Large-Eddy Simulation of a microburst

Abstract

Abstract. The three-dimensional structure and evolution of an isolated and stationary microburst are simulated using a time-dependent, high resolution Large-Eddy-Simulation (LES) model. The microburst is initiated by specifying a simplified cooling source at the top of the domain around 2 km a.g.l. that leads to a strong downdraft. Surface winds of the order of 30 m s −1 were obtained over a region of 500 m radius around the central point of the impinging downdraft, with the simulated microburst lasting for a few minutes. These characteristic length and time scales are consistent with results obtained from numerical simulations of microbursts using cloud-resolving models. The simulated flow replicated some of the principal features of microbursts observed by Doppler radars: in particular, the horizontal spread of strong surface winds and a ring vortex at the leading edge of the cold outflow. In addition to the primary surface outflow, the simulation also generated a secondary surge of strong winds that appears to represent a pulsation in the microburst evolution.These results highlight the capability of LES to reproduce complex phenomena like microbursts, indicating the potential usage of LES models to represent atmospheric phenomena of time and space scales between the convective scale and the microscale. These include short-lived convectivelygenerated damaging winds.

Transient Thunderstorm Downbursts and Their Effects on Wind Turbines

“…From an atmospheric science perspective, numerical models employed to simulate downbursts fall into two main categories: sub-cloud models and cloud-resolving downburst models [5]. Sub-cloud models include consideration of some sort of forcing that generates negative buoyancy in a storm in the upper portion of the model domain in order to initiate the downburst [6][7][8]; cloud-resolving downburst models, on the other hand, involve simulation of the full life cycle of the downburst-producing storm [9][10][11][12].…”

Investigating the impact of atmospheric stability on thunderstorm outflow winds and turbulence

“…as was done in Mason et al (2009) and Anabor et al (2011) to produce an intense downburst. The function, g(t), defines the variation of downburst intensity with time as follows:…”

“…These processes are largely responsible for generating the negative buoyancy that drives the downburst (Wakimoto, 2001) and this approach has become the state of the practice in LES simulations of downburst winds (see, for example, Orf et al (1996); Anabor et al (2011) or Oreskovic (2016)). The source (Q) equations are as follows:…”

“…to maximum downdraft (w min ) by calculating the ratio of U storm to w min (Proctor, 1989;Mason et al, 2009;Proctor, 1988;Anabor et al, 2011). Proctor (1989) showed that it was difficult to deduce any universal relationship between U storm and w min in the brief study on the low-level stability.…”

“…RANS simulations of downburst winds (Mason et al, 2009;Proctor, 1988;Mason et al, 2010a) and LES studies (Vermeire et al, 2011a;Anabor et al, 2011;Orf et al, 2012Orf et al, , 2014 both solve the Navier-Stokes equations and have been used to 15 study the dynamics of downburst winds. What separates the two methods is that RANS models estimate the entire turbulent contributions of the flow field, while LES resolves the largest turbulent eddies and parameterizes only the smaller turbulent eddies.…”