2.7.1 Thermal systems Density or gravity currents A density or gravity current is formed whenever denser air intrudes into and displaces less dense air, and (usually) flows across the surface; for example, katabatic winds, convective cloud downbursts and the New South Wales Southerly Buster. Density current motion is dependent on dynamic pressure, hydrostatic pressure and surface friction. These, in turn, are dependent on the height of the intrusion and the relative densities. The flow speed is also a function of the ambient wind flow. Two circulations evolve within the head of a density current, and provide the mass for the mixing billows and eddies. One is below the nose, or point of stagnation (as with an aerofoil), due to surface friction. The other is above the nose where the internal speed is greater than the current propagation speed. The nose tends to repeatedly collapse and reform as the current advances, thereby adding to the turbulence of the squall. A strong, opposing, ambient wind would tend to flatten the nose into a wedge shape. The advancing head of density currents, such as the NSW Southerly Buster, often have no warning cloud associated with them. On the other hand they may produce a spectacular shelf cloud, or arcus, by forcing the warmer inflow air to rise. The leading edge of the shelf may become detached to produce a horizontal cloud tube — a roll cloud. The passage of the leading edge of a density current is marked by a temperature fall, pressure jump and a strong gust-line with large, rotational shear. Other thermal systems include: thunderstorms squall lines tornadoes and sea breeze fronts. 2.7.2 Wave systems Gravity or buoyancy waves Wave motion is the basic mechanism by which local disturbances are transferred from one part of the atmosphere to another without net mass transport. Gravity waves, or buoyancy waves, are pressure waves generated by disturbances within the atmosphere, where the restoring forces (potential energy) for the wave motion are provided by buoyancy and gravity, rather than compression and expansion as in higher-frequency acoustic waves. The kinetic energy is provided by mass; i.e. an air parcel, vertically displaced by a disturbance, will be acted on by gravity because its density differs from its environment. The potential energy of displacement is converted to kinetic energy when buoyancy returns the parcel to its original level. However, kinetic energy reaches a maximum at its original position, so the parcel overshoots that position and again is returned by the restoring force of buoyancy. The air parcel tends to oscillate around its undisturbed position, at a typical frequency of 5–10 minutes. If successive parcels of air are subject to displacement then a gravity wave is generated in the direction of propagation. The source of the disturbance could be orographic effects, frontal lines, density currents, jet streams, convection penetrating a stable layer, squall lines or low level turbulence. Gravity waves can be external waves or internal waves. External waves are those propagating on a discontinuity surface such as an inversion or — in regions where the gradient is strong enough to guide the propagation in a direction perpendicular to the gradient — a solitary wave. Ocean waves are external gravity waves. Internal waves propagate horizontally or obliquely to the density strata. If propagating obliquely they transport energy to the upper atmosphere and produce clear air turbulence. If the layer in which internal waves are produced is bounded above and below by discontinuity surfaces — for example the ground, or density or wind discontinuities — then the upward oblique waves may then be deflected downward, so the waves are then effectively contained within a wave guide. Mountain waves are an example where, depending on the thickness of the layers and the intrusion of the mountain into the airstream, the deflected energy may return in phase with the following primary waves. In this case, the amplitude of the deflected waves adds to the primary wave and the wave grows by resonance. Strong convective cloud punching into a stable layer aloft may generate internal gravity waves and consequent clear air turbulence within the upper layer; e.g. turbulence near thunderstorm tops. Passage of a gravity wave is marked by a pressure jump and a wind change but no change in temperature or humidity, as there is no air mass change. The vertical lifting may initiate cloud and precipitation. Solitary waves are well-known wave systems. 2.7.3 Orographic systems The orographic systems of interest are: Slope and valley winds Low-level jets Lee wind downflow, eddies, rotors and vortices STRICT COPYRIGHT JOHN BRANDON AND RECREATIONAL FLYING (.com)
