Surface Renewal Flux Estimates from a Moving Reference Frame

The surface renewal (SR) method estimates air exchange rates from fast response measurements of temperature or other transported scalar. The concept of surface renewal is that abrupt temperature changes are associated with surface air parcels that are displaced and “renewed” from upper air (Katul et al., 1997; Castellvi, et al., 2002). Under turbulent conditions, the rate of renewal accounts for the majority portion of the sensible heat flux, and the rate is marked by characteristic time scales for given surface roughness condition. This surface renewal estimate of sensible heat flux can be used with measured net radiation and ground heat flux to calculate the latent heat flux as a residual (Paw U et al., 2005). In the SR method, the total derivative (flux) is equated with the time derivative (measured scalar trace); time averaging of measurements diminishes the impact of advection and inhomogeneity in the flux estimate. In other words, fast measurement of temperature at a stationary location (in a Eulerian sense) is treated as equal to the transport of the Lagrangian parcel. It would be useful to be able to measure a flux via the SR method in a moving reference frame, which requires validating that the rapid temperature measurements are related to the flux from the transited surface. Additionally, this estimate must be averaged over some useful time period to allow spatial resolved estimates of flux. Resolving spatially explicit estimates of surface flux has theoretical and practical applications. Moving flux estimates provide another perspective on the validity of Taylor's frozen turbulence hypothesis and the ubiquitous horizontal homogeneity assumption. Practically, the ability to spatially resolve fluxes from the surface allows mapping of energy exchange directly. Maps of surface fluxes (sensible heat and evaporation) can be used to improve water management with a currently identified need for such maps in precision irrigation. Field experiments were conducted at three times of year to establish a robust protocol for the determination of surface fluxes by the surface renewal method. Two initial experiments used simultaneous fine wire thermocouple (SR flux estimates) and eddy covariance measurements mounted on a moving (ground based) platform. A third experiment expanded on this instrumentation with two stationary 3-d arrays of four sonic anemometers each. In each case, the velocity and location of measurements were recorded with a real-time kinematic GPS. Measurements were taken for a range of stability conditions, surface roughness and wetness, air temperatures, and wind speeds. We describe a new statistical parameter describing the length scale at which a spatially resolved flux estimate can be considered valid. A moving SR method has the potential to be more cost effective than eddy covariance, Bowen ratio determinations, or Penman-Monteith methods in creating spatial maps of evaporation for irrigation management. We show the potential for this method to be deployed on aerial platforms, and the utility of moving measurements to improve our understanding of fluxes from heterogeneous surfaces.