121 Flow imbalance and gravity waves in moist baroclinic jet-front system

Thursday, 20 June 2013
Bellevue Ballroom (The Hotel Viking)
Junhong Wei, Pennsylvania State University, University Park, PA; and F. Zhang

Handout (3.4 MB)

High-resolution cloud-permitting simulations are performed to source mechanisms of flow imbalance and gravity waves (GWs) within the moist baroclinic jet-front systems. The candidate source mechanisms including the unbalanced jet streak and frontogenesis (originated from background baroclinicty), and diabatic heating from moist convection (convective instability). A series of experiments with the same initial 2-D baroclinic jet but with different moist content (zero (00EXP), reduced (20EXP) and full (100EXP) initial relative humidity ) produce different idealized life cycles of moist baroclinic waves that allows us to investigate the relative role of moist processes and baroclinicity in the generation and propagation of the mesoscale GWs. Consistent with the past dry baroclinic wave research, the dry experiment 00EXP with no moisture reproduce five GW modes. The first two modes include the northward-propagating short-scale and intermedium-scale wave packets pronounced in the exit region of the upper-tropospheric jet. The third and fourth modes are short-scale wave packets closely tied to the surface front. The fifth mode refers to the wave packets from the jet exit region in the ridge down to the jet entrance region in the trough. Having similar growth rate of the baroclinic waves in the dry experiment of 00EXP, 20EXP with some moisture and weak convective instability simulates almost the same wave packets for the first four modes as those in 00EXP. However, the fifth mode in 20EXP, located downstream of the convection, has significantly stronger amplitude than the one in 00EXP. A sixth mode in 20EXP is observed as short-scale wave packets, which may be excited by convection. Convection not only enhances the flow imbalance but also initiates a new GW mode directly. Convection-generated GWs in 20EXP propagate both upstream and downstream from the latent heating, and they are believed to interact with, strengthen, and modify the dry GW mode, such as the fifth mode in 00EXP versus that in 20EXP. The propagation effect of the background flow is then further channeling or transforming the dry GW modes in 00EXP to the moist GW modes in 20EXP. Significantly increase the moisture content to 5 times of 20EXP leads to strong convective instability in 100EXP, besides a fast growth of moist baroclinic waves, the sixth mode of gravity waves is observed that is much earlier and soon fully coupled with other dry GW modes and background flow.
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