Radar Measurement of Rainfall with and without Polarimetry

2008 ◽  
Vol 47 (7) ◽  
pp. 1929-1939 ◽  
Author(s):  
Carlton W. Ulbrich ◽  
David Atlas
Keyword(s):  
A Priori ◽  
Structural Features ◽  
Size Distributions ◽  
Tropical Ocean ◽  
Convective Storms ◽  
Toga Coare ◽  
Median Volume ◽  
Raindrop Size ◽  
Differential Reflectivity ◽  
R Relation

Abstract Raindrop size distributions (DSDs) for tropical convective storms are used to examine the relationships between the parameters of a gamma DSD, with special emphasis on their variation with the stage of the storm. Such a distinction has rarely been made before. Several storms from a variety of tropical locations are divided into storm stages according to the temporal dependence of their reflectivity factor Z, rainfall rate R, and median volume diameter D0. In most cases it is found that the DSD parameter D0 is approximately constant in time during the convective, or C, stage, which leads to a Z–R relation of the form Z = AR, that is, a linear relationship between Z and R. This finding implies the existence of equilibrium DSDs during the C stage. The convective stage is sometimes marked by pulsations in draft strength so that D0, R, and Z and associated values of the shape parameter μ decrease in a quasi-transition stage before increasing once more. Theoretical relations between the differential reflectivity ZDR and the ratio Z/R as functions of the DSD parameter μ are derived by assuming a gamma DSD and an accurate raindrop fall speed law. It is found that data derived from disdrometer observations lie along a μ = 5 isopleth for tropical continental C stages (Puerto Rico and Brazil) and along a μ = 12 isopleth for tropical maritime C stages [Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE)]. Small values of μ that occur in the weak updraft intervals do not impact the rainfall measurements because they correspond to relatively small R. The latter features imply that the measurement of rainfall for the convective stages can be performed with standard polarimetry involving only two measurables rather than three, provided knowledge of μ is available a priori. A new rain parameter diagram is presented in which isopleths of the generalized number concentration and D0 are superimposed on the Z–R plot. It is proposed that it is possible to estimate D0 from climatological and observable storm structural features, which, with Z, provide estimates of R. Such an approach is necessary for use with conventional radars until polarimetric radars are more widely available.

scholarly journals Retrieval of Arbitrarily Shaped Raindrop Size Distributions from Wind Profiler Measurements

2005 ◽  
Vol 22 (4) ◽  
pp. 433-442 ◽  
Author(s):  
Takahisa Kobayashi ◽  
Ahoro Adachi
Keyword(s):  
Drop Size ◽  
Large Data ◽  
Doppler Spectrum ◽  
Wind Profiler ◽  
Size Distributions ◽  
Vertical Profiles ◽  
Large Variability ◽  
Median Volume ◽  
Raindrop Size ◽  
Drop Size Distributions

Abstract An efficient iterative retrieval method for arbitrarily shaped raindrop size distributions (ITRAN) is developed for Doppler spectra measured with a wind profiler. A measured Doppler spectrum is a convolution of the precipitation spectrum and the turbulent spectrum. Deconvolution of the Doppler spectra is achieved through repeated convolutions. The developed method assumes no prior shape of drop size distributions and automatically obtains raindrop size distributions; additionally, it can be applied to large data volumes. Furthermore, it is insensitive to initial values. The method was applied to both simulated and observed spectra. Derived drop size distributions agree with simulated values. Narrower turbulent spectral widths yield better results. Integral values of median volume diameter (D0), liquid water content (LWC), and radar reflectivity factor are estimated with errors of less than 10%. Accurate vertical profiles of raindrop size distributions result when this method is applied to wind profiler data. The technique performed very well with most observed spectra. Some recovered spectra departed from the corresponding measured spectra, for cases in which a clear-air peak could not be accurately reproduced because of uncertainties in the location of the minimum position between the clear-air echo and the precipitation echo. Statistical relationships between LWC and integral rainfall parameters yield interesting features. The median volume diameter is statistically independent of the LWC and is associated with the large variability of the total number of drops, NT, between events. Vertical profiles from one event show a clear inverse relationship between NT and D0

scholarly journals Rain Attenuation of Radar Echoes Considering Finite-Range Resolution and Using Drop Size Distributions

2010 ◽  
Vol 27 (5) ◽  
pp. 829-842 ◽  
Author(s):  
Gerhard Peters ◽  
Bernd Fischer ◽  
Marco Clemens
Keyword(s):  
Attenuation Correction ◽  
A Priori ◽  
Real Data ◽  
Rain Attenuation ◽  
Size Distributions ◽  
Finite Range ◽  
Near Surface ◽  
Range Resolution ◽  
Raindrop Size ◽  
Drop Size Distributions

