Showing papers on "Disdrometer published in 2003"

Raindrop Size Distribution in Different Climatic Regimes from Disdrometer and Dual-Polarized Radar Analysis

Abstract: The application of polarimetric radar data to the retrieval of raindrop size distribution parameters and rain rate in samples of convective and stratiform rain types is presented. Data from the Colorado State University (CSU), CHILL, NCAR S-band polarimetric (S-Pol), and NASA Kwajalein radars are analyzed for the statistics and functional relation of these parameters with rain rate. Surface drop size distribution measurements using two different disdrometers (2D video and RD-69) from a number of climatic regimes are analyzed and compared with the radar retrievals in a statistical and functional approach. The composite statistics based on disdrometer and radar retrievals suggest that, on average, the two parameters (generalized intercept and median volume diameter) for stratiform rain distributions lie on a straight line with negative slope, which appears to be consistent with variations in the microphysics of stratiform precipitation (melting of larger, dry snow particles versus smaller, rimed ic...

The Shape–Slope Relation in Observed Gamma Raindrop Size Distributions: Statistical Error or Useful Information?

Abstract: The three-parameter gamma distribution n(D) = N0Dµ exp(–ΛD) is often used to characterize a raindrop size distribution (DSD). The parameters µ and Λ correspond to the shape and slope of the DSD. If µ and Λ are related to one another, as recent disdrometer measurements suggest, the gamma DSD model is simplified, which facilitates retrieval of rain parameters from remote measurements. It is important to determine whether the µ–Λ relation arises from errors in estimated DSD moments, or from natural rain processes, or from a combination of both statistical error and rain physics. In this paper, the error propagation from moment estimators to rain DSD parameter estimators is studied. The standard errors and correlation coefficient are derived through systematic error analysis. Using numerical simulations, errors in estimated DSD parameters are quantified. The analysis shows that errors in moment estimators do cause correlations among the estimated DSD parameters and cause a linear relation between est...

Variability of Raindrop Size Distributions in a Squall Line and Implications for Radar Rainfall Estimation

Abstract: The intrastorm variability of raindrop size distributions as a source of uncertainty in single-parameter and dual-parameter radar rainfall estimates is studied using time series analyses of disdrometer observations. Two rain-rate (R) estimators are considered: the traditional single-parameter estimator using only the radar reflectivity factor (Z) and a dual-polarization estimator using a combination of radar reflectivity at horizontal polarization (ZH) and differential reflectivity (ZDR). A case study for a squall-line system passing over the Goodwin Creek experimental watershed in northern Mississippi is presented. Microphysically, the leading convective line is characterized by large raindrop concentrations (>500 drops per cubic meter), large mean raindrop sizes (>1 mm), and wide raindrop size distributions (standard deviations >0.5 mm), as compared to the transition region and the trailing stratiform rain. The transition and stratiform phases have similar raindrop concentrations and mean raind...

An Evaluation of a Drop Distribution-Based Polarimetric Radar Rainfall Estimator

Abstract: A method for estimating the governing parameters of gamma drop size distributions (DSDs) and associated rainfall rates from polarimetric radar measurements at the S band is examined. The technique uses radar reflectivity at horizontal polarization, differential reflectivity, and an empirical constraining relationship between the DSD shape factor and slope parameter. Retrieved DSD parameters show good agreement with disdrometer observations. Retrieved rainfall estimates are insensitive to drop climatological regime. Comparison with fixed-form powerlaw estimators reveals that the constrained-gamma method outperforms reflectivity estimators and is roughly equivalent to radar reflectivity‐differential reflectivity estimators optimized for local DSDs.

