Analysis of the Systematic Errors Found in the Kipp & Zonen Large-Aperture Scintillometer

Abstract

Studies have shown a systematic error in the Kipp & Zonen large-aperture scintillometer (K&ZLAS) measurements of the sensible heat flux, H . We improved on these studies and compared four K&ZLASs with a Wageningen large-aperture scintillometer at the Chilbolton Observatory. The scintillometers were installed such that their footprints were the same and independent flux measurements were made along the measurement path. This allowed us to compare H and the direct scintillometer output, the refractive index structure parameter, C 2 n . Furthermore, spectral analysis was performed on the raw scintillometer signal to investigate the characteristics of the error. Firstly, correlation coefficients ≥0.99 confirm the robustness of the scintillometer method, and secondly we discovered two systematic errors: the low-C 2 n error and the high-C 2 n error. The low-C 2 n error is a non-linear error that is caused by high-frequency noise, and we suspect the error to be caused by the calibration circuit in the receiver. It varies between each K&ZLAS, is significant for H ≤ 50 W m −2 , and we propose a solution to remove this error using the demodulated signal. The high-C 2 n error identified by us is the systematic error found in previous studies. We suspect this error to be caused by poor focal alignment of the receiver detector and the transmitter light-emitting diode that causes ineffective use of the Fresnel lens in the current Kipp & Zonen design. It varies between each K&ZLAS (35% up to 240%) and can only be removed by comparing with a reference scintillometer in the field.

Assessing Inter-Sensor Variability and Sensible Heat Flux Derivation Accuracy for a Large Aperture Scintillometer

“…LAS units are well aligned when the transmitter is focused on the receiver aperture and vice versa . For periods of good LAS alignment the results from this study are considered comparable to the studies of Kleissl and others [ 11 ], Gowda [ 12 ] and Van Kesteren and Hartogensis [ 3 ], since there was no mention of misalignment in these references. As has been shown in the above references, we also found inter-LAS deviation in H .…”

“…Despite the common issues with failing to close the surface energy balance [ 1 , 2 ], use of the EC method is prevalent likely because of the advantage of continuous, direct measurement of turbulent sensible ( H ) and latent (λ E ) heat fluxes. It is also observed to be the common method for evaluation of Large Aperture Scintillometer-derived H and λ E [ 3 – 8 ]. Satellite- and airborne-based remote sensing methods are unique in their capability to provide land surface maps of information which can lead to production of ET raster maps.…”

“…Peak H for this study was roughly 500 W·m −2 . In addition, Van Kesteren and Hartogensis [ 3 ] showed significant inter-LAS deviation in the air refractive index structure parameter ( C n 2 ) signal for four Kipp and Zonen LAS units set up over a grass field in the UK. From the same study the authors reported sensible heat flux bias somewhat larger than 30% in the Kipp and Zonen LAS relative to a Wageningen University (Wageningen, The Netherlands) designed LAS model, where peak H was generally less than 250 W·m −2 .…”

“…From these results, it is apparent that the performance of the Kipp and Zonen LAS is variable from instrument to instrument. Kleissl and others [ 11 ] discussed some possible reasons for the inter-sensor bias, and Van Kesteren and Hartogensis [ 3 ] also explained some internal deficiencies in the Kipp and Zonen LAS model. This study was undertaken to assess the precision and accuracy of the Kipp and Zonen LAS for estimation of sensible heat flux under the environmental conditions encountered in southeastern Colorado.…”

Towards a Validation of Scintillometer Measurements: The LITFASS-2009 Experiment

“…So far, only a few scintillometer comparison results have been published in the scientific literature (e. g., Kleissl et al 2008Kleissl et al , 2009bvan Kesteren and Hartogensis 2011), and mainly consider the commercially available LAS systems from Scintec and Kipp&Zonen. Moreover, published results generally focus on the derived fluxes.…”