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Sergey Y. Matrosov
,
David E. Kingsmill
,
Brooks E. Martner
, and
F. Martin Ralph

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 (Z e) and differential attenuation effects in measurements of differential reflectivity (Z DR) 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 size distributions collected in a given geographical region. Comparisons of polarimetrically derived rainfall accumulations with data from the high-resolution rain gauges located along the coast indicated deviation between radar and gauge estimates of about 25%. The Z DR measurements corrected for differential attenuation were also used to retrieve median raindrop sizes, D 0. Because of uncertainties in differential reflectivity measurements, these retrievals are typically performed only for D 0 > 0.75 mm. The D 0 estimates from an impact disdrometer located at 25 km from the radar were in good agreement with the radar retrievals. The experience of operating the transportable polarimetric X-band radar in the coastal area that does not have good coverage by the National Weather Service radar network showed the value of such radar in filling the gaps in the network coverage. The NOAA X-band radar was effective in covering an area up to 40–50 km in radius offshore adjacent to a region that is prone to flooding during wintertime landfalling Pacific storms.

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C. W. Fairall
,
Sergey Y. Matrosov
,
Christopher R. Williams
, and
E. J. Walsh

ABSTRACT

The NOAA W-band radar was deployed on a P-3 aircraft during a study of storm fronts off the U.S. West Coast in 2015 in the second CalWater (CalWater-2) field program. This paper presents an analysis of measured equivalent radar reflectivity factor Z em profiles to estimate the path-averaged precipitation rate and profiles of precipitation microphysics. Several approaches are explored using information derived from attenuation of Z em as a result of absorption and scattering by raindrops. The first approach uses the observed decrease of Z em with range below the aircraft to estimate column mean precipitation rates. A hybrid approach that combines Z em in light rain and attenuation in stronger rain performed best. The second approach estimates path-integrated attenuation (PIA) via the difference in measured and calculated normalized radar cross sections (NRCS m and NRCS c , respectively) retrieved from the ocean surface. The retrieved rain rates are compared to estimates from two other systems on the P-3: a Stepped Frequency Microwave Radiometer (SFMR) and a Wide-Swath Radar Altimeter (WSRA). The W-band radar gives reasonable values for rain rates in the range 0–10 mm h−1 with an uncertainty on the order of 1 mm h−1. Mean profiles of Z em, raindrop Doppler velocity, attenuation, and precipitation rate in bins of rain rate are also computed. A method for correcting measured profiles of Z em for attenuation to estimate profiles of nonattenuated profiles of Z e is examined. Good results are obtained by referencing the surface boundary condition to the NRCS values of PIA. Limitations of the methods are discussed.

Open access
Matthew D. Shupe
,
Pavlos Kollias
,
Sergey Y. Matrosov
, and
Timothy L. Schneider

Abstract

In certain circumstances, millimeter-wavelength Doppler radar velocity spectra can be used to estimate the microphysical composition of both phases of mixed-phase clouds. This distinction is possible when the cloud properties are such that they produce a bimodal Doppler velocity spectrum. Under these conditions, the Doppler spectrum moments of the distinct liquid and ice spectral modes may be computed independently and used to quantitatively derive properties of the liquid droplet and ice particle size distributions. Additionally, the cloud liquid spectral mode, which is a tracer for clear-air motions, can be used to estimate the vertical air motion and to correct estimates of ice particle fall speeds.

A mixed-phase cloud case study from the NASA Cirrus Regional Study of Tropical Anvils and Cloud Layers- Florida Area Cirrus Experiment (CRYSTAL-FACE) is used to illustrate this new retrieval approach. The case of interest occurred on 29 July 2002 when a supercooled liquid cloud layer based at 5 km AGL and precipitating ice crystals advected over a ground measurement site. Ground-based measurements from both 35- and 94-GHz radars revealed clear bimodal Doppler velocity spectra within this cloud layer. Profiles of radar reflectivity were computed independently from the liquid and ice spectral modes of the velocity spectra. Empirical reflectivity- based relationships were then used to derive profiles of both liquid and ice microphysical parameters, such as water content and particle size. Although the ice crystals extended down to a height of 4 km, the radar measurements were able to distinguish the base of the cloud liquid at 5 km, in good agreement with cloud-base measurements from a collocated micropulse lidar. Furthermore, radar-derived cloud liquid water paths were in good agreement with liquid water paths derived from a collocated microwave radiometer.

Results presented here demonstrate the ability of the radar to both identify and quantify the presence of both phases in some mixed-phase clouds. They also demonstrate that, in terms of radar reflectivity, the ice component of mixed-phase clouds typically dominates the radar signal, while in terms of mean Doppler velocity, the liquid component can make a significant contribution. The high temporal resolution, 94-GHz Doppler radar observations were able to reveal a periodic cloud-top updraft that, combined with horizontal wind speeds, suggests a horizontal scale for the in-cloud circulations.

