Showing papers on "Disdrometer published in 2002"

Experiments in Rainfall Estimation with a Polarimetric Radar in a Subtropical Environment

Abstract: A unique dataset consisting of high-resolution polarimetric radar measurements and dense rain gauge and disdrometer observations collected in east-central Florida during the summer of 1998 was examined. Comparison of the radar measurements and radar parameters computed from the disdrometer observations supported previous studies, which indicate that oscillating drops in the free atmosphere have more spherical apparent shapes in the mean than equilibrium shapes. Radar‐disdrometer comparisons improved markedly when using an empirical axis ratio relation developed from observational studies and representing more spherical drop shapes. Fixedform power-law rainfall estimators for radar reflectivity ( ZH), specific differential phase (KDP), specific differential phase‐differential reflectivity ( KDP, ZDR), and radar reflectivity‐differential reflectivity ( ZH, ZDR) were then determined using the disdrometer observations. Relations were produced for both equilibrium shapes and the empirical axis ratios. Polarimetric rainfall estimators based on more spherical shapes gave significantly improved performance. However, the improvement was largely in bias mitigation. Rainfall estimates with the ZH‐ZDR measurement pair had the highest correlation with rain gauge observations, the smallest range in bias factors from storm to storm, and the smallest root-mean-square error.

Two-dimensional video disdrometer: A description

Abstract: This paper describes the design and operation of a two-dimensional video disdrometer (2DVD) for in situ measurements of precipitation drop size distribution. The instrument records orthogonal image projections of raindrops as they cross its sensing area, and can provide a wealth of information, including velocity and shape, of individual raindrops. The 2DVD is a sensitive optical instrument that is exposed to rain, high humidity, and possibly high temperatures. These and other issues such as calibration procedures impact its performance. Under low-wind conditions, the instrument can provide accurate and detailed information on drop size, terminal velocity, and drop shape in a field setting, and the instrument's advantages far outweigh its disadvantages.

A Methodology for Estimating the Parameters of a Gamma Raindrop Size Distribution Model from Polarimetric Radar Data: Application to a Squall-Line Event from the TRMM/Brazil Campaign

Abstract: A methodology is proposed for estimating the parameters of a gamma raindrop size distribution model from radar measurements of Zh, Zdr, and Kdp at S band. Previously developed algorithms by Gorgucci et al. are extended to cover low rain-rate events where both Zdr and Kdp are noisy. Polarimetric data from the S-band Dual-Polarization Doppler Radar (S-Pol) during the Tropical Rainfall Measuring Mission (TRMM)/Brazil campaign are analyzed; specifically, the gamma parameters are retrieved for samples of convective and trailing stratiform rain during the 15 February 1999 squall-line event. Histograms of Nw and Do are retrieved from radar for each rain type and compared with related statistics reported in the literature. The functional behavior of Nw and Do versus rain rate retrieved from radar is compared against samples of 2D-video and RD-69 disdrometer data obtained during the campaign. The time variation of Nw, Do, and μ averaged over a 5 km × 5 km area (within which a network of gauges and a profi...

Estimation of Raindrop Size Distribution Parameters from Polarimetric Radar Measurements

Abstract: Estimation of raindrop size distribution over large spatial and temporal scales has been a long-standing goal of polarimetric radar. Algorithms to estimate the parameters of a gamma raindrop size distribution model from polarimetric radar observations of reflectivity, differential reflectivity, and specific differential phase are developed. Differential reflectivity is the most closely related measurement to a parameter of the drop size distribution, namely, the drop median diameter (D0). The estimator for D0 as well as other parameters are evaluated in the presence of radar measurement errors. It is shown that the drop median diameter can be estimated to an accuracy of 10%, whereas the equivalent intercept parameter can be estimated to an accuracy of 6% in the logarithmic scale. The estimators for the raindrop size distribution parameters are also evaluated using disdrometer data based simulations. The disdrometer based evaluations confirm the accuracy of the algorithms developed herein.

Measurements of drop size distribution in the southwestern Amazon basin

Abstract: [1] Simultaneous observations of an optical and an impact type disdrometer and their application in radar rainfall estimation are evaluated. The disdrometers and two collocated rain gauges were operated in the southwest Amazon region of Brazil in 1999 as part of a NASA Tropical Rainfall Measuring Mission (TRMM) field campaign and the hydro-meteorological component of the Large Scale Biosphere–Atmosphere Experiment (LBA). During the experiment, we observed large drops with diameters greater than 5 mm. These large drops were not adequately detected by the impact disdrometer and resulted in differences in drop size distribution and integral rain parameters derived from the two sensors. Considering coincident observations, we calculated that the impact disdrometer recorded about 11% lower rainfall accumulations than the optical disdrometer. In addition, radar rainfall algorithms, which we derived from the impact and optical disdrometer measurements, showed instrument dependency. Out of four radar rainfall algorithms that we considered, rain rate derived from specific differential phase has the least dependency, while the rain rate derived from reflectivity at horizontal polarization and differential reflectivity combined exhibited the largest. We also observed the characteristics of rainfall and drop size distribution in two distinct wind regimes present during the TRMM–LBA field campaign. Rain was heavier in the easterly regime, with more large drops being present.

