Drop size and rain rate characteristics of Indian monsoon rainwater

Raindrops are an integral part of nature’s hydrological cycle. Information on their sizes in different seasons and climatic conditions is required in many practical applications. We report seasonal average properties of rainfall examined through raindrop sizes and rain rates measured with optical disdrometers in India. Disdrometers were operated at three sites having coastal, orographic and continental climates. Analysis has been carried out by clubbing data of each station into two groups, namely, all available and summer monsoon season-only data. The study presents contributions of different raindrop sizes and rain rates to the total rainfall. There are between 2.4 and 2.9 million raindrops (with size > 0.25 mm) in one litre of rainwater, with the orographic site having the maximum number. A sum of two exponential functions describes the size dependence of number of drops present in rainwater. Major contribution to the monsoon rainfall comes from drops having sizes in 1.5–2.5 mm range with a peak of around 2 mm. Average rain accumulation with rain rate ( R ) approximately varies as R – b (for R > 0); b depends on location and lies between 1.1 and 1.3, the former corresponding to the continental site while the latter to the orographic. Research highlights There are between 2.4 and 2.9 million raindrops (with size > 0.25 mm) in one litre of rainwater. Major contribution to the monsoon rainfall comes from drops having sizes in 1.5–2.5 mm range with a peak around 2 mm. On the other hand, all values of rain rate ( R ) are important to rain accumulation; low rain rate contributes by its sheer number (i.e., their total long duration) while large rain rate by its magnitude. Average rain accumulation varies as R – b , where b lies between 1.1 and 1.3 depending on location.