Rainfall Drop Size Distribution Research Articles

Characteristics of Rain and Sea Spray Droplet Size Distribution at a Marine Tower

The effects of sea spray on open-ocean rainfall measurements-the drop size distribution (DSD) and rainfall intensities-were studied using a state-of-the-art optical disdrometer. The number of rain droplets less than 1 mm in diameter is affected by several factors, including the type of rainfall and seasonality. Over the ocean, small rain and large sea spray droplets co-exist in the same diameter size class (0.072 to 1000 mm); hence, sea spray creates uncertainty when seeking to characterize the drop size distribution (DSD) of rain droplets over the ocean. We measured droplet sizes at a marine tower using a state-of-the-art optical disdrometer, a tipping-bucket rain gauge, a wind anemometer, and a time-lapse camera, over a period that included typhoon Krosa of 2019. The number of rain droplets of diameter less than 1 mm increased monotonically as the horizontal wind speed became stronger. Thus, the shape parameter μ of the Ulbrich distribution decreased. This decreasing trend can be recognized as an increase in sea spray. During no-rainfall hours (indicated by rain gauges on the ocean tower and nearby land), sea spray DSDs were obtained at various horizontal wind speeds. Furthermore, the proportions of sea spray to rainfall at different rainfall intensities and horizontal wind speeds were determined; at a horizontal wind speed of 16 to 20 m s−1, the average sea spray proportions were 82.7%, 19.1%, and 5.3% during total rainfall periods of 2.1 mm h−1, 8.9 mm h−1, and 32.1 mm h−1, respectively. Representation of sea spray DSDs, as well as rainfall DSDs, is a key element of calculating real rainfall intensities over the open ocean.

Semi-empirical characteristics of modified lognormal DSD inputs using rain rate distributions for radio links over the African continent

Rainfall attenuation mitigation is one of the important design factors considered in the deployment of outdoor wireless 5G networks, especially in the provision of services operating in frequency bands between 10 and 100 GHz. In this current work, previous rainfall measurement and modelling campaigns undertaken at different locations in Africa are investigated using their available rainfall Drop Size Distribution (DSD) profiles from Joss-Waldvogel (JW) disdrometer. These locations of interest are found in Western, Central and Southern Africa at the following sites: Niamey (13°30ʹN, 2°10ʹE), Boyélé (2°50ʹN, 18°04ʹE), Abidjan (5°25ʹN, 4°W), Dakar (14°34ʹN, 17°29ʹW), Ile-Ife (7°30ʹN, 18°04ʹE), Durban (29°52ʹS, 30°58ʹE) and Butare (2°36ʹS, 29°44ʹE). A variant of the lognormal DSD, categorized as a lognormal Type II DSD—is considered as a statistical tool to analyze and compare the behaviour of DSDs at these locations, using its first and second derivative conditions. Furthermore, a semi-empirical approach is proposed to formulate the input parameters of the lognormal Type II distribution using two renowned parameters: percentage of rainfall rate exceeded (P) and rainfall rate at 0.01% (R0.01). An application of power-law regression fit is then utilized to extract the coefficients of these inputs from their DSDs at different locations in Africa. A comparison of the resulting site and climate models, with previous models using self-consistent rainfall rate predictions at 20 and 100 mm/h, shows reasonable agreement. Further validation is realized using specific attenuation prediction from Mie scattering technique with computations at frequency ranges up to 100 GHz—within the microwave and millimeter wave bands. More precisely, observations from these results show that climatic and geographic characteristics largely influence the DSD profile and expected rainfall attenuation at each location.

