Showing papers on "Disdrometer published in 2005"

Correction of Radar Reflectivity and Differential Reflectivity for Rain Attenuation at X Band. Part II: Evaluation and Application

Abstract: In this paper, the attenuation-correction methodology presented in Part I is applied to radar measurements observed by the multiparameter radar at the X-band wavelength (MP-X) of the National Research Institute for Earth Science and Disaster Prevention (NIED), and is evaluated by comparison with scattering simulations using ground-based disdrometer data. Further, effects of attenuation on the estimation of rainfall amounts and drop size distribution parameters are also investigated. The joint variability of the corrected reflectivity and differential reflectivity show good agreement with scattering simulations. In addition, specific attenuation and differential attenuation, which are derived in the correction procedure, show good agreement with scattering simulations. In addition, a composite rainfall-rate algorithm is proposed and evaluated by comparison with eight gauges. The radar-rainfall estimates from the uncorrected (or observed) ZH produce severe underestimation, even at short ranges from...

Drop Axis Ratios from a 2D Video Disdrometer

Abstract: Results from an experiment to measure the drop shapes using a 2D video disdrometer (2DVD) are reported. Under calm conditions, drops were generated from a hose located on a bridge 80 m above ground, this height being sufficient to allow drop oscillations to reach a steady state. The disdrometer data had to be carefully processed so as to eliminate the drops mismatched by the instrument and to remove the system spreading function. The total number of drops analyzed was around 115 000. Their axis ratio distributions were obtained for diameters ranging from 1.5 to 9 mm. The mean axis ratio decreases with increasing drop diameter, in agreement with the upper bound of the Beard and Chuang equilibrium shape model. The inferred mode of oscillation appears to be dominated by the oblate–prolate axisymmetric mode for the diameter range of 1.5 to 9 mm. The mean axis ratio agrees well with two empirically fitted formulas reported in earlier studies. In addition, a linear fit was applied to the data for radar...

Error Characteristics of Rainfall Measurements by Collocated Joss–Waldvogel Disdrometers

Abstract: Error characteristics of rainfall measurements were studied using six collocated Joss–Waldvogel (JW) disdrometers that are located at NASA’s Wallops Flight Facility. The six disdrometer means of rain rate R, reflectivity Z, and differential reflectivity ZDR, for a given minute were considered as a reference. The maximum deviations of R, Z, and ZDR from the mean in a rain event were 0.6 mm h−1, 1.3 dB, and 0.05 dB, respectively. Rainfall statistics were then examined between disdrometer pairs. The root-mean-square (rms) difference of R, Z, and ZDR between paired disdrometers in a rain event were as high as 3.2 mm h−1, 3.7 dB, and 0.3 dB, respectively. The rms difference of R and ZDR were even higher when the disdrometer observations were stratified based on reflectivity intervals. The differences in disdrometer rainfall measurements have a potential impact when the disdrometers are considered as calibration tools for vertically pointing and scanning radars. The differences between the disdrometer ...

Intraseasonal variation of raindrop size distribution at Koto Tabang, West Sumatra, Indonesia

Abstract: [1] Intraseasonal variation of raindrop size distribution (DSD) in response to Madden Julian Oscillation (MJO) is studied using a 2D video disdrometer (2DVD), a boundary layer radar (BLR) and the Equatorial Atmosphere Radar, operated at Koto Tabang, west Sumatra, as well as GOES-9 infra-red brightness temperature. As a parameter of DSD, ΔZMP, which is defined as the difference between a measured radar reflectivity in dB and that from the Marshall-Palmer (MP) radar reflectivity (Z) - rain rate (R) relationship, Z = 200 R1.6, is used. It is found that in non-active phase of MJO, 2DVD-derived ΔZMPs are generally positive, indicating that DSDs are broad, while they decrease toward negative values as the phase of MJO shifts to active ones. Rain-top height derived from the BLR indicates that the convective processes are more intense in the non-active MJO phase than in the active phase, which would cause the difference in DSDs.

