Showing papers on "Disdrometer published in 2011"

Experimental Quantification of the Sampling Uncertainty Associated with Measurements from PARSIVEL Disdrometers

Abstract: The variability of the (rain)drop size distribution (DSD) in time and space is an intrinsic property of rainfall, which is of primary importance for various environmental fields such as remote sensing of precipitation, for example. DSD observations are usually collected using disdrometers deployed at the ground level. Like any other measurement of a physical process, disdrometer measurements are affected by noise and sampling effects. This uncertainty must be quantified and taken into account in further analyses. This paper addresses this issue for the Particle Size Velocity (PARSIVEL) optical disdrometer by using a large dataset corresponding to light and moderate rainfall and collected from two collocated PARSIVELs deployed during 15 months in Lausanne, Switzerland. The relative sampling uncertainty associated with quantities characterizing the DSD—namely the total concentration of drops Nt and the median-volume diameter D0—is quantified for different temporal resolutions. Similarly, the relativ...

Rainfall Estimation with an Operational Polarimetric C-Band Radar in the United Kingdom: Comparison with a Gauge Network and Error Analysis

Abstract: The estimate of rainfall using data from an operational dual-polarized C-band radar in convective storms in southeast United Kingdom is compared against a network of gauges. Four different rainfall estimators are considered: reflectivity–rain-rate (Z–R) relation, with and without correcting for rain attenuation; a composite estimator, based on (i) Z–R, (ii) R(Z, Zdr), and (iii) R(Kdp); and exclusively R(Kdp). The various radar rain-rate estimators are developed using Joss disdrometer data from Chilbolton, United Kingdom. Hourly accumulations over radar pixels centered on the gauge locations are compared, with approximately 2500 samples available for gauge hourly accumulations > 0.2 mm. Overall, the composite estimator performed the “best” based on robust statistical measures such as mean absolute error, the Nash–Sutcliffe coefficient, and mean bias, at all rainfall thresholds (>0.2, 1, 3, or 6 mm) with improving measures at the higher thresholds of >3 and >6 mm (higher rain rates). Error variance ...

Drop size distribution comparisons between Parsivel and 2-D video disdrometers

Abstract: . Measurements from a 2-D video disdrometer (2DVD) have been used for drop size distribution (DSD) comparisons with co-located Parsivel measurements in Huntsville, Alabama. The comparisons were made in terms of the mass-weighted mean diameter, Dm, the standard deviation of the mass-spectrum, σm, and the rainfall rate, R, all based on 1-min DSD from the two instruments. Time series comparisons show close agreement in all three parameters for cases where R was less than 20 mm h−1. In four cases, discrepancies in all three parameters were seen for "heavy" events, with the Parsivel showing higher Dm, σm and R, when R reached high values (particularly above 30 mm h−1). Possible causes for the discrepancies include the presence of a small percentage of non-fully melted hydrometers, with higher than expected fall velocity and with very different axis ratios as compared with rain, indicating small hail or ice pellets or graupel. We also present here Parsivel-to-Parsivel comparisons as well as comparisons between two 2DVD instruments, namely a low-profile unit and the latest generation, "compact unit" which was installed at the same site in November 2009. The comparisons are included to assess the variability between the same types of instrument. Correlation coefficients and the fractional standard errors are compared.

A network of disdrometers to quantify the small‐scale variability of the raindrop size distribution

Abstract: Insight into the spatial variability of the (rain) drop size distribution (DSD), and hence rainfall, is of primary importance for various environmental applications like cloud/precipitation microphysical processes, numerical weather modeling, and estimation of rainfall using remote sensing techniques. In order to quantify the small-scale variability of the DSD, a network of 16 optical disdrometers has been designed and deployed over a typical operational weather radar pixel (about 1 x 1 km(2)) in Lausanne, Switzerland. This network is fully autonomous in terms of power supply as well as data transmission and storage. The combination of General Radio Packet Service and radio communication allows a real-time access to the DSD measurements. The network is sampling at a temporal resolution of 30 s. A period representative of frontal precipitation is analyzed to illustrate the measurement capabilities of the network. The spatial variability is quantified by the coefficient of variation of the total concentration of drops, the mass-weighted diameter, and the rain rate between the 16 stations of the network. The sampling uncertainty associated with disdrometer measurements is taken into account, and the analysis of a 1.5 month rainy period shows a significant variability of these quantities, which cannot be explained by the sampling uncertainty alone, even at such a small scale.

