8. MESOSCALE DYNAMICS

To produce accurate numerical simulations of storm-scale processes in order to understand the role that planetary and synoptic scale parameters play in their generation and evolution.

To understand the dynamics of synoptic and mesoscale phenomena and their interaction with larger and smaller scales.

To improve high-resolution model simulations of synoptic and smaller scale phenomena which can be used to guide parameterization development in coarser models.

S. Garner          I. Orlanski

          The life cycle of baroclinic eddies in a controlled storm track environment has been examined by means of long model integrations with the Zeta model (7.4.1) on a hemisphere (km). A time-lag regression has been applied to the last 200 days of the model integrations in order to establish a composite view of the wave-breaking process. The eddies grow, as expected, by strong poleward heat fluxes at low levels at the entrance of the storm track, where the surface baroclinicity is strong. As they evolve to a nonlinear stage, they grow deeper by fluxing energy upward, and the characteristic westward tilt in the vorticity changes to a meridional tilt. The poleward movement of the low-level vorticity is accomplished by the flux of time-mean absolute vorticity and eddy self-advection of vorticity, while the north-south orientation of the upper-level vorticity is achieved by the projection of the surface vorticity at upper levels, combined with nonlinear advection that rotates the vorticity around to the south.

          Probably the most significant result of this study is the discovery of two distinct patterns of eddy evolution associated with two distinct types of storm track. These patterns can be referred to as baroclinic wave packets and quasi-stationary couplets. In baroclinic wave packets, which have been known for some time (1144), the age of the individual eddies increases from east to west, with the youngest eddy at the leading edge of the packet. The more mature eddies flux energy downstream to the younger eddies. The youngest eddies develop mainly by these fluxes until they are able to enhance their growth by surface heat fluxes. More mature eddies in the center of the packet flux as much energy downstream as they receive from upstream eddies, and also receive energy from the surface baroclinicity along their path. The oldest eddies decay by downstream energy fluxes and barotropic conversion.

          The second pattern arises when the downstream surface baroclinicity cannot be maintained, as in a zonally-contracted, basin-scale storm track. High baroclinicity at the entrance (to the west) is followed by abruptly weaker baroclinicity in the middle and exit. The corresponding pattern of eddy evolution quite frequently involves the simultaneous development of upper-level, downstream eddies paired with baroclinic upstream eddies. These pairs, or couplets, are self-sustaining structures in which the upstream baroclinic eddy grows through heat fluxes in the rather stronger surface baroclinicity and fluxes energy to the upper-level downstream eddy. In contrast to the wave packet, the upstream eddy is the younger. The signature of the couplet at upper levels is an omega-like pattern with a trough in the west (principal eddy) and one to the east (downstream eddy), separated by a "building" ridge. The couplet pattern is frequently observed in storm tracks of the Northern Hemisphere winter. Wave packets are more characteristic of the zonally elongated storm tracks that are a frequent feature of the Southern Hemisphere.

     8.1.2 Moist Convection in Baroclinic Life Cycles

          Latent heating has long been recognized as a fundamental process for storm development. More recently, in a study concerning the interannual variability of storm tracks (1572), it has been speculated that subtropical flux of moisture over the eastern Pacific Ocean in the warm ENSO phase can explain the observed elongated shape of the storm track in this period. It has also been argued recently (km) that the dissipation of the low-level cyclone is of paramount importance in the whole life cycle of the baroclinic system. The frontal zone is the boundary of the warm air and of the poleward heat fluxes that are crucial for sustained baroclinic growth.

          For perhaps the first time, a planetary scale circulation has been simulated without the standard moist parameterization. Actual individual clouds are explicitly resolved in new simulations of a baroclinic wave using explicit moist convection (8.4) in the non hydrostatic-compressible model ZETANC. A series of intercomparisons between the moist baroclinic and dry baroclinic life cycle are now being carried out. The successful simulation of the moist convective field around the development of the cyclonic circulation is displayed in Fig 8.1. The rain band ahead of the cold front and the cloud wrapping around the cyclonic circulation are realistic features.

          The preliminary result of the intercomparison between the moist and dry evolution of the baroclinic life-cycle shows profound differences in all aspects of its evolution. The surface front-cyclone system is considerably more extended meridionally in the dry solution than in the moist one. A similar effect was previously found to be associated with a reduction in the zonal scale (1518). The reason here seems to be that the moist air ascends more steeply than dry air, since it tends to follow the large-scale moist adiabats. As a consequence, the eddy produces vertical motion and cyclonic vorticity sooner (farther equatorward) than in the dry case.

          Perhaps the most unexpected result is the stark difference in the way the waves equilibrate. In the dry case, as has been discussed in a number of published articles (including km), the system decays via anticyclonic wave-breaking, producing poleward momentum fluxes and pulling the jet poleward from its original latitude. In the moist case, the wave seems

to break cyclonically with weak momentum fluxes in the equatorial direction, producing a westerly jet that it lies considerably more equatorward than in the dry solution. Similar results were obtained from the dry short-wave life cycle (1518), although the connection is still not yet clear. This effect of moisture could have considerable bearing on the interannual variability of storm tracks.
 

  8.2 TOPOGRAPHIC INFLUENCES IN ATMOSPHERIC FLOWS

S. Garner          I. Orlanski

          High-resolution numerical and analytical models have been used to test the feasibility of a proposed parameterization of total mountain drag due to unresolved terrain. The parameterization assumes steady, linear gravity waves and gradual horizontal variations of the basic flow. Given the low-level buoyancy frequency and topography, it yields a map of horizontal divergence due to the terrain. The total drag on the atmosphere can then be obtained by multiplying the divergent velocity by the vertical velocity due to the resolved horizontal surface wind.

          Twin experiments using the ZETA model at coarse and fine resolution showed that most of the momentum flux reaching the stratosphere from the Rocky Mountains is due to linear, hydrostatic gravity waves launched by the smallest resolved terrain feature: primarily the upstream and downstream edges of the massif. A nonlinear correction has been developed in order to parameterize the non-propagating part of the momentum forcing. However, most of the resolved drag over the Rockies in the fine-resolution experiment is linear at the source. To test the nonlinear correction, the experiments are being repeated for the Andes Mountain range.

          In Fig. 8.2, surface drag vectors from coarse- and fine-resolution simulations of wintertime flow over the Andes are displayed. The thick arrows are the quasi-steady result at fine resolution and the thin arrows are the parameterized drag arising from the coarse-resolution model run. The linear parameterization is shown in the upper panel. In the lower panel, the nonlinear correction is included. The nonlinear correction improves the drag estimate in regions of greatest drag, but slightly degrades the estimate elsewhere. The large-scale flow consists of a baroclinic jet centered at 30°S, with a maximum surface wind of 7 ms^-1 and a wind at the tropopause of about 40 ms^-1.
 

  8.3 MODEL DEVELOPMENT

S. Garner          I. Orlanski
B. Gross            L. Polinsky

          The nonhydrostatic, fully compressible version of the hydrostatic Zeta model has been used to evaluate nonhydrostatic and terrain effects in mesoscale atmospheric circulations (7.3.1) and for developing physical parameterizations (7.3.2). The model has now been coded to run on the massively parallel T3E computer at GFDL. Memory swapping and the slab-oriented array structure have been eliminated. The model can run up to 10 times faster on multiple processors, even with bulk microphysics activated.

*Portions of this document contain material that has not yet been formally published and may not be quoted or referenced without explicit permission of the author(s).