Observations of a cold air pool in a remote mountain basin

A recent panel on mountain meteorology has called for new observational and numerical modeling research on cold air pools and temperature inversions in basins and valleys. This research is considered a high priority because regions of high population are found within valleys and basins where cold air pooling can cause persistent periods of poor air quality.

This paper presents observations and numerical simulations of the evolution of a cold pool in a remote high-altitude basin. The objectives of this study are to determine the role of local slope flows on the evolution of the cold air pool, and to compare observational data to the numerical results.

Observations of the cold pool were made during a short meteorological field campaign conducted during the period from 9 to 13 September 1999 in the remote Peter Sinks basin in the Wasatch Mountains of northeastern Utah. The Peter Sinks basin, located between Logan and Bear Lake, Utah (111° 33' 30'' W and 41° 55'00''N), is a 1- by 2-km oval-shaped basin at 2500 m elevation surrounded by higher terrain of 200 m relief. The topography of the Peter Sinks is unique because the basin has no outlet, unlike most basins studied by previous investigators. In addition, this location is where the Utah state minimum temperature of -56°C was recorded in 1985, the second coldest temperature ever recorded in the contiguous United States.

Observations were made using multiple tethersondes and stationary meterological towers. The instrumentation was arranged along the fall line of a 15° west-facing slope with two tethersondes located on the slope and one at the basin center. Observations of the temperature field showed the inversion strength to be 15°C within 60 m. However, nocturnal slope flows were not observed as regularly as expected and tended to diminish after the cold pool formed. Another unique feature of the drainage flows was their extremely shallow depth of less than 2 m.

Numerical simulations at 100 m horizontal resolution were carried out using the University of Oklahoma's Advanced Regional Prediction System(ARPS)to predict the small-scale flows in and around the Peter Sinks basin. The vertical resolution ranges from 5 m near the surface to 50 m at the upper boundary. The initial temperature and wind fields in the basin, based on rawinsonde observations at Salt lake City, were assumed to be horizontally homogeneous.

The cold pool builds up quickly in the simulations. For example, by 0140 MDT the temperature is 2°C colder than at the beginning of the simulation(00 MDT). This may suggest that rapid cooling of the ground at the basin bottom, in addition to the downslope drainge flows from above, is responsible for the cold pool formation. The simulation also shows that the initial build-up of the cold pool occurs separately in both the north and south lobes of the sink. By 0050 MDT, the cold pool is deep enough to cause the flows to drain from the north lobe into and out of the south lobe. Vertical wind profiles taken from a grid point located on the north slope show typical drainage wind characteristics. Velocity maxima occur at the surface with secondary jets above which indicate that the slope flows may play a role in the advection of colder air into the basin from the upper sidewalls.