Vertical Transport and Mixing Experiment (VTMX)

Click here to see examples of the 2 primary types of scans, and an explanation of how they are interpreted.

View of the surrounding area at VTMX, from the TEACO₂ trailer.

The TEACO2 lidar trailer stationed at the Salt Lake City Municipal Airport #2. The Wasatch Range is in the background.

Lisa Darby monitors the real-time display in the TEACO2 trailer.

Doppler lidar measures wind flow in the Salt Lake City basin

TEA CO₂ Doppler lidar measurements made a significant contribution to the Vertical Transport and Mixing (VTMX) field campaign, sponsored by the Department of Energy (DOE). While deployed in Salt Lake City, Utah, TEA CO₂ simultaneously measured radial winds and backscatter during 9 intensive operating periods (IOPs) of the field project, which took place during October, 2000. These IOPs generally ran from 4 pm (local time) until 10 am the next morning, encompassing the transition in winds that occurred after sundown, the initiation and maturation of nighttime flows associated with nearby complex terrain, and the change in winds after sunrise.

The research emphasis of VTMX focused on how the regional winds affect the buildup and destruction of a cold-air layer at the surface of the Salt Lake City basin. During cooler months it is common for a layer of cooler air to form at the bottom of a basin or valley. A cold-air layer is stable (i.e., vertical mixing is restricted) inhibiting the transport of pollutants out of the layer or the entrainment of cleaner air from above. As pollutants are released from the surface into this type of layer, such as from industrial sources or vehicles, pollutant levels increase within the cold-air layer, often leading to poor air quality and low visibility. Other important consequences of the cold-air layer are on local weather, for example, where temperatures become colder and colder in the bottom of a basin during multi-day episodes, where low-level haze, clouds, or fogs hinder aviation and other human activity, and when colder air in the basin becomes separated from the larger-scale flow. Key questions involve how serious an impact such stable layers have in inhibiting exchange with air outside the basin or mixing within the basin, and how stable-layer/cold-pool occurrence can be predicted.

The ETL Doppler lidar deployed in Salt Lake City is well-suited to study this cold-air layer problem. We designed our scanning strategy to measure both the horizontal and vertical structure of the winds at regular intervals, capturing the evolution of winds as they occurred. It was typical for the basin winds to be from the north at the beginning of the IOP and shift to southerly over night. When these basin-scale winds were light, wind flow associated with gaps in the terrain to the east, south, and west sides of the basin were detected as distinct features superimposed upon the basin-scale flow. The depth and speed of these smaller-scale winds, and the intrusion of these flows into the basin, often evolved throughout the night. When the basin winds were strong, the canyon flows may not have been detected at all. Our detailed measurements, especially of the horizontal variability of the winds, will provide a context for other researchers involved in VTMX to interpret their results and analyze details of the processes involved in cool weather air pollution events. Our measurements will also help mesoscale numerical modelers assess model performance in predicting winds in the complex setting of the Salt Lake City basin.

For more information regarding the research goals of the field campaign and the other agencies involved, you can read the science plan and abstracts from accepted proposals at the VTMX home page.

Acknowledgments:

We wish to thank Jim Howell for procuring the lidar site, setting up the lidar for the field campaign, and participating in the data acquisition.

The following people worked during the field campaign to obtain the lidar measurements:
Raul Alvarez, Bob Banta, Alan Brewer, *Sherlyn Cooley, Lisa Darby, Janet Intrieri, *Brandi McCarty, Rich Marchbanks, Scott Sandberg, and Joe Shaw

We would especially like to thank Sherlyn and Brandi for staying up all night for several IOPs.

Thanks to Jenni Sweet for preparing the lidar scans for the web site.

Photographs courtesy of Scott Sandberg.