Analytical and numerical modelling of jet streaks: Barotropic dynamics

Observations suggest that vortex dipoles of mesoscale dimensions may provid e a simple yet realistic representation of the structure and dynamics of je t streaks in the extratropical upper troposphere. Moreover, the effects of horizontal divergence in the vicinity of jet streaks often may be of second ary dynamical importance, suggesting that a nondivergent barotropic framewo rk may provide a logical starting point for an idealized investigation of j et streaks. Analytical solutions of barotropic vortex dipoles are shown to exhibit char acteristic signatures similar to those identified in observational case-stu dies of jet streaks. In addition to the dipole of relative vorticity, these signatures include: (i) a localized maximum in fluid speed (i.e. a jet str eak), (ii) ageostrophic flow that is directed towards lower pressure in the entrance region and towards higher pressure in the exit region of the jet streak, (iii) a four-cell pattern of ageostrophic vorticity that is cycloni c in the entrance and exit regions and anticyclonic on the flanks of the st reak, and (iv) a translation speed that is significantly slower than the ma ximum fluid speed. On the basis of these similarities, it is suggested that vortex dipoles provide a plausible dynamical representation of the structu re and motion of jet streaks. Nevertheless, vortex dipoles in isolation are unable to account for certain observed features of jet streaks, such as the anisotropy of the wind held in the along-stream direction and the asymmetry in the relative-vorticity h eld, in which the cyclonic vortex typically is stronger than the anticyclon ic vortex. Moreover jet streaks generally are not isolated, but are embedde d in a larger-scale jet stream, which may be zonally varying or wavelike. A nalytical and numerical solutions of barotropic vortex dipoles in the prese nce of a variety of non-uniform background flows characteristic of the larg e-scale extratropical circulation are shown to account for the above featur es absent from dipoles in isolation. These solutions are also shown to prov ide idealized depictions of the Life cycles of jet streaks in the extratrop ical upper troposphere.