![]() ![]() Understanding the raindrop size distributions (DSDs) has a variety of applications, including rainfall retrievals using remote sensing instruments (e.g., ground-based and space-borne weather radars), hydrological and climate modeling (e.g., Bringi & Chandrasekar, 2001 Testik & Barros, 2007), rain scavenging of aerosol particles, soil erosion (e.g., Uijlenhoet et al., 2003 Uijlenhoet & Sempere Torres, 2006), and rain microphysics (e.g., Das et al., 2017 Konwar et al., 2014 Krishna et al., 2016). Thus, the raindrops may not fall at their terminal velocity except under calm conditions. ( 2018) also showed the existence of superterminal and subterminal raindrops from an optical probe and Two-Dimensional Video Disdrometer (2DVD) measurements. ( 2014) and Montero-Martínez and García-García ( 2016) evidenced the presence of superterminal (higher than terminal velocity) and subterminal (lower than terminal velocity) fall speeds in intense rain events. ( 2013) observed the broadening of fall speed distribution with a significant skewness during the passage of an intense convective rain event, which is due to the asymmetric horizontal mode of oscillations of raindrops. They attributed this to the raindrop breakup processes. ( 2009) found that the intermediate raindrops fall faster than the expected fall speeds. In the last decade, several studies revealed that raindrops sometimes do not necessarily fall at terminal velocities, which is calculated using relation with drop diameter (Bringi et al., 2018 Larsen et al., 2014 Montero-Martínez et al., 2009 Testik et al., 2006 Thurai et al., 2013). For instance, updrafts and downdrafts air motion within the rain cell (Battan, 1964), turbulence (Pinsky & Khain, 1997), and raindrop breakup/coalescence (Montero-Martínez et al., 2009) can have considerable influences on the raindrop fall velocity. ![]() Studies have shown that apart from raindrop size, several other factors can influence the raindrop fall velocity (Niu et al., 2009). However, in the natural laboratory-like atmosphere, the distribution of raindrops speeds likely to be slightly more complicated and always cannot be represented in terms of drop diameter (Villermaux & Eloi, 2011). ( 1973), which was formulated from the terminal velocity measurements of Gunn and Kinzer ( 1949) under calm laboratory conditions. The most extensively used form of the fall velocity ( V t)-drop diameter ( D) relationship was reported by Atlas et al. Several formulations have been developed to find the relation between raindrop diameter and terminal speed (Beard, 1976 Testik & Barros, 2007). Knowledge of raindrop terminal velocities from their size distributions is important in cloud physics for modeling different physical processes like breakup and coalescence, radar measurements, soil erosion studies, and hydrological applications (Doviak & Zrnic, 1993 Collier, 1996 Salles & Creutin, 2003 Montero-Martínez et al., 2009). The diurnal variation of these raindrops during different seasons is also studied. It is observed that the superterminal raindrops are usually small in size and prolate in shape, while the subterminal raindrops are relatively large and oblate in shape. It is found that most of the superterminal raindrops occur at rain rates below 5 mm hr −1, while subterminal raindrops are relatively higher above this rain rate. Results indicate the presence of superterminal and subterminal raindrops in definite proportion in all the season with a majority of the drops have a diameter less than 1 mm. The distribution of these skewed raindrops and its relationship with rain rate, drop diameter, and axis ratio for different seasons has been examined. ![]() The ratio between the observed velocity and calculated terminal velocity is considered for estimating superterminal (positively skewed higher than terminal velocity) and subterminal (negatively skewed lower than terminal velocity) raindrops. The observed raindrop fall velocities have been corrected for the effect of air density. To increase the reliability and accuracy of analysis, the raindrops with diameters in the range 0.5–2 mm and horizontal wind speed < 2 m s −1 are considered. The analysis is for different seasons during 2012–2015. Using the Two-Dimensional Video Disdrometer measurements, we investigated the fall velocity of raindrops at Mahabaleshwar (17.92°N, 73.6☎, ~1.4 km above mean sea level, AMSL), a tropical site on the Western Ghats of India. ![]()
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