Optimized rain drop size distribution model for microwave propagation for equatorial Africa

In radio propagation, modelling of rainfall rate, rain attenuation and drop size distribution is highly location-dependent and thus requires availability of reliable data from various locations. Models and mathematical predictions obtained based on data from one location often prove inadequate when applied to another location with either slightly or radically different rainfall pattern. In this paper, the focus is to develop a new rainfall drop size distribution model for equatorial Africa using disdrometer data obtained in Butare, Rwanda (2°35’53.88”S and 29° 44’ 31.5” E). Rainfall data was classified into annual, monthly and rainfall regimes which are drizzle, widespread, shower and thunderstorm, based on their rainfall rates. The maximum likelihood estimation technique was applied to construct estimates of input Drop Size Distribution (DSD) fit parameters of the developed statistical distribution for rainfall DSD in Butare. The newly obtained distribution was compared to the existing rainfall DSD models, namely, Lognormal, Gamma, Marshall-Palmer and Weibull distributions, and found to be an improvement over the existing ones. The Mie Scattering technique is employed to derive the scattering parameters. Thereafter, the derived scattering parameters with DSD models are used for the estimation of rainfall attenuation for the Central African region.

Effect of disdrometer type on rain drop size distribution characterisation: a new dataset for south-eastern Australia

Abstract. Knowledge of the full rainfall drop size distribution (DSD) is critical for characterising liquid water precipitation for applications such as rainfall retrievals using electromagnetic signals and atmospheric model parameterisation. Southern Hemisphere temperate latitudes have a lack of DSD observations and their integrated variables. Laser-based disdrometers rely on the attenuation of a beam by falling particles and are currently the most commonly used type of instrument to observe the DSD. However, there remain questions on the accuracy and variability in the DSDs measured by co-located instruments, whether identical models, different models or from different manufacturers. In this study, raw and processed DSD observations obtained from two of the most commonly deployed laser disdrometers, namely the Parsivel1 from OTT and the Laser Precipitation Monitor (LPM) from Thies Clima, are analysed and compared. Four co-located instruments of each type were deployed over 3 years from 2014 to 2017 in the proximity of Melbourne, a region prone to coastal rainfall in south-eastern Australia. This dataset includes a total of approximately 1.5 million recorded minutes, including over 40 000 min of quality rainfall data common to all instruments, equivalent to a cumulative amount of rainfall ranging from 1093 to 1244 mm (depending on the instrument records) for a total of 318 rainfall events. Most of the events lasted between 20 and 40 min for rainfall amounts of 0.12 to 26.0 mm. The co-located LPM sensors show very similar observations, while the co-located Parsivel1 systems show significantly different results. The LPM recorded 1 to 2 orders of magnitude more smaller droplets for drop diameters below 0.6 mm compared to the Parsivel1, with differences increasing at higher rainfall rates. The LPM integrated variables showed systematically lower values compared to the Parsivel1. Radar reflectivity–rainfall rate (ZH–R) relationships and resulting potential errors are also presented. Specific ZH–R relations for drizzle and convective rainfall are also derived based on DSD collected for each instrument type. Variability of the DSD as observed by co-located instruments of the same manufacturer had little impact on the estimated ZH–R relationships for stratiform rainfall, but differs when considering convective rainfall relations or ZH–R relations fitted to all available data. Conversely, disdrometer-derived ZH–R relations as compared to the Marshall–Palmer relation ZH=200R1.6 led to a bias in rainfall rates for reflectivities of 50 dBZ of up to 21.6 mm h−1. This study provides an open-source high-resolution dataset of co-located DSD to further explore sampling effects at the micro scale, along with rainfall microstructure.

Cross-validation of rainfall characteristics estimated from the TRMM PR, a combined PR-TMI algorithm and a C-POL ground-radar during the passage of tropical cyclone and non-tropical cyclone events over Darwin, Australia.

This study cross-validates the radar reflectivity Z, the rainfall drop size distribution parameter (median volume diameter, Do ) and the rainfall rate R estimated from the Tropical Rainfall Measuring Mission (TRMM) satellite Precipitation Radar (PR), a combined PR and TRMM Microwave Imager (TMI) algorithm (COM) and a C-band dual-polarised ground-radar (GR) for TRMM overpasses during the passage of tropical cyclone (TC) and non-TC events over Darwin, Australia. Two overpass events during the passage of TC Carlos and eleven non-TC overpass events are used in this study and the GR is taken as the reference. It is shown that the correspondence is dependent on the precipitation type whereby events with more (less) stratiform rainfall usually have a positive (negative) bias in the reflectivity and the rainfall rate whereas in the Do the bias is generally positive but small (large). The COM reflectivity estimates are similar to the PR but it has a smaller bias in the Do for most of the greater stratiform events. This suggests that combining the TMI with the PR adjusts the Do towards the "correct" direction if the GR is taken as the reference. Moreover, the association between the TRMM estimates and the GR for the two TC events, which are highly stratiform in nature, is similar to that observed for the highly stratiform non-TC events (there is no significant difference) but it differs largely from that observed for the majority of the highly convective non-TC events.

