Showing papers on "Disdrometer published in 2014"

Evaluation of the New Version of the Laser-Optical Disdrometer, OTT Parsivel2

Abstract: A comparative study of raindrop size distribution measurements has been conducted at NASA's Goddard Space Flight Center where the focus was to evaluate the performance of the upgraded laser-optical OTT Particle Size Velocity (Parsivel2; P2) disdrometer The experimental setup included a collocated pair of tipping-bucket rain gauges, OTT Parsivel (P1) and P2 disdrometers, and Joss-Waldvogel (JW) disdrometers Excellent agreement between the two collocated rain gauges enabled their use as a relative reference for event rain totals A comparison of event total showed that the P2 had a 6%absolute bias with respect to the reference gauges, considerably lower than the P1 and JW disdrometers Good agreement was also evident between the JW and P2 in hourly raindrop spectra for drop diameters between 05 and 4 mm The P2 drop concentrations mostly increased toward small sizes, and the peak concentrations were mostly observed in the first three measurable size bins The P1, on the other hand, underestimated small drops and overestimated the large drops, particularly in heavy rain rates From the analysis performed, it appears that the P2 is an improvement over the P1 model for both drop size and rainfall measurements P2 mean fall velocities follow accepted terminal fall speed relationships at drop sizes less than 1 mm As a caveat, the P2 had approximately 1ms21 slower mean fall speed with respect to the terminal fall speed near 1 mm, and the difference between the mean measured and terminal fall speeds reduced with increasing drop size This caveat was recognized as a software bug by the manufacturer and is currently being investigated

Uncertainties of GPM DPR Rain Estimates Caused by DSD Parameterizations

Abstract: A framework based on measured raindrop size distribution (DSD) data has been developed to assess uncertainties in DSD models employed in Ku- and Ka-band dual-wavelength radar retrievals. In this study, the rain rates and attenuation coefficients from DSD parameters derived by dual-wavelength algorithms are compared with those directly obtained from measured DSD spectra. The impact of the DSD gamma parameterizations on rain estimation from the Global Precipitation Measurement mission (GPM) Dual-Frequency Precipitation Radar (DPR) is examined for the cases of a fixed shape factor μ as well as for a constrained μ—that is, a μ–Λ relation (a relationship between the shape parameter and slope parameter Λ of the gamma DSD)—by using 11 Particle Size and Velocity (Parsivel) disdrometer measurements with a total number of about 50 000 one-minute spectra that were collected during the Iowa Flood Studies (IFloodS) experiment. It is found that the DPR-like dual-wavelength techniques provide fairly accurate est...

Hydrometeor classification from two-dimensional video disdrometer data

Abstract: The first hydrometeor classification technique based on two-dimensional video disdrometer (2DVD) data is presented The method provides an estimate of the dominant hydrometeor type falling over time intervals of 60 s during precipitation, using the statistical behavior of a set of particle descriptors as input, calculated for each particle image The employed supervised algorithm is a support vector machine (SVM), trained over 60 s precipitation time steps labeled by visual inspection In this way, eight dominant hydrometeor classes can be discriminated The algorithm achieved high classification performances, with median overall accuracies (Cohen's K) of 90% (088), and with accuracies higher than 84% for each hydrometeor class

Evaluation of a New Polarimetric Algorithm for Rain-Path Attenuation Correction of X-Band Radar Observations Against Disdrometer

Abstract: A new algorithm called self-consistent with optimal parameterization (SCOP) for attenuation correction of radar reflectivities at low elevation angles is developed and evaluated. The SCOP algorithm, which uses optimal parameterization and best-fitted functions of specific attenuation coefficients and backscattering differential phase shift, is applied to X-band dual-polarization radar data and evaluated on the basis of radar observables calculated from disdrometer data at a distance of 35 km from the radar. The performance of the SCOP algorithm is compared with other algorithms [reflectivity-differential phase shift (ZPHI) and full self-consistent (FSC)] presented in the literature. Overall, the new algorithm performs similarly to ZPHI for the attenuation correction of horizontal-polarization reflectivity, whereas the FSC algorithm exhibits significant underestimation. The ZPHI algorithm tends to overestimate small rain-path attenuation values. All algorithms exhibit significant underestimation at high differential rain-path attenuation values, probably due to the presence of hail along the path of the radar beam during the examined cases. The new SCOP algorithm has the potential to retrieve profiles of horizontal and differential reflectivities with better accuracy than the other algorithms due to the low error of the parameterization functions used in it. Typical radar calibration biases and measurement noise are sufficient requirements to ensure low errors of the proposed algorithm. A real-time method to calibrate the differential reflectivity without additional measurements is also described.

