The microstructure of heavy rainfall in the Netherlands

Raindrop size distributions (DSDs) are the main tool for describing and discussing rain microphysics. They play a crucial role in remote sensing of precipitation and extensive efforts have been devoted to measuring and modeling them. However, when it comes to DSDs in extreme rain, very few, reliable, results are available. Using numerical simulations, Srivastava (1978, 1982) and List (1988) theorized that for high enough rainfall intensities (>40 mm/h), DSDs should converge toward a stationary state where drop coalescence and breakup are in dynamical equilibrium with each other. In such conditions, the shape of the DSDs should be constant and the particle number concentration should be proportional to the rain rate. However, reliable evidence of such transitions toward a “number-controlled” remains scarce and many researchers have contested its existence.

In this study, high-quality DSD observations from a network of 7 optical disdrometers belonging to the Ruisdael observatory for Dutch atmospheric science are used to take a new, fresh look at the issue. The main research questions are:

• Is there empirical evidence for a transition from size to number-controlled regimes at high rainfall intensities in the Netherlands?
• What parametric model best fits DSDs at high rainfall rates?
• Can the super-CC scaling of sub-hourly rainfall extremes with temperature highlighted by Lenderink et al. (2008) be explained by changes in DSDs?

To address the questions above, we analyze characteristic drop sizes (Dm, D0), number concentrations (NT, Nw) and state variables (LWC, Z, Zdr) for different classes of rainfall intensities and temperatures and study the shape of DSDs by comparing the goodness of fit of various parametric DSD models. We look at non-parametric descriptors such as the relative number of small versus large drops and study the scaling laws linking different moments of the DSD in heavy rain using single and double-normalization frameworks to assess possible convergence toward number-controlled regime at higher intensities.