Saturation correction factor improving sensible heat flux accuracy measured by ZZLAS scintillometer

Abstract: For the purpose of investigating the influence of saturation effect on the estimation of regional sensible heat flux by using large aperture scintillometer (LAS), a field test was conducted from August to September, 2014. The dual-optical-path BLS900 scintillometer, which possesses saturation resistance, was set as a reference, and the ZZLAS type scintillometer with aperture size of 0.075 m (LAS1) and 0.15 m (LAS2) was selected as research object. In this field test, the LAS1 was set as easily as possible to be saturated. The saturation correction coefficients calculated based on the optical propagation theory, and the real-time saturation correction coefficient calculated by BLS900 were also displayed, and the air structure parameters, heat fluxes measured from scintillometer were corrected with the coefficients in this study. Variables from LAS1 and LAS2 and that from BLS900 were compared and analyzed with fitted line, as well as root mean square error before and after correction, and F-test was also used in the test of sensible heat flux. The saturation of scintillometer is often caused by the turbulence, and the scale of turbulence is thought to the main reason. Based on the optical propagation theory, inner scale of turbulence was also taken into consideration and the saturation correction coefficient of ZZLAS scintillometer was calculated. The results from the observation were corrected, and comparisons and analysis were also made. Here are the findings. Saturation correction coefficients of LAS1 and LAS2 are 1.034 and 1.019 respectively. The real-time correction coefficients given by BLS900 range from 0.70 to 1.15 depending on the developing of turbulence. The calculated coefficient of BLS900 during 10:00-12:30 shows smaller difference with the real-time coefficients, and it is thought to be more suitable for this period. During the observation, the saturation rate of the LAS1 is 24.58% and the effective saturation correction rate is 12.87%. After correction, root mean square error of air refraction index changes from 1.003×10-13to 9.74×10-14 m-2/3, while there is no change occurring in R2. There is no obvious change of the air structure parameters between LAS2 and BLS900 due to that the saturation rate of LAS2 is much less than LAS1. Sensible heat fluxes from ZZLAS type scintillometer are compared to the results from BLS900, and the root mean square error of LAS1 is 25.67 W/m2. By contrast, the saturation rate of LAS2 is 2.04% and the saturation correction rate is only 0.32%, and there is no significant difference for LAS2 before and after correction. It indicates that, the more pronounced the saturation phenomenon, the more pronounced the effect of saturation correction; this is consistent with the opinion that there is no need to do corrections when the saturation data are less than 5%. Sensible heat fluxes measured from LAS1 exceed 50 W/m2, the corrected fluxes are more close to the reference, and the system error decreases. Sensible heat fluxes from BLS900 and LAS1 are analyzed with F-test and the P value obtained is 0.15, which means there is much difference between the 2 datasets. The P value becomes to 0.004 when the saturation data are corrected with the calculated coefficient, while the P value changes to 0.06 when saturation data are corrected with the real-time coefficients. After correcting the saturation correction coefficient obtained by experiment, the error range of sensible heat flux with reference standard is 1.28-53.42 W/m2, which is closer to the reference standard than that before correction. The results of the observation over the farmland and plantation are also verified by the saturation correction method in the paper. Results also showed that the sensible heat flux observed by the scintillometer after saturation correction is closer to that of the BLS900. When saturation data of the ZZLAS type scintillometer exceed 20%, there is significant improvement on sensible heat flux after the correction with the correction coefficient calculated by the optical propagation theory.