Abstract The classical rain attenuation correction scheme of Hitschfeld and Bordan (HIBO) and the newer iterative approach by Hildebrand (HL) are reconsidered. Although the motivation for the HL algorithm was an extension into ranges, where HIBO tends to be unstable, it is shown here that the contrary is the case. The finite-range resolution causes an intrinsic instability of HL already at moderate attenuation, where HIBO would still deliver stable results. Therefore, the authors concentrate the further analysis on HIBO, and confirm that the usual implementation of HIBO does not account correctly for finite-range resolution. They suggest a modified scheme that produces exact retrievals in the ideal case of perfect measurements. For vertically pointing Doppler radars a new element is explored in the attenuation correction—namely, calculating rain attenuation κ and rainfall R from Doppler spectra via the raindrop size distributions (RSDs). Although this spectral scheme (SIBO) avoids the uncertainty of Z–R and Z–κ relations, the superiority of this approach is not a priori obvious because of its sensitivity to vertical wind. Therefore, radar rain rates, based on a Z–R relation and on RSDs, respectively, are compared with in situ measurements. The results indicate better agreement for RSD-based retrievals. Because κ is closely correlated with R, the authors assert the advantage of RSD-based retrievals of κ. The application of HIBO and SIBO to real data shows that the uncertainty of standard Z–R relations is the main source of deviation between the two versions. In addition, the comparison of profiles suggests that the parameters of Z–R relations aloft can deviate considerably from near-surface values. Although artifacts cannot be excluded with certainty, there is some evidence that this observation actually reflects microphysical processes.

Characteristics and Momentum Flux Spectrum of Convectively Forced Internal Gravity Waves in Ensemble Numerical Simulations

2007 ◽  
Vol 64 (10) ◽  
pp. 3723-3734 ◽  
Author(s):  
Hyun-Joo Choi ◽  
Hye-Yeong Chun ◽  
In-Sun Song
Keyword(s):  
Numerical Simulations ◽  
Gravity Waves ◽  
Momentum Flux ◽  
Tropical Ocean ◽  
Convective Storms ◽  
Control Simulation ◽  
Toga Coare ◽  
Flux Spectrum ◽  
Diabatic Forcing ◽  
Convective Gravity Waves

Abstract Characteristics of convectively forced gravity waves are investigated through ensemble numerical simulations for various ideal and real convective storms. For ideal storm cases, single-cell-, multicell-, and supercell-type storms are considered, and for real cases, convection events observed during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) and in Indonesia are used. For each storm case, wave perturbations and the momentum flux spectrum of convective gravity waves in a control simulation with nonlinearity and cloud microphysical processes are compared with those in quasi-linear dry simulations forced by either diabatic forcing or nonlinear forcing obtained from the control simulation. In any case, gravity waves in the control simulation cannot be represented well by wave perturbations induced by a single forcing. However, when both diabatic and nonlinear forcing terms are considered, the gravity waves and their momentum flux spectrum become comparable to those in the control simulation, because of cancellation between wave perturbations by two forcing terms. These results confirm that the two forcing mechanisms of convective gravity waves proposed by previous studies based on a single convective event can be applied generally to various types of convective storms. This suggests that nonlinear forcing, as well as diabatic forcing, should be considered appropriately in parameterizations of convectively forced gravity waves.

scholarly journals Diurnal Cycle of Raindrops Size Distribution in a Valley of the Peruvian Central Andes

Atmosphere ◽  
2019 ◽  
Vol 11 (1) ◽  
pp. 38 ◽  
Author(s):  
Elver Villalobos-Puma ◽  
Daniel Martinez-Castro ◽  
Jose Luis Flores-Rojas ◽  
Miguel Saavedra-Huanca ◽  
Yamina Silva-Vidal
Keyword(s):  
Central Andes ◽  
Amazon Forest ◽  
Size Distributions ◽  
Spectral Variability ◽  
Convective Rainfall ◽  
Convective Storms ◽  
Local Standard ◽  
Normalized Parameters ◽  
Raindrop Size ◽  
Stratiform Rainfall

In the Central Andes of Peru, convective and stratiform rainfall occurs, frequently associated with convective storms. The raindrop size distributions (RSD), measured by a Parsivel-2 optical disdrometer, were characterized by the variation of their normalized parameters. The RSD dataset includes measurements corresponding to 18 months between 2017 and 2019. As a result, it was found that the mass-weighted mean diameter Dm and the Nw parameter present respectively high and low values, in the interval of 15–20 LST (local standard time), wherein deeper and more active clouds appear. The events including convective rainfall contribute 67.5% of the accumulated total, wherein 92% corresponds to the 15–20 LST interval. It is concluded that the spectral variability of the RSD is strongly controlled by the cloudiness configuration field developing over the west (convection over highlands) and east (convection over Amazon) sides of the valley. In the afternoon, clouds develop and drift to the east, over the Andean valleys and towards the Amazon, intensified by local orographic circulation. The opposite happens at night, when the stratiform rainfall is dominant and it is controlled by clouds, located in the Inter-Andean valley, generated by the convection fields formed over the Amazon forest.

