Showing papers on "Disdrometer published in 2013"

Comparison of Raindrop Size Distribution Measurements by Collocated Disdrometers

Abstract: An impact-type Joss-Waldvogel disdrometer (JWD), a two-dimensional video disdrometer (2DVD), and a laser optical OTT Particle Size and Velocity (PARSIVEL) disdrometer (PD) were used to measure the raindrop size distribution (DSD) over a 6-month period in Huntsville, Alabama. Comparisons indicate event rain totals for all three disdrometers that were in reasonable agreement with a reference rain gauge. In a relative sense, hourly composite DSDs revealed that the JWD was more sensitive to small drops (,1 mm), while the PD appeared to severely underestimate small drops less than 0.76mm in diameter. The JWD and 2DVD measured comparable number concentrations of midsize drops (1-3mm) and large drops (3-5 mm), while the PD tended to measure relatively higher drop concentrations at sizes larger than 2.44mm in diameter. This concentration disparity tended to occur when hourly rain rates and drop counts exceeded 2.5mm/h and 400/min, respectively. Based on interactions with the PD manufacturer, the partially inhomogeneous laser beam is considered the cause of the PD drop count overestimation. PD drop fall speeds followed the expected terminal fall speed relationship quite well, while the 2DVD occasionally measured slower drops for diameters larger than 2.4mm, coinciding with events where wind speeds were greater than 4m/s. The underestimation of small drops by the PD had a pronounced effect on the intercept and shape of parameters of gamma-fitted DSDs, while the overestimation of midsize and larger drops resulted in higher mean values for PD integral rain parameters

Articulating and Stationary PARSIVEL Disdrometer Measurements in Conditions with Strong Winds and Heavy Rainfall

Abstract: The influence of strong winds on the quality of optical Particle Size Velocity (PARSIVEL) disdrometer measurements is examined with data from Hurricane Ike in 2008 and from convective thunderstorms observed during the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) in 2010. This study investigates an artifact in particle size distribution (PSD) measurements that has been observed independently by six stationary PARSIVEL disdrometers. The artifact is characterized by a large number concentration of raindrops with large diameters (>5 mm) and unrealistic fall velocities (<1 m s−1). It is correlated with high wind speeds and is consistently observed by stationary disdrometers but is not observed by articulating disdrometers (instruments whose sampling area is rotated into the wind). The effects of strong winds are further examined with a tilting experiment, in which drops are dripped through the PARSIVEL sampling area while the instrument is tilted at various angles, s...

Drop-Size Distributions in Thunderstorms Measured by Optical Disdrometers during VORTEX2

Abstract: When studying the influence of microphysics on the near-surface buoyancy tendency in convective thunderstorms, in situ measurements of microphysics near the surface are essential and those are currently not provided by most weather radars. In this study, the deployment of mobile microphysical probes in convective thunderstorms during the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) is examined. Microphysical probes consist of an optical Ott Particle Size and Velocity (PARSIVEL) disdrometer that measures particle size and fall velocity distributions and a surface observation station that measures wind, temperature, and humidity. The mobile probe deployment allows for targeted observations within various areas of the storm and coordinated observations with ground-based mobile radars. Quality control schemes necessary for providing reliable observations in severe environments with strong winds and high rainfall rates and particle discrimination schemes for distingu...

Backscatter Differential Phase—Estimation and Variability

Abstract: On the basis of simulations and observations made with polarimetric radars operating at X, C, and S bands, the backscatter differential phase d has been explored; d has been identified as an important polarimetric variable that should not be ignored in precipitation estimations that are based on specific differential phase KDP, especially at shorter radar wavelengths. Moreover, d bears important information about the dominant size of raindrops and wet snowflakes in the melting layer. New methods for estimating d in rain and in the melting layer are suggested.The method for estimatingdin rain is based on a modifiedversion of the ‘‘ZPHI’’ algorithm and provides reasonably robust estimates of d and KDP in pure rain except in regions where the total measured differential phase FDP behaves erratically, such as areas affected by nonuniform beam filling or low signal-to-noise ratio. The method for estimating d in the melting layer results in reliable estimates of d in stratiform precipitation and requires azimuthal averaging of radial profiles of FDP at high antenna elevations. Comparisons with large disdrometer datasets collected in Oklahoma and Germany confirm a strong interdependence between d and differential reflectivity ZDR. Because d is immune to attenuation, partial beam blockage, and radar miscalibration, the strong correlation between ZDR and d is of interest for quantitative precipitation estimation:dandZDRare differently affected by the particle size distribution(PSD) and thus may complement each other for PSD moment estimation. Furthermore, the magnitude of d can be utilized as an important calibration parameter for improving microphysical models of the melting layer.

