Article

Preprocessing and Assessment of Rain Drop Size Distributions Measured With a K-Band Doppler Radar and an Optical Disdrometer

Authors:
Article

Preprocessing and Assessment of Rain Drop Size Distributions Measured With a K-Band Doppler Radar and an Optical Disdrometer

If you want to read the PDF, try requesting it from the authors.

Abstract

Rain attenuation in millimeter-wave links depends on the drop size distributions (DSDs) of the raindrops. Empirical models disregard this dependence and estimate the specific attenuation using only the integrated rainfall rate [ $R$ (mm/h)]. This approach is valid for lower frequencies but it progressively losses accuracy as the frequency of interest becomes higher within the millimeter-wave range. Both the characterization of rainfall phenomena and the prediction of rain attenuation can be improved with the knowledge of DSD, which, in turn, depend on the type of rain event (stratiform or convective) and the $R$ . In this article, long-term DSD measurements from a vertical Doppler radar [Micro Rain Radar (MRR-2)] and a laser optical disdrometer (Thies laser disdrometer) are used to obtain, classify, and compare the statistics of DSD in Madrid in a period of more than ten years. The process to obtain the DSD from these advanced instruments is analyzed in detail, providing recommendations about the calibration of the radar data and the most appropriate particle filtering to apply on the laser disdrometer data.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... However, at Ku-bands, the contribution due to other factors except rain fade is not of much significance. Rain is the most dominant factor causing signal fading at higher frequencies above 10 GHz [18]. The signal attenuation due to the rain will cause a very slow variation in the spectral domain [19]. ...
Preprint
p>The received signal at the ground station for a satellite link is affected by the stochastic nature of atmospheric channel. Adverse weather events such as rain not only attenuates the signal but also increases noise, scintillation fading and multipath effects that cause rapid variations in the received signal at the ground station. In order to analyze the stochastic effects of weather phenomenon on the link performance on short-term basis, both the slowly changing signal attenuation and the rapid variations caused by channel at the receiver has to be studied. In this work, we first analyze the short-term effects of rain on the statistical and spectral properties of fast varying signal component affecting the link performance. Following this, we model such parameters using several features extracted from the slowly varying signal component with support vector machine (SVM). We show an interesting result that the statistical and spectral properties of fast varying signal under rainy channel conditions can be predicted with very high accuracy using SVM. The prediction of such parameters will lead to receiver design adaptive to varying channel dynamics affecting the link performance under rainy conditions.</p
... The application of these models requires as input data the drop size distributions (DSD) of the rain particles, which are not usually available for most sites. This research makes use of an extensive database available at Universidad Politécnica de Madrid (UPM) in Madrid, Spain with twelve years (2008-2019) of DSD data [6], which allows estimating rain attenuation with high accuracy and detail. The main conclusions are drawn in Section 4. ...
... The authors [33,34] provide detailed rainfall attenuation distribution statistics over the wireless link at a higher microwave frequency in Japan and Korean territories. Specific practical attenuation measurement and modeling-based investigation approaches to rainfall intensity effects conducted in different countries are presented [35][36][37][38][39][40][41][42][43][44]. ...
Article
Full-text available
Absorption and scattering of propagated microwave radio signals by atmospheric variables, particularly rainfall, remained a major cause of propagation attenuation losses and service quality degradation over terrestrial communication links. The International Telecommunications Union Radio (ITU-R) reports and other related works in the literature provided information on attenuation due to rain and microwave propagation data. Such propagation attenuation information in the tropical region of Nigeria is destitute, especially at lower radio waves transmission frequencies. Therefore, this study addresses this problem by employing 12-year rainfall datasets to conduct realistic prognostic modeling of rain rate intensity levels. A classification of the rainfall data into three subgroups based on the depth of rainfall in the region is presented. Additionally, an in-depth estimation of specific rain attenuation intensities based on the 12-year rainfall data at 3.5 GHz is demonstrated. On average, the three rainfall classes produced rain rates of about 29.27 mm/hr, 73.71 mm/hr, and 105.39 mm/hr. The respective attenuation values are 0.89 dB, 1.71 dB, and 2.13 dB for the vertical polarisation and 1.09 dB, 1.20 dB, and 2.78 dB for the horizontal polarisation at 0.01% time percentage computation. Generally, results indicate that higher rain attenuation of 12% is observed for the horizontal polarisation compared to the vertical polarisation. These results can provide valuable first-hand information for microwave radio frequency planning in making appropriate decisions on attenuation levels due to different rainfall depths, especially for lower frequency arrays.