Winter Precipitation Microphysics Characterized by Polarimetric Radar and Video Disdrometer Observations in Central Oklahoma

Abstract: The study of precipitation in different phases is important to understanding the physical processes that occur in storms, as well as to improving their representation in numerical weather prediction models. A 2D video disdrometer was deployed about 30 km from a polarimetric weather radar in Norman, Oklahoma, (KOUN) to observe winter precipitation events during the 2006/07 winter season. These events contained periods of rain, snow, and mixed-phase precipitation. Five-minute particle size distributions were generated from the disdrometer data and fitted to a gamma distribution; polarimetric radar variables were also calculated for comparison with KOUN data. It is found that snow density adjustment improves the comparison substantially, indicating the importance of accounting for the density variability in representing model microphysics.

Characteristics of the raindrop size distribution for freezing precipitation observed in southern China

Abstract: [1] Freezing precipitation (i.e., freezing rain or freezing drizzle) is an extremely hazardous weather that can cause severe socioeconomic loss and compromise human safety. To better document and understand the microphysics of this type of precipitation, drop size spectra were collected with an optical disdrometer during a freezing precipitation event on 27 January 2008 in southern China. The drop size distribution (DSD) characteristics, the correlations between the shape (μ), slope (Λ), and intercept (N0) of the gamma distribution, and the relations between the reflectivity (Z) and rainfall rate (R) have been investigated. It was found that the DSDs of freezing precipitation were characterized by weak stratiform rain with a small mass-weighted diameter (Dm, 0.63 mm) and a large normalized intercept (Nw, 4.25 log10 mm−1 m−3). This indicated that freezing precipitation was not formed by the melting of larger, dry snowflakes but by the melting of smaller, rimed ice particles. Furthermore, the derived μ-Λ, N0-μ, and Z-R relations are distinctly different from the convective rains or tropical stratiform rains reported in the literature.

Understanding the Variability of Z-R Relationships Caused by Natural Variations in Raindrop Size Distributions (DSD): Implication of Drop Size and Number

Abstract: In the issue of rainfall estimation by radar through the necessary relationship between radar reflectivity Z and rain rate R (Z-R), the main limitation is attributed to the variability of this relationship. Indeed, several pre-vious studies have shown the great variability of this relationship in space and time, from a rainfall event to another and even within a single rainfall event. Recent studies have shown that the variability of raindrop size distributions and thereby Z-R relationships is therefore, more the result of complex dynamic, thermody-namic and microphysical processes within rainfall systems than a convective/stratiform classification of the ground rainfall signature. The raindrop number and size at ground being the resultant of various processes mentioned above, a suitable approach would be to analyze their variability in relation to that of Z-R relation-ship. In this study, we investigated the total raindrop concentration number NT and the median volume di-ameter D0 used in numerous studies, and have shown that the combination of these two ‘observed’ parame-ters appears to be an interesting approach to better understand the variability of the Z-R relationships in the rainfall events, without assuming a certain analytical raindrop size distribution model (exponential, gamma, or log-normal). The present study is based on the analysis of disdrometer data collected at different seasons and places in Africa, and aims to show the degree of the raindrop size and number implication in regard to the Z-R relationships variability.

Analysis of raindrop size distribution characteristics in Malaysia for rain attenuation prediction

Abstract: The assessment of the variability of rainfall characteristics in the equatorial regions is a key problem in estimating adequate fade margin due to rain attenuation in satellite communication systems. Based on one year of disdrometer data that have been collected in Kuala Lumpur, Malaysia, this paper investigates the general characteristics of the raindrop size distribution (DSD) and the dependence of the rain attenuation on the DSD. Its diurnal variation and the role of critical diameter values on the estimation of the specific attenuation are also discussed. Preliminary results suggest that satellite links operating in the afternoon and early evening hours should be provided with an extra fade margin to compensate for rain attenuation impairments.

