Showing papers on "Disdrometer published in 2001"

A method for estimating rain rate and drop size distribution from polarimetric radar measurements

Abstract: Polarimetric radar measurements are sensitive to the size, shape and orientation of raindrops and provide information about drop size distribution (DSD), canting angle distribution and rain rate. The authors propose and demonstrate a method for retrieving DSD parameters for calculating rain rate and the characteristic particle size. The DSD is assumed to be a gamma distribution and the governing parameters are retrieved from radar measurements: reflectivity (Z/sub HH/), differential reflectivity (ZDR), and a constrained relation between the shape (CL) and slope (/spl Lambda/) parameters derived from video disdrometer observations. The estimated rain rate is compared with that obtained from more traditional methods and the calculated characteristic size is compared with the measured values. The calculated K/sub DP/ based on the retrieved Gamma DSD is also compared with measurements. The proposed method shows improvement over the existing models and techniques because it can retrieve all three parameters of the gamma distribution. For maintaining the continuity of earlier published results, raindrop shape is assumed to be equilibrium.

Correcting C-band radar reflectivity and differential reflectivity data for rain attenuation: a self-consistent method with constraints

Abstract: Quantitative use of C-band radar measurements of reflectivity (Z/sub h/) and differential reflectivity (Z/sub dr/) demands the use of accurate attenuation-correction procedures, especially in convective rain events. With the availability of differential phase measurements (/spl Phi//sub dp/) with a dual-polarized radar, it is now possible to improve and stabilize attenuation-correction schemes over earlier schemes which did not use /spl Phi//sub dp/. The recent introduction of constraint-based correction schemes using /spl Phi//sub dp/ constitute an important advance. In this paper, a self-consistent, constraint-based algorithm is proposed and evaluated which extends the previous approaches in several important respects. Radar data collected by the C-POL radar during the South China Sea Monsoon Experiment (SCSMEX) are used to illustrate the correction scheme. The corrected radar data are then compared against disdrometer-based scattering simulations, the disdrometer data being acquired during SCSMEX. A new algorithm is used to retrieve the median volume diameter from the corrected Z/sub h/, corrected Z/sub dr/, and K/sub dp/ radar measurements which is relatively immune to the precise drop axis ratio versus drop diameter relation. Histograms of the radar-retrieved D/sub 0/ compared against D/sub 0/ from disdrometer data are in remarkable good agreement lending further validity to the proposed attenuation-correction scheme, as well as to confidence in the use of C-band radar for the remote measurement of rain microphysics.

Comparison of Drop Size Distribution Measurements by Impact and Optical Disdrometers

Abstract: Simultaneous observations made with optical- and impact-type disdrometers were analyzed to broaden knowledge of these instruments. These observations were designed to test how accurately they measure drop size distributions (DSDs). The instruments' use in determining radar rainfall relations such as that between reflectivity and rainfall rate also was analyzed. A unique set of instruments, including two video and one Joss–Waldvogel disdrometer along with eight tipping-bucket rain gauges, was operated within a small area of about 100 × 50 m2 during a 2-month-long field campaign in central Florida. The disdrometers were evaluated by comparing their rain totals with the rain gauges. Both disdrometers underestimated the rain totals, but the video disdrometers had higher readings, resulting in a better agreement with the gauges. The disdrometers underreported small- to medium-size drops, which most likely caused the underestimation of rain totals. However, more medium-size drops were measured by the v...

What is a Raindrop Size Distribution

Abstract: It is commonly understood that the number of drops that one happens to measure as a function of diameter in some sample represents the drop size distribution However, recent observations show that rain is "patchy" suggesting that such a seemingly "obvious" definition is incomplete That is, rain consists of patches of elementary drop size distributions over a range of different scales All measured drop size distributions, then, are statistical mixtures of these patches Moreover, it is shown that the interpretation of the measured distribution depends upon whether the rain is statistically homogeneous or not It is argued and demonstrated using Monte Carlo simulations that in statistically homogeneous rain, as the number of patches included increases, the observed spectrum of drop sizes approaches a "steady" distribution On the other hand, it is argued and demonstrated using video disdrometer data that in statistically inhomogeneous rain, there is no such steady distribution Rather as long as

Classification of tropical precipitating systems and associated Z‐R relationships

Abstract: Simultaneous observations of Indian Mesosphere-Stratosphere-Troposphere (MST) radar, Lower Atmospheric Wind Profiler (LAWP) and disdrometer, located at Gadanki (13.5°N, 79.2°E), over two seasons are used to derive the Z-R relationships, of the form Z = ARb, for different types of precipitation and for different seasons. Disdrometer data are classified into three types, convection, transition, and stratiform, based on the variation of the median volume diameter D0 with the rain rate R. The disdrometer classification is verified by comparing with the profilers classification, based on reflectivity (in terms of range corrected signal-to-noise ratio), Doppler velocity, spectral width, and vertical air velocity. Overall agreement between the two classification schemes is found to be reasonable. The variations in the coefficient A and the exponent b in the Z-R relation for the case studies presented here are explained with the help of variations in drop size distribution parameters. From the total data the percentage occurrence of precipitation is found to be 55% stratiform, 9% convective, and 36% transition, whereas the total rainfall is 12, 54, and 34%, respectively. Interestingly, the exponent b (coefficient A) is found to be smaller (larger) in the case of stratiform (convective) precipitation than that in convective (stratiform) precipitation, in contrast to the earlier results. The values of A and b are also derived for different monsoon seasons.

