After successfully applying the WSMA to aerosol, radiation transfer and surface mapping studies the feasibility of EC flux measurements in the ABL was explored. The WSMA sensor package comprises the measurement of airframe motion relative to the earth (Ub, from inertial navigation system), air motion relative to the airframe (Ua, from five-hole pressure probe), atmospheric state (pressure, temperature, dew-point temperature), trace gas concentrations (CO2, H2O), radiation (actinic, global, surface temperature), infrared surface imaging and aerosol size distribution. Variables entering the EC flux calculation are stored at 10 Hz or 2.6 m horizontal resolution. Due to its flexible wing- and aircraft architecture, WSMA dynamics can be significantly enhanced as compared to fixed wing aircraft (FWA). Most of all the wind vector (U) has to be evaluated carefully, since it is calculated as vector difference U = Ub - Ua from quantities larger by one order of magnitude. Well established algorithms for FWA were adapted and corrected for the WSMAs varying motions. The system uncertainty in wind components (±0.3 ms-1) and flux measurements (±5 Wm-2 for QH and QE) was estimated by propagating sensor and calibration uncertainties. The final algorithm was applied to 50 soundings in the ABL and validated against ground truth from sonic detection and ranging, tall tower EC-, cup- and vane as well as large aperture scintillometer. Here wind components compared to 0.2 ms-1, fluxes from WSMA differed on average by -2±5 % for QH (assuming a linear Vertical Flux Gradient, VFG) and +15±17 % for QE (no VFG considered).
In succession 36 WSMA EC flux measurements have been carried out in the XRC to directly capture the regions present energy, water and CO2 exchange. Simultaneously to the WSMA soundings, half-hourly tower EC fluxes were measured at 3 control sites, two of them non-grazed (C4 dominated Leymus, C3 dominated Stipa) and one heavily grazed (3 sheep unit ha-1y-1, HG). The idea of this setup was to a) validate the spatial representativity of process-based regional flux simulation and b) allow for scenario study of advancing grassland degradation. Flight tracks over the heterogeneous terrain were planned with the aid of a geographic information system as a compromise to a) avoid topography associated local circulation, b) minimize flux statistical error and influence of VFG, and c) represent the XRCs land cover proportion. As result WSMA soundings took place around sun apex and followed multiple repetitions of 2 to 80 km long line transects, organized in vertical stacks. Operational results at lowest flight level (50 m, AC50) averaged to QH = -177±78 Wm-2, QE = -78±40 Wm-2 and QC = 0.02±0.2 mg CO2 m-2s-1 throughout the campaign. At a radiation budget QS* = -412±86 Wm-2, which is 16 % lower than the control sites, the energy balance from AC50 measurements was closed to 72±18 % on average. AC50 measured QH was found comparable in magnitude to the HG control site, whereas Leymus and Stipa sites were 30 % higher on average. The magnitude of QE was comparable to the Leymus site, whereas QE from Stipa and HG were ≥35 % higher on average. This relation is remarkable, since the soil moisture on HG was actually lower (9 %vol) compared to the non-grazed sites (14 %vol). AC50, Stipa and HG measurements indicate marginal CO2 release / uptake, only Leymus displayed notable CO2 assimilation (-0.07±0.3 mg CO2 m-2s-1).
This study proves that EC flux measurement is possible from WSMA with an accuracy comparable to conventional FWA. It shows that the WSMA, which can be easily shipped to and operated from remote areas, provides a suitable tool to close the gap from local to spatial measurements.