... Among the different methodologies to estimate the rainfall intensity, the exploitation of earth-satellite links represents a real-time cheap and valuable solution, especially considering the high density of the receivers deployed worldwide [7]. As explained in [7] and [8], alternative methods or measurement instrumentation [9], [10] (e.g., networks of rain gauges, weather radars, microwave links, satellite infrared imagery, etc.) are each affected by technical drawbacks [11] which limit their context of application: for instance, rain gauges offer inadequate coverage over wide areas, whereas remotesensing-based approaches are limited in spatial and temporal resolutions. ...
... Instead of using any model, this study makes use of the experimental DSD measured for twelve years (2008-2019) using an optical Laser disdrometer, the Thies Laser disdrometer [6], located at Escuela Técnica Superior de Ingenieros de Telecomunicación (ETSIT) of Universidad Politécnica de Madrid (UPM), Madrid, Spain, to evaluate rain specific attenuation in the 100-200 GHz band and compare with existing models. The assessment of the quality and features of the DSD was carried out previously [7]. The DSD measured by the Laser disdrometer are available in discrete bins, organized by diameter classes, Di (mm). ...
Article
The attenuation produced by raindrops in millimeter-wave links relies on the drop size distributions (DSD). However, empirical models dismiss this dependence and approximate the specific attenuation [γ (dB/km)] considering only the integrated rainfall rate [ R (mm/h)]. The use of empirical models is effective for relatively low frequencies but, as the frequency of interest becomes higher within the millimeter-wave range, empirical models lose accuracy. Moreover, the value of γ for a given R is expected to vary more than at lower frequencies, depending on the DSD. This letter aims to study this higher variability of γ, not incorporated in the current models, by calculating the specific attenuation γ from the experimental DSD measured by a vertical doppler radar (MRR-2) and a Laser optical disdrometer (Thies Laser disdrometer) in periods of more than 10 years. Both Ka-band and W-band have been considered for the calculations in order to understand the effect of increasing the frequency. Additionally, with the objective of evaluating the application of these techniques in propagation studies, the attenuation calculated from the DSD obtained from the above instruments has been compared to experimental values measured in a W-band terrestrial radio link at 75/85 GHz, yielding good results.
Article
Full-text available
A new method to determine the melting layer height using a micro rain radar (MRR) is presented. The MRR is a small vertically pointing frequency-modulated continuous-wave radar that measures Doppler spectra of precipitation. From these Doppler spectra, various variables such as Doppler velocity or spectral width can be derived. The melting layer is visible due to higher reflectivity and an acceleration of the falling particles, among others. These characteristics are fed to a neural network to determine the melting layer height. To train the neural network, the melting layer height is determined manually. The neural network is trained and tested using data from two sites that cover all seasons. For most cases, the neural network is able to detect the correct melting layer height well. Sensitivity studies show that the neural network is able to handle different MRR settings. Comparisons to radiosonde data and cloud radar data show a good agreement with respect to the melting layer heights.
