Drop size distributions and kinetic energy rates in variable intensity rainfall

Abstract

[1] Rainfall kinetic energy is a key factor in soil erosion processes. It is determined by rainfall intensity, related drop size distribution (DSD), and the drops' terminal velocity. Temporal variability in rainfall intensity is reflected in the DSD and affects the rainfall kinetic energy during the event. Smith et al. (2009) reported on 1-min interval rainfall intensity and corresponding DSD variability during a storm on 22 July 2006 at Princeton, New Jersey. They reported also on DSD characteristics of heavy convective rainfall events during the whole summer. It is shown that (1) a similar relationship between the mean drop size and the rainfall intensity characterized the local rainfall at both the seasonal and the single-storm scale, and (2) using the mean drop size as a scaling factor of the DSD removes the rainfall intensity dependence also at the intrastorm scale, providing a powerful tool to deal with temporal variability of rainfall rates during rainfall events. For a given storm characterized by a specific temporal variability of rates, three different ways of evaluating kinetic energy per unit mass or time were applied. By comparison to estimates accounting for rainfall temporal variability and related full DSDs, representing the storm by mean intensity and drop diameter tends to overestimate kinetic energy for low intensities and underestimate it for the higher ones. The relative error for the kinetic energy per unit of mass is ±45% and shifts from negative to positive sign for I > 25 mm/h. For the kinetic energy per unit of time, the relative error ranges from À100% to +210% and changes sign from negative to positive for I > 45 mm/h. When temporal variation of intensity is accounted for but drops are characterized by their mean values instead of the full DSD, kinetic energy is underestimated by 20% on average. Consequently, accounting for temporal variability in rainfall intensity during a storm has a notable impact on the erosive power of the rainfall.

A network of disdrometers to quantify the small‐scale variability of the raindrop size distribution

“…The (rain) drop size distribution (DSD) is a statistical way to summarize the information about the microstructure of rain and is consequently of primary importance for many environmental fields such as investigation on cloud/precipitation microphysical processes [e.g., Pruppacher and Klett , 1997], numerical weather modeling [e.g., Michaelides et al , 2009], weather radar applications [e.g., Marshall and Palmer , 1948; Bringi and Chandrasekar , 2001], and soil erosion due to the impact of raindrops [e.g., Salles et al , 2002; Assouline , 2009]. The DSD is highly variable in time and space [ Jameson and Kostinski , 2001], and this variability has a key influence on the uncertainty affecting radar rain rate estimates.…”

Erosion and Lateral Surface Processes

“…The primary driving force of soil erosion by water at the hillslope scale is generally referred to as rainfall erosivity. It is an important characteristic, highly variable in space and time (Panagos et al, 2016; Assouline, 2009), that determines the soil erosion intensity in most landscapes. Yin et al (2017) provide a review of the history and active research that addresses this important topic, which will hopefully lead to an improved use of the concept across all parts of the world in applications associated with erosion assessment and soil conservation planning.…”

Set-up and calibration of a portable small scale rainfall simulator for assessing soil erosion processes at interrill scale

“…Furthermore, high-intensity rainfalls are often confined to small areas as they are usually related to convective precipitation events of single storm cells. At last, the physical rainfall properties are inconstant during an event, making it extremely difficult to obtain the parameters regulating soil erosion processes, which are most importantly the drop size distribution and the terminal fall velocity, both conditioning rainfall kinetic energy (De Vente et al, 2013;Petan et al, 2010;Assouline 2009;Dunkerley, 2008;Arnáez et al, 2007;Morin et al, 2006;Usón and Ramos, 2001;Salles and Poesen, 2000;Chaubey et al, 1999).…”