Precipitation Classification Using Measurements From Commercial Microwave Links

Abstract: Commercial wireless microwave links have been recently proven to be an effective tool for precipitation monitoring, mainly for accurate rainfall estimation and high-resolution rainfall mapping. This paper focuses on the challenge of precipitation classification from the measurements of received signal level (RSL) in several commercial wireless microwave links, by suggesting a tree of classification based on the physical features that distinguish between different phenomena. Wet periods are first identified, followed by a classification of the wet periods into pure rain or sleet. The classification is based on the kernel Fisher discriminant analysis, followed by a decision-making process. The suggested procedure is tested on real data, and its performance is evaluated. It is shown that the proposed classification is in very good agreement (85%) with that of a special-purpose meteorological device called disdrometer.

Further evidence for superterminal raindrops

Abstract: A network of optical disdrometers (including laser precipitation monitors and a two-dimensional video disdrometer) was utilized to determine whether the recent reports of “superterminal” raindrops were spurious results of drop breakup occurring on instrumentation. Results unequivocally show that superterminal raindrops at small (less than 1 mm) sizes are ubiquitous, are measurable over an extended area, and appear in every rain event investigated. No evidence was found to suggest that superterminal drops are the result of drop breakup due to impact with the measurement instrument; thus, if the superterminal drops are the result of drop fragmentation, this fragmentation happens in the ambient atmosphere during all rain events measured in this study. The ubiquity of superterminal drops at small drop sizes raises natural questions regarding rain accumulation estimations, estimates of drop size distributions, and erosion characterization.

Evaluation of Gamma Raindrop Size Distribution Assumption through Comparison of Rain Rates of Measured and Radar-Equivalent Gamma DSD

Abstract: To date, one of the most widely used parametric forms for modeling raindrop size distribution (DSD) is the three-parameter gamma. The aim of this paper is to analyze the error of assuming such parametric form to model the natural DSDs. To achieve this goal, a methodology is set up to compare the rain rate obtained from a disdrometer-measured drop size distribution with the rain rate of a gamma drop size distribution that produces the same triplets of dual-polarization radar measurements, namely reflectivity factor, differential reflectivity, and specific differential phase shift. In such a way, any differences between the values of the two rain rates will provide information about how well the gamma distribution fits the measured precipitation. The difference between rain rates is analyzed in terms of normalized standard error and normalized bias using different radar frequencies, drop shape–size relations, and disdrometer integration time. The study is performed using four datasets of DSDs collec...

Nonstationarity in Intermittent Rainfall: The “Dry Drift”

Abstract: A particular aspect of the nonstationary nature of intermittent rainfall is investigated It manifests itself in the fact that the average rain rate varies with the distance to the surrounding dry areas The authors call this fundamental link between the rainfall intensity and the rainfall occurrence process the ‘‘dry drift’’ Using high-resolution radar rain-rate maps and disdrometer data, they show how the dry drift affects the structure and the variability of intermittent rainfall fields They provide a rigorous geostatistical framework to describe it and propose an extension of the concept to more general quantities like the (rain)drop size distribution

Performance of Using Cascade Forward Back Propagation Neural Networks for Estimating Rain Parameters with Rain Drop Size Distribution

Abstract: The aim of our study is to estimate the parameters M (water content), R (rain rate) and Z (radar reflectivity) with raindrop size distribution by using the neural network method. Our investigations have been conducted in five African localities: Abidjan (Cote d’Ivoire), Boyele (Congo-Brazzaville), Debuncha (Cameroon), Dakar (Senegal) and Niamey (Niger). For the first time, we have predicted the values of the various parameters in each locality after using neural models (LANN) which have been developed with locally obtained disdrometer data. We have shown that each LANN can be used under other latitudes to get satisfactory results. Secondly, we have also constructed a model, using as train-data, a combination of data issued from all five localities. With this last model called PANN, we could obtain satisfactory estimates forall localities. Lastly, we have distinguished between stratiform and convective rain while building the neural networks. In fact, using simulation data from stratiform rain situations, we have obtained smaller root mean square errors (RMSE) between neural values and disdrometer values than using data issued from convective situations.