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Sergey Y. Matrosov
,
F. Martin Ralph
,
Paul J. Neiman
, and
Allen B. White

Abstract

An evaluation of Weather Surveillance Radar-1988 Doppler (WSR-88D) KMUX and KDAX radar quantitative precipitation estimation (QPE) over a site in California’s northern Sonoma County is performed and rain type climatology is presented. This site is next to the flood-prone Russian River basin and, because of the mountainous terrain and remoteness from operational radars, is generally believed to lack adequate coverage. QPE comparisons were conducted for multiyear observations with concurrent classification of rainfall structure using measurements from a gauge and an S-band profiler deployed at the location of interest. The radars were able to detect most of the brightband (BB) rain, which contributed over half of the total precipitation. For this rain type hourly radar-based QPE obtained with a default vertical profile of reflectivity correction provided results with errors of about 50%–60%. The operational radars did not detect precipitation during about 30% of the total rainy hours with mostly shallow nonbrightband (NBB) rain, which, depending on the radar, provided ~(12%–15%) of the total precipitation. The accuracy of radar-based QPE for the detected fraction of NBB rain was rather poor with large negative biases and characteristic errors of around 80%. On some occasions, radars falsely detected precipitation when observing high clouds, which did not precipitate or coexisted with shallow rain (less than 10% of total accumulation). For heavier rain with a significant fraction of BB hourly periods, radar QPE for event totals showed relatively good agreement with gauge data. Cancelation of errors of opposite signs contributed, in part, to such agreement. On average, KDAX-based QPE was biased low compared to KMUX.

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Jonathan J. Gourley
,
David P. Jorgensen
,
Sergey Y. Matrosov
, and
Zachary L. Flamig

Abstract

Advanced remote sensing and in situ observing systems employed during the Hydrometeorological Testbed experiment on the American River basin near Sacramento, California, provided a unique opportunity to evaluate correction procedures applied to gap-filling, experimental radar precipitation products in complex terrain. The evaluation highlighted improvements in hourly radar rainfall estimation due to optimizing the parameters in the reflectivity-to-rainfall (ZR) relation, correcting for the range dependence in estimating R due to the vertical variability in Z in snow and melting-layer regions, and improving low-altitude radar coverage by merging rainfall estimates from two research radars operating at different frequencies and polarization states. This evaluation revealed that although the rainfall product from research radars provided the smallest bias relative to gauge estimates, in terms of the root-mean-square error (with the bias removed) and Pearson correlation coefficient it did not outperform the product from a nearby operational radar that used optimized ZR relations and was corrected for range dependence. This result was attributed to better low-altitude radar coverage with the operational radar over the upper part of the basin. In these regions, the data from the X-band research radar were not available and the C-band research radar was forced to use higher-elevation angles as a result of nearby terrain and tree blockages, which yielded greater uncertainty in surface rainfall estimates. This study highlights the challenges in siting experimental radars in complex terrain. Last, the corrections developed for research radar products were adapted and applied to an operational radar, thus providing a simple transfer of research findings to operational rainfall products yielding significantly improved skill.

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Sergey Y. Matrosov
,
Robert Cifelli
,
Paul J. Neiman
, and
Allen B. White

Abstract

S-band profiling (S-PROF) radar measurements from different southeastern U.S. Hydrometeorology Testbed sites indicated a frequent occurrence of rain that did not exhibit radar bright band (BB) and was observed outside the periods of deep-convective precipitation. This common nonbrightband (NBB) rain contributes ~15%–20% of total accumulation and is not considered as a separate rain type by current precipitation-segregation operational radar-based schemes, which separate rain into stratiform, convective, and, sometimes, tropical types. Collocated with S-PROF, disdrometer measurements showed that drop size distributions (DSDs) of NBB rain have much larger relative fractions of smaller drops when compared with those of BB and convective rains. Data from a year of combined DSD and rain-type observations were used to derive S-band-radar estimators of rain rate R, including those based on traditional reflectivity Z e and ones that also use differential reflectivity Z DR and specific differential phase K DP. Differences among same-type estimators for mostly stratiform BB and deep-convective rain were relatively minor, but estimators derived for the common NBB rain type were distinct. Underestimations in NBB rain-rate retrievals derived using other rain-type estimators (e.g., those for BB or convective rain or default operational radar estimators) for the same values of radar variables can be on average about 40%, although the differential phase-based estimators are somewhat less susceptible to DSD details. No significant differences among the estimators for the same rain type derived using DSDs from different observational sites were present despite significant separation and differing terrain. Identifying areas of common NBB rain could be possible from Z e and Z DR measurements.

