Synoptic and Non-Synoptic Winds

Synoptic, or atmospheric boundary layer (ABL), winds occur due to large-scale (~1000km) weather systems such as the high and low pressure regions seen on weather maps. The vertical profile of synoptic winds follows a logarithmic profile, with a rapid increase with height near the ground and an asymptotic maximum at high levels (Figure 1). In contrast, downburst winds reach a maximum close to the ground (<100m) and then reduce with height. It can also be seen from the figure that ABL winds are stationary - if short samples are taken at from different points in the series then they will be statistically negligibly different - whereas downburst winds are highly non-stationary, with a short, high magnitude peak which can be explained by the form which these winds take.

A schematic showing the differences between downburst and ABL wind profiles.

Figure 1: Comparison of Synoptic and Downburst (Non-Synoptic) Winds - Vertical Profile (adapted from [1]) and Time-Series 

Downburst winds originate in rising, warm, moist air in a thunderstorm. As it rises this air cools, becoming more dense, and the updraft reverses to become a downdraft, which becomes an ouflow upon contact with the ground. The downburst is therefore a short duration event, as once this mechanism has exhausted the original volume of warm air, the downdraft ends. There is, however, an additional source of velocity in the outflow. In many (though not all) downbursts, a ring vortex forms due to Kelvin-Helmholtz instabilities as the downdraft commences.

A schematic of a downburst showing the ring vortex

 

 

 

 

 

 

 

 

 

 

 

 

Figure 2: Downburst Structure and the Primary Ring Vortex ([2])

This ring vortex, which at ground level is rotating in the same direction as the outflow, increases the local velocity, resulting in the exceptionally high-speed, short-lived winds found in a downburst.

References

[1] LIN, W. E., ORF, L. G., SAVORY, E. & NOVACCO, C. 2007. Proposed large-scale modelling of the transient features of a downburst outflow. Wind and Structures, 10 (4), 315-346.

[2] FUJITA, T. T. 1985. Downburst: Microburst and Macroburst, Chicago, Illinois, University of Chicago Press.