Showing papers on "Disdrometer published in 2010"

PARSIVEL Snow Observations: A Critical Assessment

Abstract: The performance of the laser-optical Particle Size Velocity (PARSIVEL) disdrometer is evaluated to determine the characteristics of falling snow. PARSIVEL’s measuring principle is reexamined to detect its limitations and pitfalls when applied to solid precipitation. This study uses snow observations taken during the Canadian Cloudsat/Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) Validation Project (C3VP) campaign, when two PARSIVEL instruments were collocated with a single twodimensional disdrometer (2-DVD), which allows more detailed observation of snowflakes. When characterizing the snowflake size, PARSIVEL instruments inherently retrieve only one size parameter, which is approximately equal to the widest horizontal dimension (more accurately with large snowflakes) and that has no microphysical meaning. Unlike for raindrops, the equivolume PARSIVEL diameter—the PARSIVEL output variable—has no physical counterpart for snowflakes. PARSIVEL’s fall velocity measurement may not be accurate for a single snowflake particle. This is due to the internally assumed relationship between horizontal and vertical snow particle dimensions. The uncertainty originates from the shape-related factor, which tends to depart more and more from unity with increasing snowflake sizes and can produce large errors. When averaging over a large number of snowflakes, the correction factor is size dependent with a systematic tendency to an underestimation of the fall speed (but never exceeding 20%). Compared to a collocated 2-DVD for long-lasting events, PARSIVEL seems to overestimate the number of small snowflakes and large particles. The disagreement between PARSIVEL and 2-DVD snow measurements can only be partly ascribed to PARSIVEL intrinsic limitations (border effects and sizing problems), but it has to deal with the difficulties and drawbacks of both instruments in fully characterizing snow properties.

Distributions of Raindrop Sizes and Fall Velocities in a Semiarid Plateau Climate: Convective versus Stratiform Rains

Abstract: Joint size and fall velocity distributions of raindrops were measured with a Particle Size and Velocity (PARSIVEL) precipitation particle disdrometer in a field experiment conducted during July and August 2007 at a semiarid continental site located in Guyuan, Ningxia Province, China (36°N, 106°16′E). Data from both stratiform and convective clouds are analyzed. Comparison of the observed raindrop size distributions shows that the increase of convective rain rates arises from the increases of both drop concentration and drop diameter while the increase of the rain rate in the stratiform clouds is mainly due to the increase of median and large drop concentration. Another striking contrast between the stratiform and convective rains is that the size distributions from the stratiform (convective) rains tend to narrow (broaden) with increasing rain rates. Statistical analysis of the distribution pattern shows that the observed size distributions from both rain types can be well described by the gamma ...

A Methodology to Derive Radar Reflectivity–Liquid Equivalent Snow Rate Relations Using C-Band Radar and a 2D Video Disdrometer

Abstract: The objective of this work is to derive equivalent radar reflectivity factor–liquid equivalent snow rate (Ze–SR) power-law relations for snowfall using the C-band King City operational weather radar and a 2D video disdrometer (2DVD). The 2DVD provides two orthogonal views of each snow particle that falls through its 10 cm × 10 cm virtual sensor area. The “size” parameter used here for describing the size distribution is based on the “apparent” volume computed from the two images, and an equivolume spherical diameter Dapp is defined. The determination of fall speed is based on matching two images corresponding to the same particle as it falls through two light planes separated by a precalibrated separation distance. A new “rematching” algorithm was developed to improve the quality of the fall speed versus Dapp as compared with the original matching algorithm provided by the manufacturer. The snow density is parameterized in the conventional power-law form , where α and β are assumed to be variable...

