Showing papers on "Disdrometer published in 2004"

Drop Size Distribution Retrieval with Polarimetric Radar: Model and Application

Abstract: Polarimetric radar measurements are used to retrieve properties of raindrop distributions. The procedure assumes that drops are represented by a gamma distribution and retrieves the governing parameters from an empirical relation between the distribution shape and slope parameters and measurements of radar reflectivity and differential reflectivity. Retrieved physical characteristics of the drop size distribution (DSD) were generally well matched with disdrometer observations. The method is applied to select storms to demonstrate utility. Broad DSDs were determined for the core (high reflectivity) regions of thunderstorms. Largest drop median volume diameters were at the leading edge of the storm core and were displaced slightly downwind from updrafts. Rainy downdrafts exhibited what are believed to be equilibrium DSDs in which breakup and accretion are roughly in balance. DSDs for stratiform precipitation were dominated by relatively large drops. Median volume diameters at the ground were closely related to the intensity of an overlying bright band. The radar measurements suggest that, although DSDs in stratiform rain were also broad and nearly constant in the rain layer, they were not at equilibrium but were merely steady. DSD invariance is attributed to small total drop numbers, which result in few collisions.

Polarimetric Radar Estimators Based on a Constrained Gamma Drop Size Distribution Model

Abstract: Raindrop size distribution (DSD) retrieval from remote radar measurements or from in situ disdrometer measurements is an important area of research. If the shape (μ) and slope (Λ) of a three-parameter gamma distribution n(D) = N0Dμ exp(−ΛD) are related to one another, as recent disdrometer measurements suggest, the gamma DSD model is simplified to a two-parameter DSD, that is, a constrained gamma DSD. An empirical relation between the μ and Λ was derived using moments estimated from video-disdrometer measurements. Here, the effects of DSD truncation on a μ and Λ relation were analyzed. It was shown that characteristic size and variance of size of a constrained gamma DSD depend only on the shape parameter μ. Assuming that a constrained gamma DSD is valid, S-band polarimetric radar–based estimators for rain rate, median volume diameter, specific propagation phase, attenuation, and differential attenuation were derived. The radar-based estimators were used to obtain the spatial distribution of DSD p...

High-Resolution Rainfall Estimation from X-Band Polarimetric Radar Measurements

Abstract: The paper presents a rainfall estimation technique based on algorithms that couple, along a radar ray, profiles of horizontal polarization reflectivity (ZH), differential reflectivity (ZDR), and differential propagation phase shift (ΦDP) from X-band polarimetric radar measurements. Based on in situ raindrop size distribution (DSD) data and using a three-parameter “normalized” gamma DSD model, relationships are derived that correct X-band reflectivity profiles for specific and differential attenuation, while simultaneously retrieving variations of the normalized intercept DSD parameter (Nw). The algorithm employs an iterative scheme to intrinsically account for raindrop oblateness variations from equilibrium condition. The study is facilitated from a field experiment conducted in the period October–November 2001 in Iowa City, Iowa, where observations from X-band dual-polarization mobile radar (XPOL) were collected simultaneously with high-resolution in situ disdrometer and rain-gauge rainfall meas...

Comparison of Polarimetric Radar Drop Size Distribution Retrieval Algorithms

Abstract: Recently, two physically based algorithms, the “beta” (β) method and the “constrained-gamma” method, have been proposed for retrieving the governing parameters of the gamma drop size distribution (DSD) from polarimetric radar measurements. The β method treats the drop axis ratio as a variable and computes drop shape and DSD parameters from radar reflectivity (Z), differential reflectivity (ZDR), and specific differential phase (KDP). The constrained-gamma method assumes that the axis ratio relation is fixed and computes DSD parameters from reflectivity, differential reflectivity, and an empirical relation between the DSD slope and shape parameters. In this paper, the two approaches are evaluated by comparing retrieved rain DSD parameters with disdrometer observations and examining derived fields for consistency. Error effects on the β method retrievals are analyzed. The β approach is found to be sensitive to errors in KDP and to be inconsistent with observations. Large retrieved β values are foun...