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Sergey Y. Matrosov
,
Roger F. Reinking
,
Robert A. Kropfli
, and
Bruce W. Bartram

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 Environmental Technology Laboratory. Experimental data taken during the Winter Icing and Storms Instrument Test experiment in 1993 generally confirmed the calculations and demonstrated the applicability of the approach.

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Roger F. Reinking
,
Sergey Y. Matrosov
,
Robert A. Kropfli
, and
Bruce W. Bartram

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 competing factors such as the functional dynamic range of DR, sensitivity to low-reflectivity clouds, and insensitivity to oscillations in the settling orientations of ice crystals. A 45° slant, quasi-linear polarization state, one in which only slight ellipticity is introduced, was found to offer a very good compromise, providing considerable advantages over standard horizontal and substantially elliptical polarizations. This was determined by theoretical scattering calculations that were verified experimentally in field measurements conducted during the Mount Washington Icing Sensors Project (MWISP). A selectable-dual-polarization Ka-band (8.66-mm wavelength) radar was used. A wide variety of hydrometeor types was sampled. Clear differentiation among planar crystals, columnar crystals, and drizzle droplets was achieved. Also, differentiation among crystals of fundamentally different shapes (aspect ratios) within each of the planar and columnar families was found possible. These distinctions matched calculations of DR, usually to within 1 or 2 dB. The results from MWISP and from previous experiments with other polarizations have demonstrated that the agreement between theory and measurements by this method is repeatable. Additionally, although less rigorously predicted by theory, the field measurements demonstrated substantial differentiation among the more irregular and more spherical ice particles, including aggregates, elongated aggregates, heavily rimed dendrites, and graupel. Measurable separation between these various irregular ice particle types and drizzle droplets was also verified.

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Alexander Myagkov
,
Patric Seifert
,
Ulla Wandinger
,
Matthias Bauer-Pfundstein
, and
Sergey Y. Matrosov

Abstract

This paper presents an experimental analysis of the antenna system effects on polarimetric measurements conducted with cloud radars operating in the linear depolarization ratio (LDR) mode. Amplitude and phase of the copolar and cross-polar antenna patterns are presented and utilized. The patterns of two antennas of different quality were measured at the Hungriger Wolf airport near Hohenlockstedt, Germany, during the period from 28 January to 1 February 2014. For the measurements a test transmitter mounted on a tower and the scanning 35-GHz (Ka band) cloud radar MIRA-35, manufactured by METEK GmbH and operated in the receiving mode, were used. The integrated cross-polarization ratios (ICPR) are calculated for both antennas and compared with those measured in light rain. Correction algorithms for observed LDR and the co-cross-channel correlation coefficient ρ are presented. These algorithms are aimed at removing/mitigating polarization cross-coupling effects that depend on the quality of radar hardware. Thus, corrected LDR and ρ are primarily influenced by scatterer properties. The corrections are based on the decomposition of the coherency matrix of the received signals into fully polarized and nonpolarized components. The correction brings LDR values and the co-cross-channel correlation coefficients from two radars with different antenna systems to a close agreement, thus effectively removing hardware-dependent biases. Uncertainties of the correction are estimated as 3 dB for LDR in the range from −30 to −10 dB. In clouds, the correction of the co-cross-channel correlation coefficient ρ results in near-zero values for both vertically pointed radars.

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Edward J. Walsh
,
Ivan PopStefanija
,
Sergey Y. Matrosov
,
Jian Zhang
,
Eric Uhlhorn
, and
Brad Klotz

Abstract

The NOAA Wide-Swath Radar Altimeter (WSRA) uses 80 narrow beams spread over ±30° in the cross-track direction to generate raster lines of sea surface topography at a 10-Hz rate from which sea surface directional wave spectra are produced. A ±14° subset of the backscattered power data associated with the topography measurements is used to produce independent measurements of rain rate and sea surface mean square slope at 10-s intervals. Theoretical calculations of rain attenuation at the WSRA 16.15-GHz operating frequency using measured drop size distributions for both mostly convective and mostly stratiform rainfall demonstrate that the WSRA absorption technique for rain determination is relatively insensitive to both ambient temperature and the characteristics of the drop size distribution, in contrast to reflectivity techniques. The variation of the sea surface radar reflectivity in the vicinity of a hurricane is reviewed. Fluctuations in the sea surface scattering characteristics caused by changes in wind speed or the rain impinging on the surface cannot contaminate the rain measurement because they are calibrated out using the WSRA measurement of mean square slope. WSRA rain measurements from a NOAA WP-3D hurricane research aircraft off the North Carolina coast in Hurricane Irene on 26 August 2011 are compared with those from the stepped frequency microwave radiometer (SFMR) on the aircraft and the Next Generation Weather Radar (NEXRAD) National Mosaic and Multi-Sensor Quantitative Precipitation Estimation (QPE) system.

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