ALBERT

Description: The long-term effect of 137Cs re-suspension from contaminated soil and forests due to the Fukushima nuclear accident has been quantitatively assessed by numerical simulation, a field experiment on dust emission flux in the contaminated area (Namie, Fukushima), and air concentration measurements inside (Namie) and outside (Tsukuba, Ibaraki) the contaminated area. The assessment period is for the year 2013 just after the start of the field experiments, December 14, 2012. The 137Cs concentrations at Namie and Tsukuba were approximately 10−1–1 and 10−2–10−1 mBq/m3, respectively. The observed monthly median concentration at Namie was one to two orders of magnitude larger than that at Tsukuba. This observed difference between the two sites was consistent with the simulated difference, indicating successful modeling of 137Cs re-suspension and atmospheric transport. The estimated re-suspension rate was approximately 10−6/d, which was significantly lower than the decreasing rate of the ambient gamma dose rate in Fukushima prefecture (10−4–10−3/d) as a result of radioactive decay, land surface processes (migration in the soil and biota), and decontamination. Consequently, re-suspension contributed negligibly to reducing ground radioactivity. The dust emission model could account for the air concentration of 137Cs in winter, whereas the summer air concentration was underestimated by one to two orders of magnitude. Re-suspension from forests at a constant rate of 10−7/h, multiplied by the green area fraction, quantitatively accounted for the air concentration of 137Cs at Namie and its seasonal variation. The simulated contribution of dust re-suspension to the air concentration was 0.6–0.8 in the cold season and 0.1–0.4 in the warm season at both sites; the remainder of the contribution was re-suspension from forest. The re-suspension mechanisms, especially through the forest ecosystems, remain unknown, and thus the current study is the first but crude estimation of the long-term assessment of radiocesium re-suspension. Further study will be needed to understand the re-suspension mechanisms and to accurately assess the re-suspension mechanisms through field experiments and numerical simulations.

Description: Simulations of the global dust cycle and its interactions with a changing Earth system are hindered by the empirical nature of dust emission parameterizations in climate models. Here we take a step towards improving global dust cycle simulations by presenting a physically-based dust emission model. The resulting dust flux parameterization depends only on the wind friction speed and the soil's threshold friction speed, and can therefore be readily implemented into climate models. We show that our parameterization's functional form is supported by a compilation of quality-controlled vertical dust flux measurements, and that it better reproduces these measurements than existing parameterizations. Both our theory and measurements indicate that many climate models underestimate the dust flux's sensitivity to soil erodibility. This finding can explain why dust cycle simulations in many models are improved by using an empirical preferential sources function that shifts dust emissions towards the most erodible regions. In fact, implementing our parameterization in a climate model produces even better agreement against aerosol optical depth measurements than simulations that use such a source function. These results indicate that the need to use a source function is at least partially eliminated by the additional physics accounted for by our parameterization. Since soil erodibility is affected by climate changes, our results further suggest that many models have underestimated the climate sensitivity of the global dust cycle.

Description: Simulations of the dust cycle and its interactions with the changing Earth system are hindered by the empirical nature of dust emission parameterizations in weather and climate models. Here we take a step towards improving dust cycle simulations by using a combination of theory and numerical simulations to derive a physically based dust emission parameterization. Our parameterization is straightforward to implement into large-scale models, as it depends only on the wind friction velocity and the soil's threshold friction velocity. Moreover, it accounts for two processes missing from most existing parameterizations: a soil's increased ability to produce dust under saltation bombardment as it becomes more erodible, and the increased scaling of the dust flux with wind speed as a soil becomes less erodible. Our treatment of both these processes is supported by a compilation of quality-controlled vertical dust flux measurements. Furthermore, our scheme reproduces this measurement compilation with substantially less error than the existing dust flux parameterizations we were able to compare against. A critical insight from both our theory and the measurement compilation is that dust fluxes are substantially more sensitive to the soil's threshold friction velocity than most current schemes account for.