The Microphysical Structure of Extreme Precipitation as Inferred from Ground-Based Raindrop Spectra

Abstract: The controls on the variability of raindrop size distributions in extreme rainfall and the associated radar reflectivity–rain rate relationships are studied using a scaling-law formalism for the description of raindrop size distributions and their properties. This scaling-law formalism enables a separation of the effects of changes in the scale of the raindrop size distribution from those in its shape. Parameters controlling the scale and shape of the scaled raindrop size distribution may be related to the microphysical processes generating extreme rainfall. A global scaling analysis of raindrop size distributions corresponding to rain rates exceeding 100 mm h−1, collected during the 1950s with the Illinois State Water Survey raindrop camera in Miami, Florida, reveals that extreme rain rates tend to be associated with conditions in which the variability of the raindrop size distribution is strongly number controlled (i.e., characteristic drop sizes are roughly constant). This means that changes i...

Rain gauge and disdrometer measurements during the Keys Area Microphysics Project (KAMP)

Abstract: Four impact disdrometers and 27 tipping bucket rain gauges were operated at 11 different sites during August and September 2001, as part of the Keys Area Microphysics Project. The rain gauge and disdrometer network was designed to study the range dependency of radar calibration and rainfall verification in tropical storms. The gauges were collocated at eight sites, while three to five gauge clusters were deployed at three sites. Four disdrometers were also collocated with rain gauges. Overall the experiment was quite successful, although some problems did occur including flooding of gauge loggers, vandalism, and excessive noise at disdrometer sites. Both a south-to-north and east-to-west rainfall gradient was observed, whereby the gauges on the western and northern sides of the Lower Keys recorded more rainfall. Considering the campaign-long rain accumulations, collocated gauges agreed well, with differences generally less than 2%, except for one gauge cluster where the rain accumulation differen...

Relations between Radar Reflectivity, Liquid-Water Content, and Rainfall Rate during the MAP SOP

Abstract: SUMMARY Raindrop size distribution data obtained from two Joss‐Waldvogel disdrometers located at Locarno-Monti, Switzerland during the Mesoscale Alpine Programme (MAP) Special Observation Period are analysed to obtain appropriate relationships of radar ree ectivity, Z, with both water content, W, and surface rainfall, R, for use in MAP applications. The disdrometer data are accumulated into 10-minute samples to reduce sampling error associated with the »1 m 3 sample volume of the instruments. Based on previous studies, relations of the form W D qZ .4=7/ and Z D aR 1:5 are assumed and the coefe cients q and a are estimated from the data. The combined dataset of 10-minute samples from the two disdrometers and the 10-minute data divided into two independent subsets yielded similar mean values of the coefe cients. The recommended relationships are W D 3:4Z .4=7/ and Z D 216R 1:5 . The uncertainties in these mean relationships as expressed in terms of §1 standard deviation are approximately equivalent to a §4.4 dBZ error for the Z‐W relationship, and to a §2.4 dBZ error for the Z‐R relationship.

Instrumental Uncertainties in Z–R Relationships and Raindrop Fall Velocities

Abstract: This study demonstrates the sensitivity of reflectivity–rainfall rate (Z–R) relationships, which were derived from disdrometer-based drop size distribution measurements, to the fall velocity of the drops. The dataset used comes from the simultaneous observation of a series of five moderate rainfall events with a Joss and Waldvogel disdrometer and an optical spectropluviometer. The signal-processing software of the latter was able to measure the residence time of the drops in its sampling volume, enabling computation of the volumetric drop concentration without any assumption on the fall velocity of the drops. The Z–R relationships derived from the two instruments are significantly different. This difference is shown to essentially arise from the drop fall velocities in a twofold manner. First, it comes from the use of a theoretical terminal fall velocity in still air to compute the drop concentration from the drop flux. Second, it comes from the principle of drop size measurement by the impact-ty...