Article
Full-text available
The intensity and phase of precipitation at the ground surface can have important implications not only for meteorological and hydrological situations but also in terms of hazards and risks. In the field, Thies disdrometers are sometimes used to monitor the quantity and nature of precipitation with high temporal resolution and very low maintenance and thus provide valuable information for the management of meteorological and hydrological risks. Here, we evaluate the Thies disdrometer with respect to precipitation detection, as well as the estimation of precipitation intensity and phase at a pre-alpine site in Switzerland (1060 m a.s.l.), using a weighing precipitation gauge (OTT pluviometer) and a two-dimensional video disdrometer (2DVD) as a reference. We show that the Thies disdrometer is well suited to detect even light precipitation, reaching a hit rate of around 95 %. However, the instrument tends to systematically underestimate rainfall intensities by 16.5 %, which can be related to a systematic underestimation of the number of raindrops with diameters between 0.5 and 3.5 mm. During snowfall episodes, a similar underestimation is observed in the particle size distribution (PSD), which is, however, not reflected in intensity estimates, probably due to a compensation by snow density assumptions. To improve intensity estimates , we test PSD adjustments (to the 2DVD) and direct adjustments of the resulting intensity estimates (to the OTT pluviometer), the latter of which are able to successfully reduce the systematic deviations during rainfall in the validation period. For snowfall, the combination of the 2DVD and the OTT pluviometer seems promising as it allows for improvement of snow density estimates, which poses a challenge to all optical precipitation measurements. Finally, we show that the Thies disdrometer and the 2DVD agree well insofar as the distinction between rain and snowfall is concerned , such that an important prerequisite for the proposed correction methods is fulfilled. Uncertainties mainly persist during mixed-phase precipitation or low precipitation intensities , where the assignment of precipitation phase is technically challenging, but less relevant for practical applications. We conclude that the Thies disdrometer is suitable not only to estimate precipitation intensity but also to distinguish between rain and snowfall. The Thies disdrometer therefore seems promising for the improvement of precipitation monitoring and the nowcasting of discharge in pre-alpine areas, where considerable uncertainties with respect to these quantities are still posing a challenge to decision-making.
Article
Full-text available
A new method using the Micro Rain Radar (MRR) to determine the melting layer height is presented. The MRR is a small vertically pointing frequency modulated continuous wave radar which measures Doppler spectra of precipitation. From these Doppler spectra, various variables such as Doppler velocity or spectral width can be derived. The melting layer is visible through a higher reflectivity and an acceleration of the falling particles, among others. These characteristics are fed to a neural network to determine the melting layer height. For the training of the neural network, the melting layer height is determined manually. The neural network is trained and tested using data from two sites covering all seasons. For most cases, it is well able to detect the correct melting layer height. Sensitivity studies show that the neural network is able to handle different settings of the MRR. Comparisons to radiosonde data and cloud radar data show a good agreement in melting layer heights.
Article
Full-text available
We report on fall speed measurements of raindrops in light-to-heavy rain events from two climatically different regimes (Greeley, Colorado, and Huntsville, Alabama) using the high-resolution (50 µm) Meteorological Particle Spectrometer (MPS) and a third-generation (170 µm resolution) 2-D video disdrometer (2DVD). To mitigate wind effects, especially for the small drops, both instruments were installed within a 2∕3-scale Double Fence Intercomparison Reference (DFIR) enclosure. Two cases involved light-to-moderate wind speeds/gusts while the third case was a tornadic supercell and several squall lines that passed over the site with high wind speeds/gusts. As a proxy for turbulent intensity, maximum wind speeds from 10 m height at the instrumented site recorded every 3 s were differenced with the 5 min average wind speeds and then squared. The fall speeds vs. size from 0.1 to 2 and > 0.7 mm were derived from the MPS and the 2DVD, respectively. Consistency of fall speeds from the two instruments in the overlap region (0.7–2 mm) gave confidence in the data quality and processing methodologies. Our results indicate that under low turbulence, the mean fall speeds agree well with fits to the terminal velocity measured in the laboratory by Gunn and Kinzer from 100 µm up to precipitation sizes. The histograms of fall speeds for 0.5, 0.7, 1 and 1.5 mm sizes were examined in detail under the same conditions. The histogram shapes for the 1 and 1.5 mm sizes were symmetric and in good agreement between the two instruments with no evidence of skewness or of sub- or super-terminal fall speeds. The histograms of the smaller 0.5 and 0.7 mm drops from MPS, while generally symmetric, showed that occasional occurrences of sub- and super-terminal fall speeds could not be ruled out. In the supercell case, the very strong gusts and inferred high turbulence intensity caused a significant broadening of the fall speed distributions with negative skewness (for drops of 1.3, 2 and 3 mm). The mean fall speeds were also found to decrease nearly linearly with increasing turbulent intensity attaining values about 25–30 % less than the terminal velocity of Gunn–Kinzer, i.e., sub-terminal fall speeds.