A phenomenological relationship between vertical air motion and disdrometer derived A-b coefficients

Using the well-known Z-R power law, Z=ARb, A-b parameters derived from a single disdrometer are readily found and can provide useful information to study rainfall drop size distributions (DSDs). However, large variations in values are often seen when comparing A-b sets from various researchers. Values of b typically range from 1.25 to 1.55 for both stratiform and convective events. The values of A approximately fall into three groups: 150 to 200 for convective, 200 to 400 for stratiform, and 400 to 500 for convective. Computing the A-b parameters using the gamma DSD, coupled with a modified drop terminal velocity model, vD(D)=vT(D)−w, where D is drop diameter, vT(D) is still air drop terminal velocity, and w is an estimate of vertical velocity of the air well above the disdrometer, shows an interesting result. This model predicts three regions of A, corresponding to w<0, w=0, and w>0. Additional models that incorporate a constant vertical air velocity are also investigated. A-b sets derived from a Joss-Waldvogel (JW) disdrometer and DSD data acquired near Athalassa, Cyprus, using selected 24-hour data sets from 2011 to 2014, are compared to the above models. The data is separated into two main groups: stratiform events defined by rainfall rates that did not exceed 10mmh−1 at any time during the 24-hour period, and convective events defined by rainfall rates not flagged as stratiform. The convective rainfall is further separated into two groups: A-b pairs that fall to the left of the stratiform pairs and pairs that fall to the right. This procedure is repeated with data from other researchers that corresponds to seasonal averages. In all cases, the three vertical groupings of the A-b parameter plot seem to correlate to DSD simulations where various values of positive and negative vertical velocities are used.

Special Issue: Smart Satellite Systems

Special Issue: Smart Satellite Systems

Contrasting tropical cyclone and non-tropical cyclone related rainfall drop size distribution at Darwin, Australia

In this study the rainfall drop size distribution (DSD) during the passage of seven tropical cyclones (TCs) over Darwin is compared and contrasted with that associated with non-tropical cyclone (non-TC) events, using the impact disdrometer data at the Darwin Atmospheric Radiation and Measurement (ARM) site. The disparity of the DSD with respect to rainfall types (between TC and non-TC conditions) and distance from TC centre is also examined. It is shown that TC DSDs are statistically different from the non-TC DSDs, the former encompassing a larger concentration of small to moderate drop sizes. The TC mass-weighted mean diameter (Dm) is lower than the non-TC values at all rain rates and also for the different precipitation types (convective, transition and stratiform). The TC DSD varies with distance from the TC centre, as rainfall near the TC centre (<60km) comprises of relatively smaller drops which are strongly evident at small to moderate rain rates (<30mmh−1). Such variations in the DSD have implications for the parameters used in the algorithm that converts radar reflectivity to rainfall rate in TCs, as well as for the analytical expressions used in describing the observed DSD employed in cloud modelling parameterizations.