Raindrop Size Distribution and Radar Parameters at Cape Verde

Abstract: Precipitation measurement using passive or active microwaves from space- or ground-based radar involves hypotheses about the raindrop size distribution (DSD). A universal knowledge of DSD characteristics is needed. A 4-yr dataset collected with a disdrometer at Dakar, Senegal, on the Atlantic coast of West Africa is used to analyze the DSD at the end of the continental trajectory of Sahelian squall lines. The DSDs are stratified in eight rain-rate classes and are fitted to analytical distributions. The shape of the averaged DSDs is found to be very similar from one year to the next. From rain rates R higher than about 20 mm h−1, the slope of the DSDs tends toward a constant value. The coefficients of the Z–R relation, between the radar reflectivity factor Z and R, are different for convective and stratiform parts of the squall lines. However, because the Z–R relations for convective rain intersect the relation for stratiform rain for high rates, it is suggested that using a single Z–R relation en...

Uncertainties in Oceanic Radar Rain Maps at Kwajalein and Implications for Satellite Validation

Abstract: The Kwajalein, Marshall Islands, Tropical Rainfall Measuring Mission (TRMM) ground validation radar has provided a multiyear three-dimensional radar dataset at an oceanic site. Extensive rain gauge networks are not feasible over the ocean and, hence, are not available to aid in calibrating the radar or determining a conversion from reflectivity to rain rate. This paper describes methods used to ensure the calibration and allow the computation of quantitative rain maps from the radar data without the aid of rain gauges. Calibration adjustments are made by comparison with the TRMM satelliteborne precipitation radar. The additional steps required to convert the calibrated reflectivity to rain maps are the following: correction for the vertical profile of reflectivity below the lowest elevation angle using climatological convective and stratiform reflectivity profiles; conversion of reflectivity ( Z) to rain rate (R) with a relationship based on disdrometer data collected at Kwajalein, and a gap-filling estimate. The time series of rain maps computed by these procedures include low, best, and high estimates to frame the estimated overall uncertainty in the radar rain estimation. The greatest uncertainty of the rain maps lies in the calibration of the radar (630%). The estimation of the low-altitude vertical profile of reflectivity is also a major uncertainty ( 615%). The Z‐R and data-gap uncertainties are relatively minor (65% or less). These uncertainties help to prioritize the issues that need to be addressed to improve quantitative rainfall mapping over the ocean and provide useful bounds when comparing radar-derived rain estimates with other remotely sensed measures of oceanic rain (such as from satellite passive microwave sensors).

An Experimental Study of Small-Scale Variability of Radar Reflectivity Using Disdrometer Observations

Abstract: Analysis of data collected by four disdrometers deployed in a 1-km2 area is presented with the intent of quantifying the spatial variability of radar reflectivity at small spatial scales. Spatial variability of radar reflectivity within the radar beam is a key source of error in radar-rainfall estimation because of the assumption that drops are uniformly distributed within the radar-sensing volume. Common experience tells one that, in fact, drops are not uniformly distributed, and, although some work has been done to examine the small-scale spatial variability of rain rates, little experimental work has been done to explore the variability of radar reflectivity. The four disdrometers used for this study include a two-dimensional video disdrometer, an X-band radar-based disdrometer, an impact-type disdrometer, and an optical spectropluviometer. Although instrumental differences were expected, the magnitude of these differences clouds the natural variability of interest. An algorithm is applied to ...

A Piezoelectrical Rain Gauge for Application on Buoys

Abstract: Rain gauge systems are required to measure rainfall data on buoys at oceanic sites that are not suited for conventional rain sensors. A piezoelectrical rain gauge has been developed for use on buoys, to provide rain measurements just above the sea surface. Based on the piezoelectric effect, each drop is measured separately with regard to its size, which is derived from the momentum transfer function of the sensor. Thus, the characteristics of rain—for example, the momentum flux, the drop intensity, and the drop size distribution—can be determined. In addition to laboratory investigations, showing the principal applicability of spherical piezoelectrical ceramics for rain detection, and the calibration, a field experiment had been conducted, comparing the new rain sensor results with those of other conventionally used rain gauges (Joss–Waldvogel disdrometer, FM–CW Doppler radar, and a ship rain gauge). The results are in good agreement. Such devices are needed to obtain a better understanding of ra...