On the shape-slope relation of drop size distributions in convective rain

Abstract: The relation between the slope and shape parameters of the raindrop size distribution parameterized by a gamma distribution is examined. The comparison of results of a simple rain shaft model with an empirical relation based on disdrometer measurements at the surface shows very good agreement, but a more detailed discussion reveals some difficulties—for example, deviations from the gamma shape and the overestimation of collisional breakup.

High Temporal Resolution Path-Average Rain Gauge with 50-GHz Band Microwave

Abstract: Rain radar measures instantaneous spatial-average rainfall, while conventional rain gauges directly measure point rainfall with low temporal resolution. Thus differences in the resolution of the sensors create difficulties for rain radar validation, especially for spaceborne rain radar. Accordingly, rainfall measurement by microwave link has been proposed for several decades, as it estimates instantaneous path-average rainfall. Thus it is expected that the microwave link rain gauge will overcome, at least partly, the problems in the rain radar validation, toward which a 50-GHz band microwave link [the path-averaged rain gauge (PRG)] was developed that has been in operation since September 2000. In this paper, the authors show the potential of the PRG system by a simple model and rainfall comparison with a disdrometer and a tipping-bucket rain gauge. Differences observed by the instruments were within 15% (within 10% in half of the cases) during actual rain events in 2003. This confirmed that the ...

Monitoring the Reflectivity Calibration of a Scanning Radar Using a Profiling Radar and a Disdrometer

Abstract: This paper describes a method of absolutely calibrating and routinely monitoring the reflectivity calibration from a scanning weather radar using a vertically profiling radar that has been absolutely calibrated using a collocated surface disdrometer. The three instruments have different temporal and spatial resolutions, and the concept of upscaling is used to relate the small resolution volume disdrometer observations with the large resolution volume scanning radar observations. This study uses observations collected from a surface disdrometer, two profiling radars, and the National Weather Service (NWS) Weather Surveillance Radar-1988 Doppler (WSR-88D) scanning weather radar during the Texas–Florida Underflight-phase B (TEFLUN-B) ground validation field campaign held in central Florida during August and September 1998. The statistics from the 2062 matched profiling and scanning radar observations during this 2-month period indicate that the WSR-88D radar had a reflectivity 0.7 dBZ higher than th...

Observations and analysis of uncorrelated rain

Abstract: Most microphysical models in precipitation physics and radar meteorology assume (at least implicitly) that raindrops are completely uncorrelated in space and time. Yet, several recent studies have indicated that raindrop arrivals are often temporally and spatially correlated. Resolution of this conflict must begin with observations of perfectly uncorrelated rainfall, should such “perfectly steady rain” exist at all. Indeed, it does. Using data with high temporal precision from a two-dimensional video disdrometer and the pair-correlation function, a scale-localized statistical tool, several ∼10–20-min rain episodes have been uncovered where no clustering among droplet arrival times is found. This implies that (i) rain events exist where current microphysical models can be tested in an optimal manner and (ii) not all rain can be properly described using fractals.

Terminal settling velocity measurements of volcanic ash during the 2002-2003 Etna eruption by an X-band microwave rain gauge disdrometer

Abstract: [1] This is the first report in the scientific literature of direct measurement of the terminal settling velocity of volcanic particles during an eruption. Field measurements using a continuous wave X-band disdrometer were carried out at Mt. Etna on 18 and 19 December 2002, when the explosive activity produced a 4 km high volcanic plume. These data allow the estimation of the intensity of the fallout and the measurement of the terminal settling velocities of the volcanic particles in real-time. The main results are: (1) the tested instrument detected coherent falling volcanic particles from 0.2 to 1 mm diameter; (2) measured terminal settling velocities were in agreement with both experimental and theoretical methods; (3) however, the measured velocities were clustered around few discrete values, rather than a range of velocities as would be expected if the particles were falling simultaneously and discretely. This new methodology has many new applications for local hazard mitigation and improved understanding of fallout processes.