Derivation of parameters of Y-Z power-law relation from raindrop size distribution measurements and its application in the calculation of rain attenuation from radar reflectivity factor measurements

Abstract: The work reported herein empirically derived the coefficients of the power-law relation between the specific attenuation Y and each of two other quantities: the radar reflectivity factor Z and the rainfall rate R. The derivation was accomplished using raindrop size distributions (DSDs) measured in Montreal and Toronto using the precipitation occurrence sensor system (POSS) and a Joss-Waldvogel disdrometer (JWD). The specific attenuation was calculated for both spherical and oblate spheroidal raindrops. Prior to doing so, the effects of inaccuracies in small drop detection by the two systems on the resulting Y-Z and Y-R relations were examined. In computing the relations, the influence of grouping the DSDs according to the corresponding values of Z was assessed. The results from the two sites were then combined in a regression analysis to determine the coefficients of the power-law expressions. The final expressions were used to calculate the probability distribution of rain attenuation over several path lengths at 30 GHz. Conclusions that can be drawn from this work include that (i) the uncertainties in the measurements of small drops do not appear to be critical to the derived relations for some frequencies considered in this work, (ii) in computing rain attenuation estimates from radar reflectivity factors, different criteria for grouping the reflectivity factor measurements can be adopted without changing the statistics of the corresponding attenuation estimates, (iii) differences, which are the smallest for frequencies around 20-30 GHz, are observed in the coefficients derived from the measured DSDs compared with those recommended by the ITU-R, and (iv) the attenuation statistics at 30 GHz computed by applying the Y-Z power-law expression are resistant to spatial averaging.

On the Measurement Errors of the Joss-Waldvogel Disdrometer

Abstract: The Joss-Waldvogel (JW) disdrometer is considered to be a reference instrument for drop size distribution measurements. It has been widely used in many field campaigns as part of validation efforts of radar rainfall estimation. It has also been incorporated in radar rain gauge rainfall observation networks at several ground validation sites for NASA s Tropical Rainfall Measuring Mission (TRMM). It is anticipated that the Joss-Waldvogel disdrometer will be one of the key instruments for ground validation for the upcoming Global Precipitation Measurement (GPM) mission. The JW is an impact type disdrometer and has several shortcomings. One such shortcoming is that it underestimates the number of small drops in heavy rain due to the disdrometer dead time. The detection of smaller drops is also suppressed in the presence of background noise. Further, drops larger than 5.0 to 5.5 mm diameter cannot be distinguished by the disdrometer. The JW assumes that all raindrops fall at their terminal fall speed. Ignoring the influence of vertical air motion on raindrop fall speed results in errors in determining the raindrop size. Also, the bulk descriptors of rainfall that requires the fall speed of the drops will be overestimated or underestimated due to errors in measured size and assumed fall velocity. Long-term observations from a two-dimensional video disdrometer are employed to simulate the JW disdrometer and assess how it s shortcomings affect radar rainfall estimation. Data collected from collocated JW disdrometers were also incorporated in this study.

Measurements of raindrop size distribution over Gadanki during southwest and north-east monsoon

Abstract: Simultaneous observations of disdrometer and optical rain gauge (at National MST Radar Facility, Gadanki) and their application in rainfall estimation are evaluated. Comparison of rain rate obtained from disdrometer and optical rain gauge in different precipitation events in different seasons show reasonably good agreement between the two instruments. Southern India has two distinct rainfall seasons, namely south-west (S-W) monsoon and north-east (N-E) monsoon periods. Disdrometer data analysis shows a clear seasonal dependence in radar reflectivity factor-rainfall rate (Z-R) relations (i.e. raindrop size distribution characteristics) in S-W monsoon and N-E monsoon periods over Gadanki. It is also found that during S-W monsoon precipitation generally has bigger drops than during N-E monsoon. During the S-W monsoon most of the precipitating cloud systems are associated with mesoscale convection activities. These precipitating systems are short­ lived (1-2 h) with high intensity of rainfall.

