Kinetic energy budgets of the low-level jet during TAMEX IOP 5

Using results from numerical experiments, budgets of divergent kinetic energy (k(D)) and nondivergent kinetic energy (k(ND)), are examined for the subsynoptic-scale low-level jet (LLJ) event that occurred during 1-2 June 1987 (TAMEX IOP 5) to diagnose the energy conversion process leading to the LLJ development. Nondivergent kinetic energy (k(ND)) dominates the total kinetic energy. The intensification of the LLJ, and the subsynoptic-scale upper-level jet (ULJ) is due to the increase in k(ND). Divergent kinetic energy (kD) is rather small. In the area encompassing the LLJ, the most important energy source of k(ND) is k(D) generated from the potential energy (PE) through the cross-contour divergent winds, and then converted into k(ND) via a conversion process through the transverse secondary circulation across the jet-front system. In the upper troposphere, the cross-contour flow across the ULJ is dominated by the nondivergent winds. The major source for the increase of k(ND) associated with the intensification of ULJ is the conversion from the potential energy to k(ND) via cross-contour nondivergent winds. This conversion rate is independent of the strength of the secondary circulation. The upper-level divergence ahead of the upper-level trough is mainly contributed by the along-stream agesotrophic winds along the ULJ axis, and it provides the upper-level support needed for the LLJ development in the lower troposphere. This mechanism is at variance with the CISK (Condtional Instability of the Second Kind) process in the traditional view. Without the feedback effects from convective heating (NOLH), low-level frontal cyclone fails to fully develop. The secondary circulation is weaker with a much slower energy conversion process from PE to k(D), and then to k(ND) in the lower troposphere. As a result, the simulated LLJ in the NOLH experiment is much weaker. Nevertheless, the intensity of the simulated ULJ without latent heating is about the same as that with latent heating. This is because the conversion from the potential energy to k(ND) via cross-contour nondivergence winds, is independent of the strength of the transverse secondary circulation across the jet-front system.