A Summary of Precipitation Characteristics from the 2006–11 Northern Australian Wet Seasons as Revealed by ARM Disdrometer Research Facilities (Darwin, Australia)

Abstract: The variability of rainfall and drop size distributions (DSDs) as a function of large-scale atmospheric conditions and storm characteristics is investigated using measurements from the Atmospheric Radiation Measurement Program (ARM) facility at Darwin, Australia. Observations are obtained from an impact disdrometer with a near continuous record of operation over five consecutive wet seasons (2006–11). Bulk rainfall characteristics are partitioned according to diurnal accumulation, convective and stratiform precipitation classifications, objective monsoonal regime, and MJO phase. Findings support previous Darwin studies suggesting a significant diurnal and DSD parameter signal associated with both convective–stratiform and wet season monsoonal regime classification. Negligible MJO phase influence is determined for cumulative disdrometric statistics over the Darwin location.

Comparison of Disdrometer and X-Band Mobile Radar Observations in Convective Precipitation

Abstract: Microphysical data from thunderstorms are sparse, yet they are essential to validate microphysical schemes in numerical models. Mobile, dual-polarization, X-band radars are capable of providing a wealth of data that include radar reflectivity, drop shape, and hydrometeor type. However, X-band radars suffer from beam attenuation in heavy rainfall and hail, which can be partially corrected with attenuation correction schemes. In this research, the authors compare surface disdrometer observations to results from a differential phase-based attenuation correction scheme. This scheme is applied to data recorded by the National Oceanic and Atmospheric Administration (NOAA) X-band dual-polarized (NOXP) mobile radar, which was deployed during the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2). Results are presented from five supercell thunderstorms and one squall line (183 min of data). The median disagreement (radar–disdrometer) in attenuation-corrected reflectivity Z and...

Phase Space Parameterization of Rain: The Inadequacy of Gamma Distribution

Abstract: The authors test the adequacy of gamma distribution to describe the statistical variability of raindrop diameters in 1-min disdrometer data using the Kolmogorov–Smirnov goodness-of-fit test. The results do not support the use of this distribution, with a percentage of rejected cases that increases with the sample size. A different parameterization of the drop size distribution is proposed that does not require any particular functional form and is based on the adoption of statistical moments. The first three moments, namely the mean, standard deviation, and skewness, are sufficient to characterize the distribution of the drop diameter at the ground. These parameters, together with the drop count, form a 4-tuple, which fully describes the variability of the drop size distribution. The Cartesian product of this 4-tuple of parameters is the rainfall phase space. Using disdrometer data from 10 different locations, invariant, location-independent properties of rainfall are identified.

A video precipitation sensor for imaging and velocimetry of hydrometeors

Abstract: . A new method to determine the shape and fall velocity of hydrometeors by using a single CCD camera is proposed in this paper, and a prototype of a video precipitation sensor (VPS) is developed. The instrument consists of an optical unit (collimated light source with multi-mode fibre cluster), an imaging unit (planar array CCD sensor), an acquisition and control unit, and a data processing unit. The cylindrical space between the optical unit and imaging unit is sampling volume (300 mm × 40 mm × 30 mm). As the precipitation particles fall through the sampling volume, the CCD camera exposes twice in a single frame, which allows the double exposure of particles images to be obtained. The size and shape can be obtained by the images of particles; the fall velocity can be calculated by particle displacement in the double-exposure image and interval time; the drop size distribution and velocity distribution, precipitation intensity, and accumulated precipitation amount can be calculated by time integration. The innovation of VPS is that the shape, size, and velocity of precipitation particles can be measured by only one planar array CCD sensor, which can address the disadvantages of a linear scan CCD disdrometer and an impact disdrometer. Field measurements of rainfall demonstrate the VPS's capability to measure micro-physical properties of single particles and integral parameters of precipitation.