On the use of the log-normal particle size distribution to characterize global rain

Abstract: Although most parameterizations of the drop size distributions (DSD) use the gamma function, there are several advantages to the log-normal form, particularly if we want to characterize the large scale space-time variability of the DSD and rain rate. The advantages of the distribution are twofold: the logarithm of any moment can be expressed as a linear combination of the individual parameters of the distribution; the parameters of the distribution are approximately normally distributed. Since all radar and rainfall-related parameters can be written approximately as a moment of the DSD, the first property allows us to express the logarithm of any radar/rainfall variable as a linear combination of the individual DSD parameters. Another consequence is that any power law relationship between rain rate, reflectivity factor, specific attenuation or water content can be expressed in terms of the covariance matrix of the DSD parameters. The joint-normal property of the DSD parameters has applications to the description of the space-time variation of rainfall in the sense that any radar-rainfall quantity can be specified by the covariance matrix associated with the DSD parameters at two arbitrary space-time points. As such, the parameterization provides a means by which we can use the spaceborne radar-derived DSD parameters to specify in part the covariance matrices globally. However, since satellite observations have coarse temporal sampling, the specification of the temporal covariance must be derived from ancillary measurements and models. Work is presently underway to determine whether the use of instantaneous rain rate data from the TRMM Precipitation Radar can provide good estimates of the spatial correlation in rain rate from data collected in 5(sup 0)x 5(sup 0) x 1 month space-time boxes. To characterize the temporal characteristics of the DSD parameters, disdrometer data are being used from the Wallops Flight Facility site where as many as 4 disdrometers have been used to acquire data over a 2 km path. These data should help quantify the temporal form of the covariance matrix at this site.

A method for estimating momentum fluxes of deep precipitating convection using profiling Doppler radar

Abstract: [1] This paper outlines a novel approach for obtaining estimates of momentum fluxes for deep precipitating convection. This method relies on data collected by vertically incident profiling Doppler radars but may be applied to aircraft- or space-based radar systems. This work represents a significant step forward toward the evaluation of convective momentum transports across regional scales using data from operational satellite systems (e.g., the Tropical Rainfall Measuring Mission (TRMM) instrument). The new method is demonstrated as convective momentum flux terms and are estimated for deep convection occurring during the Third Convection and Atmospheric Moisture Experiment (CAMEX-3), which was conducted during August and September 1998. These terms contain the most information on the momentum transported in the updrafts and downdrafts of cumulus convection. Key to the method is that the tilt and orientation of the convective drafts, relative to Earth's surface, are quantified from radar information into perturbation horizontal wind components once an accurate storm motion estimate is provided. For perturbation vertical velocities, the evaluation of drop size diameters from radar reflectivities and the subsequent evaluation of terminal velocities of hydrometeors and draft motions are necessary information toward applying this technique. Doppler velocity measurements from the upward looking Rd-69 Disdromet disdrometer (Joss-Waldvogel disdrometer) calibrated 915 MHz profiler are used to test the procedures for estimating w′. The method is tested against numerically simulated convection using the University of Wisconsin-Nonhydrostatic Modeling System (UW-NMS) mesoscale and cloud-resolving model. The radar-estimated momentum fluxes are found to match in sign and relative magnitude through the depth of the convective clouds studied compared to those obtained from model simulation.

Estimation of the Z-R Relation through the Disdrometer for the Coastal Region in the Northeast of Brazil

Abstract: The preliminary results of the study on the physics of rain using disdrometer data are shown for an area located on the northern coastal board of Macei, Alagoas (933'17.24' and 3546'54.84' W), at approximately 80 meters above the sea level. The data were obtained during January 2002 using a disdrometer RD-69 (Joss-Waldvogel). After definining the criteria for determining rain type (convective and stratiform), a set of Z-R pairs was analyzed for estimating the Z-R relation for each rain type. The results were quite similar to those for other regions of the globe. This preliminary analysis will be used to study the structure of rain with the meteorological radar as well as to permit a better understanding of the physics of tropical rain.

Propagation in rain and clouds

Abstract: Water appears in the atmosphere in a variety of forms, usually referred to by the term hydrometeor, which includes particles as diverse as cloud, rain drops, snowflakes, ice crystals, hail and graupel. Of these, rain, hail, graupel and snow are generally recognised as precipitation. The effects that hydrometeors have on communications systems are dependent both on the system frequency and the type of particle present. At any given instant, of course, more than one type of particle will affect a given link. For example, an earth-space link may often encounter rain over the lower part of the path, and snow at greater heights. We will consider first the various forms of hydrometeor, together with their relevance to radiowave propagation. A brief theoretical framework will then be presented as a background to the model development process and, finally, specific effects of hydrometeors on systems will be considered.

