Showing papers on "Disdrometer published in 2007"

A Statistical and Physical Description of Hydrometeor Distributions in Colorado Snowstorms Using a Video Disdrometer

Abstract: Winter-storm hydrometeor distributions along the Front Range in eastern Colorado are studied with a ground-based two-dimensional video disdrometer. The instrument provides shape, size, and terminal velocity information for particles that are larger than about 0.4 mm. The dataset is used to determine the form of particle size distributions (PSDs) and to search for useful interrelationships among the governing parameters of assumed distribution forms and environmental factors. Snowfalls are dominated by almost spherical aggregates having near-exponential or superexponential size distributions. Raindrop size distributions are more peaked than those for snow. A relation between bulk snow density and particle median volume diameter is derived. The data suggest that some adjustment may be needed in relationships found previously between temperature and the concentration and slope parameters of assumed exponential PSDs. A potentially useful relationship is found between the slope and shape terms of the ...

Drop Shapes, Model Comparisons, and Calculations of Polarimetric Radar Parameters in Rain

Abstract: Drop shapes derived from a previously conducted artificial rain experiment using a two-dimensional video disdrometer (2DVD) are presented. The experiment involved drops falling over a distance of 80 m to achieve their terminal velocities as well as steady-state oscillations. The previous study analyzed the measured axis ratios (i.e., ratio of maximum vertical to maximum horizontal chord) as a function of equivolumetric spherical drop diameter (Deq) for over 115 000 drops ranging from 1.5 to 9 mm. In this paper, the actual contoured shapes of the drops are reported, taking into account the finite quantization limits of the instrument. The shapes were derived from the fast line-scanning cameras of the 2DVD. The drops were categorized into Deq intervals of 0.25-mm width and the smoothed contours for each drop category were superimposed on each other to obtain their most probable shapes and their variations due to drop oscillations. The most probable shapes show deviation from oblate spheroids for De...

Examination of the μ–Λ Relation Suggested for Drop Size Distribution Parameters

Abstract: Raindrop size distributions are often assumed to follow a three-parameter gamma distribution. Since rain intensity retrieval from radar observations is an underdetermined problem, there is great interest in finding physical correlations between the parameters of the gamma distribution. One of the more common approaches is to measure naturally occurring drop size distributions (DSDs) using a disdrometer and to find DSD parameters by fitting a gamma distribution to these observations. Often the method of moments is used to retrieve the parameters of a gamma distribution from disdrometer observations. In this work the effect of the method of moments and data filtering on the relation between the parameters of the DSD is investigated, namely, the shape μ and the slope Λ parameters. For this study the disdrometer observations were simulated. In these simulations the gamma distribution parameters Nw, D0, and μ were randomly selected from a wide range of values that are found in rainfall. Then, using si...

Impact of cloud microphysical processes on hurricane intensity, part 2: Sensitivity experiments

Abstract: Seven different microphysical sensitivity experiments were designed with an objective to evaluate their respective impacts in modulating hurricane intensity forecasts using mesoscale model MM5. Microphysical processes such as melting of graupel, snow and cloud ice hydrometeors, suppression of evaporation of falling rain, the intercept parameter and fall speed of snow and graupel hydrometeors are modified in the existing NASA Goddard Space Flight Center (GSFC) microphysical parameterization scheme. We studied the impacts of cloud microphysical processes by means of track, intensity, precipitation, propagation speed, kinematic and thermodynamic vertical structural characteristics of hurricane inner core. These results suggest that the set of experiments where (a) melting of snow, graupel and cloud ice were suppressed (b) melting of snow and graupel were suppressed and (c) where the evaporation of rain water was suppressed all produced most intense storms. The major findings of this study are the interconversion processes such as melting and evaporation among hydrometeors and associated feedback mechanism are significantly modulate the intensity of the hurricane. In particular an experiment where the melting of graupel, snow and cloud ice hydrometeors was eliminated from the model parameterization scheme produced the most explosively intensified storm. In the experiment where rain water evaporation was eliminated from the model, it produced a stronger storm as compared to the control run but it was not as strong as the storms produced from absence of melting processes. The impact on intensity due to variations made in intercept parameters of the hydrometeors (i.e., snow and graupel) were not that evident compared to other experiments. The weakest storm was noted in the experiment where the fall speeds of the snow hydrometeors were increased two fold. This study has isolated some of the factors that contributed to a stronger hurricane and concludes with a motivation that the findings from this study will help in further improvement in the design of sophisticated explicit microphysical parameterization for the mesoscale non-hydrostatic model for realistic hurricane intensity forecasts.

