Micrometeorological Processes and Turbulent Fluxes
Micrometeorological Processes and Turbulent Fluxes
Within Forest Canopies Over a Sloping Terrain in the
Priest River Experiment Forest, Idaho
Alejandro A Prieto, Heping Liu, Natalie Wagenbrenner, Brain Lamb Qianyu Zhang, and Zeyuan Chen
REU LAR/Civil & Environmental Engineering, Washington State University, Pullman, WA
Introduction
T he exchange of energy and water vapor (H
2
O) between land surfaces and the atmosphere has great influence on weather and climate. Climate is also affected by carbon dioxide (CO
2
) exchange through ecosystem photosynthesis and respiration, which alters CO
2
concentrations in the atmosphere. Sloping terrain changes the exchange processes of energy, H
2
O, and CO
2 between the biosphere and the atmosphere through a couple of ways. Drainage flows and upslope winds are arguably the important pairing to consider. In this project, we used two micrometeorological towers to quantify how terrain-induced flows affect micrometeorological environments within forest canopies in the Priest River Experiment Forest, Idaho. Here, we present our results to study physical processes that affect the diurnal variations of turbulent fluxes of energy, H
2
O, and CO
2
Sensible Heat Flux Vs Time (Fig. 1)
Latent Heat Flux Vs Time (Fig. 2)
Wind Speed Vs Time (Fig. 5)
Wind Direction Vs Time (Fig. 6)
Post-field Data Processing Methods
T he 10 Hz raw time-series data collected in this study were processed and corrected to obtain EC fluxes by using a postfield data processing program (e.g., Liu et al. 2009). Briefly, the virtual air temperature was converted to air temperature following the procedure suggested by Campbell Scientific Inc.’s instruction manual (2006). The raw 10 Hz time series data were checked for spikes/noise. Data points were replaced through linear interpolation when their magnitudes exceeded 5 times the half-hour mean standard deviations. The planar fit method was adopted for rotating the coordinate system to streamline
(Wilczak et al., 2001). The fluxes were computed by using a block average. H, λE, and CO
2
fluxes were obtained via 30-min mean covariance between vertical velocity and the respective air temperature, water vapor density fluctuations, and CO
2
density fluctuations. Due to air density effects, λE and CO
2
fluxes were corrected according to the Webb Pearman and Leuning (WPL) corrections (Webb et al. 1980).
Site descriptions, Instruments, and Methods
O ur experimental site is located in the Priest River Experiment
Forest, Idaho. Two eddy covariance (EC) towers were erected over a forested, south-facing slope in June 23, 2012 and will continue to collect data till mid August, 2012. The tree heights vary from approximate 15 m to 20 m. One 12-m tower has 3 EC systems at heights of 8 m (system A: CSAT3 + LI7500A), 4.8 m
(system B: CSAT3), and 2.1 m (system C: CSAT3 + LI7500A).
The second short tower, which is 14.3 m downhill from the 12m tower, has one EC system at 1.7 m (system D: CSAT3 +
LI7200).
O n the 12-m tower, there are 8 temperature and humidity probes
(HMP45C), installed at 9.0, 8.0, 6.5, 4.8, 3.4, 2.1, 1.5, and 0.6 m. Wind profiles are also measured by 3 cup anemometer/vane sets (R.M. Young 03002) at 9.5, 1.0, and 0.6 m and 2 cup anemometers (R.M. Young 03001) at 6.5 and 3.4 m. Sensor signals from the EC systems are sampled at 10 Hz and recorded with 4 data loggers (model CR5000, Campbell Scientific, Inc.).
Other microclimate variables are sampled at 1 Hz and recorded as 30 means with these same data loggers.
References
Campbell Scientific Inc. Instruction Manual, 2006: CSAT3 three-dimensional sonic anemometer, Logan, Utah,
USA.
Liu, H. P., Y. Zhang, S. Liu, H. Jiang, L. Sheng, and Q. L.
Williams, 2009: Eddy covariance measurements of surface energy budget and evaporation in a cool season over southern open water in Mississippi,
J. Geophys.
Res.
, 114, D04110, doi:10.1029/2008JD010891.
Webb, E. K., G. I. Pearman, and R. Leuning (1980),
Correction of flux measurements for density effects due to heat and water-vapor transfer,
Q. J. R.
Meteorol. Soc., 106(447)
, 85-100.
Wilczak, J. M., Oncley, S. P. and Stage, S. A.: 2001, ‘Sonic
Anemometer Tilt Correction Algorithms’,
Layer Meteorol.
99, 127-150.
Boundary-
Friction Velocity Vs Time (Fig.3)
CO
2
Flux Vs Time (Fig. 4)
Height Vs Temperature (Fig. 7)
Height Vs Humidity (Fig. 8)
Diurnal Variations
•
Even within forest canopies, H and λE exhibited diurnal variations with positive values during the daytime and negative ones (or small) at nights. Positive H and λE indicates that heat and water vapor are transferred from lower to higher levels.
•
During the daytime, H and λE reached up to 150 and 140 W m -2 , respectively.
•
During the daytime, H, λE, and CO
2
fluxes were higher at 8.0 m than at 1.7 m on the tall tower, indicating the contribution of forest canopies to heat, H
2
O, and CO
2
(photosynthesis) transfer.
•
During the daytime, H, λE, and CO
2
fluxes within the canopies were higher at 2.1 m on the short tower than at 1.7 m on the short tower, likely due to higher friction velocity at 2.1 m.
•
At nights, small sensible heat exchange occurred. Small negative λE indicates weak condensation and small positive λE indicates small evaporation, driven by increased wind speeds.
Vertical Structures
•
Presence of canopies affected the micrometeorological processes and vertical structures of winds, temperature, and humidity.
•
At nights, within-canopy temperature was lower than the above-canopy temperature.
•
During daytime, within-canopy convection developed, but it disappeared in the afternoon due to the heating rates in the canopy layer was higher than the within-canopy layer.
•
The wind blows upslope during the day with higher altitude measurements while the surface measurements show a down slope flow, with a reversed flow during the night. With several instances where the flow are close to the same, right before or after noon and midnight.
1
This work was supported by the National Science
Foundation’s REU program under grant number 43