Comparison of Estimators for Parameters of Gamma Distributions with Left-Truncated Samples

Abstract: When fitting a raindrop size distribution using a gamma model from data collected by a disdrometer, some consideration needs to be given to the small drops that fail to be recorded (typical disdrometer minimum size thresholds being in the 0.3–0.5-mm range). To this end, a gamma estimation procedure using maximum likelihood estimation has recently been published. The current work adds another procedure that accounts for the left-truncation problem in the data; in particular, an L-moments procedure is developed. These two estimation procedures, along with a traditional method-of-moments procedure that also accounts for data truncation, are then compared via simulation of volume samples from known gamma drop size distributions. For the range of gamma distributions considered, the maximum likelihood and L-moments procedures—which perform comparably—are found to outperform the procedure of method-of-moments. As these three procedures do not yield simple estimates in closed form, salient details of the...

Improving the rainfall rate estimation in the midstream of the Heihe River Basin using raindrop size distribution

Abstract: . During the intensive observation period of the Watershed Allied Telemetry Experimental Research (WATER), a total of 1074 raindrop size distribution were measured by the Parsivel disdrometer, the latest state-of-the-art optical laser instrument. Because of the limited observation data in Qinghai-Tibet Plateau, the modelling behaviour was not well done. We used raindrop size distributions to improve the rain rate estimator of meteorological radar in order to obtain many accurate rain rate data in this area. We got the relationship between the terminal velocity of the raindrop and the diameter (mm) of a raindrop: v(D) = 4.67D0.53. Then four types of estimators for X-band polarimetric radar are examined. The simulation results show that the classical estimator R (ZH) is most sensitive to variations in DSD and the estimator R (KDP, ZH, ZDR) is the best estimator for estimating the rain rate. An X-band polarimetric radar (714XDP) is used for verifying these estimators. The lowest sensitivity of the rain rate estimator R (KDP, ZH, ZDR) to variations in DSD can be explained by the following facts. The difference in the forward-scattering amplitudes at horizontal and vertical polarizations, which contributes KDP, is proportional to the 3rd power of the drop diameter. On the other hand, the exponent of the backscatter cross-section, which contributes to ZH, is proportional to the 6th power of the drop diameter. Because the rain rate R is proportional to the 3.57th power of the drop diameter, KDP is less sensitive to DSD variations than ZH.

Optimal Determination of Parameters for Gamma-Type Drop Size Distributions Based on Moments

Abstract: Measured raindrop size distributions are often approximated by analytical functions. The parameters determining such functions are usually derived from measured data. This procedure can suffer from various uncertainties. The most important of which are (i) the limited detection range of measuring devices such as, for example, disdrometers, and (ii) poor statistics resulting from the rare appearance of relatively large drops.One way to derive the parameters is the moments method that has a degree of freedom in the choice of the moments. The aim of this study is to find an optimal choice of moments. To this end, numerical experiments are performed by calculating random samples from drop populations with gamma-shaped size distributions. These samples are evaluated as they were recorded by an ideal disdrometer whose single limitation is the cutoff with respect to very small and very large raindrops. From that data the parameters mentioned above are determined by the moments method. The truncation of t...

Rain cell size statistics from rain gauge data for site diversity planning and attenuation prediction

Abstract: Cell size statistics for the rainfall types in Durban, South Africa have been derived from a five-year, one-hour integration time series rain gauge network data. The data have been converted to their equivalent one-minute integration time values and processed to determine the cell sizes for different rain rate above threshold values between 3 mm/h and 20 mm/h. The results are then compared to the cell size statistics from oneminute integration time data from disdrometer measurements taken for two years. By the use of the terrestrial link attenuation data obtained for Durban, the advection velocity for the stratiform rain has been derived as 6 m/s. The equivalent rain cell diameter model which is an input requirement in the proposed rain attenuation prediction equation and site diversity separation distance has been derived.

A Robust Error-Based Rain Estimation Method for Polarimetric Radar. Part I: Development of a Method

Abstract: The algorithms used to estimate rainfall from polarimetric radar variables show significant variance in error characteristics over the range of naturally occurring rain rates. As a consequence, to improve rainfall estimation accuracy using polarimetric radar, it is necessary to optimally combine a number of different algorithms. In this study, a new composite method is proposed that weights the algorithms by the inverse of their theoretical error. A number of approaches are discussed and are investigated using simulated radar data calculated from disdrometer measurements. The resultant algorithms show modest improvement over composite methods based on decision-tree logic—in particular, at rain rates above 20 mm h−1.

