instrumentation

A open-path eddy covariance configuration was employed to measure the local-scale surface energy balance of the study area over 366 days during 2012. The instruments are mounted near the apex of a TX-472 tower (US Tower Corporation Inc., Woodlake, CA, USA) at a height of 22.1 m AGL (above ground level). This measurement height is approximately five-fold greater than the estimated mean building roof heights in the study area. As such, the sensors are likely sited within the inertial sublayer, reflecting a spatially averaged, integrated local-scale signal of turbulent fluxes under most conditions. A CSAT3 sonic anemometer (Campbell Scientific Inc., Logan, UT, USA) is employed to measure three-dimensional wind velocities, and virtual sonic temperature. Water vapor and carbon dioxide fluxes are measured using a LI-7500 open-path infrared gas analyzer (Li-Cor Biosciences Inc., Lincoln, NE, USA), mounted ∼0.4 m away (horizontally) from the CSAT and tilted 30 degrees from vertical (to prevent moisture accumulation on the sensor). The LI-7500 was subject to laboratory zero and spanning to ensure measurement accuracy with a LI-610 dew point generator (Li-Cor Biosciences Inc.) approximately every six months. Ambient air temperature and relative humidity are measured with a HMP45AC (Vaisala Oyj, Helsinki, Finland) sensor within a radiation shield installed adjacent to the CSAT3 and LI-7500. These sensors (CSAT3, LI-7500, and HMP45AC) are mounted on a boom extending ∼1.2 m away from the tower (approximately four times the tower width at its narrowest section). The instruments are oriented to face west in the direction of greatest fetch (∼11 km). All incoming and outgoing radiation fluxes are measured at the same height with a NR01 four-way net radiometer (Hukseflux, B.V., Delft, The Netherlands). Other relevant variables including precipitation, near-surface soil heat flux, soil temperature, and moisture were measured at various depths near or below the site surface with a suite of ground-based sensors. Data from all instruments are recorded with Campbell Scientific data loggers (CR23X, CR1000) mounted near the base of the tower.

data processing

Data pre-processing included: (1) signal de-spiked, with instantaneous values ±4σ during mean 30-min measurement periods being removed iteratively prior to subsequent corrections, with no data gap filling; (2) aligned into natural wind coordinates (i.e. mean w = 0); (3) offset for sensor lag (i.e. scan delay) between CSAT3 and LI-7500; (4) block averaged into 30-min periods to enable computation of instantaneous turbulence fluctuations; (5) corrected for frequency response (high/low-pass, and for physical sensor separation); and (6) applied density fluctuations specifically for gaseous fluxes. The raw 10 Hz tower data were postprocessed through the EDiRe software platform (http://www.geos.ed.ac.uk/abs/research/micromet/EdiRe/). Ancillary hourly meteorological data corresponding with the measurement periods were obtained from the National Weather Service (NWS) station located at Phoenix Sky Harbor Airport. These were useful for stratifying flux tower data into clear-sky (i.e. no reported hourly cloud ceiling) versus non-clear, all-sky periods (excluding in situ measured storms). Further, precipitation measured at Sky Harbor was also used to quantify rainfall at the tower site due to the lack of an in situ rain gauge prior to February 2012.

For additional detail, data users should consult:

Chow, W. T. L., Volo, T. J., Vivoni, E. R., Jenerette, G. D., & Ruddell, B. L. (2014). Seasonal dynamics of a suburban energy balance in Phoenix, Arizona. International Journal of Climatology, 34(15), 3863–3880. doi:10.1002/joc.3947