Characterization of video disdrometer uncertainties and impacts on estimates of snowfall rate and radar reflectivity

Abstract: . Estimates of snow microphysical properties obtained by analyzing collections of individual particles are often limited to short timescales and coarse time resolution. Retrievals using disdrometer observations coincident with bulk measurements such as radar reflectivity and snowfall amounts may overcome these limitations; however, retrieval techniques using such observations require uncertainty estimates not only for the bulk measurements themselves, but also for the simulated measurements modeled from the disdrometer observations. Disdrometer uncertainties arise due to sampling and analytic errors and to the discrete, potentially truncated form of the reported size distributions. Imaging disdrometers such as the Snowflake Video Imager and 2-D Video Disdrometer provide remarkably detailed representations of snow particles, but view limited projections of their three-dimensional shapes. Particle sizes determined by such instruments underestimate the true dimensions of the particles in a way that depends, in the mean, on particle shape, also contributing to uncertainties. An uncertainty model that accounts for these uncertainties is developed and used to establish their contributions to simulated radar reflectivity and snowfall rate. Viewing geometry effects are characterized by a parameter, v, that relates disdrometer-observed particle size to the true maximum dimension of the particle. Values and uncertainties for v are estimated using idealized ellipsoidal snow particles. The model is applied to observations from seven snow events from the Canadian CloudSat/CALIPSO Validation Project (C3VP), a mid-latitude cold-season cloud and precipitation field experiment. Typical total uncertainties are 4 dB for reflectivity and 40–60% for snowfall rate, are highly correlated, and are substantial compared to expected uncertainties for radar and precipitation gauge observations. The dominant sources of errors are viewing geometry effects and the discrete, truncated form of the size distributions. While modeled Ze–S relationships are strongly affected by assumptions about snow particle mass properties, such relationships are only modestly sensitive to v owing to partially compensating effects on both the reflectivity and snowfall rate.

Optimum Estimation of Rain Microphysical Parameters From X-Band Dual-Polarization Radar Observables

Abstract: Modern polarimetric weather radars typically provide reflectivity, differential reflectivity, and specific differential phase shift, which are used in algorithms to estimate the parameters of the rain drop size distribution (DSD), the mean drop shape, and rainfall rate. A new method is presented to minimize the parameterization error using the Rayleigh scattering limit relations multiplied with a rational polynomial function of reflectivity-weighted raindrop diameter to approximate the Mie character of scattering. A statistical relation between the shape parameter of the DSD with the median volume diameter of raindrops is derived by exploiting long-term disdrometer observations. On the basis of this relation, new optimal estimators of rain microphysical parameters and rainfall rate are developed for a wide range of rain DSDs and air temperatures using X-band scattering simulations of polarimetric radar observables. Parameterizations of radar specific path attenuation and backscattering phase shift are also developed, which do not depend on this relation. The methodology can, in principle, be applied to other weather radar frequencies. A numerical sensitivity analysis shows that calibration bias and measurement noise in radar measurements are critical factors for the total error in parameters estimation, despite the low parameterization error (less than 5%). However, for the usual errors of radar calibration and measurement noise (of the order of 1 dB, 0.2 dB, and 0.3 $\hbox{deg}\ \hbox{km}^{-1}$ for reflectivity, differential reflectivity, and specific differential propagation phase shift, respectively), the new parameterizations provide a reliable estimation of rain parameters (typically less than 20% error).

Performance Evaluation of a New Dual-Polarization Microphysical Algorithm Based on Long-Term X-Band Radar and Disdrometer Observations

Abstract: Accurate estimation of precipitation at high spatial and temporal resolution of weather radars is an open problem in hydrometeorological applications. The use of dual polarization gives the advantage of multiparameter measurements using orthogonal polarization states. These measurements carry significant information, useful for estimating rain-path signal attenuation, drop size distribution (DSD), and rainfall rate. This study evaluates a new self-consistent with optimal parameterization attenuation correction and rain microphysics estimation algorithm (named SCOP-ME). Long-term X-band dual-polarization measurements and disdrometer DSD parameter data, acquired in Athens, Greece, have been used to quantitatively and qualitatively compare SCOP-ME retrievals of median volume diameter D0 and intercept parameter NW with two existing rain microphysical estimation algorithms and the SCOP-ME retrievals of rain rate with three available radar rainfall estimation algorithms. Error statistics for rain rate e...