The Linear Relationship Between Attenuation and Average Rainfall Rate for Terrestrial Links

Abstract: Attenuation measured on terrestrial links can be used to estimate surface rainfall rates along a link path. There is current interest in the use of inverse methods to estimate rainfall over areas inferred from terrestrial links. A power-law relationship can be used to estimate rainfall rate from attenuation measured by a terrestrial link. However, the inverse method is simplified with a linear relationship between attenuation and rainfall rate when determining path averaged rainfall rates. This paper investigates power-law and linear relationships between rainfall rate and attenuation. It examines their goodness of fit for a range of frequencies and rainfall rates. Six years of disdrometer data from Chilbolton in the U.K. has been used to study both linear and power-law fits to attenuation and quantify the associated errors.

Constraining CloudSat‐based snowfall profiles using surface observations and C‐band ground radar

Abstract: [1] The CloudSat Precipitation Radar, launched in 2006, provides vertical profiles of W-band (94 GHz) reflectivity and is sensitive to falling snow through all but the most intense precipitating cloud structures. Precipitation retrievals of falling snow are affected by a wide diversity of factors describing the medium, such as snow particle shape, size, and composition, which in turn are controlled by ambient factors including the environmental temperature and humidity. Because satellite-based radiometric sounders such as the Microwave Humidity Sounder (MHS) operate without the benefit of coordinated space radar observations, microphysical descriptions of the snow particle medium derived from CloudSat or other radar observations are beneficial to passive microwave (PMW) radiometer-only snowfall retrieval methods. At the coarse scale of these PMW observations, the radiative signal due to the snow is relatively weak compared to the contributions from the atmosphere and the land surface emissivity. Using the C-band (5 GHz) polarization-agile King City radar (WKR) operated by Environment Canada, we examined the vertical structure of winter precipitation events from coordinated overpasses of CloudSat and NOAA 18 (MHS). Two-dimensional video disdrometer observations are used to limit (constrain) the range of the drop-size distribution parameters that are provided through a priori databases to dual-frequency (C/W-band) radar retrieval. Bayesian retrievals using the constrained database produce water content profiles that more closely replicate the observed radar reflectivity profiles and transition smoothly between the single-frequency (CloudSat only) and dual-frequency regions.

A Maximum Entropy Modelling of the Rain Drop Size Distribution

Abstract: This paper presents a maximum entropy approach to Rain Drop Size Distribution (RDSD) modelling. It is shown that this approach allows (1) to use a physically consistent rationale to select a particular probability density function (pdf) (2) to provide an alternative method for parameter estimation based on expectations of the population instead of sample moments and (3) to develop a progressive method of modelling by updating the pdf as new empirical information becomes available. The method is illustrated with both synthetic and real RDSD data, the latest coming from a laser disdrometer network specifically designed to measure the spatial variability of the RDSD.

Comparison between empirical lognormal and gamma rain drop-size distribution models for Indian region

Abstract: The disdrometer data on rain drop sizes, collected at Ahmedabad (India), has been analyzed to develop empirical model on drop-size distribution (DSD). Analyses has been carried out to find suitability of lognormal or gamma distribution. The lognormal distribution has been found to give better correlation. The data has been analysed on annual and monthly basis to find out seasonal variability. Statistical analysis on rate of rain fall was carried out on monthly and annual basis to derive rain rate exceedance characteristic. August appears to be worst month from the attenuation point of view.

Rain research with disdrometers: a bibliometric review

Abstract: . This study analyses the research on disdrometers based on published studies. To do so, a wide data base of bibliographic references has been used: the Web of Science (published by Thomson Reuters). The search was carried out for all of the articles whose "TOPIC" was disdrometer. The more than 300 articles found were analysed according to various criteria: countries with research using disdrometers; publication dates; evolution of the number of articles; concepts studied and research lines followed in each article; and finally, a bibliometric analysis of the more than 60 journals where these articles have been published. Since 1963, there has been an increase in the number of articles published on disdrometers, which in the last 20 yr has been more than ten times higher than the increase in the number of articles on meteorology.