Microwave link dual-wavelength measurements of path-average attenuation for the estimation of drop size distributions and rainfall

Abstract: Microwave attenuation measurements at 25 and 38 GHz made on a 2.3-km microwave link are employed to estimate drop size distributions (DSD), rainfall rate, and rainfall accumulation. A theoretical model for the propagation of microwaves in a link system sets forth the basis for the development of a dual-wavelength analytical technique to invert two parameters of a path-average gamma DSD. The DSDs obtained from the technique are evaluated in conjunction with point measurements performed with a 2-D video disdrometer. Additionally, the DSDs yield path-average rainfall rates and rainfall accumulation which are compared with path-average measurements from a network of optical and tipping bucket rain gauges located beneath the link path, and with estimates based on empirical power law relations.

Simultaneous ambient air motion and raindrop size distributions retrieved from UHF vertical incident profiler observations

Abstract: [1] The raindrop size distribution is a fundamental quantity used to describe the characteristics of rain. Vertically pointing Doppler radar profilers are well suited to retrieve the raindrop size distributions because of their operating frequency and data collection methodology. Doppler radar profilers operating at UHF are sensitive to both Bragg scattering from the radio refractive index of turbulence and Rayleigh scattering from distributed targets. During light precipitation, both scattering processes are resolved in the Doppler velocity spectra. During moderate to heavy precipitation the ambient air motion is not resolved in the Doppler velocity spectra. The sans air motion (SAM) model is introduced in this study and uses only the Rayleigh scattering portion of the Doppler velocity spectrum to estimate the ambient vertical air motion, the spectral broadening, and the raindrop size distribution. The SAM model was applied to 915 MHz profiler observations in central Florida. There was good agreement between the SAM-model-retrieved rain rate and mass-weighted mean diameter at an altitude of 300 m with simultaneous surface disdrometer observations. The SAM model was applied to the profile of Doppler velocity spectra to yield estimates of rain rate, mass weighted mean diameter, and ambient vertical air motion from 300 m to just under the melting level at 4 km.

The use of the Joss-type disdrometer for the derivation of Z-R relationships

Abstract: Fourteen rainfall events have been recorded and analysed by the RD-69 (Joss-type) Disdrometer. The range of drop diameters that can be measured spans from 0.3 mm to 5.0 mm. The current position of the instrument was chosen after taking into account: a) the quiet surroundings, since high acoustic noise levels will impair the measurement of small drops, b) the effect caused by strong winds, produc- ing turbulence at the edges of the transducer, c) the preven- tion from flooding and d) the resonance and splashing by raindrops. This instrument measures the raindrop size dis- tributions continuously and automatically having the ability to transform the vertical momentum of an impacting rain- drop into an electric pulse, whose amplitude is a function of the drop diameter. The knowledge of drop size distributions in rain is of importance in radar meteorology, precipitation physics, microwave propagation etc. and can be used for the estimation of other hydrometeorological parameters, such as rainfall rate R, liquid water content in a given volume W , radar reflectivity factor Z, etc. To calculate a drop size distri- bution, the distribution of drops of the diameter correspond- ing to size class i per unit volume must first be calculated from the data for every drop size class. Different Z-R rela- tionships were derived and compared with the one describing the disdrometer data for the specific event, as well as, all rain- fall events. volume of air may be easily calculated. The range of drop diameters that can be measured spans from 0.3 mm to 5.0 mm. Drops smaller than 0.3 mm cannot be measured due to practical limits of the measuring principle and are usually of minor importance in applications for which the instrument is intended. Drops larger than 5.0 mm are very rare because of drop breakup due to the instability of large drops. The RD-69 Disdrometer for raindrops consists of three main units (Distromet LTD, 1997): - The transducer which is exposed to the rain - The processor and - The analog to digital converter - adapter, Analyser ADA - 90

Drop size distribution retrieval from polarimetric radar measurements

Abstract: Retrieval of drop size distribution (DSD) param- eters is one of the longstanding goals of polarimetric radar measurements. Using a gamma model, estimates for the pa- rameters of the DSD model are developed for S, C, and X band frequencies. The model for mean shape of raindrops is critical to the interpretation of polarimetric radar measure- ments and DSD retrievals. The shape size relation for rain- drops is modeled as an equivalent linear form and the slope of the prevailing shape size relation is also derived. The DSD retrievals are validated using disdrometer data. It is shown that the parameters of the DSD namely drop median diameter (D0) and the intercept parameter log10 Nw can be estimated to an accuracy of 10%.