A comparison of rainfall measurements from multiple instruments

Abstract: . Simultaneous observations of rainfall collected by a tipping bucket rain gauge (TBRG), a weighing rain gauge (WRG), an optical rain gauge (ORG), a present weather detector (PWD), a Joss–Waldvogel disdrometer (JWD), and a 2-D video disdrometer (2DVD) during January to October 2012 were analyzed to evaluate how accurately they measure rainfall and drop size distributions (DSDs). For the long-term observations, there were different discrepancies in rain amounts from six instruments on the order of 0% to 27.7%. The TBRG, WRG, and ORG have a good agreement, while the PWD and 2DVD record higher and the JWD lower rain rates when R > 20 mm h−1, the ORG agrees well with JWD and 2DVD, while the TBRG records higher and the WRG lower rain rates when R > 20 mm h−1. Compared with the TBRG and WRG, optical and impact instruments can measure the rain rate accurately in the light rain. The overall DSDs of JWD and 2DVD agree well with each other, except for the small raindrops (D 15 mm h−1. The small raindrops tend to be omitted in the more large-size raindrops due to the shadow effect of light. Therefore, the measurement accuracy of small raindrops in the heavy rainfall from 2DVD should be handled carefully.

In-Place Estimation of Wet Radome Attenuation at X Band

Abstract: The effect of wet radome attenuation is estimated on a French operational X-band weather radar deployed in the Maritime Alps of southeastern France. As the radar is deployed in a remote location, the reflectivity factor in the immediate vicinity of the radar is used as a proxy for rain rate at the radar and by extension, to the radome wetting. By means of intercomparison with a neighboring radar that lacks a radome, a wet radome correction is deduced. The correction is reasonably consistent with theoretical expectations and with other evaluations done, for example, via disdrometer. The improvement is evaluated by comparison to a Micro Rain Radar located under the point of comparison, and the impact on quantitative precipitation estimation (QPE) retrievals is positive. The intercomparison of such observations permits a routine means of monitoring radome attenuation.

A New Method for Identifying the Main Type of Solid Hydrometeors Contributing to Snowfall from Measured Size-Fall Speed Relationship

Abstract: This paper presents a new method for identifying the type of solid hydrometeor mainly contributing to snowfall from the measured size and fall speed data. The main type is determined from the relationship between measured size and fall speed by considering the contributions of various hydrometeor types to precipitation, including graupel, graupel-like snow, aggregates at different riming stages, and small particles such as single snow crystals. The mass flux of each hydrometeor, defined as the product of its mass and fall speed, is needed to evaluate its contribution; however, it is practically difficult to measure. In this study, we estimate mass flux from the empirical relationships between size and mass and between size and fall speed. The mass flux distribution in the size̶fall speed coordinates for all measured hydrometeors is found to accurately reflect the characteristics of types of hydrometeors and their contribution to observed precipitation. Considering these results, we introduce a new variable, the center of mass flux distribution (CMF), in the size̶fall speed coordinates. The CMF, which is the average of size and fall speed weighted by the mass flux, can be obtained in the same way as the center of gravity in mechanics. We believe that it indicates the size and fall speed of the principal hydrometeors among all particles in the observation period. This new method allows the quantitative identification of the main hydrometeor types from the locations of CMFs in the coordinates of size and fall speed. We verify this method by its application to different types of observed snowfall events. Although there is some ambiguity in estimating the mass flux, the method is expected to be useful for identifying the main hydrometeor types in snowfall events and for quantitatively interpreting returned radar power.

On the Estimation of Surface Runoff through a New Plot Scale Rainfall Simulator in Sardinia, Italy☆

Abstract: Artificial rainfall is widely used to study the surface runoff process but several problems are related to the reproducibility of natural rainstorms. A new rainfall simulator and a collection system were designed and tested in the laboratory and in the field. The rainfall simulator consists of four independent lines of low-cost pressure washing nozzles operated at a pressure of 80 mbar which number and position causes the rainfall intensity delivered on the plot. The spatial rainfall distribution and his intensity were measured with 63 rainfall gauges covering the whole plot The Joss Waldvogel Disdrometer was used to characterize the rainfall produced by the rainfall simulator. The drop size distribution was obtained. The drop size spectrum ranges from 0.25 mm to 3.3 mm and its shape is the same to that one produced by a natural rainfall. The rainfall intensity varies from approximately 31 to 62 mm/h and it is sufficiently spatially uniform (Christiansen's coefficient of uniformity is 0.62 to 0.75) over the plot. Field tests were carried out in on a grassy field with silt-loam soil in Orroli, Sardinia in July and August 2010. The values of the mean rainfall intensities obtained from field data are in accord with the laboratory values. The field site measurement includes the surface runoff, evaluated using a dedicated tipping bucket flow meter, and the soil water content measured throughout the field experiments. The results showed the good performance of this new rainfall simulator that offers the possibility to reproduce natural rainfall to gather parameters needed for hydrologic modeling. The entire designed system offers the possibility to carry out reliable measurements of the surface runoff under different rain intensities and also allows one to measure this on different temporal scales by taking into account the differen environmental conditions.