scholarly journals Raindrop Size Distribution (DSD) Retrieval for X-Band Dual-Polarization Radar

2014 ◽  
Vol 31 (2) ◽  
pp. 387-403 ◽  
Author(s):  
Eiichi Yoshikawa ◽  
V. Chandrasekar ◽  
Tomoo Ushio
Keyword(s):  
Size Distribution ◽  
Attenuation Correction ◽  
Statistical Fluctuation ◽  
Dual Polarization ◽  
Horizontal Polarization ◽  
Axis Ratio ◽  
Raindrop Size Distribution ◽  
Median Volume ◽  
Raindrop Size ◽  
Differential Reflectivity

Abstract A raindrop size distribution (DSD) retrieval method for a dual-polarization radar at attenuating frequency is proposed. The proposed method is developed such that the range profiles of the gamma DSD parameters, an intercept parameter Nw (mm−1 m−3), and a median volume diameter D0 (mm) can be estimated to match the dual-polarization measurements, measured equivalent reflectivity at horizontal polarization ZHm, measured differential reflectivity ZDRm, and measured differential propagation phase ΦDPm, where the forward scattering and backscattering are formulated simultaneously to avoid the two-step process of attenuation correction and DSD retrieval. Additionally, the proposed method does not have the attenuation-correction errors accumulated along range that traditional forward and backward processes have, since the range profiles of the DSD parameters are optimized in a radar beam simultaneously. In the simulation, the proposed algorithm showed fairly good accuracies for retrievals Nw and D0. Errors with the different axis ratio models or calibration biases in ZHm and ZDRm, which contaminate assumptions of the proposed method in real observational data, were also evaluated. Under a Gaussian fluctuation model, the estimation process, known as an iterative maximum-likelihood estimator, derives the best estimation in the statistical fluctuation conditions. This scheme could be extended to duplicative observation such as a radar network environment.

scholarly journals Microphysics of Raindrop Size Spectra: Tropical Continental and Maritime Storms

2007 ◽  
Vol 46 (11) ◽  
pp. 1777-1791 ◽  
Author(s):  
Carlton W. Ulbrich ◽  
David Atlas
Keyword(s):  
Size Spectra ◽  
Quantitative Measurements ◽  
Time Space ◽  
Gamma Distributions ◽  
Raindrop Size Distribution ◽  
Median Volume ◽  
Drop Number ◽  
C Segments ◽  
Raindrop Size ◽  
Differential Reflectivity

Abstract This work uses raindrop size spectra measured at the surface in tropical continental storms to determine the associated parameters of the best-fit gamma distributions. The physical processes responsible for those parameters and their relations to the measurable radar reflectivity Z and differential reflectivity ZDR are then explored. So too are their relations to quantitative measurements of rain. Comparison is then made with corresponding features previously reported in tropical maritime regimes. The storms observed in Brazil and Arecibo, Puerto Rico, have been divided into convective (C), transition (T), and stratiform (S) segments. The raindrop size distribution (DSD) parameters are clearly defined on a gamma parameter diagram (GPD) that shows 1) how median volume drop size D0 increases from S to T to C segments of the rain while 2) the range of the spectrum breadth parameter μ increases, and the range of the slope parameter Λ decreases in the same sequence of S to C. Drop growth occurs predominantly below the 0°C level by collision, coalescence, and breakup in the C rains. The median volume diameter D0 grows as more of the water is concentrated near that size and so the DSD narrows; that is, both μ and Λ increase. In both maritime and continental storms the DSD in the convective portion of the storm approaches equilibrium. The coefficient A in the Z = ARb relation increases with D0 while the exponent b approaches unity. The D0 and A pair increase with, and appear to be determined largely by, the updraft strength, thus providing a possible means of determining the appropriate algorithms for rainfall measurement. Although the small drop number samples measured by the surface disdrometer relative to the large volumes sampled by a radar tend to truncate the DSD at both small and large drop sizes, narrow distributions with μ = 5 to 12 cannot be attributed to such an effect. Such narrow DSDs accord with common experience of monodispersed large drops at the beginning of a convective storm. There is also remarkable agreement of the surface-based observations of ZDR–Z–D0 with the time–space variations from C to T to S rain types observed by radar in England and elsewhere. Because the C region of a storm often accounts for a major share of the rain accumulation despite its shorter duration, it is particularly important to measure that region more accurately. There are distinctive clusters of the generalized number parameter NW versus D0 between maritime and continental storms. Methods for remote sensing and parameterization must partition the rainstorms into convective, transition, and stratiform segments.