Raindrop size distribution and rainfall attenuation modeling in equatorial and subtropical Africa: the critical diameters

Abstract: Raindrop sizes were obtained from Butare, Rwanda (2.6°S, 29.74°E) and Durban, South Africa (29°52′S, 30°58′E) using the Joss–Waldvogel RD-80 disdrometer. The obtained data is used for the analysis of the drop size distribution (DSD) and specific rainfall attenuation modeling in these African countries identified as equatorial (Butare) and subtropical (Durban) regions. The influence of the raindrop diameters that are critical to the DSD and specific rain attenuation at operating frequencies of 10–150 GHz is investigated using the estimated R 0.01 values for Butare and Durban. Parameter fittings for the proposed DSD models at these locations for different rain rate values are investigated. The proposed drop size distribution models are also compared with those of other countries. At operating frequency range 10 ≤ f ≤ 40 GHz, the specific attenuation in Rwanda tends to be higher when compared to Durban. However, at frequency above 40 GHz, Durban shows a higher specific attenuation than Rwanda. The largest contributions to the overall specific rain attenuation are formed by drop diameters in the range 1.5 ≤ D ≤ 3.5 and 1.0 ≤ D ≤ 3.0 mm for Rwanda and South Africa, respectively, especially at higher frequencies. A minimal contribution is observed at the larger diameters. The influence of the disdrometer bins on the attenuation due to rain is also analyzed for these locations. The estimation will be useful to properly design adequate fade margin levels and for the purpose of link budget design by service providers and system engineers in the regions.

Applicability of satellite rainfall estimates for erosion studies in small offshore areas: A case study in Cape Verde Islands

Abstract: To assess the usability of satellite rainfall estimates for erosion studies in Cape Verde, the 3B42 and 3B43 products from TRMM, and the Multi-Sensor Precipitation Estimate MPE from Meteosat are compared to daily and monthly ground rainfall measured between 1998 and 2010. TRMM estimates from 1998 and MPE from 2009 were studied and it was found that they detect the general trend and direction of rainy clouds but underestimate the amount of rainfall compared to rain gauges. To obtain an improved rainfall estimate from Meteosat data, the cloud top temperature derived from the 10.8 μm infrared channel was correlated to rainfall intensity measured at a 3-minute interval between 2008 and 2010 by a Parsivel optical disdrometer. A modified exponential equation of daily rainfall intensity as a function of cloud top temperature was obtained. The equation was applied to 15-minute intervals from the infrared channel to obtain 15‐minute rainfall and summed up as daily rainfall. The daily rainfall depths estimated from cloud top temperature showed a better correspondence with the amounts measured by ground gauges on the island (r = 0.75), and are a first approach to cope with lack of short duration rainfall data for erosion studies in island states such as Cape Verde.

Examination and Application of Doppler Spectral Density Data in Drop Size Distribution Retrieval in Weak Precipitation by Cloud Radar

Abstract: The effects of Mie Scattering, air turbulence, and air vertical speed on drop size distribution(DSD) retrieval from Doppler spectral density data observed by cloud radar are discussed in this study The processing algorithm for the Doppler spectral density data and retrieval algorithm for DSD, liquid water content(LWC), rain rate, and air vertical speed with Doppler spectral density data are presented The two weak precipitation cases observed by vertical-pointing Ka-band cloud radar, Ku-band micro-rain radar, and disdrometer in July 2012 at Tengchong, Yunnan Province, are used to examine the cloud radar data quality and retrieval algorithm The vertical profiles of reflectivity and velocity observed by cloud radar and micro-rain radar are compared The Doppler spectral density data at an altitude of 780 m observed by cloud radar, micro-rain radar, and disdrometer calculation are compared In addition, the DSD and rain rate are retrieved by Doppler spectral density data and compared with disdrometer data The effect of water over the cloud radar antenna on reflectivity measurement is also discussed The results indicate that although the effects of air turbulence on precipitation microphysical parameters are negligible, those of air vertical speed and Mie scattering are obvious The velocity, Doppler spectral density data, and variational pattern of reflectivity with altitude observed by cloud radar and micro-rain radar show good agreement; however, the reflectivity measured by the cloud radar was weaker than that observed by the other methods The water over the cloud radar antenna significantly reduced the reflectivity The Doppler spectral density data and DSD observed by the cloud radar were similar with that observed by disdrometer Therefore, this study has verified the effectiveness of Doppler spectral density data measurement by cloud radar and retrieval parameters