Theoretical and empirical scale dependency of Z-R relationships: Evidence, impacts, and correction

Abstract: Estimation of rainfall intensities from radar measurements relies to a large extent on power-laws relationships between rain rates R and radar reflectivities Z, i.e., Z = a*R^b. These relationships are generally applied unawarely of the scale, which is questionable since the nonlinearity of these relations could lead to undesirable discrepancies when combined with scale aggregation. Since the parameters (a,b) are expectedly related with drop size distribution (DSD) properties, they are often derived at disdrometer scale, not at radar scale, which could lead to errors at the latter. We propose to investigate the statistical behavior of Z-R relationships across scales both on theoretical and empirical sides. Theoretically, it is shown that claimed multifractal properties of rainfall processes could constrain the parameters (a,b) such that the exponent b would be scale independent but the prefactor a would be growing as a (slow) power law of time or space scale. In the empirical part (which may be read independently of theoretical considerations), high-resolution disdrometer (Dual-Beam Spectropluviometer) data of rain rates and reflectivity factors are considered at various integration times comprised in the range 15 s - 64 min. A variety of regression techniques is applied on Z-R scatterplots at all these time scales, establishing empirical evidence of a behavior coherent with theoretical considerations: a grows as a 0.1 power law of scale while b decreases more slightly. The properties of a are suggested to be closely linked to inhomogeneities in the DSDs since extensions of Z-R relationships involving (here, strongly nonconstant) normalization parameters of the DSDs seem to be more robust across scales. The scale dependence of simple Z = a*R^b relationships is advocated to be a possible source of overestimation of rainfall intensities or accumulations. Several ways for correcting such scaling biases (which can reach >15-20% in terms of relative error) are suggested. Such corrections could be useful in some practical cases where Z-R scale biases are significant, which is especially expected for convective rainfall.

Multi-technique observations on precipitation and other related phenomena during cyclone Aila at a tropical location

Abstract: A severe cyclone, named Aila, passed over Kolkata on 25 May 2009. The strong convective activities manifested through rainfall during the cyclone were studied with multi-technique observations involving micro rain radar, disdrometer, rain gauges, and a Ku-band satellite signal receiving system. A number of features of precipitation, namely the presence of large rain drops, the large vertical extent of the precipitating layer, an unusual enhancement of cross-polar component, and strong scintillations of the co-polar component of the satellite signal due to strong turbulences associated with the cyclone, were observed. This study leads to a more comprehensive understanding of the precipitation associated with a cyclonic storm.

Diurnal variation of slant path Ka-band rain attenuation at four tropical locations in India

Abstract: Rain attenuation is very severe at Ka and higher frequencies especially in tropical regions. The conventional fade mitigation techniques are not be able to mitigate this severe fade and hence, suitable diversity technique is required for this purpose. In this paper, rain characteristics and slant path rain attenuation at 30 GHz using synthetic storm technique has been presented. Three years of high resolution rain rate data obtained from disdrometer at four tropical and equatorial locations in India have been utilized. The results indicate that the fade margin requirement is very high (above 50 dB at 30 GHz) for 99.99% link availability for these locations. However, the diurnal analysis shows that in most of the places, the rain occurrence is much less in morning/early-morning hours and therefore, the fade margin requirement will be considerably lower (around 30 dB at 30 GHz) in the morning hours compared to the afternoon hours. The result indicates the suitability of using selective time period for high link availability data communication over the locations studied.

Error in the Sampling Area of an Optical Disdrometer: Consequences in Computing Rain Variables

Abstract: The aim of this study is to improve the estimation of the characteristic uncertainties of optic disdrometers in an attempt to calculate the efficient sampling area according to the size of the drop and to study how this influences the computation of other parameters, taking into account that the real sampling area is always smaller than the nominal area. For large raindrops (a little over 6 mm), the effective sampling area may be half the area indicated by the manufacturer. The error committed in the sampling area is propagated to all the variables depending on this surface, such as the rain intensity and the reflectivity factor. Both variables tend to underestimate the real value if the sampling area is not corrected. For example, the rainfall intensity errors may be up to 50% for large drops, those slightly larger than 6 mm. The same occurs with reflectivity values, which may be up to twice the reflectivity calculated using the uncorrected constant sampling area. The - relationships appear to have little dependence on the sampling area, because both variables depend on it the same way. These results were obtained by studying one particular rain event that occurred on April 16, 2006.