Modeling the Variability of Drop Size Distributions in Space and Time

Abstract: The information on the time variability of drop size distributions (DSDs) as seen by a disdrometer is used to illustrate the structure of uncertainty in radar estimates of precipitation. Based on this, a method to generate the space–time variability of the distributions of the size of raindrops is developed. The model generates one moment of DSDs that is conditioned on another moment of DSDs; in particular, radar reflectivity Z is used to obtain rainfall rate R. Based on the fact that two moments of the DSDs are sufficient to capture most of the DSD variability, the model can be used to calculate DSDs and other moments of interest of the DSD. A deterministic component of the precipitation field is obtained from a fixed R–Z relationship. Two different components of DSD variability are added to the deterministic precipitation field. The first represents the systematic departures from the fixed R–Z relationship that are expected from different regimes of precipitation. This is generated using a simp...

Improving Drop Size and Velocity Estimates of an Optical Disdrometer: Implications for Sprinkler Irrigation Simulation

Abstract: Optical disdrometers measure the attenuation of an infrared beam when water drops pass between the emitter and the receptor. The duration and intensity of the attenuation are used to estimate drop size and time of passage. These variables are used to calibrate and validate ballistic sprinkler simulation models. Two experimental problems affect the quality of the measurements: first, drops can pass to the side of the detector, so that only part of the drop attenuates the luminous flow; and second, several drops can overlap as they pass through the beam. This work presents a statistical treatment of the observed time of passage that can be used to eliminate a large part of the erroneous measurements, significantly improving the accuracy of disdrometer data. Furthermore, drop velocities can be estimated from the corrected times of passage. Simulation with the ballistic model shows that the minimum drop size accurately measured by the disdrometer is too large to characterize the fine diameters typical of drops landing close to the emitter. For farther landing distances, the discrepancies between measurements and simulations using ballistic theory can be large. Differences in drop velocity, drop size, and maximum sprinkler reach are discussed in this article. From our results, it can be concluded that the ballistic model (assuming independent movement of drops) constitutes an excessive simplification of reality. We believe that group displacement of the drops, resulting in a reduced air drag and in an increased probability of drop collision, is responsible for a relevant part of the reported differences.

Evaluation of X-band polarimetric radar estimates of drop size distributions from coincident S-band polarimetric estimates and measured raindrop spectra

Abstract: Recent research has demonstrated the value of polarimetric measurements for the correction of rain-path attenuation at X-band radar frequency and the estimation of rain parameters including drop-size distributions (DSD). The issue this paper is concerned with is to what degree uncertainties in attenuation correction can affect the estimation of DSD. Since attenuation-correction uncertainty enhances with rain path, our hypothesis is that DSD retrieval uncertainty at X-band may deteriorate with range. In this paper, we evaluate the relative accuracy of X-band DSD retrieval against DSD estimates from S-band radar observations and in situ disdrometer spectra. We present comparisons of various techniques for estimating DSD model parameters from attenuation-corrected X-band dual-polarization radar data. Coincident X-band polarimetric-radar (XPOL) and S-band polarimetric-radar dual-polarized radar measurements from the International H2O Project experiment as well as coincident XPOL (MP-X) measurements over disdrometer during a typhoon storm case in Japan are used to assess the accuracy of the different DSD retrieval algorithms applied to X-band radar measurements.

Measurement of solid precipitation with an optical disdrometer

Abstract: . A study about measurements of solid precipitation using an optical disdrometer is presented. The optical disdrometer is an improved version of the ODM 470 disdrometer. It allows to measure hydrometeors within a size range of 0.4 to 22 mm in diameter. The main advantage of this instrument is its ability to estimate accurately precipitation even under strong wind conditions (Grosklaus, 1996). To measure solid precipitation a geometrical model was developed to determine the mean cross-sectional area of snow crystals for different predefined shapes and sizes. It serves to develop an algorithm, which relates the mean cross sectional area of snow crystals to their maximum dimension, liquid water content, and terminal velocity. The algorithm was applied to disdrometer measurements during winter 1999/2000 in Uppsala/Sweden. Resulting precipitation was compared to independent measurements of a Geonor gauge and to manual measurements. In terms of daily precipitation the disdrometer shows a reliable performance.