Article
Full-text available
The Laser Precipitation Monitor (LPM) manufactured by Thies is a Present Weather sensor based on an optical disdrometer. Thus rainfall amounts can be derived from the disdrometric information. The factory calibration process for the volume measurement is described and the resulting measurement uncertainty is given. In this paper rainfall amount measured by the LPM is compared with measurements of reference rainfall gauges such as a pit gauge. Rainfall intensities are evaluated in comparison with post-processed high resolution data of a Pluvio (OTT) weighing rain gauge.
Article
Full-text available
Because knowledge of the melting level is critical to river forecasters and other users, an objective algorithm to detect the brightband height from profiles of radar reflectivity and Doppler vertical velocity collected with a Doppler wind profiling radar is presented. The algorithm uses vertical profiles to detect the bottom portion of the bright band, where vertical gradients of radar reflectivity and Doppler vertical velocity are negatively correlated. A search is then performed to find the peak radar reflectivity above this feature, and the brightband height is assigned to the altitude of the peak. Reflectivity profiles from the off-vertical beams produced when the radar is in the Doppler beam swinging mode provide additional brightband measurements. A consensus test is applied to subhourly values to produce a quality-controlled, hourly averaged brightband height. A comparison of radar-deduced brightband heights with melting levels derived from temperature profiles measured with ra-winsondes launched from the same radar site shows that the brightband height is, on average, 192 m lower than the melting level. A method for implementing the algorithm and making the results available to the public in near–real time via the Internet is described. The importance of melting level information in hydrological prediction is illustrated using the NWS operational river forecast model applied to mountainous watersheds in California. It is shown that a 2000-ft increase in the melting level can triple run off during a modest 24-h rainfall event. The ability to monitor the brightband height is likely to aid in melting-level forecasting and verification.
Article
Full-text available
Detailed measurements of the rain phenomena can be obtained from modern equipment that provides experimental drop size distributions (DSDs), which can be used to analyze the effects of past rain events or to predict their influence on colocated radio links. In this letter, the use of experimental DSDs to predict rain effects on millimeter-wave propagation is discussed from a practical point of view, taking advantage of the availability of measurements from various instruments. The derived results show that predictions can be calculated with reasonable accuracy, provided that some practical considerations are taken into account.
Article
Full-text available
Because of its simplicity, the empirical relation A = aR^{b} between the specific attenuation A and the rainrate R is often used in the calculation of rain attenuation statistics. Values for the frequency-dependent parameters a and b are available, however, for only a limited number of frequencies. Some of these values, furthermore, were obtained experimentally, and may contain errors due to limitations in the experimental techniques employed. The aR^{b} relation is shown to be an approximation to a more general relation, except in the low-frequency and optical limits. Because the approximation is a good one, however, a comprehensive and self-consistent set of values for a and b is presented in both tabular and graphical form for the frequency range f = 1-1000 GHz. These values were computed by applying logarithmic regression to Mie scattering calculations. The dropsize distributions of Laws and Parsons, Marshall and Palmer, and Joss et al., were employed to provide calculations applicable to "widespread" and "convective" rain. Empirical equations for some of the curves of a(f) and b(f) are presented for use in systems studies requiring calculations at many frequencies. Some comparison is also made with experimental results, and suggestions are given regarding application of the various calculations.