A new algorithm for classification of tropical convective precipitating clouds over north- eastern region of india

Abstract: The North East (NE) region of India is prone to severe thunderstorm during pre monsoon period. These premonsoon precipitations are measured at Guwahati (26 o 17’ N, 91 o 77’ E) using a laser based particle size and velocity (PARSIVEL) disdrometer from 15 April to 31 May 2010 under a national field campaign named Severe Thunderstorm Observational and Regional Modeling (STORM). For the improved understanding of these premonsoon thunderstorm a new cloud classification hybrid algorithm is developed based on thermodynamics and microphysical characterstics of precipitation. This algorithm can classify the premonsoon precipitaitng clouds into thuderstrom (TS), non- thuderstrom (NTS) and futher into convective & stratiform cloud fractions based on thermodynamic indices and rain integral parameters. The observation results showed that raindrops of Small and mid (large) size are having same concentration in convective (stratiform) regions of both TS and NTS precipitations. There is a large spread in the mean diameter (Dm) and total concentration (NT) at higher rainrate of TS than NTS. The coefficient (A) of the radar reflectivity and rainrate relation (Z-R) is found to be smaller for TS than NTS. There is a significant difference in Raindrop concentration in stratiform, convective regions of TS and NTS precipitation.

Measurements of DSD Second Moment Based on Laser Extinction

Abstract: Using a technique recently developed for estimating the density of surface dust dispersed during a rocket landing, measuring the extinction of a laser passing through rain (or dust in the rocket case) yields an estimate of the 2nd moment of the particle cloud, and rainfall drop size distribution (DSD) in the terrestrial meteorological case. With the exception of disdrometers, instruments that measure rainfall make in direct measurements of the DSD. Most common of these instruments are the rainfall rate gauge measuring the 1 1/3 th moment, (when using a D(exp 2/3) dependency on terminal velocity). Instruments that scatter microwaves off of hydrometeors, such as the WSR-880, vertical wind profilers, and microwave disdrometers, measure the 6th moment of the DSD. By projecting a laser onto a target, changes in brightness of the laser spot against the target background during rain, yield a measurement of the DSD 2nd moment, using the Beer-Lambert law. In order to detect the laser attenuation within the 8-bit resolution of most camera image arrays, a minimum path length is required, depending on the intensity of the rainfall rate. For moderate to heavy rainfall, a laser path length of 100 m is sufficient to measure variations in optical extinction using a digital camera. A photo-detector could replace the camera, for automated installations. In order to spatially correlate the 2nd moment measurements to a collocated disdrometer or tipping bucket, the laser's beam path can be reflected multiple times using mirrors to restrict the spatial extent of the measurement. In cases where a disdrometer is not available, complete DSD estimates can be produced by parametric fitting of DSD model to the 2nd moment data in conjunction with tipping bucket data. In cases where a disdrometer is collocated, the laser extinction technique may yield a significant improvement to insitu disdrometer validation and calibration strategies

Direct and indirect measurement of rain drop size distributions using an acoustic water tank disdrometer

Abstract: Several rain drop size distribution (DSD) point measurement technologies exist, but all are unable to sample either short timescales or the large drop tail of the DSD due to inherent instrumental limitations. The development of an acoustic water tank disdrometer (AWTD) is described, which improves the sampling statistics by increasing the catchment area. This is achieved by distinguishing individual drops, locating them on the surface of the tank then converting the impact pressure into a drop size. Wavelet decomposition is used to distinguish the broadband, short duration impact events and a fast multilateration method is used to position the drop. Issues relating to the different types of noise are also investigated and mitigated. Also, further work on inverting the measured acoustic intensity into a DSD, by fitting sampling distributions, is presented. Six months of data were collected in the Eastern UK. The AWTD then converted the data into DSDs and the results were compared to a commercially available co-located laser precipitation monitor. The sampling errors are far lower due to the increased catchment size, and hence the large drop sized tail of the DSD is greatly improved. DSD results compare favourably to other disdrometers for drop diameters greater than 1.8 mm. Below this size individual drops become increasingly difficult to detect and are underestimated.

Recent measurements and inferences of drop shapes in rain from 2D-video disdrometer and polarimetric radar

Abstract: Two events with contrasting drop fall velocity versus drop shape characteristics are discussed. The events were captured by two collocated 2D video disdrometers, as well as a C-band polarimetric radar, 15 km away. Contoured images of the drops as well as their fall velocities from the disdrometer measurements, together with the analysis of simultaneously recorded radar observations, point towards the possibility of mixed mode drop oscillations including the horizontal mode occurring for the second event. The first event --which occurred 7 days prior-did not show any unusual characteristics, and represents the majority of the recorded events. Results are presented and discussed.

Rain-induced erosion: a study in badlands national park

Abstract: Factors that may influence the severity of erosion at a given location include soil composition, annual rainfall, climate, and vegetation type and coverage. This study addresses the ongoing erosion occurring in the Badlands National Park (BNP) in South Dakota. The research work is aimed to determine a better measurement of the rain erosion in the BNP using a laser optical disdrometer and erosion scales. A Parsivel disdrometer instrument was installed during May-September, 2011, to measure raindrop sizes and their fall velocities. Using these variables, we calculated the rainfall intensities and kinetic energy fluxes of individual rain events, the main parameters needed to estimate the rain in duced erosion rate. Rain events were categorized as light, moderate, and heavy based on rainfall intensity, as well as either convective or stratiform based on radar data. Heavy events that also produced hail were considered an additional category. Comparisons between kinetic energy fluxes associated with hail- and non-hail producing events showed a clear separation: an average of 1574 J/m 2

