Vertical evolution of raindrop size distribution: Impact on the shape of the DSD

Models of coalescence and breakup lead to equilibrium of the raindrop size distribution (DSD) after a fall through sufficient vertical height. At equilibrium, the DSD no longer evolves, and its shape is unique whatever the rain rate or Liquid Water Content (LWC). This implies that the DSD is known, to within a multiplication constant. In the past, numerous measurements using disdrometers revealed that the slope of the DSD tail is close to 20–22 cm−1 when equilibrium is reached, whereas models based on the Low and List experiment predict values of approximately 65 cm−1. The present paper proposes a simple modification of the coalescence efficiency in the Low and List parameterization, leading to a DSD tail with a slope of 24 cm−1. To evaluate the relevance of this modification, some of the DSD parameters such as slope, mean volume diameter, and the relationship between moments are calculated, and compared with experimental DSD. The modified parameterization is then used to study the evolution of an initially gamma-like DSD in a 1D vertical rain shaft.