Article
Analysis of drop size distributions (DSD) measured by collocated Meteorological Particle Spectrometer (MPS) and a third-generation, low-profile, 2D-video disdrometer (2DVD) are presented. Two events from two different regions (Greeley, Colorado, and Huntsville, Alabama) are analyzed. While the MPS, with its 50-μm resolution, enabled measurements of small drops, typically for drop diameters below about 1.1 mm, the 2DVD provided accurate measurements for drop diameters above 0.7 mm. Drop concentrations in the 0.7-1.1-mm overlap region were found to be in excellent agreement between the two instruments. Examination of the combined spectra clearly reveals a drizzle mode and a precipitation mode. The combined spectra were analyzed in terms of the DSD parameters, namely, the normalized intercept parameter NW, the mass-weighted mean diameter Dm, and the standard deviation of mass spectrum σM. The inclusion of small drops significantly affected the NW and the ratio σM/Dm toward higher values relative to using the 2DVD-based spectra alone. For each of the two events, polarimetric radar data were used to characterize the variation of radar-measured reflectivity Zh and differential reflectivity Zdr with Dm from the combined spectra. In the Greeley event, this variation at S band was well captured for small values of Dm (<0.5 mm) where measured Zdr tended to 0 dB but Zh showed a noticeable decrease with decreasing Dm. For the Huntsville event, an overpass of the Global Precipitation Measurement mission Core Observatory satellite enabled comparison of satellite-based dual-frequency radar retrievals of Dm with ground-based DSD measurements. Small differences were found between the satellite-based radar retrievals and disdrometers.
Article
Empirical analyses are shown to imply variation in the shape or analytical form of the raindrop size distribution consistent with that observed experimentally and predicted theoretically. These natural variations in distribution shape are demonstrated by deriving relationships between pairs of integral rainfall parameters using a three parameter gamma drop size distribution and comparing the expressions with empirical results.-from Author
Article
An impact-type Joss-Waldvogel disdrometer (JWD), a two-dimensional video disdrometer (2DVD), and a laser optical OTT Particle Size and Velocity (PARSIVEL) disdrometer (PD) were used to measure the raindrop size distribution (DSD) over a 6-month period in Huntsville, Alabama. Comparisons indicate event rain totals for all three disdrometers that were in reasonable agreement with a reference rain gauge. In a relative sense, hourly composite DSDs revealed that the JWD was more sensitive to small drops (<1 mm), while the PD appeared to severely underestimate small drops less than 0.76 mm in diameter. The JWD and 2DVD measured comparable number concentrations of midsize drops (1-3 mm) and large drops (3-5 mm), while the PD tended to measure relatively higher drop concentrations at sizes larger than 2.44 mm in diameter. This concentration disparity tended to occur when hourly rain rates and drop counts exceeded 2.5 mm h(-1) and 400 min(-1), respectively. Based on interactions with the PD manufacturer, the partially inhomogeneous laser beam is considered the cause of the PD drop count overestimation. PD drop fall speeds followed the expected terminal fall speed relationship quite well, while the 2DVD occasionally measured slower drops for diameters larger than 2.4 mm, coinciding with events where wind speeds were greater than 4 m s(-1). The underestimation of small drops by the PD had a pronounced effect on the intercept and shape of parameters of gamma-fitted DSDs, while the overestimation of midsize and larger drops resulted in higher mean values for PD integral rain parameters.