An experiment to measure the spatial variability of rain drop size distribution using sixteen laser disdrometers

Abstract: [1] This paper reports on the results of an experiment designed to measure the spatial variability of rain drop size distribution (RDSD) at kilometer scale. Eight dual instruments (16 Parsivel disdrometers) were used to record the RDSD from October 2009 to January 2010. The spatial variability of the RDSD in terms of cross-correlation and changes in the reflectivity-rainfall (Z–R) relationship was calculated. The results provide an estimate of the variability range at spatial scales relevant for spatial radars such as TRMM-PR and GPM-DPR. It was found that the spatial variability of the RDSD in a single episode can exceed the inter-episode variability. This implies that estimates of the RDSD using a few disdrometers are not enough to capture the evolution of the RDSD, and that more detailed areal estimates are needed in order to fully analyze the RDSD.

An Experimental Study of Small-Scale Variability of Raindrop Size Distribution

Abstract: An experimental study of small-scale variability of raindrop size distributions (DSDs) has been carried out at Wallops Island, Virginia. Three Joss-Waldvogel disdrometers were operated at a distance of 0.65, 1.05, and 1.70 km in a nearly straight line. The main purpose of the study was to examine the variability of DSDs and its integral parameters of liquid water content, rainfall, and reflectivity within a 2-km array: a typical size of Cartesian radar pixel. The composite DSD of rain events showed very good agreement among the disdrometers except where there were noticeable differences in midsize and large drops in a few events. For consideration of partial beam filling where the radar pixel was not completely covered by rain, a single disdrometer reported just over 10% more rainy minutes than the rainy minutes when all three disdrometers reported rainfall. Similarly two out of three disdrometers reported5%more rainy minutes than when all three were reporting rainfall. These percentages were based on a 1-min average, and were less for longer averaging periods. Considering only the minutes when all three disdrometers were reporting rainfall, just over one quarter of the observations showed an increase in the difference in rainfall with distance. This finding was based on a 15-min average and was even less for shorter averaging periods. The probability and cumulative distributions of a gamma-fitted DSD and integral rain parameters between the three disdrometers had a very good agreement and no major variability. This was mainly due to the high percentage of light stratiform rain and to the number of storms that traveled along the track of the disdrometers. At a fixed time step, however, both DSDs and integral rain parameters showed substantial variability. The standard deviation (SD) of rain rate was near 3 mm/h, while the SD of reflectivity exceeded 3 dBZ at the longest separation distance. These standard deviations were at 6-min average and were higher at shorter averaging periods. The correlations decreased with increasing separation distance. For rain rate, the correlations were higher than previous gauge-based studies. This was attributed to the differences in data processing and the difference in rainfall characteristics in different climate regions. It was also considered that the gauge sampling errors could be a factor. In this regard, gauge measurements were simulated employing existing disdrometer dataset. While a difference was noticed in cumulative distribution of rain occurrence between the simulated gauge and disdrometer observations, the correlations in simulated gauge measurements did not differ from the disdrometer measurements.

Snow Studies. Part II: Average Relationship between Mass of Snowflakes and Their Terminal Fall Velocity

Abstract: This study uses a dataset of low-density snow aggregates measurements collected by a ground-based optical disdrometer that provides particle size and terminal fall speed for each size interval from which the velocity–size and area ratio–size relationships can be derived. From these relationships and relations between the Best and Reynolds numbers proposed in the literature, the mass power-law coefficients are obtained. Then, an approximate average relation between the coefficients in the experimentally determined velocity–size power law (with exponent fixed at 0.18) and the coefficients in the estimated mass power law (with exponent fixed at 2) is obtained. The validation of the retrieved relation is made by comparing, for each snowfall event, the time series of the reflectivity factor calculated from the derived mass–size relationship for a snowflake and from the size distribution measured by the optical disdrometer, with the reflectivity obtained from measurements. Using the measured snow size ...