Variation of multimodality in rainfall drop size distribution with wind speeds and rain rates

In the coming years, there will be more usage of the millimetre/sub-millimetre frequencies due to congestion of the lower frequencies. At these frequencies, precipitation greatly affects the quality of service, by attenuating signals, hence the need for a thorough study and understanding in order to design mitigation techniques for improved signal quality. Previous studies modelled rainfall data using mostly unimodal statistical distributions, which may not fit multimodality encountered in the data. This paper looks at the prediction of the number of modes, given rain rates and wind speeds by looking at the occurrence of multimodality in rainfall data captured at Chilbolton Observatory, southern England from 2003 to 2009. From the drop size distributions, it develops a novel model based on the Gaussian mixture model. This enables the multimodal distributions observed by various researchers to be modelled. It provides expressions for calculating model parameters as a function of rain rate, R (mm/h) and wind speed, W (m/s). The model parameters include number of modes N m , standard deviation σ 1 –σ m of each mode along with corresponding means, μ 1–μ m . The study concludes that multimodality exists, and the average number of modes tends to increase with increasing wind speeds and rain rates.

Modeling rainfall drop size distribution in southern England using a Gaussian Mixture Model

AbstractUnderstanding and modeling the rainfall drop size distribution is important in a number of applications, in particular predicting and mitigating attenuation of satellite signals in the millimeter band. Various standard statistical distributions have been proposed as suitable models, the first widely accepted being the exponential distribution. Subsequently, gamma and lognormal distributions have been shown to provide better rainfall rate computations. Some empirical studies have revealed bimodal distributions under some circumstances. A natural question to ask therefore is how often gamma and lognormal distributions fit the empirical data. In this paper we fit lognormal and gamma distributions to 1 min slices of rainfall drop size distributions taken from 7 year data from the Chilbolton Observatory in southern England. The chi‐square goodness of fit of the models against the data is calculated, and it is found that failure to fit is greater than would normally be expected. This failure to fit is broken down and examined against seasonal variations, different rain rates, atmospheric temperature, and wind speed. Possible reasons for the lack of fit are explored, and alternative fits using models based on Gaussian Mixture Models are developed and found to be an improvement.

PARSIVEL 측정 자료를 활용한 대관령 산악지역 강수입자분포 모형 연구

본 연구에서는 대관령 지역에서의 광학우적계(PARSIVEL disdrometer) 강수관측으로부터 산출된 강수율에 따른 강수입자분포 자료를 바탕으로 기존의 강수입자분포 모형을 개선하였다. 선행 연구에서 제안한 다양한 강수입자분포 모형과 측정 자료와의 상관성을 분석한 결과, 대관령 지역에 적용 가능한 원형 모형은 개선된 <TEX>${\Gamma}$</TEX> 분포 모형임을 확인하였다. 원형 모형을 대관령 지역에 적용할 수 있도록, 민감도 실험을 통해 최적의 매개변수들(<TEX>${\alpha}$</TEX>, A, B)을 산정하였으며, 다섯 가지 강수율에 대한 강수입자분포 모형을 제안하였다. 강수율에 따른 강수입자분포 모형의 결과는 관측에서 측정된 값과 높은 상관성(<TEX>$R^2=0.975$</TEX>)을 보였다. 강수율에 따라 표현되는 강수입자분포 모형을 일반화 형태로 개선하기 위해 강수율과 매개변수의 상관성을 도출하여 일반식을 결정하였다. 일반화된 강수입자분포 모형은 대관령 지역의 강수입자분포 측정 자료와 높은 상관성(<TEX>$R^2=0.953$</TEX>)을 보였으며, 이는 본 연구에서 제안한 모형이 대관령 지역의 강수입자분포를 모의하는데 효과적임을 의미한다. 그러나 본 연구에서 제안된 강수입자분포 모형은 대관령 지역의 강수입자분포에만 최적화 되었다는 한계성이 있어, 따라서 한반도를 대표하는 모형을 개발하기 위해서는 다른 지역에 대한 광범위한 측정이 필요하다. In this study, a model of rainfall drop-size distribution was modified using PARSIVEL-retrieved rainfall drop-size distribution over Daegwanryeong mountainous area. A prototype model (Modified <TEX>${\Gamma}$</TEX> distribution model) applicable for this area was decided through the comparative analysis between results from models proposed by preceding research and PARSIVEL-retrieved data over Daegwanryeong mountainous area. In order to apply the prototype model for Daegwanryeong region, the parameters (<TEX>${\alpha}$</TEX>, A, B) were made via sensitivity experiments and models of the rainfall drop-size distributions for five cases of rainfall rate were proposed. Results from the proposed five models showed high correlations with PARSIVEL-retrieved data (<TEX>$R^2=0.975$</TEX>). In order to suggest a generalized form of rainfall drop-size distribution, interaction equations between rainfall rates and parameters (<TEX>${\alpha}$</TEX>, A, B) were investigated. The generalized model of the rainfall drop-size distribution was highly correlated with PARSIVEL-retrieved data (<TEX>$R^2=0.953$</TEX>), which means that the proposed model from this study was effective for simulating the rainfall drop-size distribution over Daegwanryeong region. However, the proposed model was optimized for rainfall drop-size distribution over Daegwanryeong region. Therefore, broad observations of other regions are necessary in order to develop the representative model of the Korean peninsula.