A Statistical and Physical Description of Hydrometeor Distributions in Colorado Snowstorms Using a Video Disdrometer

2007 ◽  
Vol 46 (5) ◽  
pp. 634-650 ◽  
Author(s):  
Edward A. Brandes ◽  
Kyoko Ikeda ◽  
Guifu Zhang ◽  
Michael Schönhuber ◽  
Roy M. Rasmussen
Keyword(s):  
Terminal Velocity ◽  
Size Distributions ◽  
Front Range ◽  
Particle Size Distributions ◽  
Snow Density ◽  
Winter Storm ◽  
Physical Description ◽  
Median Volume ◽  
Velocity Information ◽  
Raindrop Size

Abstract Winter-storm hydrometeor distributions along the Front Range in eastern Colorado are studied with a ground-based two-dimensional video disdrometer. The instrument provides shape, size, and terminal velocity information for particles that are larger than about 0.4 mm. The dataset is used to determine the form of particle size distributions (PSDs) and to search for useful interrelationships among the governing parameters of assumed distribution forms and environmental factors. Snowfalls are dominated by almost spherical aggregates having near-exponential or superexponential size distributions. Raindrop size distributions are more peaked than those for snow. A relation between bulk snow density and particle median volume diameter is derived. The data suggest that some adjustment may be needed in relationships found previously between temperature and the concentration and slope parameters of assumed exponential PSDs. A potentially useful relationship is found between the slope and shape terms of the gamma PSD model.

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

2008 ◽  
Vol 9 (3) ◽  
pp. 408-425 ◽  
Author(s):  
Brooks E. Martner ◽  
Sandra E. Yuter ◽  
Allen B. White ◽  
Sergey Y. Matrosov ◽  
David E. Kingsmill ◽  
...  
Keyword(s):  
Size Distributions ◽  
Coast Range ◽  
Rain Intensity ◽  
Size Spectra ◽  
Winter Storms ◽  
Mean Values ◽  
Bright Band ◽  
Raindrop Size ◽  
Drop Size Distributions ◽  
Differential Reflectivity

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 = 168R1.58 for BB periods and Z = 44R1.91 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.

scholarly journals Raindrop Size Distribution and Rain Characteristics during the 2013 Great Colorado Flood

2015 ◽  
Vol 17 (1) ◽  
pp. 53-72 ◽  
Author(s):  
Katja Friedrich ◽  
Evan A. Kalina ◽  
Joshua Aikins ◽  
Matthias Steiner ◽  
David Gochis ◽  
...  
Keyword(s):  
Drop Size ◽  
Doppler Radar ◽  
Liquid Water Content ◽  
Size Distributions ◽  
Intense Rainfall ◽  
Wind Fields ◽  
Raindrop Size Distribution ◽  
Median Volume ◽  
Drop Size Distributions ◽  
Analysis System

Abstract Drop size distributions observed by four Particle Size Velocity (PARSIVEL) disdrometers during the 2013 Great Colorado Flood are used to diagnose rain characteristics during intensive rainfall episodes. The analysis focuses on 30 h of intense rainfall in the vicinity of Boulder, Colorado, from 2200 UTC 11 September to 0400 UTC 13 September 2013. Rainfall rates R, median volume diameters D0, reflectivity Z, drop size distributions (DSDs), and gamma DSD parameters were derived and compared between the foothills and adjacent plains locations. Rainfall throughout the entire event was characterized by a large number of small- to medium-sized raindrops (diameters smaller than 1.5 mm) resulting in small values of Z (<40 dBZ), differential reflectivity Zdr (<1.3 dB), specific differential phase Kdp (<1° km−1), and D0 (<1 mm). In addition, high liquid water content was present throughout the entire event. Raindrops observed in the plains were generally larger than those in the foothills. DSDs observed in the foothills were characterized by a large concentration of small-sized drops (d < 1 mm). Heavy rainfall rates with slightly larger drops were observed during the first intense rainfall episode (0000–0800 UTC 12 September) and were associated with areas of enhanced low-level convergence and vertical velocity according to the wind fields derived from the Variational Doppler Radar Analysis System. The disdrometer-derived Z–R relationships reflect how unusual the DSDs were during the 2013 Great Colorado Flood. As a result, Z–R relations commonly used by the operational NEXRAD strongly underestimated rainfall rates by up to 43%.

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