Abstract—This paper compares the behaviour of existing rainfall drop-size distribution models from 2-year disdrometer measurements in Durban (29 o 97’S, 30 o 95’E). The measurements were classified into four rainfall types: drizzle, widespread, shower and thunderstorm. Initial results indicate that the tropical 2-parameter lognormal model has the best fit for the location at different rainfall rates, especially at high rainfall rate. Consequently, the Method of Moment estimation technique is applied to derive the three input parameters of the lognormal model for rainfall types in Durban. The proposed model was seen to compare well with our measurements for various rainfall types. Index Terms—Rainfall drop size distribution, rainfall types, method of moment, lognormal DSD model. I. INTRODUCTION Satellite and terrestrial communication systems deployed at microwave and millimetric frequencies (up to 10 GHz and above) are prone to attenuation due to precipitation [1]. Precipitations (or hydrometeors) are known to be a major concern to link budget engineers particularly with respect to bandwidth availability and efficiency [2]. In past research involving studies of precipitation effects on signal transmission [1]–[8], rainfall has been identified as the active phenomena responsible for signal outages and losses during transmission. Instances of outages (either short or long) due to rainfall in packet-oriented networks often result in irreplaceable loss of time and resources. Hence, it is necessary to reduce these losses by embarking on corrective schemes which eliminate to a great extent the effects of rainfall on communication systems. The two well-known microstructural parameters used by researchers in the determination of rainfall attenuation are: rainfall rate and rain drop size distribution (or rainfall DSD) [2]. In this work, we approach rainfall attenuation studies for various rainfall types by modeling the rainfall DSD for Durban, evaluating their respective performances and deriving input parameters for the best model. The models under consideration include negative exponential model by Marshall and Palmer [3], modified gamma model by Ulbrich [4] and tropical lognormal model by Ajayi et al. [5]. In this paper, we investigate rainfall DSD with the aid of Joss-Waldvogel impact disdrometer measurements available at the University of KwaZulu-Natal, Durban, South Africa between the period of January 2009 and December 2010. II. THEORY OF RAIN DROPSIZE DISTRIBUTION Rainfall is made up of tiny droplets of spherical or oblate- shaped particles whose number density contribute to attenuation in a microwave or satellite link [6]. The mechanism of rain drop attenuation are of two kinds: reflection mechanism and absorption mechanism. The two mechanisms are dependent on the transmission frequency and polarization sequence of a microwave link. Usually, rain droplets tend to produce reflection of transmitted signals at wavelengths larger than its diameter and absorption at smaller wavelengths [7]. In theory, the reflection encountered during rainfall account for the scattering mechanisms of signals due to rainfall – this results in destructive coupling signals (noise) being added to the transmitted signal. The coefficients produced by scattering can range from real to complex values, which are very helpful, in the calculations of propagation coefficients. Mulangu et al. [7] and Odedina et al. [6] in their recent study determined the scattering parameters for locations in Botswana and South Africa respectively. The knowledge of these scattering parameters is helpful in the computation of the specific attenuation on signals due to rainfall droplets. Generally, the contribution of rain droplets to attenuation depends on other parameters such as rainfall rate, drop diameter, drop temperature, number of rain drops, fall velocity (or terminal velocity) of drops and drop diameter interval [2]. While the drop diameter and drop temperature influence the scattering coefficients, other parameters help in the determination of the rainfall DSD and the resulting attenuation. The presence of these microphysical rainfall parameters can be used in the estimation of rainfall specific attenuation, and thus path attenuation, due to rain drops. The specific attenuation, A s , due to rainfall drops is given as:    4.343 × 10 3           ⁄  (1) where N(D) is the rainfall drop-size distribution in m -3 mm -1 and Q t (D) is the extinction cross section (ECS) of the arriving rain droplets in mm 2 . As seen in (1), rainfall DSD is an integral parameter in the attenuation function, hence, it is important to get a suitable model that fits the measurement. III. DETERMINATION OF RAINFALL DROPSIZE DISTRIBUTION An important process in the estimation of rainfall attenuation involves the determination of rainfall drop-size distribution. The disdrometer computation for rainfall DSD, N(D), is given as:            ∆       (2) where n i represents the number of available rain drops per Estimation of Parameters for Lognormal Rainfall DSD Model for Various Rainfall Types in Durban Akintunde A. Alonge, Student Member, IEEE and Thomas J. Afullo, Senior Member, SAIEE School of Electrical, Electronics and Computer Engineering, University of KwaZulu-Natal, Durban, South Africa email: {210546050, afullot}@ukzn.ac.za