Rainfall rate modeling for various rainfall types in South Africa

Abstract: This paper models the rainfall rate in Durban (29° 97′S, 30°95′E), South Africa for four different rainfall classifications: drizzle, widespread, shower and thunderstorm. Twelve-months data obtained in 2009 via the Joss_Waldvogel disdrometer, gave a total of 729 independent rain events and 5,542,147 raindrops. The measured rain rate probability functions were compared with the Moupfouma model using 1-minute integration time. The maximum likelihood estimators were adopted for the parameters of two other statistical models, the lognormal and gamma models. The results show that gamma model had the closest performance for the various rain classifications under consideration.

A study of the raindrop size distribution and its effect on microwave attenuation

Abstract: Current frequency allocations below Ku- band are becoming increasingly congested. The problem continues to grow as the use of telecommunications becomes more pop- ular. In order to compensate for increased in demand, telecommunication operating frequencies have to be raised, yielding larger channel capacity. However, raising the operating frequency has the adverse result of intensifying the attenuation effects of the troposphere. At high frequencies, such as V-band, rain is known to cause the most severe attenuation. It has been shown that model data for numerical weather prediction, combined with propagation prediction models, can be used to forecast telecommunication link atten- uation. The forecast of attenuation can be used to improve the effectiveness of fade mitigation techniques. A key problem in determining rain attenuation from rainfall rate is the considerable variability of the raindrop size distribution (DSD). Based on the analysis of disdrometer and numerical weather prediction model data, the aim of this work is to constrain the parameters of the DSD and ultimately generate proba- bilistic forecasts of attenuation. Using disdrometer analysis, a relationship between mean raindrop size and rainfall rate is established, and a link between drop concentration and rainfall rate is also demon- strated. The DSD is shown to vary with season and shows some small relationships with meteorological parameters. It is concluded that, despite some relation of the DSD with rainfall rate and meteorological parameters, there are too many variables to conclusively recreate the DSD. Analysis of the attenuation-rainfall rate relationship illustrates that variability due to the DSD increases with frequency above 40GHz. At 30-40GHz the DSD is shown to have little impact on the attenuation-rainfall rate relationship. Finally, this thesis examines the horizontal structure of rain cells in the UK, demonstrating the suitability of the EXCELL model and illustrating that rain cells are more elliptical in the UK.

Sensitivity of C-Band Polarimetric Radar-Based Drop Size Distribution Measurements to Maximum Diameter Assumptions

