The effect of curvature
Commonly, jet streaks show either cyclonic or anticyclonic curvature, which modifies the position and intensity of the divergence and convergence fields aloft and at surface level.
In a geostrophic flow, the pressure gradient and Coriolis force balance. This is usually the case at the jet level. In the case of a curved jet stream, centrifugal forces disturb the geostrophic balance.
For cyclonic curvature, if the pressure gradient force remains constant along the jet stream, then the speed of the air parcel has to decrease in order to balance the Coriolis and centrifugal forces. This is usually the case at the southern tip of an upper-level trough. Hence, wind speed convergence is strongest just behind the trough axis. This upper-level convergence leads to low-level divergence (see Figure 1). Conversely, on the leading edge of the trough, curvature decreases and wind speed increases. This causes upper-level divergence and low-level convergence.
Figure 1: Typical positions of upper-level convergence and divergence in a trough. © Lutgens, F.K. and E.J. Tarbuck
On the eastern side of a trough, downstream divergence causes low-level convergence and hence initiates cyclogenesis when it collocates with a low level baroclinic boundary. This is the region where most mid-latitude cyclones form.
The strength of upper-level divergence on the eastern flank of a short-wave trough is influenced by:
- The amplitude of the short-wave trough (the higher the amplitude, the stronger the divergence).
- The wavelength of the trough (the shorter the wavelength, the stronger the divergence).
- The wind speed at 300 hPa (the higher the wind speed, the stronger the divergence).
- The wind shear (the higher the shear, the stronger the divergence).