Recent developments in the use of Doppler radar profilers for the remote sensing of precipitating clouds

Abstract: During the past decade Doppler radar profilers that operate near 1 GHz and 3 GHz have been developed at the NOAA Aeronomy Laboratory for use in dynamics and precipitation research. The profilers have been used extensively in numerous field campaigns during the past decade. In the presence of precipitating clouds, backscattering from hydrometeors is dominant and the Doppler velocity provides a measure of the fall velocity of hydrometeors. Profiler observations yield time height cross-sections of equivalent reflectivity, Doppler velocity and spectral width that illustrate the evolution of precipitating clouds systems. The vertical structure of these parameters has been used to classify the precipitating cloud systems into several different categories. These observations document the prevalence of deep anvil cloud systems over the Pacific warm pool region. They also show the relative abundance of rainfall from stratiform and convective components of precipitating cloud systems and the continuous observations reveal the diurnal evolution of the precipitating clouds over the profiler. The profiler observations provide important information for the calibration and validation of precipitation measurements by other instruments and platforms. For example, direct comparisons of profiler reflectivities with scanning radar reflectivities provide a direct means for calibration of scanning radars. The profilers are calibrated with a collocated disdrometer. An important objective of the profiler observations is to retrieve drop-size distributions and to determine the variability of the drop-size distributions in diverse precipitating cloud systems. Recent developments provide optimism that drop-size distribution retrievals can be made by profilers operating at 1 GHz or 3 GHz without complementary measurement of vertical air motions.

Campaign mode observation of tropical convection using ground-based radar systems

Abstract: Tropical convection plays an important role in enhancing rainfall and also creates uncertainty in the model-based predictions of weather in tropics due to the latent heat released into the troposphere. Ground-based radar systems are important tools available for the effective characterization of convective events. Availability of different radar systems ideally suited to study tropical convection in an area popularly known as the Golden triangle for weather observations in southern part of India, led to the organization of an interagency program for a campaign mode of observations using the different radar systems and associated rain gauges etc. to observe tropical convection during the period Oct-Dec, on a few days when the north east monsoon was prevalent over the east coast of peninsular India. The Golden triangle consists of Sriharikota Island where the Space Launch Complex of ISRO is located at the Satish Dhavan Space Center, the Cyclone Detection Radar site of IMD at Chennai and the National MST Radar Facility (NMRF) at Gadanki in close proximity to the temple city of Tirupati. An indigenously developed S band Doppler weather radar is commissioned at Sriharikota Island on the east coast of India in December 2002, as an interagency program between India Meteorological Department and Indian Space Research Organization. A siphoning type fast response rain gauge and a tipping bucket rain gauge are located within 10 km from the radar. Another S band DWR is located in Chennai, India by IMD. These radars have the capability to measure precipitation and Doppler velocity and provide in real time the 3 base products viz., reflectivity, velocity, and spectral width of the hydrometeors within radar sample volume with good accuracy up to 250 km. From these base products, other meteorological products like rainfall rate, rainfall accumulation, Cappi, echotop etc. are derived. Indian MST radar, a VHF profiler (at 53 MHz) normally used for estimating the winds and turbulence and an L band lower atmospheric wind profiler operating at 1357.5 MHz for estimating the winds, turbulence, and precipitating weather systems in tropical latitudes are operating at NMRF since 1990. Apart from these radar systems, a disdrometer and an optical rain gauge are also located at this facility as collocated instrumentations for the measurement of rainfall rate and rainfall accumulation. All these three locations are geographically located within 80-100 km from each other and form a triangle. These instrumentation systems provide an excellent ground-based network for the characterization of tropical convection. The paper describes the campaign details including the detailed characteristics of the radar systems used and provide intercomparison of the data obtained as the convective systems transited over the terrain which is essentially coastal for Chennai and SHAR and mountainous for Gadanki.