Raspodjela kapi tropskih kiša po veličini nad južnom Indijom

Abstract: Drop size distributions (DSD) associated with tropical rainfall at Cuddalore in the south-eastern part of India have been measured by a JossWaldvogel disdrometer (RD–80 model) during September to November 2002. The rainfall data corrected for instrumental error, matches very well with rainfall rates measured by a self recording raingauge, at the same site. For further analysis of the DSD, the rainfall events were separated into convective and stratiform rainfall by an algorithm based on variation of DSD parameters. One rain event in the form of a squall line of 15 September 2002, was analysed in greater detail to investigate the validity of the classification scheme as well as to study the variation of the DSD parameters during the course of a rain event. It was observed that, the algorithm was robust and had quite good correspondence with other independent rainfall separation algorithms. During the rain event, at low rainrates, the convective phase of the rainfall event was marked by DSD spectra that have greater population of small droplets as compared to stratiform DSDs at the same rainrates. At higher rainrates, the convective regime is characterised by narrow spectra centred at higher diameters. At the transition region between convective and stratiform spectra, mixed large and small drop spectra are observed. Similar variation was also observed in the averaged drop spectra. In addition, the averaged spectra also reveal an equilibrium distribution of the drop population in DSDs at higher rainrates (>39 /hr) for diameter range (>1.91 mm) corresponding to nearly constant values of the slope of the distribution, the intercept and the mean mass diameter. The value of the shape parameter, which for small rainrates varies the same as the slope parameter, starts to increase with increasing rainrate as the other two parameters of the gamma distribution approach a constant value corresponding to equilibrium shape. The value of the intercept parameter is highest for low to moderate convective rainfall and decreases as the rainrate increases.

DSD characterization and computations of expected reflectivity using data from a two-dimensional video disdrometer deployed in a tropical environment

Abstract: A two-dimensional video disdrometer (2DVD) has been deployed in Puerto Rico northern coastal zone since mid- August 2004. An initial drop-size distribution (DSD) characterization has been performed to compare with previous results of other studies made in the island of Puerto Rico. The event studied was Tropical Storm Jeanne, affecting the region on 15-16 September 2004. Preliminary results confirmed that DSDs are highly variable between coastal and mountainous regions, even in a small island (~9000 km 2 ), as suggested by Ulbrich in (1). In addition, this work intends to improve the reliability of rain algorithms currently used in tropical regions, contributing to better estimation of rainfall rates (R). The expected radar reflectivity Z is calculated from 2DVD-measured DSDs and compared with measured Z from the National Weather Service WSR-88D radar (S-band Doppler radar a.k.a. NEXRAD). Different Z-R relationships - used to determine the rainfall amount from measured Z - for each of the two days of the storm are obtained from these calculations. Discrepancies between storm days are adduced to differences in stratiform and convective rain components.

Climatological analysis of DSDs in Oklahoma as revealed by 2D-video disdrometer and polarimetric WSR-88D radar

Abstract: Since the Spring of 1998, over 47,000 oneminute drop size distribution (DSD) measurements have been made by the National Severe Storms Laboratory 2D-video disdrometer (Schuur et al., 2001) in Norman, OK. Collected over many seasons and precipitation regimes, these DSDs reveal much information about natural DSD variability over the southern Great Plains. In this study, we use this large dataset to examine the relationship of the measured DSDs to season, a variety of storm and precipitation system types, continental and tropical precipitation, warm and cold season precipitation, surface temperature, and the height of the bright band. Measured DSDs are used to compute average dependencies of differential reflectivity (ZDR) and specific differential phase (KDP) on radar reflectivity (Z) for different storm types. Special consideration is given to the relative frequency and importance of DSDs dominated by big drops, characterized by an unusually large median volume diameter (D0), and their impact on polarimetric rainfall estimation.