Drop Size Distributions Measured by a 2D Video Disdrometer: Comparison with Dual-Polarization Radar Data

Abstract: An analysis of drop size distributions (DSDs) measured in four very different precipitation regimes is presented and is compared with polarimetric radar measurements. The DSDs are measured by a 2D video disdrometer, which is designed to measure drop size, shape, and fall speed with unprecedented accuracy. The observations indicate that significant DSD variability exists not only from one event to the next, but also within each system. Also, despite having vastly different storm structures and maximum rain rates, large raindrops with diameters greater than 5 mm occurred with each system. By comparing the occurrence of large drops with rainfall intensity, the authors find that the largest median diameters are not always associated with the heaviest rainfall, but are sometimes located either in advance of convective cores or, occasionally, in stratiform regions where rainfall rates are relatively low. Disdrometer and polarimetric radar measurements of radar reflectivity Z, differential reflectivity ...

Rainfall Estimation from Polarimetric Radar Measurements: Composite Algorithms Immune to Variability in Raindrop Shape–Size Relation

Abstract: Polarization diversity radar measurements such as reflectivity factor, differential reflectivity, and differential propagation phase are extensively used in rainfall estimation. Algorithms to estimate rainfall from polarimetric radar measurements are based on a model for the raindrop shape as a function of drop diameter. Most of the current algorithms use an equilibrium shape‐size model for raindrops. Variation of the prevailing mean raindrop shapes from an assumed model has a direct impact on the accuracy of radar rainfall estimates. This paper develops composite algorithms to estimate rainfall from polarimetric radar data without an a priori assumption about the specific form of mean raindrop shape‐size model such as equilibrium shape model. The accuracy of rainfall estimates is evaluated in the presence of random measurement errors as well as systematic bias errors. The composite algorithms, independent of a prespecified raindrop shape model, were applied to radar parameters simulated from disdrometer data collected over 3 months, and the corresponding rainfall estimates were found to be in good agreement with disdrometer estimates. The composite algorithms were also tested with Colorado State University CHILL radar observations of the 28 July 1997 Fort Collins (Colorado) flood event. The storm total precipitation estimates based on the composite algorithms developed in this paper were in much better agreement with rain gauge estimates in comparison with conventional algorithms.

Listening to Raindrops from Underwater: An Acoustic Disdrometer

Abstract: Different sized raindrops splashing on a water surface produce sound underwater that is distinctive and can be used to measure the drop size distribution in the rain. Five acoustically significant raindrop sizes are described. An inversion of the underwater sound to measure the drop size distribution in the rain is described and demonstrated. Limitations to the inversion include problems associated with the relative loudness of the largest drops (diameter over 3.5 mm), the relative quietness of the medium drops (diameter 1.2–2.0 mm), and the influence of wind to suppress the signal from the otherwise remarkably loud small drops (diameter 0.8–1.2 mm). Various measures of rainfall, including rainfall rate, equivalent radar reflectivity, median drop size, and other integrated moments of the drop size distribution are measured acoustically and used to examine rainfall research issues. The relationship between equivalent reflectivity and rainfall rate, the Z–R diagram, is partitioned acoustically show...

Estimation of the Equivalent Radar Reflectivity Factor from Measured Snow Size Spectra.

Abstract: In this paper, a method for the estimation of radar reflectivity from measured snow particle size distributions is presented based on earlier works of Marshall and Gunn and of Smith. During two snowfalls, the method was applied to estimate the equivalent reflectivity factor from measured snow size distributions obtained by the Particle Size and Velocity (PARSIVEL) optical disdrometer. The results are compared with the data of conventional C-band Doppler radar. Here, two snowfalls are presented as case studies. In addition, a comparison during one rainfall is included, which shows good agreement between the two instruments. In the case of snow, the calculation of the equivalent reflectivity factor from the PARSIVEL data is based on a relation between the mass and the size of the snow particles. In this study, a mass–size relation for graupel-like snow was used for all snowfalls. Because this is a crude description of naturally occurring snow, which can be of any other type (e.g., dendrites), the d...