A New Characterization of Rain and Clouds: Results from a Statistical Inversion of Count Data

Abstract: Most variables in meteorology are statistically heterogeneous. The statistics of data from several different locations, then, can be thought of as an amalgamation of information contained in several contributing probability density functions (PDFs) having different sets of parameters, different parametric forms, and different mean values. The frequency distribution of such data, then, will often be multimodal. Usually, however, in order to achieve better sampling, measurements of these variables over an entire set of data gathered at widely disparate locations are processed as though the data were statistically homogeneous, that is, as though they were fully characterized by just one PDF and one single set of parameters having one mean value. Is there, instead, a better way of treating the data in a manner that is consistent with this statistical heterogeneity? This question is addressed here using a statistical inversion technique developed by Tarantola based upon Bayesian methodology. Two examples of disdrometer measurements in real rain, one 16 h and the other 3 min long, reveal the presence of multiple mean values of the counts at all the different drop sizes. In both cases the heterogeneous rain can be decomposed into five–seven statistically homogeneous components, each characterized by its own steady drop size distribution. Concepts such as stratiform versus convective rain can be given more precise meaning in terms of the contributions each component makes to the rain. Furthermore, this discovery permits the explicit inclusion of statistical heterogeneity into some analytic theories.

Analysis of Terminal Velocity and VHF Backscatter of Precipitation Particles Using Chung-Li VHF Radar Combined with Ground-Based Disdrometer

Abstract: The backscatter from precipitation particles observed by the vertically pointed antenna beam of the Chung-Li VHF radar and the drop size distributions measured by a ground-based disdrometer co-located at the radar site are analyzed and studied in this article. We find that the disdrometermeasured drop size distribution can be well approximated to a Gamma distribution. On the basis of this property and a power law approximation to the fallspeed-diameter relation V D = AD B , we derive the theoretical relation between terminal velocity V D and range-corrected VHF backscatter P of the precipitation particles. We find that the V D - P relation follows a power law in the form of VD = α β P , where α and β are both the functions of the precipitation parameters. Chu et al. (1999) first found that the relation between α and β can be empirically approximated to an exponential form of α = Ae ξβ , where β is a function of B and ξ is a factor associated with precipitation. In this article, under the assumptions of the Gamma distribution of the drop size distribution and the power-law relation between VD and D, we theoretically show that the analytical relation between α and β indeed follows an exponential form of α = Ae ξβ , where ξ is a function of the drop size distribution. The experimental results obtained by the Chung-Li VHF radar combined with the ground-based disdrometer measurements validate the exponential approximation to the αβ - relation. The uses of the αβ - relation for the investigations of the rainfall rate and properties of drop size distribution are presented and discussed.

Specific attenuation and depolarisation in rain from 2-dimensional video disdrometer data

Abstract: Data from 2-dimensional video disdrometers (2DVD) taken in different locations are used to derive the specific attenuation for horizontal and vertical polarisations at 20 GHz. The data include both drop-size distribution and their axis ratio distributions. T-matrix calculations using data over 1 min integration time show a power-law dependence on rain rate. The best-fit coefficients are compared with those given in the ITU-R Recommendation P. 838, both versions 2 and 3. Closest agreement is seen for vertical polarisation for version 2 and the largest discrepancies are seen for horizontal polarisation for version 3. The discrepancies are attributed to the different assumptions in the drop-size distributions, as well as the upper limit of integration for deriving the specific attenuation. The variation of the crosspolar discrimination (XPD) against copolar attenuation (CPA) is also examined. The 2DVD data are used to simulate a beacon experimental scenario at 20 GHz, the actual locations of the two instruments being different but climatically similar. Good agreement is seen with one year of beacon measurements, but the latter shows more spread in the variation, partly attributed to drop oscillations. Calculations also show that XPD-CPA variation is sensitive to the assumed drop shapes.

Sampling Errors in the Measurement of Rainfall Parameters Using the Precipitation Occurrence Sensor System (POSS)

Abstract: The Precipitation Occurrence Sensor System (POSS) is a small Doppler radar originally designed by the Meteorological Service of Canada (MSC) to report the occurrence, type, and intensity of precipitation in automated observing stations. It is also used for real-time estimation of raindrop size distributions (DSDs). From the DSD, various rainfall parameters can be calculated and relationships established, such as between the radar reflectivity factor (Z) and the rainfall rate (R). Earlier work presented first-order estimates of the sampling errors for some POSS rainfall parameter estimates. This work combines a Monte Carlo simulation and “inverse problem” analysis to better estimate errors due to the specific sampling problems of this disdrometer type. The uncertainties are necessary to determine the statistical significance of differences between DSD estimates by the POSS and other collocated disdrometers, or between POSS measurements in different climatologies. Additionally, confidence limits ca...