Observations and Comparative Analysis of the Precipitation in the Beiluhe Region,Tibetan Plateau

Abstract: According to the precipitation observed by Thies disdrometer,T-200B and TE525 rain gauges from November 2011 to September 2012 in Beiluhe,Tibetan Plateau.It is found that during the monitoring period precipitation in the study area was approximately 466.56 mm,much higher than that in the previous years.It means that precipitation in 2012 was a maximum.Comparing three records observed by the three instruments,one can see that: For observing solid precipitation,the observation of TE525 rain gauge is the worst.The precipitation observed by T-200B is slightly smaller than that by Thies disdrometer.Solid precipitation observed by T-200B was corrected with Smith's and Jimmy's correction formulas,respectively.After correcting,it is found that precipitation observed by Thies disdrometer has a good consistency with that from the Jimmy corrected T-200B.For measuring sleet,Thies disdrometer obtains higher precipitation owing to misreading the type of precipitation and the diameter of particles.Anyway,the liquid precipitations observed by the three instruments are in good agreement.However,when the hourly rainfall is higher than 2 mm,precipitation obtained from the disdrometer is about 20% smaller than that from the rain gauge.The impact of blowing snow on the observation of Thies disdrometer was analyzed also.The preliminary view is that the particle diameter of the blowing snow is mainly less than 0.5 mm.

Estimating the Concentration of Large Raindrops from Polarimetric Radar and Disdrometer Observations

Abstract: Estimation of rainfall integral parameters, including radar observables, and empirical relations between them are sensitive to the truncation of the drop size distribution (DSD), particularly at the large drop end. The sensitivity of rainfall integral parameters to the maximum drop diameter (D(sub max)) is exacerbated at C‐band since resonance effects are pronounced for large drops in excess of 5 mm diameter (D). Due to sampling limitations, it is often difficult to reliably estimate D(sub max) with disdrometers. The resulting uncertainties in D(sub max0 potentially increase errors in radar retrieval methods, particularly at C‐band, that rely on disdrometer observations for DSD input to radar models. In fact, D(sub max) is typically an assumed DSD parameter in the development of radar retrieval methods. Because of these very uncertainties, it is difficult to independently confirm disdrometer estimates of D(sub max) with polarimetric radar observations. A couple of approaches can be taken to reduce uncertainty in large drop measurement. Longer integration times can be used for the collection of larger disdrometer samples. However, integration periods must be consistent with a radar resolution volume (RRV) and the temporal and spatial scales of the physical processes affecting the DSD therein. Multiple co‐located disdrometers can be combined into a network to increase the sample size within a RRV. However, over a reasonable integration period, a single disdrometer sample volume is many orders of magnitudes less than a RRV so it is not practical to devise a network of disdrometers that has an equivalent volume to a typical RRV. Since knowledge of DSD heterogeneity and large drop occurrence in time and space is lacking, the specific accuracy or even general representativeness of disdrometer based D(sub max) and large drop concentration estimates within a RRV are currently unknown. To address this complex issue, we begin with a simpler question. Is the frequency of occurrence of large rain drops (D > 5 mm) in disdrometer observations, either stand alone or networked, generally representative and consistent with polarimetric radar observations? We first show from simulations that the concentration of large (D > 5 mm) rain drops (N(sub T5)) can be estimated from polarimetric observations of specific differential phase (K(sub dp)) and differential reflectivity (Z(sub dr)), N(sub T5)=F(K(sub dp),Z(sub dr)), or horizontal reflectivity (Z(sub h)) and Z(sub dr), N(sub T5)=(Z(sub h),Z(sub dr)). We assess the error associated with polarimetric retrieval of N(sub T5), including sensitivity to D(sub max) parameterization assumptions and measurement error in the radar simulations. Polarimetric measurements at S‐band and C‐band will then be used to retrieve estimates of N(sub T5) and compared to disdrometer estimates of N(sub T5). After careful consideration of retrieval error, we will check consistency between disdrometer and polarimetric radar estimates of N(sub T5) and the frequency of occurrence of large rain drops in a variety of precipitating regimes using data from NASA's Global Precipitation Measurement (GPM) Ground Validation (GV) program, including field campaigns such as MC3E (Oklahoma) and IFloodS (Iowa) and extended measurements over Huntsville, Alabama and NASA Wallops Flight Facility in coastal Virginia.

