Size Distributions and Rain Characteristics in California Coastal Rainfall for Periods with and without a Radar Bright Band

Abstract Recent studies using vertically pointing S-band profiling radars showed that coastal winter storms in California and Oregon frequently do not display a melting-layer radar bright band and inferred that these nonbrightband (NBB) periods are characterized by raindrop size spectra that differ markedly from those of brightband (BB) periods. Two coastal sites in northern California were revisited in the winter of 2003/04 in this study, which extends the earlier work by augmenting the profiling radar observations with collocated raindrop disdrometers to measure drop size distributions (DSD) at the surface. The disdrometer observations are analyzed for more than 320 h of nonconvective rainfall. The new measurements confirm the earlier inferences that NBB rainfall periods are characterized by greater concentrations of small drops and smaller concentrations of large drops than BB periods. Compared with their BB counterparts, NBB periods had mean values that were 40% smaller for mean-volume diameter, 32% s...

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...