Estimation of vertical profiles of raindrop size distribution from the VHF wind pro filer radar Doppler spectra

Abstract: This paper describes research on a new form of gamma drop size distribution (DSD) model that employs two arbitrary moments as free DSD parameters to enhance the flexibility in studying the characteristics of gamma DSD model to fit the spectrum. The validity of a DSD model is evaluated in terms of the stability in solving non-linear least-squares (NLLS) problem and the accuracy in DSD moment estimates. The microphysical [i.e., raindrop size distributions (DSD)] parameters are retrieved during stratiform precipitating cloud system passed (on 23 Oct.! 997) overhead of 53-MHz VHF wind pro filer radar at Gadanki. The retrieved rain integral parameters were compared to the corresponding disdrometer data and a reasonably good agreement between the measurements has been found, lending credence to the VHF wind profiler radar retrievals of DSD parameters. Wind profi ler radar estimated DSD during stratiform precipitation has been compared with the lognormal Indian climate model. It is observed that the pattern of DSD agrees closely with both models.

Quantification of the radar reflectivity sampling error in non-stationary rain using paired disdrometers

Abstract: Knowledge of the raindrop size distribution (DSD) is essential for understanding the physics of precipitation and for interpreting remotely sensed observations of rain. Disdrometer measurements of DSDs are affected by uncertainties due to the limited sampling volumes or areas of the sensors. Determining this sampling error directly from disdrometer observations is of primary importance for the practical application of DSD analyses. Gage et al. (2004) proposed an estimator of the sampling error affecting the radar reflectivity estimates based on pairs of collocated disdrometers. We provide an interpretation of this estimator and assess its accuracy through controlled experiments using a Monte Carlo framework. Our simulation model of the disdrometer sampling process closely mimics the observations reported by Gage et al. (2004). Using this model, we demonstrate that the estimator proposed by Gage et al. (2004) provides a reliable quantification of the reflectivity sampling error. However, we also show that its accuracy depends on the ratio between the length of the disdrometer time series involved and the characteristic time scale of the rainfall

Constant Raindrop Fall Speed Profiles Derived from Doppler Radar Data Analyses for Steady Nonconvective Precipitation

Abstract: For nonconvective, steady light rain with rain rates <5 mm h−1 the mean Doppler velocity of raindrop spectra was found to be constant below the melting band, when the drop-free fall speed was adjusted for pressure. The Doppler radar–weighted raindrop diameters varied from case to case from 1.5 to 2.5 mm while rain rates changed from 1.2 to 2.9 mm h−1. Significant changes of advected velocity moments were observed over periods of 4 min. These findings were corroborated by three independent systems: a Doppler radar for establishing vertical air speed and mean terminal drop speeds [using extended Velocity Azimuth Display (EVAD) analyses], a Joss–Waldvogel disdrometer at the ground, and a Particle Measuring System (PMS) 2-DP probe flown on an aircraft. These measurements were supported by data from upper-air soundings. The reason why inferred raindrop spectra do not change with height is the negligible interaction rate between raindrops at low rain rates. At low rain rates, numerical box models of dr...

Variation of Rainrate and Radar Reflectivity in Busan Area and Its Measurement by Clouds Type

Abstract: Z=200R 1.6 by Marshall and Palmer which is applied at stratiform rainfall type has contained many uncertainties for the rainfall estimation of convective clouds. To calculate and forecast the precipitation amount more accurate, measurement of adequate Z-R relationship is needed at coastal area, since the variation and movement of clouds and precipitation system are faster in Busan than other areas. In this study the variations of daily Z-R relationship and its calculation with respect to new classified precipitation types were accomplished using the DSDs obtained by a disdrometer, POSS (Precipitation Occurrence Sensor System) at each 5 and 11 precipitation event in 2001 and 2002 year. The variation of precipitation amount by power law regression was expressed depending on the daily radar reflectivities, duration time and rain rates. As a result, coefficient A and b values showed at 98~669 and 1.31~1.67 ranges. Using gamma DSD factors N0 and Λ by Tokay and Short (1996), rain rates R were obtained at stratiform and convective clouds type each other. Generally, A values of convective clouds had 78~91 and b values had mean 1.6±0.1, however, their values were shown to the 386~481 and 1.5 at stratiform clouds type, respectively. As a consideration of the threshold value of 5.5 ㎜/h which is not occurred in the case of stratiform rainfall, A values of Z-R relationship had a tendency of increase at both clouds type in contrast to non-classification rain rates.