Stable isotopic differences between summer and winter monsoon rains over southern India

Abstract: Rain drop size and isotopic composition of rain, important parameters that shed light on rain formation processes, are highly sensitive to the ambient weather. We reported earlier a significant correlation between them in individual rain events (with limited sampling), but this is yet to be tested with better, longer term sampling. Here we attempt to do so over a tropical region (i.e. Tirupati, India). Rain samples were collected at short time intervals (<1 h) to capture even small variations in their stable oxygen (δ18O) and hydrogen (δD) isotopic compositions. Isotopic analyses were made using an isotope ratio mass spectrometer, and a disdrometer measured the drop size distribution. Summer rains show a progressive 18O & D depletion with time, while the winter rains fluctuate about a mean value. We find no definite correlation between the drop size and stable isotope ratios as was reported earlier, based on a smaller number of samples: the complexity of rain formation process and varying ambient weather conditions for individual rain events could be the reason. Further, there is no significant difference between the local meteoric water lines (δ18O- δD line) of summer and winter monsoon rains, though the intercepts in both the cases were significantly smaller than global meteoric waterline, suggesting significant strong influence of secondary evaporation. However, the winter rains are more depleted in D & 18O. Paleoclimate proxies such as δ18O of cave calcite or teak cellulose form this region need to be interpreted in terms of the relative seasonality of the rainfall rather than the total annual rain.

Consistency in Z-R Relationship Variability Regardless Precipitating Systems, Climatic Zones Observed from Two Types of Disdrometer

Abstract: Data from rain Drop Size Distributions gathered on five sites in Africa as well as those of the pilot site in Kourou (French Guyana, South America), located in different climatic zones, and collected by two types of disdrometer (the impact JW RD-69 disdrometer and the Optical Spectro-Pluviometer, OSP) are used to study the consistency of the reflectivity factor-rain rate at the ground (Z-R) relationship variability. The results clearly confirm that the relationship Z-R knows a large spatial variability, from a type of precipitation to another and within the same precipitation regardless the type of disdrometer used for DSD measurements. Base on the similarity of the relations reflectivity factor-rain rate and ratio median volume diameter over the total number of drops-rain rate, the variability of the Z-R coefficients (A, b) through the simultaneously implication of the size and number of drops which characterize the DSD was exhibited. It was shown that the relationships A-α and b-β designed to understand the involvement of parameters D0 and NT of DSD in the variability of the relationship Z-R are similar regardless the types of disdrometer used. However, the relations A-α in the Sahelian region appear to deviate from those of Guinean, equatorial and Soudanian zones. The plausible reasons were discussed.

Very large rain drops from 2D video disdrometers and concomitant polarimetric radar observations

