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Assessing inter-sensor variability and sensible heat flux derivation accuracy for a large aperture scintillometer.

Rambikur EH, Chávez JL - Sensors (Basel) (2014)

Bottom Line: Average H fluxes were compared between LAS units and between LAS and EC.It is possible that the LAS physical misalignment may have caused edge-of-beam signal noise as well as vulnerability to signal noise from wind-induced vibrations, both having an impact on the solution of H.In addition, there were some uncertainties in the solutions of H from the LAS and EC instruments, including lack of energy balance closure with the EC unit.

View Article: PubMed Central - PubMed

Affiliation: Department of Civil and Environmental Engineering Colorado State University, 1372 Campus Delivery, Fort Collins, CO 80523, USA. evan.rambikur@gmail.com.

ABSTRACT
The accuracy in determining sensible heat flux (H) of three Kipp and Zonen large aperture scintillometers (LAS) was evaluated with reference to an eddy covariance (EC) system over relatively flat and uniform grassland near Timpas (CO, USA). Other tests have revealed inherent variability between Kipp and Zonen LAS units and bias to overestimate H. Average H fluxes were compared between LAS units and between LAS and EC. Despite good correlation, inter-LAS biases in H were found between 6% and 13% in terms of the linear regression slope. Physical misalignment was observed to result in increased scatter and bias between H solutions of a well-aligned and poorly-aligned LAS unit. Comparison of LAS and EC H showed little bias for one LAS unit, while the other two units overestimated EC H by more than 10%. A detector alignment issue may have caused the inter-LAS variability, supported by the observation in this study of differing power requirements between LAS units. It is possible that the LAS physical misalignment may have caused edge-of-beam signal noise as well as vulnerability to signal noise from wind-induced vibrations, both having an impact on the solution of H. In addition, there were some uncertainties in the solutions of H from the LAS and EC instruments, including lack of energy balance closure with the EC unit. However, the results obtained do not show clear evidence of inherent bias for the Kipp and Zonen LAS to overestimate H as found in other studies.

No MeSH data available.


Related in: MedlinePlus

Aerial image overview of the Timpas grassland site. An access (dirt) road ran parallel to the LAS paths, in between the LAS units and the EC and SAT towers. LAS#T represents LAS transmitter and LAS#R represents LAS receiver.
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f1-sensors-14-02150: Aerial image overview of the Timpas grassland site. An access (dirt) road ran parallel to the LAS paths, in between the LAS units and the EC and SAT towers. LAS#T represents LAS transmitter and LAS#R represents LAS receiver.

Mentions: During the summer of 2011, three Kipp and Zonen LAS units were installed at a relatively flat grassland site near Timpas, CO, USA (latitude 37.8173, longitude −)103.82304, elevation 1,350 m above sea level) and near the Comanche National Grasslands. The vegetation cover was mostly dry and did not seem to change significantly over the study period. There was a mix of short grass (approximately 9 cm) and tall grass (approximately 25 cm), along with occasional shrubs and cactus bushes (approximately 0.4–1.2 m;. The grass types in the historical climax plant community for the area were predominantly western wheatgrass, blue grama, and galleta grasses [14]. Approximately 51 mm of rainfall were recorded over the study period. An overview of the instrument deployment is shown in Figure 1. All LAS units were installed side by side at the Timpas site from 2 July 2011 to 3 August 2011. The transmitter and receiver units were mounted on top of a tripod with a custom extension, and anchored using four guy wires (Figure 2). The path length was approximately 600 m and LAS height as determined from LAS transmitter and receiver heights was approximately 2.25 m. There was approximately 20 m separation between each LAS transect, and in addition, the LAS-2 transect (transmitter to receiver) was inverted relative to that of LAS-1 and LAS-3, such that no risk of beam contamination was expected (Figure 1) (LAS beam width widens to approximately 1% of the path length upon reaching receiver [11]). Measurements from a surface aerodynamic profile (SAT) tower with six levels (each cross-arm about 1 m apart) were used to provide air temperature, relative humidity (Vaisala, Inc. HMP45C, Campbell Scientific Inc. (CSI), Logan, UT, USA), and wind speed (R.M. Young Wind Sentry 03101, CSI) as input for LAS data processing. In addition, an eddy covariance (EC, described later) system was installed adjacent to the SAT tower, both towers being approximately 40 m west of the closest LAS path at the approximate north-south path center (Figures 1 and 2). Both the SAT and EC systems were operational first on 8 July. At the SAT tower and at the LAS-1 receiver, ancillary instrumentation was installed to measure net radiation (net radiometer NR-Lite, Kipp and Zonen, CSI), radiometric surface temperature (infra-red thermometer IRT SI-111, Apogee, CSI), soil heat flux (soil heat flux plates, REBS HFT3, CSI,), shallow soil temperature (thermocouple T107, CSI), and shallow soil moisture (volumetric water content, CS616, CSI). Finally, barometric pressure (barometer CS106, Vaisala BAROCAP, CSI) and precipitation (rain gauge TE525, CSI) were measured at the LAS-1 receiver location.


