Research Item (2)
- Nov 2014
Mountains (and hills) modify the weather experienced both locally and further afield. Much of the variation in the geographical patterns of rainfall in the United Kingdom potentially can be associated with orography: for example, there are local maxima of rainfall over the Lake District and the Welsh Hills. Convective systems can be initiated by mechanical ascent upstream, over and downwind of steep terrain, and by thermally driven ascent due to elevated heating. When these systems become terrain-locked they have the potential to focus rainfall over preferred areas, enhancing the risk of flash-flooding.
How does wind shear affect gravity currents in the atmosphere? It is known that shear has a controlling effect on gravity current dynamics, which influence the propagation of thunderstorms for example, but there have been very few systematic studies of this process. The main aim was to identify if there is a relationship between how the gravity currents propagate and the ambient wind shear. Gravity currents can occur in both natural and man-made environments. One way in which gravity currents are formed in the atmosphere is through convective storm downdraughts of cold air interacting with warmer air closer to the surface. Gravity currents in practice are highly complex phenomena, generally featuring a great deal of turbulence and significant mixing of the two bodies. The total effect of wind shear on these gravity currents is not fully understood as previous research has not heavily included this parameter. Downdraughts are an important natural hazard in many parts of the world, including the continental USA, Europe, and large parts of the tropical oceans and continents. Being able to fully understand the relationship between shear and the development and propagation of gravity currents will help minimise these hazards. By performing extremely simplified numerical simulations of gravity currents in wind shear environments, designed to represent cold pool outflow from a cumulonimbus storm cell, it has been found in this study that the gravity currents are heavily influenced by a change in the low-level shear profile. The results are organised into four regimes; increasing wind shear, decreasing wind shear, constant wind and a contrasting shear wind profile. These regimes were then simulated in three model environments. The initial experiment used applied constant temperature over the domain. To enhance these results simulations were completed including a temperature inversion profile and altering the domain to generate three-dimensional grid. It has been found that with increasing average wind shear the up-shear gravity current slows and the down-shear accelerates at the same rate. There appears to be a relationship with the strength of the wind shear and the propagation distance of the gravity current. However, the gravity current perpendicular to the wind shear in the three-dimensional model was significantly dampened in front speed and does not form like a typical gravity current. As such this computational study gives a general indication of the importance of wind shear in the development of gravity currents.