Dependency of rain integral parameters on specific rain drop sizes and its seasonal behaviour Saurabh Das a,n , Debaleena Ghosh b a Center for Soft Computing Research, Indian Statistical Institute, 203 Barrackpore Trunk Road, Kolkata 700108, WB, India b Institute of Radio Physics and Electronics, University of Calcutta, 92, A. P. C. Road, Kolkata 700009, WB, India article info Article history: Received 24 March 2016 Received in revised form 8 September 2016 Accepted 9 September 2016 Available online 10 September 2016 Keywords: Rain drop size distribution Radar reflectivity Rain attenuation Z-R relation Tropical region abstract This paper investigates the variability of raindrop size distribution (DSD) and rain integral parameters at Ahmedabad, a tropical location, in relation to the radar estimation of rainfall. Rain DSDs for the years 2006–2007 at Ahmedabad (23°04′N, 72°38′E) have been measured using a disdrometer. Variability of DSD is evaluated for different seasons and its effect on the integral rain parameters like radar reflectivity, rainfall intensity and attenuation are examined. A percentage contribution of different drop diameters on rain integral parameters is studied to understand the seasonal behaviour of rain attenuation and radar reflectivity. It is observed that drops with diameter around 3 mm contribute maximum to the radar reflectivity while drops having a diameter around 2 mm contribute the maximum to the rainfall intensity for the present location. The critical diameter range responsible for the maximum contribution in rain attenuation found to shift towards large drops with an increase in rain rate for a fixed frequency. Linear and non-linear regression analysis between radar reflectivity and rainfall intensity show significant variations in different seasons but does not differ much for different regression techniques. Results point to the necessity of considering the seasonal variability of rain DSD in radar remote sensing and will be helpful for better characterizing of rain parameters from radar measurements. & 2016 Elsevier Ltd. All rights reserved. 1. Introduction Meteorological or climate studies nowadays rely heavily on remote sensing of precipitation systems, either from space-borne or using ground-based radars. Use of high frequencies, such as X (8–12 GHz), Ku (12–18 GHz) or Ka (18–40 GHz) band as radar sig- nals, improve the capability of detection of very small rain rates (Das et al., 2015; Thurai and Bringi, 2008). An excellent example of improved capability of high-frequency signals in radars is the precipitation radars onboard Global Precipitation Mission (GPM) satellite, one at Ku band (13.6 GHz) and another in Ka-band (35.5 GHz) (Hamada and Takayabu, 2016). Radar reflectivity factor is an important parameter for quanti- tative estimation of rain rate using radar. It is completely in- dependent of the radar electronics and dependents only on the raindrop size distributions (DSD) (Uijlenhoet, 2001). However, high frequencies used in space-borne radars as well conventional meteorological radars used for characterization of hydrometeors like rain, snow, hail or cloud are affected by the atmospheric phenomena to a great extent (Crane, 1996; Iguchi et al., 2009). Particularly, rain severely attenuates millimeter and microwave signals. This is also a crucial factor determining the link availability in high-frequency satellite communication (Sarkar et al., 2014; Das et al., 2010). Since drops concentrations are not same for all drop sizes, the rain attenuation (as well the rain rate and radar re- flectivity factor) will be primarily influenced by a specific drop size, known as the critical diameter (Lakshmi et al., 2007; Adetan and Afullo, 2013 , , 2014; Lee et al., 2007). Hence, for proper func- tioning and correct estimation of weather conditions, it's im- portant to know the correspondence between the DSD, radar re- flectivity factor, and rain attenuation. The drop size distribution is not unique to any place and it also varies from season to season and time to time (Tokay and Short, 1996; Bringi et al., 2003; Thurai and Bringi, 2008; Das and Maitra, 2010). For example, high rain rates associated with convective activities are much more frequent in tropical regions than the temperate regions which mostly have the stratiform type of rain. It is, therefore, of utmost importance to understand the features of raindrop size distribution for improving radar estimation of rain- fall for any location. Besides, the study of DSD is of practical im- portance in a variety of research areas such as satellite meteorol- ogy, microwave communication, cloud physics, soil erosion, tele- communication and broadcast areas and so on (Caracciolo, 2012; Tokay et al., 1996; Das et al., 2010). Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jastp Journal of Atmospheric and Solar-Terrestrial Physics http://dx.doi.org/10.1016/j.jastp.2016.09.003 1364-6826/& 2016 Elsevier Ltd. All rights reserved. n Corresponding author. E-mail address: das.saurabh01@gmail.com (S. Das). Journal of Atmospheric and Solar-Terrestrial Physics 149 (2016) 15–20