Rain attenuation prediction based on raindrop size distribution measurement in Malaysia

Attenuation due to rain at frequencies above 10 GHz in temperate climates and above 7 GHz in tropical ones is a critical factor for both terrestrial and satellite link system designers. Knowledge of the rain drop size distribution (DSD) is essential for an accurate estimate of the attenuation experienced by electromagnetic waves traveling through the rain. Large uncertainties remain in the variability of DSDs and their dependence on rainfall types and climatological regimes. Such uncertainties are much more critical in the equatorial region, where there are only limited experimental results of DSD data. In this study, a two-year measurement of DSD, using a 2D video distrometer (2DVD) installed for the first time in UTM Johor Malaysia, has been used. The 2DVD is an advanced instrument that not only can measure large ranges of DSD but also can capture the raindrop shape, axial ratio, oscillation mode and drop fall velocity, so it can provide a higher accuracy of estimations than any other instrument. A millimeter wave (mmwave) link operating at 38 GHz and a meteorological station installed at the same location, as well as earlier DSD data from Kuala Lumpur, are used to validate the findings. Based on the statistical analysis of the measured data samples, DSD parameters are computed using T-Matrix calculations. Specific attenuation of mmwave signals is presented for vertical and horizontal polarisations. Satisfactory results are achieved in comparison with other prediction models. Further, the study separates stratiform and convective rain types using the characteristics of the main DSD parameters. Seasonal variations are studied to elucidate characteristics of DSD in the Asian monsoon region. It is found that DSDs are affected by diurnal convective cycles and seasonal variations in precipitation characteristics. The implications of the variations on specific attenuation are presented. The results of this study will be helpful for the proper design and allocation of the wireless communication system to achieve the expected quality of service (QoS) in Malaysia

Refinements to data acquired by 2-dimensional video disdrometers

The 2-Dimensional Video Disdrometer (2DVD) is a commonly used tool for exploring rain microphysics and for validating remotely sensed rain retrievals. Recent work has revealed a persistent anomaly in 2DVD data. Early investigations of this anomaly concluded that the resulting errors in rain measurement were modest, but the methods used to flag anomalous data were not optimized, and related considerations associated with the sample sensing area were not fully investigated. Here, we (i) refine the anomaly-detecting algorithm for increased sensitivity and reliability and (ii) develop a related algorithm for refining the estimate of sample sensing area for all detected drops, including those not directly impacted by the anomaly. Using these algorithms, we explore the corrected data to measure any resulting changes to estimates of bulk rainfall statistics from two separate 2DVDs deployed in South Carolina combining for approximately 10 total years of instrumental uptime. Analysis of this data set consisting of over 200 million drops shows that the error induced in estimated total rain accumulations using the manufacturer-reported area is larger than the error due to considerations related to the anomaly. The algorithms presented here imply that approximately 4.2% of detected drops are spurious and the mean reported effective sample area for drops believed to be correctly detected is overestimated by ~8.5%. Simultaneously accounting for all of these effects suggests that the total accumulated rainfall in the data record is approximately 1.1% larger than the raw data record suggests

A Rain Taxonomy for Degraded Visual Environment Mitigation

This Technical Memorandum (TM) provides a description of a rainfall taxonomy that defines the detailed characteristics of naturally occurring rainfall. The taxonomy is based on raindrop size measurements collected around the globe and encompasses several different climate types. Included in this TM is a description of these rainfall observations, an explanation of methods used to process those data, and resultant metrics comprising the rain taxonomy database. Each of the categories in the rain taxonomy are characterized by a unique set of raindrop sizes that can be used in simulations of electromagnetic wave propagation through a rain medium

A video precipitation sensor for imaging and velocimetry of hydrometeors

A new method to determine the shape and fall velocity of hydrometeors by using a single CCD camera is proposed in this paper, and a prototype of a video precipitation sensor (VPS) is developed. The instrument consists of an optical unit (collimated light source with multi-mode fibre cluster), an imaging unit (planar array CCD sensor), an acquisition and control unit, and a data processing unit. The cylindrical space between the optical unit and imaging unit is sampling volume (300 mm × 40 mm × 30 mm). As the precipitation particles fall through the sampling volume, the CCD camera exposes twice in a single frame, which allows the double exposure of particles images to be obtained. The size and shape can be obtained by the images of particles; the fall velocity can be calculated by particle displacement in the double-exposure image and interval time; the drop size distribution and velocity distribution, precipitation intensity, and accumulated precipitation amount can be calculated by time integration. The innovation of VPS is that the shape, size, and velocity of precipitation particles can be measured by only one planar array CCD sensor, which can address the disadvantages of a linear scan CCD disdrometer and an impact disdrometer. Field measurements of rainfall demonstrate the VPS's capability to measure micro-physical properties of single particles and integral parameters of precipitation

An analytical representation of raindrop size distribution in a mixed convective and stratiform precipitating system as revealed by field observations

This study investigated a rainfall event under a typhoon influence using a 2D video disdrometer and weather radar observations to characterize raindrop size distribution (DSD) in a mixed convective and stratiform precipitating system. During the time period when both convective and stratiform rainfalls existed, the DSDs generally indicated a monotonically decreasing shape with increasing particle size, with a relatively gradual decrease at intermediate particle size observed at certain times; this feature is attributed to the combined effect of convective and stratiform rainfalls. During the transitional period between convective and stratiform rainfalls, the DSDs exhibited a bimodal shape. The DSDs were well approximated by a newly proposed gamma raindrop distribution combined with exponential (GRACE) distribution function, which was defined as the sum of the exponential distribution and the gamma distribution. A comparison of the volume ratio of the exponential and gamma components of the GRACE distribution revealed that the exponential component of the DSD was larger than the gamma component in the bimodal DSD. These results suggest that the DSD became bimodal during the period when stratiform rainfall predominated because of the weakening of convective rainfall. The GRACE distribution is useful for understanding cloud-microphysical processes in mixed stratiform and convective precipitation conditions

Binning effects on in-situ raindrop size distribution measurements

This paper investigates the binning effects on drop size distribution (DSD) measurements obtained by Joss-Waldvogel disdrometer (JWD), Precipitation Occurrence Sensor System (POSS), Thies disdrometer (Thies), Parsivel OTT disdrometer, two-dimensional video disdrometer (2DVD) and optical spectro-pluviometer (OSP) instruments, therefore the evaluation comprises non-regular bin sizes and the effect of minimum and maximum measured sizes of drops. To achieve this goal, 2DVD measurements and simulated gamma size distributions were considered. The analysis of simulated gamma DSD binned according each instrument was performed to understand the role of discretisation and truncation effects together on the integral rainfall parameters and estimators of the DSD parameters. In addition, the drop-by-drop output of the 2DVD is binned to simulate the raw output of the other disdrometers which allowed us estimate sampling and binning effects on selected events from available dataset. From simulated DSD it has been found that binning effects exist in integral rainfall parameters and in the evaluation of DSD parameters of a gamma distribution. This study indicates that POSS and JWD exhibit underestimation of concentration and mean diameter due to binning. Thies and Parsivel report a positive bias for rainfall and reflectivity (reaching 5% for heavy rainfall intensity events). Regarding to DSD parameters, distributions of estimators for the shape and scale parameters were analyzed by moment, truncated moment and maximum likelihood methods. They reported noticeable differences between instruments for all methodologies of estimation applied. The measurements of 2DVD allow sampling error estimation of instruments with smaller capture areas than 2DVD. The results show that the instrument differences due to sampling were a~relevant uncertainty but that concentration, reflectivity and mass-weighted diameter were sensitive to binning