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.

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.

Abstract

A remote sensing method is proposed to derive vertical profiles of the visible extinction coefficients in ice clouds from measurements of the radar reflectivity and Doppler velocity taken by a vertically pointing 35-GHz cloud radar. The extinction coefficient and its vertical integral, optical thickness τ, are among the fundamental cloud optical parameters that, to a large extent, determine the radiative impact of clouds. The results obtained with this method could be used as input for different climate and radiation models and for comparisons with parameterizations that relate cloud microphysical parameters and optical properties. An important advantage of the proposed method is its potential applicability to multicloud situations and mixed-phase conditions. In the latter case, it might be able to provide the information on the ice component of mixed-phase clouds if the radar moments are dominated by this component. The uncertainties of radar-based retrievals of cloud visible optical thickness are estimated by comparing retrieval results with optical thicknesses obtained independently from radiometric measurements during the yearlong Surface Heat Budget of the Arctic Ocean (SHEBA) field experiment. The radiometric measurements provide a robust way to estimate τ but are applicable only to optically thin ice clouds without intervening liquid layers. The comparisons of cloud optical thicknesses retrieved from radar and from radiometer measurements indicate an uncertainty of about 77% and a bias of about −14% in the radar estimates of τ relative to radiometric retrievals. One possible explanation of the negative bias is an inherently low sensitivity of radar measurements to smaller cloud particles that still contribute noticeably to the cloud extinction. This estimate of the uncertainty is in line with simple theoretical considerations, and the associated retrieval accuracy should be considered good for a nonoptical instrument, such as radar. This paper also presents relations between radar-derived characteristic cloud particle sizes and effective sizes used in models. An average relation among τ, cloud ice water path, and the layer mean value of cloud particle characteristic size is also given. This relation is found to be in good agreement with in situ measurements. Despite a high uncertainty of radar estimates of extinction, this method is useful for many clouds where optical measurements are not available because of cloud multilayering or opaqueness.

Abstract

Model calculations and measurements of the specific propagation and backscatter differential phase shifts (K _DP and δ _o , respectively) in rain are discussed for X- (λ ∼ 3 cm) and K_a-band (λ ∼ 0.8 cm) radar wavelengths. The details of the drop size distribution have only a small effect on the relationships between K _DP 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 K _DP 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 K _DP 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 K_a band is significantly larger than at X band. This makes propagation and backscatter phase shift contributions comparable for most practical cases and poses difficulties in estimating rainfall rate from K_a-band measurements of the differential phase.

Experimental studies of rain using X-band differential phase measurements were conducted near Boulder, Colorado, in a stratiform, intermittent rain with a rate averaging about 4–5 mm h⁻¹. The differential phase shift approach proved to be effective for such modest rains, and finer spatial resolutions were possible in comparison to those achieved with similar measurements at longer wavelengths. A K _DP–R relation derived for the mean drop aspect ratio (R = 20.5 K ^0.80 _DP ) provided a satisfactory agreement between rain accumulations derived from radar measurements of the differential phase and data from several nearby high-resolution surface rain gauges. For two rainfall events, radar estimates based on the assumed mean drop aspect ratio were, on average, quite close to the gauge measurements with about 38% relative standard deviation of radar data from the gauge data.

Abstract

Polarimetric radar can be used to identify various types of hydrometeors. Ice crystals of the varied growth habits depolarize and backscatter millimeter-wavelength radiation according to crystal aspect ratio, bulk density, and orientation, and the polarization state of the incident radiation. In this paper model calculations of the depolarization caused by various crystal types are extended from previous work, and K_a-band (8.66 mm) radar measurements of linear and elliptical depolarization ratios (LDR and EDR) from various ice hydrometeors are presented. The measurements for regular crystals are related to the models. Drizzle drops, which are quasi-spherical, serve as a reference. Signature discrimination in cloud systems with more than one type of hydrometeor is addressed.