Snow scattering signals in ground-based passive microwave radiometer measurements

Abstract: [1] This paper investigates the influence of snow microphysical parameters on the enhancement of ground-based passive microwave brightness temperature (TB) measurements. In addition to multispectral passive microwave observations between 20 and 150 GHz, a 35 GHz cloud radar and a 2-D video disdrometer for in situ measurements of snowfall were deployed as part of the “towards an optimal estimation-based snowfall characterization algorithm” campaign in the winter season of 2008–2009 at an Alpine environment located at 2650 m mean sea level. These observations are combined with nearby radiosonde ascents and surface standard meteorological measurements to reconstruct the atmospheric state (i.e., fields of temperature, humidity, snow, and liquid water contents) and are subsequently used as input for a microwave radiative transfer (RT) model. We investigate the sensitivity of the missing information about snow shape and snow particle size distribution (SSD) on the microwave TB measurements using the disdrometer data as a rough constraint. For an extended case study, we found that TBs at 90 and 150 GHz are significantly enhanced because of scattering of surface radiation at snow crystals and that this enhancement is clearly correlated with the radar derived snow water path (SWP < 0.2 kg m−2). RT simulations highlight the strong influence of the vertical distribution of cloud liquid water (liquid water path LWP < 0.1 kg m−2) on the TB, which in extreme cases, can fully obscure the snow scattering signal. TB variations of the same magnitude can also be caused by typical variations in SSD parameters and particle shape similar to results obtained by space-borne studies. Ground-based stations with their infrastructural capabilities in combining active and passive microwave observations have the potential to disentangle the influences of different snow shape, SSD, and SWP in snow retrievals, thus supporting current and future satellite missions.

Polarimetric Radar Rain Estimation through Retrieval of Drop Size Distribution Using a Bayesian Approach

Abstract: This study proposes a Bayesian approach to retrieve raindrop size distributions (DSDs) and to estimate rainfall rates from radar reflectivity in horizontal polarization ZH and differential reflectivity ZDR. With this approach, the authors apply a constrained-gamma model with an updated constraining relation to retrieve DSD parameters. Long-term DSD measurements made in central Oklahoma by the two-dimensional video disdrometer (2DVD) are first used to construct a prior probability density function (PDF) of DSD parameters, which are estimated using truncated gamma fits to the second, fourth, and sixth moments of the distributions. The forward models of ZH and ZDR are then developed based on a T-matrix calculation of raindrop backscattering amplitude with the assumption of drop shape. The conditional PDF of ZH and ZDR is assumed to be a bivariate normal function with appropriate standard deviations. The Bayesian algorithm has a good performance according to the evaluation with simulated ZH and ZDR. ...

Evaluating Polarimetric X-Band Radar Rainfall Estimators during HMT

Abstract: Different relations between rainfall rate R and polarimetric X-band radar measurables were evaluated using the radar, disdrometer, and rain gauge measurements conducted during the 4-month-long field experiment. The specific differential phase shift KDP–based estimators generally show less scatter resulting from variability in raindrop size distributions than with the power-based relations. These estimators depend on model assumptions about the drop aspect ratios and are not applicable for lighter rainfalls. The polynomial approximation for the mean drop aspect ratio provides R–KDP relations that result overall in good agreement between the radar retrievals of rainfall accumulations and estimates from surface rain gauges. The accumulation data obtained from power estimators that use reflectivity Zeh and differential reflectivity ZDR measurements generally exhibit greater standard deviations with respect to the gauge measurements. Unlike the phase-based estimators, the power-based estimators have a...

Retrieval of Three-Dimensional Raindrop Size Distribution Using X-Band Polarimetric Radar Data

Abstract: An improved algorithm based on the self-consistent principle for rain attenuation correction of reflectivity ZH and differential reflectivity ZDR are presented for X-band radar. The proposed algorithm calculates the optimum coefficients for the relation between the specific attenuation coefficient and the specific differential phase, every 1 km along a slant range. The attenuation-corrected ZDR is calculated from reflectivity at horizontal polarization and from reflectivity at vertical polarization after attenuation correction. The improved rain attenuation correction algorithm is applied to the range–height indicator (RHI) scans as well as the plan position indicator (PPI) volume scan data observed by X-band wavelength (MP-X) radar, as operated by the National Research Institute for Earth Science and Disaster Prevention (NIED) in Japan. The corrected ZH and ZDR values are in good agreement with those calculated from the drop size distribution (DSD) measured by disdrometers. The two governing parameters of a normalized gamma DSD, normalized number concentration NW, and drop median diameter D0 are estimated from the corrected ZH and ZDR, and specific differential phase KDP values based on the ‘‘constrained-gamma’’ method. The method is applied to PPI and RHI data of a typhoon rainband to retrieve the three-dimensional distribution of DSD. The retrieved DSD parameters show reasonable agreement with disdrometer data. The present results demonstrate that high-quality correction and retrieval DSDs can be derived from X-band polarimetric radar data.