Rainfall Measurement on Ship Revisited: The 1997 PACS TEPPS Cruise

Abstract: Fifteen rain measurement instruments were deployed on the National Oceanic and Atmospheric Administration Ship Ronald H. Brownduring the 1997 Pan American Climate Studies (PACS) Tropical Eastern Pacific Process Study (TEPPS). To examine differences in rainfall catchment related to instrument design, three types of disdrometers, an optical rain gauge, a ship rain gauge, and a siphon gauge were clustered in one location to ensure similar exposure. To address exposure effects, eight siphon rain gauges were deployed on different sides of the ship and on several different levels. Cross-ship differences in hourly rainfall accumulation were negligible when relative wind speeds were less than 3 m s21 and became significant at greater than 5 m s21, especially when the relative wind direction was 208 or greater from the bow. Instruments with both horizontal and vertical catchment surfaces yielded a measurable collection advantage over instruments with only horizontal catchment surfaces. Analysis of data collected during TEPPS using a multiple-instrument, multiple-location approach yields the following recommendations for reducing uncertainty in rain measurement at sea. The first two of the four recommendations apply to rain measurements on buoys as well as on ships. 1) Deploy experimental rain measurement instrumentation paired with a baseline minimum siphon gauge or other trusted instrument. Comparison of the rain-rate time series between the baseline gauge measurements and the experimental instrument data permits detection of erratic behavior and bias. 2) Apply an appropriate wind correction. To do this step properly, both a wind correction formula derived for the specific gauge type and a nearby measurement of relative wind are needed. These features are already incorporated into the ship rain gauge. 3) Locate gauges where distortion of the airflow by the ship is locally minimized and relative wind speeds are as low as possible. This analysis confirms previous recommendations for placement of rain instrumentation at lower locations as long as the location is protected against direct spray from the sea without being shadowed by higher objects. 4) Place instrumentation on both sides of ship and along centerline. Airflow distortion by the ship itself can induce significant differences between port and starboard accumulations at high wind speeds and high angle of wind attack to the bow. Multiple locations aid in constraining error, because relative wind direction and speed vary during a cruise and there is no one perfect location on ship for rain instrumentation.

Experimental Test of the Effects of Z–R Law Variations on Comparison of WSR-88D Rainfall Amounts with Surface Rain Gauge and Disdrometer Data

Abstract: Reflectivity factors and rainfall rates found from Level II WSR-88D data for the National Weather Service (NWS) radar in Greer, South Carolina (KGSP), are compared with similar parameters found from disdrometer data collected at the Clemson Atmospheric Research Laboratory. These comparisons are used to determine experimentally the sensitivity of rainfall amounts found from the WSR-88D data to variations in the parameters A and b of the Z–R law (Z = ARb) that is used in analysis of the data. Analyses of data for nine storms in upstate South Carolina are described. These nine cases encompass a variety of rainfall types including stratiform rain, airmass thunderstorms, and strong cold front convective activity. It is found, after correction of the radar reflectivity factors for obvious calibration offset, that the rainfall depths found by radar are in good agreement with those found from the disdrometer when the NWS default values of A and b (A = 300, b = 1.4) are used. If the values of A and b foun...

An example of computational approach used for aerodynamic design of a rain disdrometer

Abstract: The present work reports on the application of a computational fluid dynamics-based method as a tool to improve the aerodynamic design of rainfall measurement devices. The focus is on a new instrument, a two-dimensional video disdrometer that provides information about raindrop size distribution. The distorted wind field around and inside the instrument's body is simulated using a three-dimensional numerical model. A modified geometry of the instrument, suggested for operational purposes, is tested numerically. Trajectories of raindrops are simulated to investigate the wind effect on the catchment efficiency of the instrument. A stochastic Lagrangian particle-tracking model that accounts for the turbulence effect is examined. General guidelines related to aerodynamic aspects of the design of in-situ rainfall measuring devices are discussed.

Ground Based Rainfall Measurements at the NASA Wallops Flight Facility

Abstract: The rainfall measurements collected at the NASA Wallops Flight Facility during the first 4 months of 2001 were analyzed to study the small-scale variability of rainfall and to evaluate the instruments performance with respect to each other. The instrument platform included impact and optical type disdrometers, tipping bucket rain gauges, and sonic anemometers. An X-band polarametric (X-POL) radar that was situated approximately 3.8 km from the disdrometer site was also operated for two months period starting from February 20, 2001. The disdrometers helped to calibrate the X-POL radar. Winds played a crucial role on the performance of both rain gauges and disdrometers. In the presence of strong winds, disdrometers, particularly 2-dimensional video disdrometer (2DVD), severely underestimated the rainfall. The underestimation of rainfall by impact disdrometers was due to sampling errors of the drops larger than 5.0 to 5.5 mm in diameter. The drops less than 2 mm in diameter were also undersampled due to disdrometer dead time and background noise generated by the winds. The design of the 2DVD was problematic to collect the drop samples falling at an angle from the vertical axis. The rain amounts between the same types of disdrometers and of rain gauges were also varied as a function of wind speed. The variations in drop size distribution measured within a radar pixel of 2 x 2 km resulted in 1 to 1.7 dB difference in reflectivities. This should be considered a source of error in radar rainfall estimates. It was shown that the horizontal reflectivity was insensitive to variations in drop shape while observed axis ratios resulted in lower differential reflectivity than equilibrium axis ratios. The X-POL radar measurements of horizontal reflectivity was in good agreement with the reflectivity calculated by the disdrometer while the radar differential reflectivities were consistently lower than the disdrometer differential reflectivites.