Abstract: Drop size distribution (DSD) measurements using ground-based disdrometers (point measurements) have often been used to derive equations to relate radar observations to the integral rainfall parameters (Atlas et al. 1999, Bringi et al., 2003, Kozu et al., 2006, Tokay and Short, 1996, Ajayi and Owolabi, 1987, Battan, 1973). Disdrometers such as JWD, MRR and several others have a major limitation in measuring drops with equi-volume diameters (D(sub eq)) larger than 5 mm because they often rely on the velocity-diameter relationship which plateaus beyond this diameter range (Atlas et al., 1973, Gunn & Kinzer, 1949). Other disdrometers such as Parsivel also lack accuracy beyond this diameter range. The 2D video disdrometer (2DVD: Schonhuber et al., 2008) on the other hand gives drop-shape contours and velocities for each individual drop/hydrometeor falling through its sensor area; this provides a unique opportunity to study the role of very-large drops on radar measurements in particular those with polarimetric radar capability where DSDs with a significant component of very large drops may require special consideration given that the differential reflectivity and other polarimetric radar parameters including attenuation-correction methods will be sensitive to the concentrations of these large drops. A recent study on the occurrence of large drops by Gatlin et al. (2014) has compiled a large and diverse set of measurements made with the 2D video disdrometers from many locations around the globe. Some of the largest drops found in this study were 9 mm D(sub eq) and larger, and in this paper, we report on three such events, with maximum D(sub eq's) of 9.0, 9.1 and 9.7 mm, which occurred in Colorado, Northern Alabama, and Oklahoma, respectively. Detailed examination of the 2DVD data - in terms of shapes and fall velocities - has confirmed that these are fully-melted hydrometeors, although for the last case in Oklahoma, a bigger and non-fully-melted hydrometeor was also observed. All three events were also captured by polarimetric radars, namely the S-band CHILL radar operated by Colorado State University (Brunkow et al., 2000), the C-band ARMOR radar (Petersen et al., 2007) operated by University of Alabama in Huntsville, and NEXRADKVNX, operated by the US National Weather Service, respectively. For the last event, several other radar observations were also made, including two X-band radars operated by the US Dept. of Energy. Analyses of 2DVD data in conjunction with the corresponding radar observations are presented, along with some discussion on sampling issues related to the measurements of such large rain drops. The latter is addressed using maximum diameter D(sub max) measurements from 1-minute DSDs using two collocated 2DVDs for 37 events in Huntsville.

In situ disdrometer calibration using multiple DSD moments

Abstract: In situ calibration is a proposed strategy for continuous as well as initial calibration of an impact disdrometer. In previous work, a collocated tipping bucket had been utilized to provide a rainfall rate based ∼11/3 moment reference to an impact disdrometer’s signal processing system for implementation of adaptive calibration. Using rainfall rate only, transformation of impulse amplitude to a drop volume based on a simple power law was used to define an error surface in the model’s parameter space. By incorporating optical extinction second moment measurements with rainfall rate data, an improved in situ disdrometer calibration algorithm results due to utilization of multiple (two or more) independent moments of the drop size distribution in the error function definition. The resulting improvement in calibration performance can be quantified by detailed examination of the parameter space error surface using simulation as well as real data.

Binning effects on in-situ raindrop size distribution measurements

Abstract: . This paper investigates the binning effects on drop size distribution (DSD) measurements obtained by Joss-Waldvogel disdrometer (JWD), Precipitation Occurrence Sensor System (POSS), Thies disdrometer (Thies), Parsivel OTT disdrometer, two-dimensional video disdrometer (2DVD) and optical spectro-pluviometer (OSP) instruments, therefore the evaluation comprises non-regular bin sizes and the effect of minimum and maximum measured sizes of drops. To achieve this goal, 2DVD measurements and simulated gamma size distributions were considered. The analysis of simulated gamma DSD binned according each instrument was performed to understand the role of discretisation and truncation effects together on the integral rainfall parameters and estimators of the DSD parameters. In addition, the drop-by-drop output of the 2DVD is binned to simulate the raw output of the other disdrometers which allowed us estimate sampling and binning effects on selected events from available dataset. From simulated DSD it has been found that binning effects exist in integral rainfall parameters and in the evaluation of DSD parameters of a gamma distribution. This study indicates that POSS and JWD exhibit underestimation of concentration and mean diameter due to binning. Thies and Parsivel report a positive bias for rainfall and reflectivity (reaching 5% for heavy rainfall intensity events). Regarding to DSD parameters, distributions of estimators for the shape and scale parameters were analyzed by moment, truncated moment and maximum likelihood methods. They reported noticeable differences between instruments for all methodologies of estimation applied. The measurements of 2DVD allow sampling error estimation of instruments with smaller capture areas than 2DVD. The results show that the instrument differences due to sampling were a~relevant uncertainty but that concentration, reflectivity and mass-weighted diameter were sensitive to binning.