Rain drop size distribution variability and impact on X-band polarimetric radar retrieval: Results from the AMMA campaign in Benin

Abstract: During the African Monsoon Multidisciplinary Analysis (AMMA) field campaign, polarimetric weather radars, a rain-gauge network and disdrometers were available to analyse the convective systems and the rainfall in Benin. In this paper a three-year dataset of drop size distributions is used to compute theoretical relationships between X-band polarimetric variables and the rain rate, adjusted for the region of interest. The variability of these relationships from storm to storm and between rain types is studied. With an X-band polarimetric radar the rain rate can be retrieved from the differential specific phase shift KDP, the specific attenuation AH, the reflectivity Z or differential reflectivity ZDR corrected for attenuation, or a combination of these variables. The set of coefficients needed to run the rain retrieval or the attenuation correction schemes under the conditions found in Benin are computed. The rain retrieval schemes are compared on the basis of simulations. The retrievals based on one single propagative variable like KDP or AH perform well. Preliminary results from direct comparisons between X-band polarimetric radar estimates and rain-gauges are consistent with the disdrometer-based analysis. Copyright © 2010 Royal Meteorological Society

Automated Retrievals of Precipitation Parameters Using Non-Rayleigh Scattering at 95 GHz

Abstract: Automated retrievals of vertical air motion and the drop size distribution (DSD) slope parameter from the surface to the base of the melting layer are presented using a technique for W-band (95 GHz) profiling radars. The technique capitalizes on non-Rayleigh resonance signatures found in the observed Doppler spectra to estimate the mean vertical air motion. The slope parameter of the DSD for an assumed exponential form is retrieved through an inversion of the Doppler spectra. Extended testing is performed in central Oklahoma for a monthlong period of observation that includes several midlatitude convective line trailing stratiform events featuring low to moderate rainfall rates (<1 to 30 mm h−1). Low-level DSD slope parameter retrievals are shown in agreement (bias of −1.48 cm−1 and rms error of 4.38 cm−1) with collocated surface disdrometer DSD observations. Velocity retrievals indicate a net downward motion in stratiform rain of 0.05 m s−1 with a standard deviation of 0.24–0.3 m s−1. Time–heigh...

Radar/rain-gauge comparisons on squall lines in Niamey, Niger for the AMMA

Abstract: Massachusetts Institute of Technology C-band radar observations are integrated with rainfall measurements from an extensive network of gauges in Niamey, Niger, West Africa, for the African Monsoon and Multidisciplinary Analysis (AMMA). The large number of gauges available enabled Ze -R power-law relationships for the convective and stratiform regions of individual squall lines. The Ze -R relationships based solely on radar measurements directly over the gauges were developed for the estimate of rainfall and attendant latent heat release (by other AMMA investigators) where gauges were unavailable. The low prefactor values of the Ze -R power laws relative to like values for Z -R disdrometer power laws have contributions of order 1-2 dB from the use of the lowest beam tilt (0.57 ◦ )a nd∼1-2 dB by the radar reading low. (The sphere calibration and the Tropical Rainfall Measuring Mission TRMM - radar calibration are inconsistent at the 1-2 dB level for unknown reasons.) Radar/gauge comparisons are also shown for individual storms. Accurate, unbiased results for the convective regime require adjustment of the radar-to-gauge radials for attenuation. Beam filling problems and aliasing issues can often be identified in the case of outlier points. Copyright c � 2010 Royal Meteorological Society

Ground validation of oceanic snowfall detection in satellite climatologies during LOFZY