Abstract: The estimation of rain drop size distribution (DSD) parameters from polarimetric radar observations is accomplished by first establishing a relationship between differential reflectivity (Z(sub dr)) and the central tendency of the rain DSD such as the median volume diameter (D0). Since Z(sub dr) does not provide a direct measurement of DSD central tendency, the relationship is typically derived empirically from rain drop and radar scattering models (e.g., D0 = F[Z (sub dr)] ). Past studies have explored the general sensitivity of these models to temperature, radar wavelength, the drop shape vs. size relation, and DSD variability. Much progress has been made in recent years in measuring the drop shape and DSD variability using surface-based disdrometers, such as the 2D Video disdrometer (2DVD), and documenting their impact on polarimetric radar techniques. In addition to measuring drop shape, another advantage of the 2DVD over earlier impact type disdrometers is its ability to resolve drop diameters in excess of 5 mm. Despite this improvement, the sampling limitations of a disdrometer, including the 2DVD, make it very difficult to adequately measure the maximum drop diameter (D(sub max)) present in a typical radar resolution volume. As a result, D(sub max) must still be assumed in the drop and radar models from which D0 = F[Z(sub dr)] is derived. Since scattering resonance at C-band wavelengths begins to occur in drop diameters larger than about 5 mm, modeled C-band radar parameters, particularly Z(sub dr), can be sensitive to D(sub max) assumptions. In past C-band radar studies, a variety of D(sub max) assumptions have been made, including the actual disdrometer estimate of D(sub max) during a typical sampling period (e.g., 1-3 minutes), D(sub max) = C (where C is constant at values from 5 to 8 mm), and D(sub max) = M*D0 (where the constant multiple, M, is fixed at values ranging from 2.5 to 3.5). The overall objective of this NASA Global Precipitation Measurement Mission (GPM/PMM Science Team)-funded study is to document the sensitivity of DSD measurements, including estimates of D0, from C-band Z(sub dr) and reflectivity to this range of D(sub max) assumptions. For this study, GPM Ground Validation 2DVD's were operated under the scanning domain of the UAHuntsville ARMOR C-band dual-polarimetric radar. Approximately 7500 minutes of DSD data were collected and processed to create gamma size distribution parameters using a truncated method of moments approach. After creating the gamma parameter datasets the DSD's were then used as input to a T-matrix model for computation of polarimetric radar moments at C-band. All necessary model parameterizations, such as temperature, drop shape, and drop fall mode, were fixed at typically accepted values while the D(sub max) assumption was allowed to vary in sensitivity tests. By hypothesizing a DSD model with D(sub max) (fit) from which the empirical fit to D0 = F[Z(sub dr)] was derived via non-linear least squares regression and a separate reference DSD model with D(sub max) (truth), bias and standard error in D0 retrievals were estimated in the presence of Z(sub dr) measurement error and hypothesized mismatch in D(sub max) assumptions. Although the normalized standard error for D0 = F[Z(sub dr)r] can increase slightly (as much as from 11% to 16% for all 7500 DSDs) when the D(sub max) (fit) does not match D(sub max) (truth), the primary impact of uncertainty in D(sub max) is a potential increase in normalized bias error in D0 (from 0% to as much as 10% over all 7500 DSDs, depending on the extent of the mismatch between D(sub max) (fit) and D(sub max) (truth)). For DSDs characterized by large Z(sub dr) (Z(sub dr) > 1.5 to 2.0 dB), the normalized bias error for D0 estimation at C-band is sometimes unacceptably large (> 10%), again depending on the extent of the hypothesized D(sub max) mismatch. Modeled errors in D0 retrievals from Z(sub dr) at C-band are demonstrated in detail and comparedo similar modeled retrieval errors at S-band and X-band where the sensitivity to D(sub max) is expected to be less. The impact of D(sub max) assumptions to the retrieval of other DSD parameters such as Nw, the liquid water content normalized intercept parameter, are also explored. Likely implications for DSD retrievals using C-band polarimetric radar for GPM are assessed by considering current community knowledge regarding D(sub max) and quantifying the statistical distribution of Z(sub dr) from ARMOR over a large variety of meteorological conditions. Based on these results and the prevalence of C-band polarimetric radars worldwide, a call for more emphasis on constraining our observational estimate of D(sub max) within a typical radar resolution volume is made

Optimal rain rate estimation algorithm for light and heavy rain using polarimetric measurements

Abstract: . In this paper, we propose an ameliorated physically-based rain rate estimation algorithm for semi-arid regions using the Rayleigh approximation. The proposed algorithm simultaneously uses the reflectivity and the specific differential phase to provide an accurate estimation for both small and large rain rates. In order to validate the proposed estimator, simulated polarimetric rain rate data based on a dual approach, referring to both physical and statistical models of the rain target, are used. Moreover, experimental radar data (the same as used in Matrosov et al., 2006) taken in light to moderate stratiform rainfalls with rain rates varying between 2 and 15 mm h−1 were collected as part of the GPM pilot experiment. It is shown that the proposed algorithm for rain rate estimation based on the full set of polarimetric radar measurements agree better with in situ disdrometer ones.

Progress of Measurement of Raindrop Size Distribution

Abstract: The main parameters in detecting raindrop size distribution are overviewed,and the traditional measuring methods are discussed separately.Most of them possess the shortcomings of low measurement precision,big error,workload and poor real-time response.New type measurement raindrops spectrum toollaser disdropmeter are developed,and the measurement principle of disdropmeter are analyzed in detail. Compared to traditional measurements,the virtues of disdropmeter in detecting raindrop size distribution are pointed out.The applications of disdropmeter,such as raindrop size distribution,rainfall measurement and the correction of weather radar et al are given.The developments of laser disdrometer technology at home and abroad are displayed.The research work progress on laser disdrometer of the Key Laboratory of Atmospheric Composition and Optical Radiation,CAS,is introduced.The experiments carried out with laser disdrometer in recent years are simply discussed.