연직강우레이더와 광학우적계 관측자료를 이용한 2012년 여름철 남해안 강우사례 분석

Abstract: To investigate properties of cloud and rainfall occurred at Boseong on 10 July 2012, Raindrop Size distributions (RSDs) and other parameters were analyzed using observation data collected by Micro Rain Radar (MRR) and PARticle SIze and VELocity (PARSIVEL) disdrometer located in the National center for intensive observation of severe weather at Boseong in the southwest of the Korean peninsula. In addition, time series of RSD parameters, relationship between reflectivity-rain rate, and vertical variation of rain rates-fall velocities below melting layer were examined. As a result, good agreements were found in the reflectivity-rain rate time series as well as their power relationships between MRR and PARSIVEL disdrometer. The rain rate was proportional to reflectivity, mean diameter, and inversely proportional to shape (μ), slope (Λ), intercept (N0) parameter of RSD. In comparison of the RSD, as rain rate was increased, the slope of RSD became less steep and the mean diameter became larger. Also, it was verified that reflectivities are classified in three categories (Category 1: Z (reflectivity) > 40 dBZ, Category 2: 30 dBZ 0 ) parameter of RSD were increased. We expected that these results will lead to better understanding of microphysical process in convective rainfall system occurred during short-term period over Korean peninsula.

Rainfall variability in the Netherlands from radars, rain gauges, and disdrometers

Abstract: Chapter 1 . This thesis presents studies on the variability of precipitation in the Netherlands from datasets collected by radars, rain gauges and disdrometers. Accurate rainfall estimates are highly relevant in hydrology, meteorology and climatology as precipitation has a large impact on society. Precipitation has been studied extensively in the past, although it is impossible to describe all processes and behavior involved. This thesis attempts to add to the knowledge on precipitation. In the first chapter a short overview of rainfall variability at different scales is presented together with the most common instruments for measuring precipitation. Chapter 2 . The spatial variability of daily rainfall accumulations is studied. Ninety-day averaged semi-variograms are created based on a 30-year data set gathered by automatic stations operated by the Royal Netherlands Meteorological Institute (KNMI). This is complemented by a one-year dataset of 10 gauges within a 5 km radius around CESAR (Cabauw Experimental Site for Atmospheric Research) in the center of the Netherlands. It is shown that it is possible to derive an average semi-variogram that describes the climatology of daily precipitation for each day of the year. Chapter 3 . The study of chapter 2 is extended by investigating accumulation intervals shorter than daily scales. These are at 12, 8, 6, 4, 3, 2 and 1-hour accumulation intervals. It is shown that at shorter temporal scales the behavior of semi-variograms of precipitation still shows a clear seasonal trend. At hourly and two-hourly accumulation intervals the signal of the range becomes fairly constant during the summer due to the limited accumulation period, the frequent occurrence of convective precipitation, and measurement errors. This illustrates the lower limit of using cosine functions to describe variogram parameters. By fitting a power-law function through the different cosine parameters it is possible to describe the semi-variance of precipitation at scales between 1 and 24 hours using a limited set of equations. Chapter 4 . Different sources of error affecting rainfall estimates by weather radar are identified. By focussing on precipitation near a C-band radar some of these sources are reduced, which allows to focus on a limited set of error sources. These are radar calibration, ground clutter, wet radome attenuation and variations in rain drop size distribution. An event that caused high precipitation amounts in a band along the center of the Netherlands and more than 50~mm near the radar between the 25th and the 27th of August 2010 is studied. Without any correction and by applying a standard Marshall-Palmer Z-R relation the radar is found to underestimate by approximately 50% with respect to the rain gauge measurements. Using the sun for calibration a correction of 1 dB is applied. Clutter is corrected by subtracting a clear sky clutter map as this proves to provide better results than the standard doppler filter. Wet-radome attenuation is corrected by looking at the amount of attenuation at a known strong clutter pixel near the radar. Disdrometer data near the radar are used to derive accurate Z-R relations specific to the precipitation of the event. These corrections combined provide very promising results with a slight overestimation of the quantitative precipitation estimates (QPE) from the radar by 5 to 8%. Chapter 5 . An extensive dataset of 195 precipitation events measured by an X-band radar (SOLIDAR) is used to study precipitation at a high spatial resolution of 120 m and a high temporal resolution of 16 s. This study shows the benefit of using such high resolution X-band radars over flat terrain. The errors in the radar measurements are first assessed and corrected as well as possible by considering different techniques. These errors are calibration, ground clutter and attenuation. Finally, five strongly different precipitation events are studied in detail to illustrate the strengths and weaknesses of the X-band weather radar. Chapter 6 . The variability and possible measurement methods of precipitation have been studied. It was shown that precipitation spatial and temporal variability has a clear statistical signal by analyzing variograms for different accumulation intervals. Weather radars were also shown to be able to give good estimates of precipitation at ground level as well as detailed information on the spatial variability. Some recommendations are given to perform follow up studies. For chapters 2 & 3 it is recommended to use a larger and more detailed dataset, which also incorporates Belgian and German data. This would allow the study of anisotropy in the semi-variograms as well as extending the analysis to accumulation times shorter than 1 hour and longer than 24 hours. For chapter 4 it is recommended to study pixels located further away from the radar. While other error sources would become more pronounced it would be possible to study the applicability of the proposed corrections at longer ranges. Studying the wet-radome attenuation with several strong clutter pixels near the radar would allow the study of wind-effects on wet-radome attenuation, possibly allowing corrections using measurements of (Doppler) wind-speed and direction. Finally, in chapter 5 it is recommended to study the successor of SOLIDAR, IDRA, which is currently operational at CESAR. This radar is a polarimetric radar, allowing a more detailed study of precipitation together with the data from other instruments at this location and the C-band radar of KNMI, which is located close to this location at approximately 23 km.

