Abstract:

A short meteorological field campaign will be conducted in the remote Peter Sinks basin in the Wasatch Mountains of northwestern Utah to examine the role of slope flows on the evolution of cold air pools in basins. The Peter Sinks basin is a shallow high altitude basin where the Utah state minimum temperature of -56 °C was recorded in 1985, the second coldest temperature ever recorded in the contiguous United States. Field measurements planned for the period between 9-13 September 1999 will be followed by extensive analysis of the data.

Background:

A recent panel on mountain meteorology called for new observational and numerical modeling research on cold air pools and temperature inversions in basins and valleys.This research was 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 in which pollution concentrations can reach hazardous levels. Large population increases are expected in many Rocky Mountain valleys and basins during the next several decades and there is an increasing level of interest in meteorological mechanisms responsible for air quality deterioration. Considerable fundamental work on these meteorological mechanisms is required to address present and future air quality problems in mountainous areas.

While an extensive literature now exists on inversion buildup and destruction in valleys, there has been relatively little effort expended on gaining an understanding of cold pool evolution in basins. This proposed research is intended as an adjunct to a new DOE research program entitled Vertical Transport and Mixing (VTMX). Field campaigns in urban basins such as Salt Lake City or Phoenix are planned as part of the VTMX program, which will begin in Fiscal Year 2000. The goal is to contrast the slope flow in a simple, small, rural basin to the large complex basins of the VTMX program. The problem this research will address  is the mechanics and role that slope flows have on cold air pool formation and destruction within a rural basin.

Objectives:

The objectives of this study are:

1) to perform a short field experiment to collect essential data on the role of slope flows in the
     buildup and breakdown of temperature inversions in a simple basin,

2) to analyze the field experiment data to gain an understanding of the dynamical and thermody-
     namical processes that control the diurnal formation and dissipation of the basin cold pool,

3) to contrast the results of this study in a simple basin to results from a larger-scale study to be
     undertaken under DOE sponsorship in the more complex urban environment of the Salt Lake
     Valley as part of DOE's VTMX program.

Approach:

The methods used for this research will be based on a short meteorological field campaign in the Peter Sinks basin. The Peter Sinks area, 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 200m relief. The elevated terrain surrounding the Sink leads to nocturnal drainage flows that pool in the basin and lead to unusually cold nighttime temperatures. An extensive array of instrumentation will be deployed as part of this experiment to achieve an understanding of the role of slope winds in the buildup and destruction of the basin cold pool. This instrumentation will be provided by DOE and the University of Utah. The instrumentation includes an array of 15 temperature dataloggers, which will be placed around the basin's periphery and on slope transects. In addition, approximately five 3-m meteorological masts will be placed around the basin and on its sidewalls. The masts will be instrumented with wind, temperature, and solar radiation sensors. Three tethered balloon systems will be flown simultaneously from different locations in the basin to collect vertical profiles of temperature, humidity and wind speed and direction. One tethersonde will be flown from the center of the basin while the two others will be flown from the basin sidewalls. The sidewall soundings will observe the slope wind structure evolution on the sidewalls as the slope flows leave the slope and enter the cold pool. Two additional masts (heights ~ 10 m) will be equipped with low threshold anemometer/ vane sensors (R. M. Young Windsentry 3001) and thermistors at different levels. The masts will provide continuous measurements of the atmospheric boundary layer adjacent to the sidewall to complement the occasional tethered balloon soundings.

The goal of the field program is to obtain data sets for three clear-sky, undisturbed days, with the observations focused on the morning and evening transition periods. Once the data sets are available, analyses will be performed collaboratively by investigators at the University of Utah and at Pacific Northwest National Laboratory with the goal of understanding the role of the slope winds on formation, maintenance, and destruction of the basin's cold pool. To obtain the volumetric characteristics of the basin as compared to other basins, the basin's topographic amplification factor (TAF) will be calculated from topographic maps. This information will be used in determining the energetics of the basin to establish the mechanisms that are responsible for heat losses and gains of the basin atmosphere and the rates of mass buildup and removal by slope wind convergence in the basin.