Measurements of drop size distribution in the southwestern Amazon basin : Large-scale biosphere-atmosphere experiment in Amazonia (LBA)

Abstract: [1] Simultaneous observations of an optical and an impact type disdrometer and their application in radar rainfall estimation are evaluated. The disdrometers and two collocated rain gauges were operated in the southwest Amazon region of Brazil in 1999 as part of a NASA Tropical Rainfall Measuring Mission (TRMM) field campaign and the hydrometeorological component of the Large Scale Biosphere-Atmosphere Experiment (LBA). During the experiment, we observed large drops with diameters greater than 5 mm. These large drops were not adequately detected by the impact disdrometer and resulted in differences in drop size distribution and integral rain parameters derived from the two sensors. Considering coincident observations, we calculated that the impact disdrometer recorded about 11% lower rainfall accumulations than the optical disdrometer. In addition, radar rainfall algorithms, which we derived from the impact and optical disdrometer measurements, showed instrument dependency. Out of four radar rainfall algorithms that we considered, rain rate derived from specific differential phase has the least dependency, while the rain rate derived from reflectivity at horizontal polarization and differential reflectivity combined exhibited the largest. We also observed the characteristics of rainfall and drop size distribution in two distinct wind regimes present during the TRMM - LBA field campaign. Rain was heavier in the easterly regime, with more large drops being present.

Influence of disdrometer deadtime correction on self-consistent analytical parameterizations for raindrop size distributions

Abstract: The influence of the deadtime correction on raindrop size distribution parameters inferred from Joss- Waldvogel disdrometer observations is investigated. The parameters are estimated using a procedure to adjust self- consistent analytical parameterizations to normalized empir- ical raindrop size distributions. A case study for a squall line system passing over the Goodwin Creek experimental watershed in northern Mississippi (USA) is presented. The data consist of a 2.5-hour time series of one-minute raindrop size distributions collected with a Joss-Waldvogel disdrom- eter. Application of the deadtime correction increased the estimated rainfall accumulation for the event by almost 15% ( 35 mm before vs. 40 mm after correction), mainly due to a nearly 35% increase of the peak rain rate during the convective phase of the squall line ( 140 mm h 1 before vs. 190 mm h 1 after correction). The exponents of the Z R relationships inferred using the normalization proce- dure show a slight decrease for all storm phases (convection, transition, stratiform), whereas the corresponding prefactors increase for all but the transition phase (where the prefactor decreases slightly).

Vertical air motion estimates from the disdrometer flux conservation model and related experimental observations

Abstract: The use of meteorological radar reflectivity Z to estimate rainfall rate R is approached using a different perspective from the classical Z-R relation. Simultaneous rain measurements from different sensors are combined to construct a model that estimates the vertical air velocity by minimizing the error in reflectivity between the different sensors. This model is based on the fact that rain rate and reflectivity are both dependent on the integrals of rain drop size distribution (DSD) but only R depends on vertical air velocity. This study attempts to validate the vertical air velocity estimates and quantify their affects on the rainfall rate estimation. Disdrometer Flux Conservation Model (DFC) uses measurements from disdrometers and other sensors such as vertically pointing radar profilers and scanning radars. Disdrometers measure a drop size flux (Phi) (D), defined as the number of drops passing a horizontal surface per unit time, per unit area, per drop size. The flux is equal to the product of the drop size distribution near the ground NG(D) and drop velocity near the ground vG(D). The drop velocity is the difference between the droplet terminal velocity and the vertical component of the wind velocity, which varies with altitude. The estimates derived from the DFC model using two pair wise selected sensors are used to study the change of reflectivity and vertical air velocity with altitude. Sensitivity tests for the DFC model are also discussed and these outcomes are validated by comparison with independent profiler vertical velocity observations.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A 3D drop-size distribution model based on the convolution of raindrops at terminal velocity

Abstract: Using a matrix of drop size distributions (DSDs), measured by a microscale array of disdrometers, a method of spatial and temporal DSD interpolation is presented. The goal of this interpolation technique is to estimate the DSD above the disdrometer array as a function of three spatial coordinates, time and drop diameter. This interpolation algorithm assumes simplified drop dynamics, based on cloud advection and terminal velocity of raindrops. Once a 3D DSD has been calculated, useful quantities such as radar reflectivity Z and rainfall rate R can be computed and compared with corresponding rain gauge and weather radar data.

Comparison of Two Disdrometers Based On Different Principles

Abstract: The performances of two sensors that measure drop size distributions (DSDs) using different principles and their estimates of rainfall amounts were compared A classical Joss-Waldvogel disdrometer (JW) and a new instrument named PLUDIX (an X-band raingauge-disdrometer) were used; rainfall rates were also measured by a tipping-bucket raingauge The instruments were operated during the winter of 2001/2002 at the Department of Physics-University of Ferrara (Ferrara, Italy) Four stratiform rainfall events, one snow event and a rain-hail event were analyzed It was demonstrated that the number concentration of small diameter drops is underestimated by JW as the rainfall-rate increases and that the long-time-period averaged DSD followed well a gamma distribution It was also found that the Pludix DSD is better parametrized by an exponential distribution Pludix for the light rain

Relationships between rainfall rate, attenuation, and reflectivity at 14 and 35 GHz frequencies

Abstract: The specific attenuation and reflectivity factors are simulated at 14, and 35 GHz frequencies from disdrometer measured raindrop spectra. Scatter plots showing their relationships with the simulated rainfall rates are presented. Power fits to these relationships are also presented for drop size distributions measured at three different geographical locations around the world.