The raindrop size distribution - the unknown that holds everything together

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The raindrop size distribution (DSD) is a statistical description of the number and
size distribution of raindrops within a specified volume of air. DSDs play a central
role in radar remote sensing and are essential for understanding the scattering and
absorption of electromagnetic radiation as it travels through the air and interacts with
falling rain. They form the mathematical backbone for linking radar observations
to physical quantities such as rainfall intensities, liquid water content, and kinetic
energy. Yet, in most remote sensing applications, the DSD in the target volume is
likely to be unknown. Therefore, parameterized DSD models have been created to act
as substitutes for direct measurementswhen observations are unavailable, incomplete,
or impractical. Over time, a large number of models have been proposed, each with
their own intricacies and constraints. Understanding these mathematical constructs
and the assumptions behind them is essential for interpreting radar measurements and
improving quantitative rainfall estimation.