Microphysical Origin of Raindrop Size Distributions During the Indian Monsoon

• Published in: Geophysical research letters 2021-08, Vol.48 (16), p.n/a
• Main Authors: Raut, Bhupendra A., Konwar, Mahen, Murugavel, P., Kadge, Dhananjay, Gurnule, Dinesh, Sayyed, Imran, Todekar, Kiran, Malap, Neelam, Bankar, Shivdas, Prabhakaran, Thara
• Format: Article
• Language: English
• Subjects: bright band; CAIPEEX; Dual Polarization KDP; GPM; hydrometeor classification; Quantitative Precipitation Estimation (QPE); TRMM; unsupervised machine learning; warm rain; ZDR
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2021GL093581

Surface raindrop size distributions over a rain shadow region during the Indian summer monsoon are clustered using the k‐means algorithm. The rainfall for five dominant clusters has distinct vertical features in polarimetric radar and micro rain radar. The deep convection with low cloud bases and high liquid water is associated with sharply increasing radar reflectivity at low levels and the largest drops at the surface. The large drops, originated by ice processes, break while falling below the melting layer, causing a peak in raindrops smaller than 0.5 mm diameter near the cloud base, where the falling raindrops rapidly grow by collision‐coalescence without breaking. The number concentration in the stratiform rain is relatively moderate; hence a gradual raindrop growth, and uniform vertical reflectivity. However, few raindrops in stratiform rain grow to a larger diameter than in convective rain, for the same rain rate due to the absence of collisional breakup. Plain Language Summary Different clouds produce raindrops of various sizes in different concentrations. Therefore, we classified ground‐based measurements of raindrop sizes to study the rainfall characteristics during the monsoon. Generally, raindrops grow inside the cloud as they collide and combine with smaller drops, and break when they collide with larger drops. These processes affect the rain intensity and the sizes of raindrops. We found the abundance of tiny raindrops in convective clouds at lower levels. The larger drops collect them and grow while falling from this region of the cloud. The concentration of the tiny drops is higher when the cloud is taller and the cloud base is lower with sufficient water. The growing raindrops are detected by the radar with an increasing reflectivity signal. On the other hand, stratiform rain has a lower concentration of raindrops and less water. Therefore, the raindrop growth is slow, and the radar reflectivity generally appears uniform from cloud to ground. Despite this, due to a low concentration of large drops in the stratiform cloud, some raindrops gradually grow without breaking. Therefore, larger raindrops are often found in stratiform rain than convective rain, for the same rain intensity. Key Points Five drop size distribution clusters at the surface during the Indian monsoon have distinct rainfall features in polarimetric radar and micro rain radar A low‐level peak in tiny drops helps collisional growth of raindrops in convective rain, increasing