Measurement of Time-Varying Rainfall Rate in Surabaya and Estimation of Attenuation Statistics by Synthetic Storm Technique

Abstract: Measurement of time-varying rain rate has been performed in Surabaya using an optical disdrometer. The results suggest that consecutive samples of rain rate in Surabaya possesses higher correlation than those measured in such non-tropical sites like Barcelona. Attenuation statistics obtained from a simulation study by means of the synthetic storm technique applied to the rain rate time series measurements agree with those predicted by ITU-R only for links of 1-2 km length at outage probabilities in the range of 0.01-0.1%. The simulation results also show that the use of daily and monthly averages of wind velocity does not yield significant differences in the attenuation statistics.

Diagnosing the Intercept Parameter for Exponential Rain Drop Size Distribution Based on Video Disdrometer Observations

Abstract: Information about the drop size distribution (DSD) is essential for understanding precipitation physics, estimating rainfall, and improving microphysics parameterizations in numerical weather prediction (NWP) models (Steiner et al. 2004). The characteristics of rain DSDs are often associated with the types of storms (e.g., convective versus stratiform rain) and their stages of development (e.g., the developing versus decaying stage, Brandes et al. 2006). Strong convective rain usually contains both large and small drops and has a broad DSD while the decaying stage is often dominated by small drops. Stratiform rain usually contains relatively larger drops but has a low number concentration for a given rain rate (Zhang et al. 2006). Rain DSDs are usually represented by distribution models such as the exponential distribution, gamma distribution, and lognormal distribution. A DSD model usually contains a few free parameters that should be easy to determine and the model should be capable of capturing the main physical processes and properties. The exponential distribution is the most commonly used DSD model that has some of these properties, and it is given by

Processing disdrometer raindrop spectra time series from various climatological regions using estimation and autoregressive methods

Abstract: A large data set of rain drop size distribution (RSD) measurements collected with Joss-Waldvogel (JWD) and 2D video disdrometers (2DVD) in UK, Athens, Japan and USA are analyzed. The objective of this work are manifold: i) show the differences of a wide climatological DSD-derived moments; ii) retrieve from this disdrometer data set the driving parameters of the normalized gamma RSD and perform a sensitivity analysis of these results by using different best-fitting techniques; iii) exploit the correlation structure of the estimated RSD parameters as input of a vector autoregressive stationary model in order to simulate time series (or horizontal profiles) of RSDs and, consequently, of either rain rate or path attenuation; iv) characterize the distribution of the inter-rain duration (or dry periods: DP) and rain duration (or wet periods: WP) to design a simple semi-Markov chain to represent the intermittency feature of rainfall process. The overall stochastic procedure to randomly synthetize (or generate) RSD time series is named Vector Autoregressive Raindrop Markov Synthesizer (VARMS) model. This stochastic RSD generation tool may find useful applications both in hydro-meteorology and radio-propagation.

Drop shape and orientation angle distributions in rain from 2-D video disdrometer

Abstract: In an earlier study, the drop shapes were investigated using the 2-D video disdrometer (2DVD; Randeu et al 2002, Kruger and Krajewski, 2002) in artificial rain (Thurai and Bringi, 2005). Contoured shapes (which filters the quantization noise of the instrument) were derived for over 115,000 drops (Thurai et al, 2007) and were shown to be consistent with the BeardChuang (non-oblate) shape model (1987). Drops with equivalent diameter (Deq) greater than 4 mm were shown to deviate more and more from oblate shapes. A fitted equation for the mean shapes was derived based only on Deq.