Article
The influence of strong winds on the quality of optical Particle Size Velocity (PARSIVEL) disdrometer measurements is examined with data from Hurricane Ike in 2008 and from convective thunderstorms observed during the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) in 2010. This study investigates an artifact in particle size distribution (PSD) measurements that has been observed independently by six stationary PARSIVEL disdrometers. The artifact is characterized by a large number concentration of raindrops with large diameters (>5 mm) and unrealistic fall velocities (<1 m s−1). It is correlated with high wind speeds and is consistently observed by stationary disdrometers but is not observed by articulating disdrometers (instruments whose sampling area is rotated into the wind). The effects of strong winds are further examined with a tilting experiment, in which drops are dripped through the PARSIVEL sampling area while the instrument is tilted at various angles, suggesting that the artifact is caused by particles moving at an angle through the sampling area. Most of the time, this effect occurs when wind speed exceeds 20 m s−1, although it was also observed when wind speed was as low as 10 m s−1. An alternative quality control is tested in which raindrops are removed when their diameters exceed 8 mm and they divert from the fall velocity–diameter relationship. While the quality control does provide more realistic reflectivity values for the stationary disdrometers in strong winds, the number concentration is reduced compared to the observations with an articulating disdrometer.
Article
Optical disdrometers have the potential to be low maintenance instruments that can measure drop-size distributions and drop-size distribution moments such as rainfall rate, radar reflectivity, optical extinction, and others. As with any other measurement device, their output is affected by different sources of uncertainty. To better understand these uncertainties, we compared rainfall accumulations that were measured by three dual tipping bucket stations, three Vaisala WXT510 compact weather stations, and four Thies optical disdrometers. We detected considerable bias between disdrometers and tipping buckets, as well as among disdrometers, that we attempted to minimize through calibrating diameter measurements. Although the calibration successfully decreased bias among disdrometers, it increased bias between disdrometer rainfall accumulations and tipping bucket accumulations, indicating that there are other relevant sources of error in the estimation of rainfall accumulation from disdrometer measurements. We developed a simulation capable of mimicking the disdrometer's operation during the calibration procedure and studied the effects of using three arbitrary laser beam patterns into the measurement of the spheres' diameters. The simulation successfully reproduced some of the behavior observed in calibration data, showing the dependence of the bias and the spread of sphere diameter measurements on the studied beam patterns. Furthermore, we designed a Monte-Carlo-based simulation to propagate the uncertainties of the measurement of spheres' diameters into the estimation of rainfall accumulations. We used the calculated uncertainties to correct rainfall accumulations which improved the agreement among disdrometers, although apparently overcorrected the accumulation for one of the instruments. Moreover we found the average correction to be related to both average and standard deviation of the diameter error and insensitive to the diameter error skewness. Overall we observed considerable bias between instruments with different principles of measurements, which could not be solely explained by uncertainty in the evaluation of the diameters, or by wind effects, indicating that difficulties in the determination of the optical disdrometer's sensing area could be the cause.
Article
Measurements have been made of the fall speeds and masses of a large number of different types of solid precipitation particles. Particular attention is paid to the effects of riming and aggregation on the fall speeds and masses. Empirical expressions are given for the relationships between fall speeds and maximum dimensions and between masses and maximum dimensions for the particles studied. The results are compared with other experimental observations when they exist. The rate of increase in the mass of an ice particle due to collisions with supercooled cloud droplets (riming) and other ice particles (aggregation) as it moves through a cloud is dependent on its mass, dimensions, and fall speed. Also, in a given wind field the trajectory of a particle is determined by its fall speed, and the contribution that it makes to the precipitation rate is proportional to the product of its mass and fall speed. Consequently, as theoretical models of cloud and precipitation