Rain-rate estimate algorithm evaluation and rainfall characterization in tropical environments using 2DVD, rain gauges and TRMM data

Abstract: Precipitation is an important environmental parameter which affects the hydrology of the land s urface, coastal processes, terrain stability, and climate a nd global heat circulation. Understanding rainfall distribution a nd intensity can improve protection of environmental and human resources, and knowledge of geophysical process of land, ocean and atmosphere. Rain measurements have been historically verified using traditional rain-gauges in high detail or mic rowave radars that cover vast areas. Nevertheless, in order to d evelop more accurate rainfall forecast algorithms and validate them, the drop size distribution (DSD) of rainfall events nee d to be studied. Using measurements from NASA TRMM satellite and rain gauges, the raindrop size distribution will be studied and used in analyzing disdrometer rain retrieval. The comparison took place on September 15 th to 17 th , 2004 in San Juan, Puerto Rico; when the tropical storm Jeanne passed by the island of Puerto Rico, in the Caribbean. Only 4 out of 21 locations worldwide where 2DVDs have been deployed in the past are in the tropics, therefore we expect this work to provide further insight into the rainfall s tatistics of tropical regions.

Disdrometer calibration using an adaptive signal processing algorithm

Abstract: Disdrometers are considered exotic instruments and provide valuable information As such, their price tag is also high Impact disdrometers are instruments that produce an electrical impulse output related to the mass of a rain drop colliding at terminal velocity with a sensor The produced electrical impulse signal amplitude and energy are related to the drop diameters This relation is in general nonlinear and depends heavily on the type of transducer used mechanical structure imperfections and electrical tolerances dictate the need for the individual calibration of each instrument in an attempt to create calibration curves that convert impulse amplitudes to equivalent drop diameters Conventional calibration techniques using drop towers have been a tedious process to say the least A proposed alternative calibration technique utilizing an adaptive signal processing algorithm eliminates the need of a single drop calibration An accumulation rain gauge provides a reference signal to the disdrometer that is used for adaptive training and optimization of a model based calibration function In this paper we describe a prototype low-cost disdrometer implementation at the University of Central Florida A prototype impact sensor was built using an array of piezoelectric elements encapsulated in water resistant material For the data acquisition and processing we use the soundboard of a general purpose computer The signal processing algorithms and Matlab implementation will be described Data have been collected and processed and results will be presented Future plans on developing a low cost disdrometer will also be discussed The availability of affordable disdrometers will benefit NASA's upcoming GPM program, as well as many other meteorological agencies

Observations of Winter Storms with a 2-D Video Disdrometer and Polarimetric Radar

Abstract: The Winter Icing and Storms Project 2004 (WISP04) was conducted from February to April 2004 in north central Colorado. Program objectives were to evaluate remote sensing techniques for icing detection and for quantifying winter precipitation in support of airport deicing operations. Measurements from a Sband dual-polarization radar and a two-dimensional video disdrometer are being used to develop radarbased algorithms to discriminate between rain and snow, quantify winter precipitation, and improve parameterization of winter precipitation in numerical forecast models. An ability to match radar-measured and disdrometer-based radar parameters is essential when developing algorithms for winter precipitation. Video disdrometers provide important information regarding hydrometeor size, shape, terminal velocity, and number concentration at high temporal resolution. As a first step in this project, we verify that the radar detects subtle changes in the character of winter precipitation. Here radar reflectivity factor (reflectivity or ZH, hereafter) and differential reflectivity (ZDR) are calculated from disdrometer data collected on 5 March 2004, an event in which precipitation changed from rain to snow. The calculations are based on scattering amplitudes of raindrops and snow computed with the T-matrix method. The scattering matrix during the transition and snow phases was allowed to vary according to an empirical relation between particle size and bulk snow density developed from disdrometer observations. Reflectivity and differential reflectivity calculations based on disdrometer observations show good agreement with the radar measurements. The radar measurements of ZH and ZDR are then used to retrieve snow particle size distributions for comparison with the disdrometer observations. A case study from a precipitation event on 20 February 2004 will be also presented at the conference. The dataset and a brief description of the disdrometer are given in section 2. Section 3 discusses the disdrometer calculations for reflectivity and ____________________________________________ * Corresponding author address: Kyoko Ikeda, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307 E-mail: kyoko@ncar.ucar.edu differential reflectivity followed by a comparison of the estimates with the radar measurements in Section 4. Results from retrievals of particle size distributions are presented in section 5. A summary and concluding remarks are given in section 6.

