Vertical winds: a tutorial

Vertical winds in the thermosphere exist at far greater speeds than expected from a first-principles modelling approach. Early observations by ground-based high resolution Doppler spectroscopy and tracking of vapor trails indentified the phenomenon but there was little understanding of how the thermosphere could be stably stratified and yet sustain upward and downward winds of up to 100 m s(-1) It is clear that such strong vertical motions must have important dynamic and thermodynamic coupling effects as well as inevitable disruption of diffusive equilibrium. Up to the present, little has been done to study such activity in thermospheric models. More recent observations show that regions of large vertical winds tend to be restricted in time and space and some can be qualitatively understood in terms of adjacent upward and downward moving air columns driven directly by localized heating or indirectly by gravity waves emanating from some distant disturbance. The horizontal scale of these motions is thought to be tens to a few hundred kilometers and the time scale tens of minutes to a few hours. They are not well represented in global-scale models because they are small compared to the grid size. Some attempts have been made to describe vertical winds in local and in nested-grid models. However, a full description of the nature of vertical winds and their comprehensive and self-consistent inclusion in thermospheric models remains to be done. (C) 1998 Elsevier Science Ltd. All rights reserved.