RAINFALL MICROSTRUCTURAL ANALYSIS FOR MICROWAVE LINK NETWORKS: COMPARISON AT EQUATORIAL AND SUBTROPICAL AFRICA

The quest to understand the variation of rainfall microstructures at subtropical and equatorial regions is vital to rain attenuation studies. In this study, point rainfall datasets obtained at Butare (2 - 36 0 S, 29 - 44 0 E) and Durban (29 - 52 0 S, 30 - 58 0 E), are compared at the re∞ectivity threshold of 38dBZ. Joss-Walvogel (JW) distrometer measurements collected from these two locations represent physical rainfall data from equatorial and subtropical climates respectively. The re∞ectivity threshold enables the classiflcation of rainfall datasets into stratiform and convective (S-C) precipitation regimes. These thresholds, Rth, at Durban and Butare are analysed based on three known rainfall microphysical parameters: rain rate, rainfall Drop Size Distribution (DSD) and radar re∞ectivity. The results from rain rate distributions at the both regions are similar for both stratiform and convective classes. However, the sampled DSDs indicate the dominance of larger rain droplets at Butare compared to observations at Durban, irrespective of the rain classes. In addition, it is found that the re∞ectivity distributions at both regions, under stratiform and convective conditions, are distinct in their probability proflles. The overall S-C analysis implied that the re∞ectivity and DSD proflles at both regions | result in signiflcant variation of predicted speciflc attenuation | at microwave and millimeter band. In comparison with other global locations, it is a-rmed that the S-C transition occurs globally at rain rates between 6mm/h and 13mm/h.

Regime analysis of rainfall drop-size distribution models for microwave terrestrial networks

In this study, parameters of statistical models for estimating the rainfall specific attenuation in Durban, South Africa are derived. The rainfall data in this work are based on 2-year measurements obtained via the Joss-Waldvogel RD-80 disdrometer between January 2009 and December 2010. Four rainfall regimes are applied: drizzle, widespread, shower and thunderstorm types. Using the third, fourth and sixth moments of the measured data, estimators for four rainfall drop-size distribution (DSD) models – 3-parameter lognormal model, negative exponential model, 3-parameter Weibull model and the modified gamma model – are computed. The specific attenuation estimates from our model are compared with the values from predicted measurements alongside the International Telecommunication Union (ITU) estimates for frequencies between 2 and 100 GHz. The results from the specific attenuation estimation are compared with the semi-empirical measurements for their respective regimes using root-mean-square and χ2 error tests. From the results, the best models for the various regimes are: Weibull model for drizzle regime, modified gamma model for widespread and shower regimes and lognormal model for the thunderstorm regime.