Rain Observations with Micro Rain Radar (MRR) over a Tropical Station

Abstract: The Micro Rain Radar (MRR) a high resolution radar operates at a frequency of 24 GHz installed at Thumba (8.5°N, 76.9°E) under Ka band propagation experiment is used extensively to characterize the tropical rain. This radar measurements of rain were obtained with fine spatial and temporal resolutions like One minute time resolution and 200 m height resolution. With this radar for the first time classification of precipitating systems are studied. With the presence or absence of bright band a radar signature of melting layer one can classify particular rain type as convective or stratiform. For present study MRR data from September 2005 onwards are collected. The main objective is to classify precipitation system into Stratiform and Convective with the presence or absence of Bright band. Another potential of this radar is ability to give information of vertical structure of fall velocity of hydrometeors. This also gives profiles of number concentration of various ranges of Drop sizes, liquid water content and rain rate for different heights. There is a dearth of rain drop Size data and distribution models for the tropics, especially over Indian continent. Models for drop size distribution are required for the evaluation of microwave and millimeter wave propagation effects due to rainfall. In the present paper various DSD models namely exponential, gamma model and lognormal model with different combination of moments for observing the characteristic features of tropical rain are studied. These results are compared with the collocated ground based Disdrometer. Attenuation at Microwave frequencies during the presence of rain is a serious concern to the communication. Once temporal and spatial information of DSD is known microwave attenuation can be studied. These results will be presented in this paper.

Variability of the Rain Drop Size Distributions within a Storm

Abstract: The variability of drop size distributions (DSDs) within a storm lasted for 14 hours in Montreal, Canada, is investigated utilizing 1) 840 one-minute DSDs observed by a disdrometer, and 2) vertical profiles of reflectivity (Z) and vertical Doppler velocity (Vr) from a vertically pointing radar. Vertical Doppler velocity indicates that riming exists throughout the rain event. Based on the vertical structures of Z and Vr, we have identified three periods as a function of the degree of riming and examine the characteristics of the DSD variability. The fluctuation of the rain intensity shows the coherent structure with the decorrelation time of about 40 minutes. This emphasizes the nonstationary nature of the rain field. When riming and aggregation coexist, DSDs are narrower and show the pivoting around smaller diameters. As the degree of riming increases, DSDs become broader and then steeper when strong updraft appears. In addition, the average DSD shows peaks in small diameters associated with break-up of large raindrops. The exponent of R-Z relationships decrease with increasing degree of riming. The decorrelation time of the R-Z uncertainty is about 20 minutes. When aggregation and riming coexist, the characteristic number density N' 0 and diameter D' m are small. D' m increases rapidly and N' 0 remains its value when riming is dominant. Finally, when strong updraft exists, N' 0 dramatically increases whereas D' m slightly decreases.

Estimation of drop size distribution from the low frequency components in the rain generated underwater acoustic noise

Abstract: Drop size distribution (DSD) is an important parameter that completely characterizes the rain and rain generated acoustic energy. This paper presents a simple and efficient method for measuring the DSD of rain by analyzing the low‐frequency components in the spectrum of rain generated acoustic noise underwater. A specially designed sensor assembly captures the acoustic noise produced due to the raindrop impacts on the water surface. Every individual drop produces acoustic noise comprising the low frequency components which has distinct characteristics in the time domain. It has been found that these distinct features vary with the size of the droplets and its velocity. Experimental evidences show that the acoustic energy of the low frequency signal is related to the size of the drops that caused the acoustic noise and, hence, the energy of the drops generated acoustic signal is a measure of the drop size. An algorithm for detecting and computing the energy of these low frequency components leading to the estimation of DSD of rain is presented in this paper. This approach has been employed in developing a Disdrometer based automatic rain gauge, which is simple, reliable, and cost effective.

Analyzing relationships between lightning and rain in order to improve estimation accuracy of rain

Abstract: The remote estimation of rainfall rate R is essential for the aviation industry, agriculture, and flood warning. Radar, the current means of R estimation, is not available in much of the world. In addition, this measurement involves a level of inaccuracy. Using lightning to detect rain is a relatively inexpensive alternative to radar systems and can be done from existing satellites. Previous research has revealed correlations between lightning and rain, suggesting either that it is possible to estimate R using lightning, or that it is possible to use it to correct for a portion of the radar inaccuracies. These correlations are not only between the amount of lightning and the amount of rain, but also between other parameters, including statistics describing raindrop size. Rain, lightning, and radar data were collected in Central Florida over a two month period in the summer of 2005. Rain data, including raindrop size statistics, were collected from a single point using a disdrometer. Lightning data were collected using the Los Alamos Sferic Array. Radar data were obtained from the WSR-88D radar network. Rain rate R and the raindrop size statistics were compared to lightning statistics to determine which rain/lightning parameter pairs were most correlated. The degree of correlation between rain and lightning parameters was evaluated using the correlation coefficient r. Different lightning types (Cloud-to-Ground, Intra-Cloud, Narrow-Bipolar-Event, Total) were considered, and various circular areas were used for lightning collection to optimize the strength of the correlations. Four models using lightning and/or radar for the estimation of R were developed and then compared for accuracy. The first model is based on the relationship between R and the radar reflectivity factor Z, as is the current practice. Two models using only lightning for the estimation of R were evaluated, and a final model used both radar and lightning data to estimate R. The performance of each model was evaluated using the RMS error. The correlations between rain and lightning parameters were generally weak (r < 0.5), although some pairs clearly produced stronger correlations than others. Results show that the strongest correlations are between lightning density (strokes/km/hr) and Λ, a parameter of the raindrop size distribution. This correlation was strongest for Intra-Cloud (IC) lightning measured on a 75 km