Determining reflectivity measurement error from serial measurements using paired disdrometers and profilers

Abstract: [1] Serial reflectivity measurements from paired instruments are examined during two field campaigns in order to examine the precision of the measurements. The instruments studied are two collocated Joss-Waldvogel disdrometers (JWD) at Wallops Island, VA and two collocated profilers deployed at Ji-Parana, Brazil during Tropical Rainfall Measuring Mission (TRMM) Large-Scale Biosphere-Atmosphere Experiment. Differencing the measured reflectivity from the instrument pairs eliminated most of the temporal and large-scale precipitation variability, reducing the error fluctuations to those of the instrument precision plus fluctuations due to precipitation variability over the small differences in sample volume and distances between the instruments. For both pairs of calibrated instruments we found that the observed time-series of one-minute dBZ differences were not autocorrelated and exhibited a Gaussian-like distribution. Consequently, the difference time-series could be meaningfully characterized by their standard statistics, including the rms difference or standard deviation, and the standard error about the mean. While the disdrometer pair exhibited an rms difference of 2.1 dBZ, a standard error about the mean of less than 0.1 dBZ for the 12-hour rain event was achieved. The profiler pair exhibited an rms difference of 0.4 dBZ, with a standard error of only 0.05 dBZ for the 90-minute stratiform rain event. Since it is currently difficult to routinely calibrate radars in an absolute sense to better than 1–3 dBZ, the precisions of a few tenths of a dBZ obtained here suggest the potential for substantially improving these calibrations, and open the door to examination of subtle sampling and stability effects.

Toward a Global Map of Raindrop Size Distributions. Part I: Rain-Type Classification and Its Implications for Validating Global Rainfall Products

Abstract: Variability in the global distribution of precipitation is recognized as a key element in assessing the impact of climate change for life on earth. The response of precipitation to climate forcings is, however, poorly understood because of discrepancies in the magnitude and sign of climatic trends in satellite-based rainfall estimates. Quantifying and ultimately removing these biases is critical for studying the response of the hydrologic cycle to climate change. In addition, estimates of random errors owing to variability in algorithm assumptions on local spatial and temporal scales are critical for establishing how strongly their products should be weighted in data assimilation or model validation applications and for assigning a level of confidence to climate trends diagnosed from the data. This paper explores the potential for refining assumed drop size distributions (DSDs) in global radar rainfall algorithms by establishing a link between satellite observables and information gleaned from regional validation experiments where polarimetric radar, Doppler radar, and disdrometer measurements can be used to infer raindrop size distributions. By virtue of the limited information available in the satellite retrieval framework, the current method deviates from approaches adopted in the ground-based radar community that attempt to relate microphysical processes and resultant DSDs to local meteorological conditions. Instead, the technique exploits the fact that different microphysical pathways for rainfall production are likely to lead to differences in both the DSD of the resulting raindrops and the three-dimensional structure of associated radar reflectivity profiles. Objective rain-type classification based on the complete three-dimensional structure of observed reflectivity profiles is found to partially mitigate random and systematic errors in DSDs implied by differential reflectivity measurements. In particular, it is shown that vertical and horizontal reflectivity structure obtained from spaceborne radar can be used to reproduce significant differences in Zdr between the easterly and westerly climate regimes observed in the Tropical Rainfall Measuring Mission Large-scale Biosphere‐Atmosphere (TRMM-LBA) field experiment as well as the even larger differences between Amazonian rainfall and that observed in eastern Colorado. As such, the technique offers a potential methodology for placing locally observed DSD information into a global framework.

