Simulated density currents in idealized stratified environments

The effects of three distinct stratifications on density current dynamics a re investigated using a nonhydrostatic numerical model: (i) a stably strati fied layer underneath a deep neutrally stratified flow. representing a noct urnal boundary layer over land: (ii) a neutrally stratified layer underlyin g a deep stably stratified flow representing a daytime boundary layer; and (iii) a continuously stratified atmosphere. In the first case, a weak or intermediate stratification decreases the heig ht of density currents and increases the propagation speed. The same result holds in strongly stratified situations as long as the generated disturban ces in the neighborhood of the head do not propagate away. Classical densit y currents occur in weak stratification, multiheaded density currents in in termediate stratification, and multiheaded density currents with solitary w avelike or borelike disturbances propagating ahead of rile current in stron g stratification. In the second case. the upper-layer stratification consistently reduces the density-current height and its propagation speed. The simulated system res embles laboratory density currents and is nor much affected by the overlyin g stratification. Finally, in continuously stratified flow, the effect of stratification is s imilar to the second case. The density current becomes shallower and moves more slowly as the stratification is increased. The modeled system ha the b asic features of density currents if the stratification is weak, or moderat e, but it becomes progressively less elevated as stratification increases. In strong stratification the density current assumes a wedgelike structure. The simulation results are compared with the authors' previously obtained a nalytical results, and the physical mechanisms for the effect of stratifica tion are discussed.