Abstract: A thorough knowledge of global ocean precipitation is an indispensable prerequisite for the understanding of the water cycle in the global climate system. However, reliable detection of precipitation over the global oceans, especially of solid precipitation, remains a challenging task. This is true for both, passive microwave remote sensing and reanalysis based model estimates. The optical disdrometer ODM 470 is a ground validation instrument capable of measuring rain and snowfall on ships even under high wind speeds. It was used for the first time over the Nordic Seas during the LOFZY 2005 campaign. A dichotomous verification of precipitation occurrence resulted in a perfect correspondence between the disdrometer, a precipitation detector and a shipboard observer's log. The disdrometer data is further point-to-area collocated against precipitation from the satellite based Hamburg Ocean Atmosphere Parameters and fluxes from Satellite data (HOAPS) climatology. HOAPS precipitation turns out to be overall consistent with the disdrometer data resulting in a detection accuracy of 0.96. The collocated data comprises light precipitation events below 1 mm h–1. Therefore two LOFZY case studies with high precipitation rates are presented that indicate plausible HOAPS satellite precipitation rates. Overall, this encourages longer term measurements of ship-to-satellite collocated precipitation in the near future.

The effect of reported high-velocity small raindrops on inferred drop size distributions and derived power laws

Abstract: . It has recently been shown that at high rainfall intensities, small raindrops may fall with much larger velocities than would be expected from their diameters. These were argued to be fragments of recently broken-up larger drops. In this paper we quantify the effect of this phenomenon on raindrop size distribution measurements from a Joss-Waldvogel disdrometer, a 2-D Video Distrometer, and a vertically-pointing Doppler radar. Probability distributions of fall velocities have been parameterized, where the parameters are functions of both rainfall intensity and drop size. These parameterizations have been used to correct Joss-Waldvogel disdrometer measurements for this phenomenon. The effect of these corrections on fitted scaled drop size distributions are apparent but not major. Fitted gamma distributions for three different types of rainfall have been used to simulate drop size measurements. The effect of the high-velocity small drops is shown to be minor. Especially for the purpose of remote sensing of rainfall using radar, microwave links, or optical links, the errors caused by using the slightly different retrieval relations will be masked completely by other error sources.

Calibration of radar reflectivity measurements from the KMA operational radar network

Abstract: A systematic procedure for calibrating system gain bias (so-called “calibration error”) of radar reflectivity measurements from the Korea Meteorological Administration (KMA) operational radar network is presented. First, the RJNI radar located at Jindo Island is calibrated by comparing with radar reflectivities simulated theoretically by a scattering algorithm using drop spectra collected by a disdrometer from June 19 to 29, 2009. Once the RJNI radar is calibrated, the reflectivity measurements from nearby radars are compared with the RJNI radar reflectivities to determine existing gain biases of nearby radars. This radar-radar calibration procedure was repeated with the other radars within the network. For isolating a system gain bias, echoes affected by partial beam blockage due to ground clutter and by attenuation due to precipitation were removed. The system gain biases of the RJNI and RPSN radars were −3 and −4.2 dB, respectively, during the experimental period. The RBRI and RDNH radars revealed relatively large biases, above −8 dB. The other radars (RKSN, RGSN, RSSP, RKWK, RGDK, RIIA, and RMYN) revealed biases from −6 to −7 dB. Thus, the reflectivity measurements from all of the KMA radars were severely biased. New R-Z relations of R = 3.350 × 10−2Z0.624 (Z = 231.1R1.6) for stratiform and R=1.546 × 10−2Z0.714 (Z = 342.4R1.4) for convective precipitations were derived using disdrometer data. Using these R-Z relations, the radar-derived total rainfall amounts from the reflectivity measurements without calibration produced significant underestimations, compared to gauge measurements at about 80 sites, with a normalized bias of about −56%. On the other hand, after calibrating the above system biases, the radar-derived rainfall amounts corresponded well with the gauge measurements, with a normalized bias of about −3%. In conclusions, the radar reflectivity measurements from the KMA radar network are severely biased and the procedure presented in this study can be used to resolve the system gain biases.