Toward a Global Map of Raindrop Size Distributions

Abstract: Variability in the global distribution of precipitation is recognized as a key element in assessing the impact of climate change for life on earth. The response of precipitation to climate forcings is, however, poorly understood because of discrepancies in the magnitude and sign of climatic trends in satellite-based rainfall estimates. Quantifying and ultimately removing these biases is critical for studying the response of the hydrologic cycle to climate change. In addition, estimates of random errors owing to variability in algorithm assumptions on local spatial and temporal scales are critical for establishing how strongly their products should be weighted in data assimilation or model validation applications and for assigning a level of confidence to climate trends diagnosed from the data. This paper explores the potential for refining assumed drop size distributions (DSDs) in global radar rainfall algorithms by establishing a link between satellite observables and information gleaned from regional validation experiments where polarimetric radar, Doppler radar, and disdrometer measurements can be used to infer raindrop size distributions. By virtue of the limited information available in the satellite retrieval framework, the current method deviates from approaches adopted in the ground-based radar community that attempt to relate microphysical processes and resultant DSDs to local meteorological conditions. Instead, the technique exploits the fact that different microphysical pathways for rainfall production are likely to lead to differences in both the DSD of the resulting raindrops and the three-dimensional structure of associated radar reflectivity profiles. Objective rain-type classification based on the complete three-dimensional structure of observed reflectivity profiles is found to partially mitigate random and systematic errors in DSDs implied by differential reflectivity measurements. In particular, it is shown that vertical and horizontal reflectivity structure obtained from spaceborne radar can be used to reproduce significant differences in Z(sub dr) between the easterly and westerly climate regimes observed in the Tropical Rainfall Measuring Mission Large-scale Biosphere-Atmosphere (TRMM-LBA) field experiment as well as the even larger differences between Amazonian rainfall and that observed in eastern Colorado. As such, the technique offers a potential methodology for placing locally observed DSD information into a global framework.

Investigation of rainfall microstructure and variability using vertically pointing radar and disdrometer

Abstract: In this study variability in the drop size distribu- tion inside an area of 200 by 600 m is analyzed with a tempo- ral resolution of 30 s using 3 Micro Rain Radars/disdrometers and a 2D-Video Disdrometer Comparison of the instruments at different temporal scales allowed the estimation of mea- surement errors and necessary integration time Deviations resulting from spatial and temporal variability could success- fully be isolated from sampling effects and other errors This opened the door for combining the instruments to measure inhomogeneity inside a volume corresponding to that illumi- nated by a conventional weather radar beam, and to study the temporal evolution of precipitation microstructure in single rain events

A New Videosonde with a Particle Charge Measurement Device for In Situ Observation of Precipitation Particles

Abstract: A new videosonde designed for microphysical soundings inside thunderclouds is described. This sensor makes use of a charge-coupled device (CCD) camera and can provide the phase (liquid or ice), the shape, the size, and the electric charge of each precipitating particle detected singly, the size of which ranges from 0.5 mm to 2 cm and the electric charge from ±1 to ±400 pC. The performances of the videosonde are analyzed and evaluated. It is found that the accuracy on the size measurement varies between about 13% for the smaller sizes and less than 2.6% for the larger sizes; meanwhile the average accuracy on the charge measurement is 3.2%. The determination of several large-scale parameters deduced from the videosonde data and comparable with radar observations and electrical soundings is presented. An intercomparison experiment with a disdrometer at the ground shows that the size distribution is perfectly restituted for large drops, even though the video permits filming at a maximum rate of only ...

Raindrop size distribution measurements and associated rain parameters at a tropical location in the indian region

Abstract: Measurements of rain drop size distributions (DSD) at Kolkata (22°34' N, 88°29' E), India, have been carried out using a Josstype disdrometer since June 2004. The size distribution during the growing phase of a rain event is found biased towards larger drops compared to that during the later phase of the event for identical rain rates. The three-parameter distributions, lognormal and gamma, are fitted to the disdrometer data to model DSD for a number of rain events. The same data are also used to find integral rainfall parameters (IRP), such as liquid water content, radar reflectivity factor and specific attenuation which are compared to that obtained with the modelled DSD and the Marshall-Palmer distribution.

Radar-disdrometer comparison

Abstract: A THIES-CLIMA disdrometer has been installed in November 2003 in the vicinity o f the C-band Trappes (near Paris) radar of M´ etFrance. 1-minute data repre- senting the number of particles detected per class diameter and class fall speed have been continuously recorded since then allowing the retrieval of the reflectivity and rain rate at ground level. Also archived are the dominant particle types among 8 different types (rain, snow, graupel). In a first part, the disdrometer performances have been assessed using vari- ous precipitating events (rain, snow, hail). The outputs of the particle type identification were compared with two other mi- crophysical sensors (PWD11 and PWD21) and the rain rates were compared to those measured with rain gauges. Overall, the quality of the disdrometer was in agreement with PWD sensors. In a second part, the rain rate and reflectivity mea- surements were compared to co-located radar measurements for a number of rain events and analysed in terms of calibra- tion biases and fluctuations of the Z-R relationship.