Model-based iterative approach to polarimetric radar rainfall estimation in presence of path attenuation

Abstract: . A new model-based iterative technique to correct for attenuation and differential attenuation and retrieve rain rate, based on a neural-network scheme and a differential phase constraint, is presented. Numerical simulations are used to investigate the efficiency and accuracy of this approach named NIPPER. The simulator is based on a T-matrix solution technique, while precipitation is characterized with respect to shape, raindrop size distribution and orientation. A sensitivity analysis is performed in order to evaluate the expected errors of this method. The performance of the proposed methodology on radar measurements is evaluated by using one-dimensional Gaussian shaped rain cell models and synthetic radar data derived from disdrometer measurements. Numerical results are discussed in order to evaluate the robustness of the proposed technique.

Seasonal variability of raindrop size distribution over Cuddalore, a coastal station in Tamilnadu of southern peninsular India

Abstract: Cuddalore (11.46° N / 79.46° E), a tropical coastal station in Tamilnadu of southern peninsular India receives precipitation from pre-monsoon (March – May), southwest monsoon (June – September) and northeast monsoon (October – December). While the precipitation during pre-monsoon (PM) and southwest monsoon (SWM) is mostly convective, that received during northeast monsoon (NEM) is mostly stratiform albeit a juxtaposition of both convective and stratiform is also feasible. The seasonal variability of raindrop size distribution (DSD) has been studied using the data obtained from electro-mechanical disdrometer (Joss-Waldvogel type) at Cuddalore. The modal drop size is less than 2.0 mm diameter in stratiform precipitation whereas drops of higher diameter (more than 3 mm) is quite probable in convective precipitation events. The mean concentration of rain drops of size more than 3 mm is highest during pre-monsoon followed by southwest monsoon in rain rates exceeding 10 mm h-1 due to rapid collision and coalescence taking place in afternoon mixing and convective currents. The concentration of smaller size drops (of size less than 2 mm dia) especially in rain rates exceeding 8 mm h-1 is more during NEM than the SWM because the condensed particles could not grow effectively into larger drops due to the prevalence of either weak instability or nocturnal stability conditions during NEM. Convective type precipitation has higher rain rates than the stratiform type. Inverse relationship between drop concentration and rain rate is seen during convective situations, while the relationship is linear during stratiform conditions. Lognormal distribution fits the DSD of northeast monsoon (mostly stratiform precipitation) extremely well. However, this fitting has some deviation in the rain rate 10-50 mm h-1 during pre-monsoon and southwest monsoon season (mostly convective precipitation) based on the limited data sample obtained during 2003.

Microphysical evidence of the transition between predominant convective/stratiform rainfall associated with the intraseasonal oscillation in the

Abstract: The distinction between convective and stratiform precipitation profiles around various precipitating systems existent in tropical regions is very important to the global atmospheric circulation, which is extremely sensitive to vertical latent heat distribution. In South America, the convective activity responds to the Intraseasonal Oscillation (IOS). This paper analyzes a disdrometer and a radar profiler data, installed in the Ji-Parana airport, RO, Brazil, for the field experiment WETAMC/LBA & TRMM/LBA, during January and February of 1999. The microphysical analysis of wind regimes associated with IOS showed a large difference in type, size and microphysical processes of hydrometeor growth in each wind regime: easterly regimes had more turbulence and consequently convective precipitation formation, and westerly regimes had a more stratiform precipitation formation.

