Characteristics of colliding sea breeze gravity current fronts: a laboratory study

Abstract

Sea and land breeze circulations driven by surface temperature differences between land and sea often evolve into gravity currents with sharp fronts. Along narrow peninsulas, islands and enclosed seas, sea/land breeze fronts from opposing shorelines may converge and collide and may initiate deep convection and heavy precipitation. Here we investigate the collision of two sea breeze gravity current fronts in an analogue laboratory setting. We examine these collisions by means of 'lock-exchange' experiments in a rectangular channel. The effects of differences in gravity current density and height are studied. Upon collision, a sharp front separating the two currents develops. For symmetric collisions (the same current densities and heights) this front is vertical and stationary. For asymmetric collisions (density differences, similar heights) the front is tilted, changes shape in time and propagates in the same direction as the heavier current before the collision. Both symmetric and asymmetric collisions lead to upward displacement of fluid from the gravity currents and mixing along the plane of contact. The amount of mixing along the collision front decreases with asymmetry. Height differences impact post-collision horizontal propagation: there is significant propagation in the same direction as the higher current before collision, independent of density differences. Collisions of two gravity current fronts force sustained ascending motions which increase the potential for deep convection. From our experiments we conclude that this potential is larger in stationary collision fronts from symmetric sea breeze collisions than in propagating collision fronts from asymmetric sea breeze collisions.

On Cold Pool Collisions in Tropical Boundary Layers

“…For example, Simpson (1997) examined the collision of two gravity currents in a tank of water, showing the creation of two bores traveling in opposite directions following the collision. This work was recently extended by van der Wiel et al (2017) who, in order to better understand the collision between sea breeze fronts, investigated the effects of height as well as initial density differences between gravity currents colliding in a water tank. This work was recently extended by van der Wiel et al (2017) who, in order to better understand the collision between sea breeze fronts, investigated the effects of height as well as initial density differences between gravity currents colliding in a water tank.…”

“…Along with theoretical work to predict the propagation speed of the bores resulting from the collision, Shin (2001) also presented results of experiments on colliding gravity currents of different heights but equal densities. This work was recently Geophysical Research Letters 10.1029/2018GL080501 extended by van der Wiel et al (2017) who, in order to better understand the collision between sea breeze fronts, investigated the effects of height as well as initial density differences between gravity currents colliding in a water tank. More recently still, collisions were at the center of two studies: Cafaro and Rooney (2018) considered the behavior of the interfacial slope of two colliding currents with an idealized numerical and a theoretical approach, whereas Zhong et al (2018) conducted lock-exchange experiments with equal gravity currents to quantify the turbulent mixing at the time of collision.…”

Structures of the Sea‐Breeze Front in Dual‐Doppler Lidar Observation and Coupled Mesoscale‐to‐LES Modeling

“…The scenario differs somewhat from that of a moving SBF with sustained density currents, in which the lobes/clefts can continue to form at the frontal head (Miller et al, 2003;Sha et al, 1991Sha et al, , 1993Simpson, 1997). Enhanced frontal structures and ascending motions due to two colliding density currents are observed in a recent laboratory study (van der Wiel et al, 2017).…”

“…An important aspect of the SBF is the three-dimensional (3-D) fine-scale structures that form and grow along the front. At its leading edge, the SBF is sometimes divided into a number of lobes and clefts, which are similar to the features of gravity current in laboratory tanks (Härtel et al, 2000;Mitsumoto et al, 1983;Simpson, 1969;Simpson & Britter, 1980;van der Wiel et al, 2017). At the shear zone between low-level onshore winds and ambient flows aloft, a train of Kelvin-Helmholtz billows (KHBs) may form and propagate seaward (Plant & Keith, 2007;Sha et al, 1991Sha et al, , 1993.…”

Characteristics of colliding density currents: A numerical and theoretical study

“…More recently, van der Wiel et al. s*() have performed laboratory experiments on colliding density currents with different densities and depths. These parameters have an important influence on the characteristics of the collision (also (Shin et al.…”