processes have become more refined, the need has increased for more detailed measurements of the relationships between the fall speeds, masses, and dimensions of various types of solid precipitation particles. Although several sets of measurements of the fall speeds and masses of solid precipitation particles of various types and sizes have been reported [e.g., Nakaya and Terada, 1935; Magono, 1951, 1954; Langleben, 1954; Litvinov, 1956; Bashkirova and Pershina, 1964; Brown, 1970; Zikmunda, 1972; Zikmunda and Vali, 1972], the available data are still scanty and inadequate for many purposes. Moreover, some of the previous measurements show inconsistencies, and a complete pattern to the results has not emerged. This lack of pattern is not surprising in view of the fact that Magono and Lee [1966] classify snow crystals into 80 different types, and each of these types may exist over a wide range of sizes and with various degrees of riming and aggregation. In this paper we present the results of a new set of measurements of the fall speeds and masses of a wide variety of solid precipitation particles obtained during the winter months of 1971-1972 and 1972-1973 in the Cascade Mountains of Washington. The effects of size, riming, aggregation, and density on the fall speeds and masses of different types of solid precipitation particles are considered. INSTRUMENTATION AND PROCEDURES The instrument used for measuring the fall speeds of solid precipitation particles is shown in Figure 1. The light sources consist of two incandescent lamps (18 W each), transmitted by fiber optics as two parallel beams of light separated by 4.1 cm. These two light beams are received by a similar set of fiber optics on the other side of the instrument, and the intensity of the two signals is recorded with two photomultiplier tubes. Decreases in the intensities of the beams caused by the fall of a precipitation particle through them are detected by the photomultiplier tubes and can be displayed on a storage oscilloscope. The time difference between the changes in intensity of the upper beam and those changes in the lower beam is
Article
The terminal velocities for distilled water droplets falling through stagnant air are accurately determined. More than 1500 droplets of mass from 0.2 to 100,000 micrograms, embracing droplets so small that Stokes' law is obeyed up to and including droplets so large that they are mechanically unstable, were measured by a new method employing electronic techniques. An apparatus for the production of electrically charged artificial water droplets at a controllable rate is described. The over-all accuracy of the mass-terminal-velocity measurements is better than 0.7 per cent.
Article
The inadequacy of previous calculations of terminal velocities at other than sea level conditions is discussed. Attention is called to actual measurements of terminal velocities at different air densities, and empirical formulae are presented which fit the data very closely.
Article
This paper presents a scheme that classifies reflectivity measurements from ground-based volume scanning radar into convective and stratiform precipitation types. The algorithm developed in this paper uses the neural network approach as a tool for classification. The algorithm is trained and validated on radar data obtained from 18 Weather Surveillance (WSR-88D) Doppler Radar sites in the southern United States, sampled over the period December 1997 to October 1999. For the training and validation of the neural network, classification information from a hybrid approach based on Tropical Rainfall Measurement Mission (TRMM) Precipitation Radar (PR) classification products and low-level ground radar reflectivity fields is taken as reference. The approach utilises PR's ability to observe the vertical structure of storms with high resolution (250 metres) and WSR-88D's ability to observe the low-level horizontal reflectivity magnitude and spatial variability. The method was applied to an extended dataset consisting of storm cases associated with squall line convection, mesoscale convective system, and widespread stratiform precipitation with embedded convection. The scheme is shown to have an overall 75% probability of detection, 18% false alarm rate, 67% critical success ratio, and 0.554 in prediction correlation (Cramer's V coefficient). It has an overall 4% overestimation and 27% underestimation of stratiform and convective rain areas, respectively. The classification results are compared to two current methods and improvements are shown in terms of the above statistical measures and through visual comparison with PR classification fields. The improved classification scheme derived herein will help establish more accurate radar reflectivity-rainfall relationships and improve the retrieval of diabatic heating in different cloud systems. Copyright © 2004 Royal Meteorological Society.