In Situ Disdrometer Calibration Using Multiple DSD Moments

Abstract: In situ calibration is a proposed strategy for continuous as well as initial calibration of an impact disdrometer. In previous work, a collocated tipping bucket had been utilized to provide a rainfall rate based ~11/3 moment reference to an impact disdrometer's signal processing system for implementation of adaptive calibration. Using rainfall rate only, transformation of impulse amplitude to a drop volume based on a simple power law was used to define an error surface in the model's parameter space. By incorporating optical extinction second moment measurements with rainfall rate data, an improved in situ disdrometer calibration algorithm results due to utilization of multiple (two or more) independent moments of the drop size distribution in the error function definition. The resulting improvement in calibration performance can be quantified by detailed examination of the parameter space error surface using simulation as well as real data.

Real-Time Radar Reflectivity Calibration from Differential Phase Measurements

Abstract: An algorithm based on the self-consistency between the horizontal reflectivity ZH and the specific differential phase KDP has been devised for the calibration of the reflectivity measurements of the McGill S-band dual-polarization radar and implemented in real time in the fall of 2012. By combining pairs of measured and theoretical differential propagation phases (ΦDP) along rain paths from several azimuths, elevation angles, and radar cycles, a robust calibration estimate is obtained even in relatively light precipitation, provided the number of pairs is of the order of at least 103. It confirmed the stability of the radar system as further corroborated by disdrometer and ground echo comparisons. However, the two-parameter ZH–KDP technique proved to be inadequate in convective situations because it overestimates ΦDP differences of paths with heavy precipitation. An ex post facto analysis has revealed that a three-parameter (ZH–KDP–ZDR) relationship provides a much better agreement with the measur...

Estimation of Z-R relationship and comparative analysis of precipitation data from colocated rain-gauge, vertical radar and disdrometer

Abstract: The estimation of precipitation rates and accumulations constitutes an essential input data for distributed hydrological models and for many hydrometeorological applications, such as short term hydro-scheduling, forecasting and monitoring of river floods and inflow forecasting into a catchment, its peak flow and response time. In order to improve quantitative precipitation estimates from C-band weather radar, variable Z-R relationships that represent the local precipitation characteristics and conditions are derived from measured precipitation drop size distribution (DSD) by two ground-based instruments: vertical pointing Doppler radar MRR and the optical disdrometer OTT Parsivel. The variability and robustness of those local Z-R relationships are analyzed, as well as their level of dependency to the storm type (stratiform, convective, air mass convection), to the precipitation phase (rain, snow, mixed precipitation), to each event and to each month. Comparative analysis of the precipitation accumulations measured or estimated by three different local instruments (tipping bucket rain gauge, disdrometer OTT Parsivel, vertical pointing radar MRR) is also performed in order to assess under which conditions those instruments are able to provide reliable precipitation rates and robust local Z-R relationships. MRR provides the most variable and uncertain local Z-R parameters that are highly dependent on the precipitation phase. This dependency leads to high event-to-event variability of DSD-measured Z-R relations and large differences in estimated precipitation rates and accumulations (especially for snow) between the local instruments and compared to precipitation values estimated from the weather radar, using the standard Z-R relationship. The optical disdrometer OTT Parsivel provides the most stable and robust Z-R parameters that are independent of the precipitation phase and the season. In the only case of rain events, both instruments derive similar local Z-R parameters that are comparable to the standard Z-R relationship. The high variability and uncertainty related to Z-R parameters concern mainly mixed precipitations. There is no evident dependency of Z-R parameters on storm type. When comparing the three local precipitation instruments, they all provide similar rain accumulations. In cases of snow and mixed precipitation, accumulations derived from those instruments are quite different. However, the disdrometer OTT Parsivel and the tipping bucket rain gauge agree relatively well in terms of accumulations. In addition that OTT Parsivel may provide robust local Z-R relationships for any kind of precipitation that correspond well to standard Z-R relationship, it may compensate for precipitation losses, catch deficit and low temporal resolution of the conventional rain gauge. However, long periods of instrument instability and breakdowns for the disdrometer reduce significantly the number of valid precipitation data available at any time of the year and any precipitation conditions, hence the data representativity of this instrument. In case of MRR, more investigations and possibly better measurement filtering and correction prior to the precipitation estimation may reduce the uncertainties around the MRR-derived Z-R parameters and precipitation estimates.