Seasonal variations of raindrop size distribution over a coastal station Thumba

Variation of raindrop size distribution (DSD) are investigated using long-term (2007-2014) measurements made at Thumba (8.5°N, 76.9°E) by Joss-Waldvogel disdrometer. The DSD is observed to be distinctly different for NE and SW monsoon seasons. Results show a significant difference in the diurnal pattern of rainfall with large amplitude in the diurnal variation of rainfall in the monsoon with an evening (19:00 LT) to midnight (04:00 LT) peak and a weak diurnal variation in the Pre-monsoon and Post monsoon seasons. Probability of occurrence of rain is minimum during 10:00 – 13:00 LT for all seasons. The diurnal variation of DSD parameter Dm (mass-weighted mean diameter) also shows a distinct pattern with smaller values in monsoon compared to pre-, and post monsoon seasons. During monsoon season due to the presence of large number of small drops reduces the Dm value. While the presence of fewer small drops and relatively more big drops in the post and pre monsoon increases the values of Dm.