Preliminary analysis of ITU-R rain attenuation time series synthesizers in equatorial Kuala Lumpur

Abstract: This contribution investigates the performance of the model currently recommended by ITU-R for the synthesis of rain attenuation time series (recommendation P1853) The Synthetic Storm Technique (SST) is used to generate reference rain attenuation time series from one year of 1-minute integrated rain rate samples collected by means of a disdrometer located at Kuala Lumpur, Malaysia First-and second-order statistics calculated from the reference and synthetic rain attenuation time series are compared In addition, fade duration statistics as estimated by the prediction model in ITU-R recommendation P1623 are included in the comparison Preliminary results suggest an acceptable performance of the time series synthesizer included by ITU-R in recommendation P1853 However, further investigations on the input parameters to such model (eg higher operating frequencies, lower elevation angles and dynamic parameterβ) are necessary to confirm its applicability in this specific equatorial region

Drop Size Distribution Measurements Supporting the NASA Global Precipitation Measurement Mission: Infrastructure and Preliminary Results

Abstract: Global Precipitation Measurement Mission (GPM) retrieval algorithm validation requires datasets that characterize the 4-D structure, variability, and correlation properties of hydrometeor particle size distributions (PSD) and accumulations over satellite fields of view (5 -- 50 km). Key to this process is the combined use of disdrometer and polarimetric radar platforms. Here the disdrometer measurements serve as a reference for up-scaling dual-polarimetric radar observations of the PSD to the much larger volumetric sampling domain of the radar. The PSD observations thus derived provide a much larger data set for assessing DSD variability, and satellite-based precipitation retrieval algorithm assumptions, in all three spatial dimensions for a range of storm types and seasons. As one component of this effort, the GPM Ground Validation program recently acquired five 3rd generation 2D Video disdrometers as part of its Disdrometer and Radar Observations of Precipitation Facility (DROP), currently hosted in northern Alabama by the NASA Marshall Space Flight Center and the University of Alabama in Huntsville. These next-generation 2DVDs were operated and evaluated in different phases of data collection under the scanning domain of the UAH ARMOR C-band dual-polarimetric radar. During this period approximately 7500 minutes of PSD data were collected and processed to create gamma size distribution parameters using a truncated method of moments approach. After creating the gamma parameter datasets the DSDs were then used as input to T-matrix code for computation of polarimetric radar moments at C-band. The combined dataset was then analyzed with two basic objectives in mind: 1) the investigation of seasonal variability in the rain PSD parameters as observed by the 2DVDs; 2) the use of combined polarimetric moments and observed gamma distribution parameters in a functional form to retrieve PSD parameters in 4-D using the ARMOR radar for precipitation occurring in different seasons and for different rain system types. Preliminary results suggest that seasonal variations in the DSD parameters do occur, but are most pronounced when comparing tropical PSDs to either winter or summer convective precipitation. For example the previously documented shift to relatively smaller drop diameters in higher number concentrations for equivalent rain rate bins was observed in tropical storm rainbands occurring over Huntsville. On a more inter seasonal basis empirical fits between parameters such as D0 and ZDR do not appear to exhibit robust seasonal biases- i.e., one fit seems to work for all seasons within acceptable standard error (O[10%]) for estimates of D0. In polarimetric retrievals of the vertical variability in PSD (rain layer) for a tropical rainband we find that the Do varies with height when partitioned by specified precipitation categories (e.g., convective or stratiform, heavy and light stratiform etc.) but this variation is of order 10-20% and is smaller than the difference in D0 observed between the basic delineation of convective and stratiform precipitation types. Currently we are expanding our analysis of the vertical structure of the PSD to include several seasonally and/or dynamically-different storm system types (e.g., winter convection and stratiform events; summer mid-latitude convective etc.) sampled by ARMOR. The study will present the results of our combined analyses.

An Introduction to Rain Gauges and Disdrometers

Abstract: An introduction to the main ground-based instruments to measure rain and distribution of drop size is illustrated. Rain gauge and disdrometer principles will be briefly described to provide a useful overview of the local in situ measurement techniques. The measurements of drop size distribution provide a powerful opportunity to investigate the storm microphysics to be used in synergy with ground-based remote sensing instrument like radars. For this reason, in this chapter, the significance of drop size distribution is explained, the most common instruments to measure it are described and methods and models to interpret the derived quantities are given.