Assessing inter-sensor variability and sensible heat flux derivation accuracy for a large aperture scintillometer.

Rambikur EH, Chávez JL - Sensors (Basel) (2014)

Aerial image overview of the Timpas grassland site. An access (dirt) road ran parallel to the LAS paths, in between the LAS units and the EC and SAT towers. LAS#T represents LAS transmitter and LAS#R represents LAS receiver.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3958211&req=5

f1-sensors-14-02150: Aerial image overview of the Timpas grassland site. An access (dirt) road ran parallel to the LAS paths, in between the LAS units and the EC and SAT towers. LAS#T represents LAS transmitter and LAS#R represents LAS receiver.
Mentions: During the summer of 2011, three Kipp and Zonen LAS units were installed at a relatively flat grassland site near Timpas, CO, USA (latitude 37.8173, longitude −)103.82304, elevation 1,350 m above sea level) and near the Comanche National Grasslands. The vegetation cover was mostly dry and did not seem to change significantly over the study period. There was a mix of short grass (approximately 9 cm) and tall grass (approximately 25 cm), along with occasional shrubs and cactus bushes (approximately 0.4–1.2 m;. The grass types in the historical climax plant community for the area were predominantly western wheatgrass, blue grama, and galleta grasses [14]. Approximately 51 mm of rainfall were recorded over the study period. An overview of the instrument deployment is shown in Figure 1. All LAS units were installed side by side at the Timpas site from 2 July 2011 to 3 August 2011. The transmitter and receiver units were mounted on top of a tripod with a custom extension, and anchored using four guy wires (Figure 2). The path length was approximately 600 m and LAS height as determined from LAS transmitter and receiver heights was approximately 2.25 m. There was approximately 20 m separation between each LAS transect, and in addition, the LAS-2 transect (transmitter to receiver) was inverted relative to that of LAS-1 and LAS-3, such that no risk of beam contamination was expected (Figure 1) (LAS beam width widens to approximately 1% of the path length upon reaching receiver [11]). Measurements from a surface aerodynamic profile (SAT) tower with six levels (each cross-arm about 1 m apart) were used to provide air temperature, relative humidity (Vaisala, Inc. HMP45C, Campbell Scientific Inc. (CSI), Logan, UT, USA), and wind speed (R.M. Young Wind Sentry 03101, CSI) as input for LAS data processing. In addition, an eddy covariance (EC, described later) system was installed adjacent to the SAT tower, both towers being approximately 40 m west of the closest LAS path at the approximate north-south path center (Figures 1 and 2). Both the SAT and EC systems were operational first on 8 July. At the SAT tower and at the LAS-1 receiver, ancillary instrumentation was installed to measure net radiation (net radiometer NR-Lite, Kipp and Zonen, CSI), radiometric surface temperature (infra-red thermometer IRT SI-111, Apogee, CSI), soil heat flux (soil heat flux plates, REBS HFT3, CSI,), shallow soil temperature (thermocouple T107, CSI), and shallow soil moisture (volumetric water content, CS616, CSI). Finally, barometric pressure (barometer CS106, Vaisala BAROCAP, CSI) and precipitation (rain gauge TE525, CSI) were measured at the LAS-1 receiver location.

Bottom Line: Average H fluxes were compared between LAS units and between LAS and EC.It is possible that the LAS physical misalignment may have caused edge-of-beam signal noise as well as vulnerability to signal noise from wind-induced vibrations, both having an impact on the solution of H.In addition, there were some uncertainties in the solutions of H from the LAS and EC instruments, including lack of energy balance closure with the EC unit.

View Article: PubMed Central - PubMed

Affiliation: Department of Civil and Environmental Engineering Colorado State University, 1372 Campus Delivery, Fort Collins, CO 80523, USA. evan.rambikur@gmail.com.

ABSTRACT
The accuracy in determining sensible heat flux (H) of three Kipp and Zonen large aperture scintillometers (LAS) was evaluated with reference to an eddy covariance (EC) system over relatively flat and uniform grassland near Timpas (CO, USA). Other tests have revealed inherent variability between Kipp and Zonen LAS units and bias to overestimate H. Average H fluxes were compared between LAS units and between LAS and EC. Despite good correlation, inter-LAS biases in H were found between 6% and 13% in terms of the linear regression slope. Physical misalignment was observed to result in increased scatter and bias between H solutions of a well-aligned and poorly-aligned LAS unit. Comparison of LAS and EC H showed little bias for one LAS unit, while the other two units overestimated EC H by more than 10%. A detector alignment issue may have caused the inter-LAS variability, supported by the observation in this study of differing power requirements between LAS units. It is possible that the LAS physical misalignment may have caused edge-of-beam signal noise as well as vulnerability to signal noise from wind-induced vibrations, both having an impact on the solution of H. In addition, there were some uncertainties in the solutions of H from the LAS and EC instruments, including lack of energy balance closure with the EC unit. However, the results obtained do not show clear evidence of inherent bias for the Kipp and Zonen LAS to overestimate H as found in other studies.

No MeSH data available.


Related in: MedlinePlus