Bias correction of radar rainfall estimates based on a geostatistical technique

Various types of rainfall characteristic have different rainfall drop size distributions (DSDs). DSDs that are measured by radar have a fundamental influence on parameters of the Z-R relationship; using a climatological Z-R relationship to estimate radar rainfall can lead to bias in radar rainfall estimates. This paper attempts to remove the source of bias in radar rainfall estimates due to an uncertain Z-R relationship by applying a local bias adjustment factor to a region that has the same climatological rainfall characteristic. Recorded historical daily rainfall data from 188 uniformly distributed rain gauges located under the radar umbrella and its vicinity were used for describing the climatological spatial pattern of rainfall in the study area based on kriging approaches. It was found that a simple kriging technique with the isotropic Bessel-J semivariogram model was the best method to classify climatological patterns of rainfall characteristic of the study area and therefore it has been used for identifying local bias correction areas of the proposed hourly local bias (HLB) correction method. Performances of different bias correction methods with various levels of complexity were evaluated from 500 of the calibrated and cross-validated gauges of the validated data set, selected randomly. These methods include mean field bias correction (MFB), hourly mean field bias correction (HMFB), hourly range dependent mean field bias correction (HRMFB), and HLB correction. Forty-four rainfall events recorded during 2003-2005 from the S-band Pimai radar located in Nakhon-Ratchasima Province, Thailand, and 50 automatic rain gauges were used in this study. The results of this study showed that, on average, the proposed HLB method could improve accuracy of radar rainfall estimates by 16.7%, 14.3%, 2.8%, 0.4% for the calibrated gauges, and by 11.8%, 10.2%, 9.4%, 4.1% for the cross-validated gauges when compared to non-bias corrected, MFB, HMFB, and HRMFB methods, respectively.

SEASONAL ANALYSIS AND PREDICTION OF RAINFALL EFFECTS IN EASTERN SOUTH AFRICA AT MICROWAVE FREQUENCIES

Network fade countermeasures for link budget can be better implemented based on the knowledge of seasonal variability of rainfall attenuation in a locality. Therefore, in this study, a seasonal approach is applied to estimate the efiects of spatial rainfall attenuation in Durban (29 - 52'S, 30 - 58'E), South Africa using two- year rainfall data obtained from the RD-80 Joss-Waldvogel (J-W) distrometer. An analysis is undertaken for difierent seasons to obtain the rainfall rate exceedences at 0.001%, 0.01%, 0.1% and 1% of time. Consequently, rainfall drop-size distribution (DSD) models are developed for the control site at difierent seasons for the same period. The probability density analysis for each model indicates that the lognormal distribution best flts the summer and autumn season with percentage root-mean-square errors (RMS) of 30% and 26% respectively; gamma distribution flts winter season with RMS error of 16% and Weibull distribution flts spring season with RMS error of 26%. The results from the rainfall rate and rainfall DSD are combined to estimate the rainfall speciflc attenuation, by applying spherical droplet assumption for Mie scattering techniques, between 2GHz and 1000GHz. With this, the seasonal k and fi coe-cients for speciflc attenuation are derived from the best rainfall DSD models, using regression technique at 2.5GHz, 25GHz, 40GHz and 100GHz. At these frequencies, the results show that the predicted speciflc attenuation coe-cients for all seasonal rainfall rates at the control site are lower, when compared to those from ITU-R models. It is concluded that speciflc attenuation levels may be similar and more intense in summer and autumn seasons, while, lower and less intense in autumn and winter seasons at similar rainfall rates.

Assessing the sources of uncertainty associated with the calculation of rainfall kinetic energy and erosivity – application to the Upper Llobregat Basin, NE Spain

Abstract. The diverse sources of uncertainty associated with the calculation of rainfall kinetic energy and rainfall erosivity, calculated from precipitation data, were investigated at a range of temporal and spatial scales in a mountainous river basin (504 km2) in the south-eastern Pyrenees. The sources of uncertainty analysed included both methodological and local sources of uncertainty and were (i) tipping-bucket rainfall gauge instrumental errors, (ii) the efficiency of the customary equation used to derive rainfall kinetic energy from intensity, (iii) the efficiency of the regressions obtained between daily precipitation and rainfall erosivity, (iv) the temporal variability of annual rainfall erosivity values, and the spatial variability of (v) annual rainfall erosivity values and (vi) long-term erosivity values. The differentiation between systematic (accuracy) and random (precision) errors was taken into account in diverse steps of the analysis. The results showed that the uncertainty associated with the calculation of rainfall kinetic energy from rainfall intensity at the event and station scales was as high as 30%, because of insufficient information on rainfall drop size distribution. This methodological limitation must be taken into account for experimental or modelling purposes when rainfall kinetic energy is derived solely from rainfall intensity data. For longer temporal scales, the relevance of this source of uncertainty remained high if low variability in the types of rain was supposed. Temporal variability of precipitation at wider spatial scales was the main source of uncertainty when rainfall erosivity was calculated on an annual basis, whereas the uncertainty associated with long-term erosivity was rather low and less important than the uncertainty associated with other model factors such as those in the RUSLE, when operationally used for long-term soil erosion modelling.

