The influence of synoptic conditions on flow between mountain basins

The Salt Lake City basin was the location of the Vertical Transport and Mixing (VTMX) program field experiment in October of 2000, and it is an area that has experienced urban air quality problems. During most the experiment's Intensive Observation Periods (IOPs), air flow through the Jordan Narrows, the gap in the Traverse Range that divides the Salt Lake City basin from the Utah Lake basin to its south, was found to be significant, in addition to the known night time drainage winds from canyons that enter the valley from the east. Simulations of IOP's 7 and 8 of the field experiment with the Regional Atmospheric Modeling System (RAMS) have yielded results, which suggest that synoptic weather conditions influence the timing and location of the interaction of the flow through the Jordan Narrows with canyon drainage flows, and thus the relative strength and locations of vertical motions that can lead to vertical transport and mixing within the basin.

Although the large scale weather was relatively weak during both IOP's, the rawinsonde soundings from the airport indicate that the low level vertical temperature gradient was more stable during IOP 7 and the winds above ridge top were light and from the east. In contrast, the ridge top winds during IOP 8 were somewhat stronger and from the west. Lidar and surface observations in the Salt Lake City basin indicate that the flow through the Jordan Narrows, from the south, begins earlier in the night and is stronger during IOP 7 than during IOP 8.

The model results agree with the observations by producing stronger and earlier down valley (southerly) flow through the Jordan Narrows during IOP 7 than during IOP 8. The strength and the timing of the canyon outflows were also different during the two IOP's. This suggests that they are also influenced by synoptic conditions. The down-valley flow converges with the canyon outflows earlier in the simulation of IOP 7, producing localized areas of vertical motion in the Salt Lake City basin earlier in the evening. By just before sunrise, both simulations showed down-valley flow in the basin, converging with the canyon outflows, but the locations of the associated vertical motions are somewhat different and the magnitudes are much larger for IOP 7.

These simulations also indicate that the flow over the Traverse Mountains and through the Narrows generated a gravity wave that contributes to vertical transport and mixing in the basin. The synoptic conditions are more favorable for the formation of a gravity wave during IOP 7, resulting in a wave with higher amplitude and greater vertical motions.