Silicon Strain Gauge Load Cell for Weighting Disdrometer

Abstract: In this paper, the usability of a compact silicon strain gauge load cell in a weighting disdrometer for measuring the impact load of afalling raindrop is introduced for application in a multi-meteorological sensor. The silicon strain gauge load cell is based on thepiezoresistive effect, which has a high linearity output from the momentum of the raindrop and the simplicity of signal processing. Theweighting disdrometer shows a high sensitivity of 7.8 mV/g in static load measurement when the diaphragm thickness of the load cell is250
um. Keywords : Silicon strain gauge, Center boss diaphragm, Load cell, Disdrometer 1 Dept. of Sensor and Display Eng., Kyungpook National University 2 School of Electronics Eng., Kyungpook National University1370 Sangyik-dong, Buk-gu, Daegu 702-701, Korea 3 Daeyang Instrument Co., LTD. 503 Sinpyeong-dong, Saha-gu, Busan 604-030, Korea 4 Dept. Of Computer Eng., Yeungnam University 280 Daehak-Ro, Gyeongsan-si, Gyeongbuk-do 712-749, Korea + Corresponding author: sychoi@ee.knu.ac.kr(Received : Sep. 6, 2013, Revised : Sep. 24, 2013, Accepted : Sep. 24, 2013)This is an Open Access article distributed under the terms of the Creative CommonsAttribution Non-Commercial License(http://creativecommons.org/licenses/by-nc/3.0)which permits unrestricted non-commercial use, distribution, andreproduction in any medium, provided the original work is properly cited.

Analysis of Summer Rainfall Case over Southern Coast Using MRR and PARSIVEL Disdrometer Measurements in 2012

Abstract: To investigate properties of cloud and rainfall occurred at Boseong on 10 July 2012, Raindrop Size distributions (RSDs) and other parameters were analyzed using observa- tion data collected by Micro Rain Radar (MRR) and PARticle SIze and VELocity (PAR- SIVEL) disdrometer located in the National center for intensive observation of severe weather at Boseong in the southwest of the Korean peninsula. In addition, time series of RSD parame- ters, relationship between reflectivity-rain rate, and vertical variation of rain rates-fall veloci- ties below melting layer were examined. As a result, good agreements were found in the reflectivity-rain rate time series as well as their power relationships between MRR and PAR- SIVEL disdrometer. The rain rate was proportional to reflectivity, mean diameter, and inversely proportional to shape (µ), slope (Λ), intercept (N0) parameter of RSD. In comparison of the RSD, as rain rate was increased, the slope of RSD became less steep and the mean diameter became larger. Also, it was verified that reflectivities are classified in three categories (Cate- gory 1: Z (reflectivity) > 40 dBZ, Category 2: 30 dBZ < Z < 40 dBZ, Category 3: Z < 30 dBZ). As reflectivity was increased, rain rate was intensified and larger raindrops were existed, while reflectivity was decreased, shape (µ), slope (Λ), intercept (N0) parameter of RSD were increased. We expected that these results will lead to better understanding of microphysical process in convective rainfall system occurred during short-term period over Korean peninsula.

Mobile Radar Network Measurements for Flood Applications During the Field Campaign of HydroRad Project

Abstract: The main aim of the HydroRad European project was to develop an innovative dual-polarization X-band mini-radar system and software support tools like rainfall estimation, nowcasting, precipitation classification and integration with hydrological and meteorological models for the use in weather and flood applications. These mini-radars are low cost, easy to deploy and, thus, ideal for the setup of radar networks to cover areas with complex terrain. In order to test the system an experimental campaign took place during autumn 2011 in Moldova. A network of three mini-radars was setup and tested against an advanced mobile polarimetric radar (XPol) in the center of the network and in-situ rain measurements from a video disdrometer and raingauges. Original polarimetric algorithms for attenuation correction and rainfall estimation were applied and their results were compared to the reference in-situ data for moderate widespread and intense convective rain events. The results show that mini-radars can produce high quality and accurate rain fields in difficult to cover complex terrain areas.