Calculations of XPD Spread for 20 GHz Fixed Satellite Systems using 2-D Video Disdrometer Data

Abstract: Individual drop images in natural rain from the 2-dimensional video disdrometer have been processed to obtain their shapes and orientation angles, which in turn have been used to derive their complex forward scattering amplitudes at 19.701 GHz. Using these calculations, the XPA versus co-polar attenuation variation has been derived for a simulated experimental link. The variation is compared with 1-year of beacon data and shown to give rise to significantly higher XPD spread than the previous computations which used mean shapes and orientation models for the measured drop sizes. (5 pages)

Microwave Modelling of Rain Attenuation Fields using Disdrometer Measurements and Stochastic Methods

Abstract: Hydro-meteorological and radio-propagation applications can benefit from the capability to model time evolution of raindrop size distribution (RSD). In this respect this work presents a study to simulate synthetic time series of RSDs using a vectorial autoregressive approach and a semi-Markov chain. A large set of disdrometer measurements, acquired in different climatological sites, is analyzed in order to drive the RSD temporal simulator. Climatological differences of RSD data are quantitatively discussed. A spectral technique, based on an iterative adaptive fast-Fourier-transform technique, is also applied to transform the temporal series into rainfall spatial fields. Numerical examples of radar and radio- propagation parameters, such as co-polar reflectiviy and at Ka-band co-polar specific attenuation, are discussed by employing the rainfall space-time RSD simulator coupled with a raindrop scattering model valid for oblate raindrop ensembles. (5 pages)

20 GHz Specific attenuation calculations using drop size distributions and drop shape measurements from 2D video disdrometer data in different rain climates

Abstract: Specific attenuation calculations using drop size and drop shape data are presented for horizontal and vertical polarizations at 20 GHz. The data were obtained from several 2- dimensional video disdrometers located around the globe, ranging from mid-latitude to equatorial regions. The results are compared with predictions from the ITU- R recommendation P. 838 -3.

Studies on Ku‐band Propagation over Earth‐Space Paths and Related Rain Parameters at a Tropical Location

Abstract: Propagation measurements at Ku‐band over an earth‐space path have been carried out at Kolkata (22°34′N, 88°29′E) by receiving a signal at 11.172 GHz from the satellite NSS‐6 (geostationary at 95°E) since June 2004. The amplitudes of the co‐polar signal and the cross‐polar component have been monitored along with rain rate and drop size distribution measurements by an optical raingauge and a Joss‐type disdrometer, respectively. The relationship between rain attenuation over the earth‐space path and the rain rate gives an assessment of raining conditions along the satellite path. Fast fluctuations of the satellite signal amplitude are studied to indicate the effect of small scale irregularities associated with the rain events. TRMM satellite data obtained with PR and TMI indicate that the rain rate has a distinct control of rain height which increases with the former, the relationship depending on the type of rain. The propagation measurements and TRMM data have been used to study the features of rain at Ko...

Quantitative precipitation estimation and error analysis with a UHF wind profiling radar and a two-dimensional video disdrometer

Abstract: During periods of precipitation, vertically pointed wind profiling radars can be used to directly measure the drop size distribution (DSD) through the Doppler velocity spectrum. This is achieved by directly mapping the Doppler spectrum from velocity space into diameter space. In the absence of vertical ambient air motion and if Rayleigh scatter from discrete particles is the dominant contribution to the radar signal, the velocities detected by the profiler are primarily due to falling hydrometeors. Under these conditions, the DSD is retrieved from the Doppler spectrum by applying an appropriate expression that relates drop diameter to terminal fall speed. Precipitation parameters such as rainfall rate, radar reflectivity factor, liquid water content, massweighted mean drop diameter, and median volume drop diameter can be calculated from the retrieved DSD. Unlike in-situ instruments located at the surface, measurements from the profiler can be used to investigate the evolution of these parameters with height.