Development of an Inexpensive Raindrop Size Spectrometer

Abstract: The deployment of weather radar, notably in mountainous terrain with many microclimates, requires the use of several or even many drop size spectrometers to provide confidence in the quantitative relation between radar reflectivity and rainfall. While there are several different commercial disdrometers available they are all expensive, large, or fragile, which militates against multiple deployment in the field. The design brief was for a reasonably accurate and sensitive, low-cost and rugged disdrometer to support field work. A design based on piezoceramic disks normally used in hydrophones is described. Calibration and typical field results are presented.

Errors in the radar calibration by gage, disdrometer, and polarimetry: theoretical limit and application to operational radar.

Abstract: A long time record of drop size distributions (DSDs) is used to evaluate the effect of the DSD variability on the accuracy of radar calibratin by a) comparison with a rain gage on a daily basis and b) with polarimetric information. A calibration of reflectivity can be done if a disdrometer is available. Good correlations between radar and disdrometric reflectivities indicate that this could be an excellent way of calibrating radar on a daily basis. The information from operational S-band polarimetric radar is also used for calibration. The sensitivity of a polarimetric calibration with respect to the drop deformation is tested. Furthermore, the consistency in the disdrometric and polarimetric calibration suggests that the use of both calibrations allows to estimate the mean drop deformation. Key Words: Radar calibration, rain gage, disdrometer, operational polarimetric radar, drop size distributions (DSD), drop deformation Introduction When considering radar calibration there are several areas of interest. The stability of the electronic equipment is a concern in its own right. Possible effects of radome on the measurements are a problem that could depend on the state of the radome, whether dry, wet, or with rain streaks. In hydrological applications calibration of the radar may imply some adjustment with ground truth. The differences between the radar measurements and precipitation intensity at the ground may be caused by the height of the radar measurement coupled with the vertical profile of reflectivity, contamination by non-meteorological target, etc (Wilson and Brandes 1979; Zawadzki 1984). The transformation of radar reflectivity into rain rate is another source of discrepancy. Instead of adjusting radar information with ground truth, the radar calibration with radar data themselves is another possibility that can provide an independent monitoring of the performance of system. Due to its inherent characteristics, the specific differential phase shift

Maximisation numérique et mesures acoustiques des précipitations

Abstract: Knowledge of extreme precipitation phenomena is of crucial importance to the safety of civil engineering works and for electricity production management in a country of lakes and mountains like Switzerland. In order to study the distribution in space and the evolution of strong rain episodes, the work presented here relies on the complementary approaches of field observation and numerical simulation. The experimental portion of this project relies on a novel, acousticbased, rain metrology instrument. Based on the results, a methodology for the determination of raindrop size distribution (disdrometer facility) and rainfall rate (rain gauge facility) has been developed and is described in the present dissertation. In addition, numerical modelling and simulation methods were developed with the aim of calculating — for a given watershed topography — the Probable Maximum Precipitation (PMP). The method relies on the separation of the different phenomenological contributions and on the climatic characterization of atmospheric situations leading to extreme rain events. Boundary and initial conditions are represented by theoretical profiles of the wind speed, wind direction, temperature, and water contents, turbulent energy and dissipation rate variables. The numerical model calculates the consequent wind and rain fields within the simulation domain for the desired atmospheric situation. The hydrodynamic code (CFX4) is based on the finite volume approach and is particularly adapted to complex geometries, allowing an excellent representation of the topography. The code is partially open and several specific atmospheric models were implemented. Microphysics schemes considered are Kessler's warm classic scheme (1969) and the Caniaux detailed scheme (1993). The latter includes solid ice particles, aggregates and graupel and allows the simulation of convective as well as orographic cloud system precipitation cycles. Sensitivity studies of the results with respect to the dominant parameters of each situation, lead to a maximisation procedure successfully applied to convective as well as frontal precipitation. The work shows that the maximisation method consisting of maximising severe events into critical events can be more effective than using statistical approaches. Use of this method compensates for the relative lack of measurement facilities in many regions.

