Tomoyoshi Hirota

There is an increasing need for accurate winter agrometeorological forecasts, which is facilitated by a better understanding of the evolution process of nighttime air temperature distribution. However, studies on how air temperature distributions evolve in mesoscale plains have been limited. To clarify how the low temperatures in winter nights form, we analyzed the effects of topography and boundary-layer wind on the temperature distribution of the Tokachi region for a winter night using numerical simulations by the Japan Meteorological Agency Nonhydrostatic Model (JMA-NHM）with horizontal grid spacing of 2 and 5 km. We also analyzed vertical profiles of boundary-layer atmospheric conditions. The results show that, although boundary-layer wind is expected to affect the temperature distribution over the entire Tokachi region, the effects were generally confined to the northwestern part. Widespread effects over the Tokachi region were found only under strong wind conditions. We found that the mountain pass in the northwestern part of the Tokachi region is an important wind path, and the downslope winds as well as the sensible heat transfer by turbulent mixing in the boundary layer also was important in the evolution of the air temperature distribution. On the night we considered, a moderate boundary-layer wind was maintained throughout the night, but the surface wind speed decreased from the northern and southern parts of the Tokachi region; this can be attributed to the development of an inversion layer. A drainage flow was observed to originate from the southern part of the Tokachi Plain, reaching the central part of the Tokachi region in the night. We find that radiative cooling and sensible heat transfer by turbulent mixing in the surface layer do not adequately explain the temporal change in observed surface air temperatures. The development of an inversion layer and katabatic drainage flow drastically change the temperature distribution, despite a moderately strong wind condition in the boundary layer.

On-farm soil frost control has been used for the management of volunteer potatoes (Solanum tuberosumL.), a serious weed problem caused by climate change, in northern Japan. Deep soil frost penetration is necessary for the effective eradication of unharvested small potato tubers; however, this process can delay soil thaw and increase soil wetting in spring, thereby delaying agricultural activity initiation and increasing nitrous oxide emissions from soil. Conversely, shallow soil frost development helps over-wintering of unharvested potato tubers and nitrate leaching from surface soil owing to the periodic infiltration of snowmelt water. In this study, we synthesised on-farm snow cover manipulation experiments to determine the optimum soil frost depth that can eradicate unharvested potato tubers without affecting agricultural activity initiation while minimising N pollution from agricultural soil. The optimum soil frost depth was estimated to be 0.28–0.33 m on the basis of the annual maximum soil frost depth. Soil frost control is a promising practice to alleviate climate change effects on agriculture in cold regions, which was initiated by local farmers and further promoted by national and local research institutes.

Winter air temperatures strongly affect crop overwintering and cold resource usage. To clarify how winter air temperature distributions are formed in a mesoscale plain, field observations and simulations were conducted for the Tokachi region in Japan. Results elucidating the winter climate within the plain revealed that the winter mean air temperature at each site was correlated closely with the mean daily minimum air temperature. The daily minimum air temperature was not correlated with altitude, suggesting that local variation of the daily minimum temperature influences the temperature distribution. Observations at different distances from the upwind mountains revealed that nocturnal air temperatures were higher for stronger winds closer to the mountain foot. Low temperatures associated with wind speed suggest that radiative cooling strongly affects the temperature distribution. Wind and temperature conditions in the boundary layer influence the degree of drop in nocturnal air temperature and its distribution. The wind speed and direction, respectively, affect the extent and direction of the high-temperature zone from the northwest mountain foot. Simulations with a spatial resolution of 2 km reproduced the observed temperatures, but the error exceeded 5°C at sites having complex terrain under moderate or strong wind conditions. A higher-resolution model of 0.5 km showed that simulated temperatures approach the observed temperatures in association with a local wind system of down-valley drainage flow. In conclusion, the synoptic background, wind strength and direction over the plain, and microscale valleys affect boundary layer mixing and, thereby, determine the winter air temperature distribution.

Short‐term N ₂ O emission occurs in relation to snowmelt within seasonally frozen soil. To understand the effects of changing winter climates on the N ₂ O flux, snow cover manipulation experiments are useful. In Japan, snow cover manipulation is practiced by farmers to improve agricultural yield and is executed either by applying a broadcast of blackish agent onto the snow cover, which leads to faster snow‐melting thereby extending the crop‐growing season, or by snow cover removal/re‐accumulation, leading to an enhanced soil frost depth for weed management. Implementation of these practices involves using an amount of fossil fuel, in addition to influencing soil‐derived N ₂ O emissions, therefore, the load factors of snow cover management practices per unit area of agricultural field were estimated in this study. Field data including micrometeorological conditions, ground surface flux of N ₂ O, and amount of fossil fuel consumed during machinery operation for management practices, were obtained at two sites in Hokkaido over 2 years (2008–2010). Fuel consumption for the field spreading was found to be unexpectedly small (0.017 Mg CO ₂ eq ha ⁻¹ ). It was therefore suggested that acceleration of snowmelt may have the potential to reduce net greenhouse gas emissions if the agent used is a low‐degradable C‐rich material, such as charcoal. For soil frost control, the fossil fuel consumption during a set of snow cover removal/re‐accumulation (estimated as 0.052 Mg CO ₂ eq ha ⁻¹ ) is discussed, together with the relationship between possible mechanisms causing stimulation of N ₂ O production in frozen soil and inherent large differences in N ₂ O flux among sites.

Climate change often causes unexpected problems for agricultural production. In the Tokachi region of Hokkaido, northern Japan, soil-frost depth has been decreasing since the late 1980’s, facilitating the winter survival of small potato (Solanum tuberosum L.) tubers that remained unharvested and emerge as weeds in the following cropping season. Soil frost control by snow plowing (yukiwari), which removes snow cover and allows deep soil frost to kill the potato tubers, is spreading as a practical countermeasure. However, the technique has largely been empirically based, and the level of expertise varies among farmers. We present the final limit for yukiwari based on a numerical model of soil temperature for Tokachi and other potato-producing regions in Hokkaido, with annual fluctuations taken into account. An average of 60800 unharvested potato tubers was present per hectare, mostly distributed in the top soil to a depth of 0.15 m. As the survival probability of potato tubers was almost zero where the maximum soil-frost depth reached deeper than 0.30 m, the objective frost depth was set to 0.30 m. In the Tokachi region, soil-frost depth reached 0.30 m by yukiwari in most areas except Hiroo on the coast in the southernmost area. The final limit of yukiwari in a 30-year return period was estimated to be around 20 February in mountainous areas in the north and from late January to early February in other areas. For other potato-producing zones in Hokkaido, soil froze to 0.30 m deep by yukiwari in the inland areas from the Pacific Ocean, the Sea of Okhotsk, and the Sea of Japan, but soil frost was not as deep in the southwestern part of Hokkaido. The numerical model for estimating soil temperature facilitates the decision on the work schedule of the practice of yukiwari in order that the damage from volunteer potatoes may be minimized.