The model calculations illustrate the interplay of the parameters that control depolarization. They predict that in the depolarization signatures, crystals of the various basic planar and columnar habits should generally be most separable, one habit group from another and, to a degree, within each group when they occur in common, mature size distributions. It is verified in this and related papers that measurements of depolarization with a K_a-band dual-polarization radar provide good estimates of hydrometeor identity to separately distinguish drizzle, pristine crystals of various growth habits, graupel, and aggregates in winter storm clouds that have reasonable horizontal homogeneity over short distances (∼10–20 km). Characterization of the mix of two or three hydrometeor types is also possible, once the individual types are identified in some part of the cloud. Quantitative agreement between the measurements and the models, supported by snow crystal samples, was much better for EDR than for LDR; that is, EDR enabled more specific hydrometeor identification. However, LDR provided indications of randomness of crystal orientation and a wider decibel gap differentiating graupel from drizzle.

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.

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 (Z–R) 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 Z–R 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.

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.

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% smaller for rain intensity, 87% larger for total drop concentration, and 81% larger (steeper) for slope of the exponential DSDs. The differences are statistically significant. Liquid water contents differ very little, however, for the two rain types. Disdrometer-based relations between radar reflectivity (Z) and rainfall intensity (R) at the site in the Coast Range Mountains were Z = 168R ^1.58 for BB periods and Z = 44R ^1.91 for NBB. The much lower coefficient, which is characteristic of NBB rainfall, is poorly represented by the Z–R equations most commonly applied to data from the operational network of Weather Surveillance Radar-1988 Doppler (WSR-88D) units, which underestimate rain accumulations by a factor of 2 or more when applied to nonconvective NBB situations. Based on the observed DSDs, it is also concluded that polarimetric scanning radars may have some limited ability to distinguish between regions of BB and NBB rainfall using differential reflectivity. However, differential-phase estimations of rain intensity are not useful for NBB rain, because the drops are too small and nearly spherical. On average, the profiler-measured echo tops were 3.2 km lower in NBB periods than during BB periods, and they extended only about 1 km above the 0°C altitude. The findings are consistent with the concept that precipitation processes during BB periods are dominated by ice processes in deep cloud layers associated with synoptic-scale forcing, whereas the more restrained growth of hydrometeors in NBB periods is primarily the result of orographically forced condensation and coalescence processes in much shallower clouds.

Abstract

Ice cloud microphysical parameters derived from a remote sensing method that uses ground-based measurements from the Environmental Technology Laboratory’s K_a-band radar and an IR radiometer are compared to those obtained from aircraft sampling for the cirrus priority event from the FIRE-II experiment. Aircraft cloud samples were taken not only by traditional two-dimensional probes but also by using a new video sampler to account for small particles. The cloud parameter comparisons were made for time intervals when aircraft were passing approximately above ground-based instruments that were pointed vertically. Comparing characteristic particle sizes expressed in terms of median mass diameters of equal-volume spheres yielded a relative standard deviation of about 30%. The corresponding standard deviation for the cloud ice water content comparisons was about 55%. Such an agreement is considered good given uncertainties of both direct and remote approaches and several orders of magnitude in natural variability of ice cloud parameters. Values of reflectivity measured by the radar and calculated from aircraft samples also showed a reasonable agreement; however, calculated reflectivities averaged approximately 2 dB smaller than those measured. The possible reasons for this small bias are discussed. Ground-based and aircraft-derived particle characteristic sizes are compared to those available from published satellite measurements of this parameter for the cirrus priority case from FIRE-II. Finally, simultaneous and collocated, ground-based measurements of visible (0.523 nm) and longwave IR (10–11.4 μm) ice cloud extinction optical thickness obtained during the 1995 Arizona Program are also compared. These comparisons, performed for different cloud conditions, revealed a relative standard deviation of less than 20%;however, no systematic excess of visible extinction over IR extinction was observed in the considered experimental events.