Article
A new rain gauge network was installed in the Great Smoky Mountains National Park (GSMNP) in the Southern Appalachians since 2007 to investigate the space–time distribution of precipitation in the inner mountain region. Exploratory Intense Observing Periods (IOPs) have been conducted in the summer and fall seasons to devise optimal long-term monitoring strategies, and Micro Rain Radars (MRR) were deployed twice in July/August and October/November 2008 at a mountain ridge location and a nearby valley. Rain gauge and MRR observations were analyzed to characterize seasonal (summer/fall) and orographic (valley/ridge) precipitation features. The data show that summer precipitation is characterized by large event-to-event variability including both stratiform and convective properties. During fall, stratiform precipitation dominates and rainfall is two times more frequent at the ridge than in the valley, corresponding to a 100% increase in cumulative rainfall at high elevation. For concurrent rain events, the orographic enhancement effect is on the order of 60%. Evidence of a seasonal signature in the drop size distribution (DSD) was found with significantly heavier tails (larger raindrops) for summer DSDs at higher elevations, whereas no significant differences were observed between ridge and valley locations during fall deployment. However, physically-based modeling experiments suggest that there are inconsistencies between the reflectivity profiles and MRR DSD estimates when large raindrop sizes are present. The number of very small drops is very high (up to two orders of magnitude) at high elevations as compared to the typical values in the literature, which cannot be explained only by fog and drizzle and suggest an important role for mixed phase processes in determining the shape of the DSD below the brightband. Because numerical modeling experiments show that coalescence is the dominant microphysical mechanism for DSD evolution for the relatively low to moderate observed rain rates characteristic of mountainous regions, it is therefore critical to clarify the shape and parameters that characterize the left-hand side of the DSD in mountainous regions. Finally, whereas low cost Micro Rain Radars (MRR) were found particularly useful for qualitative description of precipitation events and to identify rain/snow melting conditions, when compared against collocated rain gauges, MRR Quantitative Precipitation Estimation (QPE) is not reliable. Place-based calibration and reliance upon physically-based QPE retrieval algorithms can improve their utility.
Article
The first Field Intercomparison of Rainfall Intensity (RI) gauges was organised by WMO (the World Meteorological Organisation) from October 2007 to April 2009 in Vigna di Valle, Rome (Italy). The campaign is held at the Centre of Meteorological Experimentations (ReSMA) of the Italian Meteorological Service. A group of 30 previously selected rain gauges based on different measuring principles are involved in the Intercomparison. Installation of the instruments in the field was preceded by the laboratory calibration of all submitted catching-type rain gauges at the University of Genoa. Additional meteorological sensors (ancillary information) and the observations and measurements performed by the Global Climate Observing System/Global Atmosphere Watch (GCOS/GAW) meteorological station of Vigna di Valle were analyzed as metadata. All catching-type gauges were tested after installation using a portable calibration device specifically developed at the University of Genoa, simulating an ordinary calibration inspection in the field.This paper is dedicated to the summary of preliminary results of the Intercomparison measurements. It offers a view on the main achievements expected from the Intercomparison in evaluating the performance of the instruments in field conditions. Comparison of several rain gauges demonstrated the possibility to evaluate the performance of RI gauges at one-minute resolution in time, as recommended by the WMO Commission for Instruments and Methods of Observations (WMO-CIMO). Results indicate that synchronised tipping-bucket rain gauges (TBR), using internal correction algorithms, and weighing gauges (WG) with improved dynamic stability and short step response are the most accurate gauges for one-minute RI measurements, since providing the lowest measurement uncertainty with respect to the assumed working reference.
Article
Empirical analyses are shown to imply variation in the shape or analytical form of the raindrop size distribution consistent with that observed experimentally and predicted theoretically. These natural variations in distribution shape are demonstrated by deriving relationships between pairs of integral rainfall parameters using a three parameter gamma drop size distribution and comparing the expressions with empirical. There comparisons produce values for the size distribution parameters which display a systematic dependence of one of the parameters on another between different rainfall types as well as from moment to moment within a given rainfall type. The implications of this finding are explored in terms of the use of a three-parameter gamma distribution in dual-measurement techniques to determine rainfall rate.
Separation of convective and stratiform precipitation using polarimetric radar data with a support vector machine method
  • Y Wang
  • L Tang
  • P.-L Chang
  • Y.-S Tang
Physical properties of hydrometeors
  • brussaard
Assessment of drop size distributions obtained by different instruments and their application to W-band propagation studies
  • S Pérez-Peña
  • J M Riera
  • A Benarroch
  • M Munilla-Diez
  • J García-Rubia
Separation of convective and stratiform precipitation using polarimetric radar data with a support vector machine method
  • wang
Assessment of drop size distributions obtained by different instruments and their application to W-band propagation studies
  • pérez-peña