Evaluation of Rainfall Estimation Feasibility Using Ka-band and W-band Cloud Radars

Abstract: Two cloud radars working at Ka-band (35㎓) and at W-band (95㎓) together with radiosonde observations (RaOb) and disdrometer were used to retrieve physical parameters such as liquid water content and median volume diameter in rain. These instruments were deployed at the National Research Institute for Earth Science and Disaster Prevention (NIED) in Tsukuba, Japan where rain data was collected on June 21. In order to retrieve these parameters, the data was processed and analyzed dual wavelength ratio through by attenuation correction with assumptions of light precipitation, spherical rain droplets, the Marshall-Palmer drop size distribution, and ideal gas model. Compared the measured reflectivity with the corrected reflectivity by considering the gas attenuation and the variation of dielectric factor at each band during the weak rain, it suggests that W-band seriously affects the gas and dielectric factor which are neglected at centimeter wavelength radar. The wavelength is shorter, the correction of attenuation and dielectric factor is more important. And the feasibility test give some information about the location of the maximum height at which the return signal can't be detected by the serious rain attenuation at Ka- and W-band radar. W-band radar used in this study is available less than 6 ㎜/hr to get the vertical profile of some information up to 5 ㎞. Median volume diameters derived from dual wavelength method were found to go from 0.3 ㎜ to 1.3 ㎜ and liquid water content has up to 0.5 g/㎥. Median volume diameter derived from cloud radars and disdrometer have a good agreement at the peak point, otherwise the pattern of D0 from dual wavelength radar showed some fluctuation.

High-Frequency Single-Board Doppler Minisodar for Precipitation Measurements. Part I: Rainfall and Hail

Abstract: A high-frequency Doppler sodar for precipitation measurements has been developed. Such a Doppler sodar (6–20 kHz) can almost always measure precipitation and turbulence spectra simultaneously. Therefore, the mean vertical wind and spectral broadening effects can be directly removed. As the acoustic refractive indices for ice and liquid water are almost the same, the acoustic retrieval of precipitation can also be applied to rain with small hail (e.g., diameter D 10 mm) or large hail, but for the latter, neglecting the effects of different orientations and shapes of hailstones. The authors’ single-board minisodar is based on the digital signal processing (DSP) technique. The first prototype has been continuously operated at a coastal weather station since 25 October 2002. For stratiform rain events, the minisodar showed good agreement with a Joss–Waldvogel disdrometer and an optical rain gauge. However, for convective heavy showers, the minisodar always observed higher rain rates. The continuous, nonattended automatic operation of the minisodar has shown its capability for all kinds of precipitation measurements. The retrieval of precipitation rates for snow and graupel will be provided in a subsequent paper.

Simulation of rain attenuation time series

Abstract: The simulation of a fine space-time structure of rain has great potential applications in many fields of research such as radar-meteorology, hydrology, and telecommunications. A rainfall simulator has been developed based on simple assumptions on the release of raindrops from cloud bases, and on a model of raindrop interarrival time and size distributions at ground level, derived from a statistical analysis of actual measurements using CETP's disdrometer over more than a year. Among the simple assumptions used, the probability of precipitating drops follows an exponential law related to the drop size. This Monte-Carlo simulation can be performed in two dimensions giving the spatial distribution of rain drops in a vertical plane. From this simulation, time series of rain rates, but also of microwave attenuations at a given frequency and elevation angle can be derived. The simulator is designed so that the main characteristics of rain observed on the ground are preserved: size distribution, interarrival time distribution and autocovariance of drop sizes. Results show that quantities derived from the series of raindrops generated by the simulator, such as time series of rain rate or attenuation, are in good agreement with observations.

Disdrometer and Radar Observation-Based Microphysical Parameterization to Improve Weather Forecast

Abstract: where the slope parameter Λ relates to a characteristic size of the raindrops such as the mean diameter () or median volume diameter (D0). N0 is an intercept parameter, which was fixed at 10000 m mm by Kessler. When N0 = 8000 m mm, Eq. (1) becomes the Marshall−Palmer (M-P) drop size distribution (Marshall and Palmer, 1948). For the M-P DSD model and Kessler’s parameterization scheme, microphysical processes for evaporation rate (Re in g m s) for a unit water vapor saturation deficit, accretion rate (Rc in g m s) for a unit cloud water content, and mass-weighted terminal velocity (Vtm in m s) can be represented in terms of rain water content (W in g m) as follows