Characteristics of raindrop spectra as normalized gamma distribution from a Joss–Waldvogel disdrometer

Abstract

The raindrop spectra observed in a precipitation system is a complex phenomenon that can help to explain the underlying physical processes of rainfall. This paper explores the characteristics of raindrop spectra in terms of drop size distributions (DSD) using seven years of Joss–Waldvogel disdrometer data within the mid-latitude UK region climatology. A total of 162,415 one-minute “filtered” raindrop spectra obtained from the disdrometer are fitted into a normalized gamma DSD model describing DSDs by the concentration parameter (Nw), the drop diameter (Dm and D0), and the shape parameter (μ). The results show that the rain rates retrieved from the normalized gamma DSD model are in good agreement with the disdrometer measured rain rates, implying the appropriateness of the raindrop spectra as normalized gamma distributions. The DSD characteristics are studied in different seasonal (“cold” and “warm”), atmospheric (“dry” and “wet”) as well as rain type (“stratiform” and “convective”) contexts in a long-term perspective. It has been revealed that the normalized gamma DSD parameters are very sensitive to the rain intensities. The mass weighted mean drop diameter Dm clearly increases exponentially with respect to the rain intensities. Variation of the DSDs in different contexts is also exposed reflecting seasonal, atmospheric and rain type consequence on raindrop spectra. Particularly, the scatterplot between the concentration parameter log10Nw and the median drop diameter D0 exhibits clear separation index between stratiform and convective DSDs. There is a large difference in averaged mass weighted mean drop diameters among stratiform and convective segments (stratiform <Dm>=0.988mm versus convective <Dm>=1.99mm). The DSD inconsistency in different contexts have been further examined in terms of the Z–R relationships (Z=aRb) variability. Additionally, 10 selected events taken place during the study period are also investigated, in which each of the events has shown unique DSD characteristics.