Synergetic use of millimeter- and centimeter-wavelength radars for retrievals of cloud and rainfall parameters

Abstract: . A remote sensing approach for simultaneous retrievals of cloud and rainfall parameters in the vertical column above the US Department of Energy's (DOE) Climate Research Facility at the Tropical Western Pacific (TWP) Darwin site in Australia is described. This approach uses vertically pointing measurements from a DOE Ka-band radar and scanning measurements from a nearby C-band radar pointing toward the TWP Darwin site. Rainfall retrieval constraints are provided by data from a surface impact disdrometer. The approach is applicable to stratiform precipitating cloud systems when a separation between the liquid hydrometeor layer, which contains rainfall and liquid water clouds, and the ice hydrometeor layer is provided by the radar bright band. Absolute C-band reflectivities and Ka-band vertical reflectivity gradients in the liquid layer are used for retrievals of the mean layer rain rate and cloud liquid water path (CLWP). C-band radar reflectivities are also used to estimate ice water path (IWP) in regions above the melting layer. The retrieval uncertainties of CLWP and IWP for typical stratiform precipitation systems are about 500–800 g m−2 (for CLWP) and a factor of 2 (for IWP). The CLWP retrieval uncertainties increase with rain rate, so retrievals for higher rain rates may be impractical. The expected uncertainties of layer mean rain rate retrievals are around 20%, which, in part, is due to constraints available from the disdrometer data. The applicability of the suggested approach is illustrated for two characteristic events observed at the TWP Darwin site during the wet season of 2007. A future deployment of W-band radars at the DOE tropical Climate Research Facilities can improve CLWP estimation accuracies and provide retrievals for a wider range of stratiform precipitating cloud events.

Stratiform and convective rain discrimination for equatorial region

Abstract: This paper presents the application of the Lowered EXCELL model to discriminate between stratiform and convective precipitation in Kuala Lumpur, Malaysia, which is located in the equatorial region. The model generates two longterm cumulative distribution functions (CDFs) that separately account for the two different types of rain, based on the input rainfall statistics reflecting the local climatology. The aim of this paper is to present the applicability of the model in equatorial climates. The model performance in estimating stratiform and convective CDFs is carried out using 3 years of time series of rainfall intensity data collected in Kuala Lumpur by a disdrometer. The stratiform and convective rain rate CDFs are finally shown to be well predicted by the Lowered EXCELL model. Hence, the outcome of this paper seems encouraging for further application of the model to improve the prediction of rain attenuation for satellite communication, especially in equatorial region.

Vertical structure of raindrop size distribution in lower atmospheric boundary layer

Abstract: [1] We examine the vertical structure of the raindrop size distribution (DSD) in the lower atmospheric boundary layer (ABL) below altitudes of 300 m, where conventional radars typically do not observe. The DSD in the lower ABL is retrieved using Ku-band broadband radar (BBR) having an observational range of 50 m to 15 km, and a high range resolution of several meters and a 3-dB beam width of 3 deg. The radar-retrieved DSD are in excellent agreement with the DSD measured with a co-located, 2 dimensional video disdrometer with correlation coefficients over 0.96 in both stratiform and convective rain events. While the DSD reveals no significant change in the stratiform event, the growth process increases about 2 times in the number of raindrops larger than 0.5 mm in diameter in the convective event. This growth process in the ABL is important when we discuss the rainfall rate from radar reflectivity factor.

Disdrometer system having a three-dimensional laser array unit

Abstract: Provided is a disdrometer system having a three-dimensional laser array unit, including: a plurality of laser transmitting parts which are arranged on an inner side of a cylindrical body to generate a laser beam; a plurality of laser receiving parts which are arranged on the inner side of the cylindrical body to correspond to the laser transmitting parts; and a laser control part which converts a cut-off signal of the laser beam, which is cut-off by precipitation drops flowing into the inner side of the cylindrical body, into an electrical signal, and records the converted electrical signal. Thus, the disdrometer system has an effect that the shape, volume, number, falling velocity of precipitation drops, and the intensity, density and weight of rainfall can be integrally measured.