Application of radar profilers in multisensor hydrologic field campaigns

Abstract: Precipitation profiling from vertically-looking ground-based radar profilers operating at frequencies of 915- and 2835-MHz have been demonstrated to be useful tools in several field campaigns during the past decade When combined with a surface disdrometer and a nearby scanning radar, the calibrated profiling radar provides high resolution details of the precipitation vertical structure while the scanning radar provides the horizontal context of the precipitation relative to the profiler site Profiling radars provide detailed information of reflectivities and drop-size distributions that are essential for quantitative precipitation estimation (QPE) One role that profiling radars have in QPE is monitoring the calibration of the scanning radar reflectivity used to map the precipitation over a large area The concept of up-scaling uses a surface disdrometer to calibrate the profiling radar which is then used to calibrate the scanning radar This method of up-scaling the reflectivities observed by the surface disdrometer to the scanning radar reflectivities eliminates some of the uncertainties of Z-R relationships inherent in surface rain gauge to scanning radar calibrating and monitoring techniques© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering Downloading of the abstract is permitted for personal use only

An attempt to calibrate the UHF strato-tropospheric radar at Arecibo using NexRad radar and disdrometer data

Abstract: . The goal of this paper is to present a methodology to calibrate the reflectivity of the UHF Strato-Tropospheric (ST) radar located at NAIC in Puerto Rico. The UHF lower relevant altitude is at 5.9km, the melting layer being at around 4.8km. The data used for the calibration came from the observations of clouds, carried out with Strato-Tropospheric dual-wavelength (UHF and VHF) radars and a disdrometer; those instruments being located on the NAIC site in Arecibo, Puerto Rico. The National Weather Service operates other instruments like the radiosondes and the NexRad Radar in other sites. The proposed method proceeds in two steps. The first consists of the comparison between the NexRad reflectivity and the reflectivity computed from the drop size distributions measured by the disdrometer for one day with a noticeable rainfall rate. In spite of the distance of both instruments, the agreement between the reflectivities of both instruments is enough good to be used as a reference for the UHF ST radar. The errors relative at each data set is found to be 2.75dB for the disdrometer and 4dB for the NexRad radar, following the approach of Hocking et al. (2001). The inadequacy between the two sampled volume is an important contribution in the errors. The second step consists of the comparison between the NexRad radar reflectivity and the UHF non-calibrated reflectivity at the 4 altitudes of common observations during one event on 15 October 1998. Similar features are observed and a coefficient is deduced. An offset around 4.7dB is observed and the correlation factor lies between 0.628 and 0.730. According to the errors of the data sets, the precision on the calibration is of the order of 2dB. This method works only when there are precipitation hydrometeors above the NAIC site. However, the result of the calibration could be applied to other data obtained during the campaign, the only constraint being the same value of the transmitter power. Key words. Meteorology and atmospheric dynamics (tropical meteorology; remote sensing; instruments and techniques)

Time evolution of rain drop spectra over Cuddalore in coastal Tamilnadu

Abstract: A disdrometer has been installed at Meteorological Office, Cuddalore in Coastal Tamilnadu during first week of April 2002. This paper attempts to describe an overview of the technicalities and facilities available in this disdrometer besides describing the results of the rain drop size distribution(DSD) from May to August 2002 covering parts of pre-monsoon (March – May) and southwest monsoon (June – September) season. The rain accumulation as measured by the disdrometer matches perfectly with the surface rain gauge measured amount for the rain rate upto 66 mm/hr. The contrasting features of rain spells over Cuddalore, a coastal station in southern peninsular India are clearly brought out in terms of DSDs albeit the rain amount during these period in the year 2002 was very subdued. The modal rain rate was less than 6 mm/hr (84% frequency) followed by 10-40 mm/hr (11%). The concentration of drops, specifically the concentration of larger drops, per rain spell is higher during June - August than during May. The modal class of drop size has an average diameter of 1.116 and 1.912 mm. Both exponential and log-normal distribution seem to be fitting well with the DSD. The rain rate derived from the moment 3.67 (m3.67) of the moment generating function (MGF) of the lognormal distribution function agrees reasonably well with the disdrometer measured rain rate for both the seasons considered. The variability of rain rate, in one minute interval, clearly reveals how the radar derived rainfall accumulation may go wrong even presuming that one uses a perfect Z – R relationship. This suggests the need for understanding the DSD and the micro-physical processes that cause the variability of concentration of drops more precisely to estimate the rain rate and warns about the need for very frequent low elevation radar scans for estimating the rain accumulation.