|
Figure 1. The spatial distribution of the CO2 concentration composited between Julian days 240 to 250 at 1m AGL (left) and 2m (right). Here the CO2 measurements over Como Creek were not included in both plots.
Figure 2. Composite CO2 concentration at 1 m AGL across Como Creek during CME-04. |
During the period of October 1, 2005 to September 30, 2006, Sean Burns and Jielun Sun have analyzed the observational data collected from Carbon in the Mountain Experiment (CME-04) and Airborne Carbon in the Mountain Experiment (ACME-04). In addition, both of us were involved in a small field experiment at Niwot Ridge, which is designed to test instruments and to investigate low CO2 over Como Creek. The experiment will last beyond this report period.
Based on our Niwot Ridge Pilot Experiment (NRPE), we found that the respirated CO2, in general, drained to low grounds at night. We found that even slightly low ground such as Como Creek meandering along the main slope might collect high CO2 at night. During CME-04, we designed CO2 measurements across Como Creek and expended the tower coverage area. Our data analysis in CME-04 confirmed what we discovered during NRPE (Figure 1). We found that the respirated CO2 was transported down the main slope and also funneled into Como Creek low ground. Furthermore we also discovered that the CO2 concentration was high toward Como Creek but was significantly reduced right above Como Creek (Figure 2). (The observed CO2 concentration over the creek was not included in Figure 1 since the graphic package cannot handle the narrow low CO2 concentration along Como Creek). This spatial variation of CO2 was even observable at 4 m above Como Creek. The low concentration could not be explained by the variation of absolute measurement heights. We hypothesis that the low CO2 concentration is caused by low CO2 respiration from the water surface upstream of Como Creek, and the vertical mixing due to the convergence of the creek-ward drainage flow from both banks. This hypothesis will be investigated from the small-scale experiment we are doing at Niwot Ridge in collaboration with Lynette Laffae and Russ Monson at CU.
 |
Figure 3. Relationship between the terrain altitude and the CO2 concentration along the race track from flight 11 between 8 and 9 am LST on July 22, 2004. Each point represents 5 Hz data, and the red line represents the bin average CO2 as a function of the terrain altitude. |
In addition, we found that vertical structure of CO2, water vapor, temperature, and turbulence characteristics vary significantly with land surface during daytime (contact Jielun Sun for more details).
We also designed a flight track, called the race track, aimed to understand CO2 transport by vertical circulations induced by topography. The track was flown 11 out of the total 16 flights during ACME-04. We found that around 7 am when wind was weak and from northwest, CO2 concentration and terrain attitude are negatively correlated (Figure 3). We also found that the high CO2 and water vapor were correlated to upward vertical velocity in the morning (not shown), implying that the high CO2 accumulated at low ground at night was transported out of the low ground, partly by the local upslope flow induced by topography. |