Application of piezoelectric transducers in simulated rainfall erosivity assessment

Rainfall simulators have often been used to mimic natural rainfall for studies of various land-surface and water interaction processes. The characteristics of the simulated rainfall are the main indicators used to judge the performance of the rainfall simulators. The aim of this study is to investigate the potential of piezoelectric transducers for measuring and evaluating a dripper-type simulated rainfall drop-size distribution (DSD) and kinetic energy (KE). The directly measured KE was significantly correlated with the estimated KE using the drop-size distribution and empirical rain drop fall velocity relationships. This result emphasizes the potential use of the piezoelectric sensor to directly measure and evaluate rainfall kinetic energy. Also, the relationship between rainfall intensity and KE showed good patterns of agreement between simulated rainfall and natural rainfall. Citation Abd Elbasit, M. A. M., Yasuda, H. & Salmi, A. (2011) Application of piezoelectric transducers in simulated rainfall erosivity assessment. Hydrol. Sci. J. 56(1), 187–194.

Comparison of Raindrop Size Distribution Estimates From X-Band and S-Band Polarimetric Observations

This letter evaluates the consistency of rainfall drop size distribution (DSD) parameters that were estimated from attenuation-corrected X-band dual-polarization (XPOL) radar measurements with estimates from a colocated S-band dual-polarization radar (S-Pol). The DSD retrieval technique uses reflectivity and differential reflectivity, and a constrained relationship to estimate the three parameters of a ldquonormalizedrdquo gamma distribution model. The more definitive S-Pol DSD parameter estimates are used as a reference to assess the performance of the corresponding XPOL estimates for different rain-path attenuation values. Results show that XPOL attenuation-corrected profiles can provide rainfall DSD estimations that are consistent to an S-band dual-polarization radar, even in cases of moderate to high rain-path attenuation.

TECHNICAL NOTE: The representation of rainfall drop-size distribution and kinetic energy

Abstract. To relate observed rainfall rates (R) to the kinetic energy flux (E) that affects soil erosion it is necessary to develop relationships between the two. This paper explores theoretical E–R relationships based on gamma distributions of drop size. The relationship is poorly defined unless assumptions are made about changes in the shape of the drop-size distribution (DSD) with rainfall rate. The study suggests that the assumption of an exponential DSD leads to overestimation of kinetic energy flux. Further, incorporation of a horizontal component of kinetic energy allows for a clearer relationship between kinetic energy and rainfall intensity to be defined, but a question remains regarding the most appropriate definition of the horizontal component of drop velocity. Keywords: drop-size distribution, drop kinetic energy, soil erosion

Estimation of Rainfall Drop Size Distributions from Dual-Frequency Wind Profiler Spectra Using Deconvolution and a Nonlinear Least Squares Fitting Technique

Abstract This paper compares two different analysis approaches for dual-frequency (VHF and UHF) wind profiler precipitation retrievals. The first technique is based on a general deconvolution process to remove broadening effects from the Doppler spectra due to turbulence and finite radar beamwidth. The second technique is based on a nonlinear least squares fitting approach, where an analytical function that describes the drop size distribution is convolved with a clear air distribution to model the broadening of the observed spectra. These techniques are tested with simulated data, where the true drop size distribution is known and represented by a gamma or exponential distribution, and with observations. The median volume diameter D0, which is based on the third moment of the drop size number distribution, is used to test these retrieval techniques. This parameter is closely related to the rain rate, which is approximately the 3.67th moment of the drop size number distribution. Methods for extracting D0 ...