Effects of deforestation and weather on diurnal frost heave processes on the steep mountain slopes in south central Japan Kenichi Ueno, 1 * Kousei Kurobe, 2 Fumitoshi Imaizumi 3 and Ryoko Nishii 4 1 Life and Environmental Sciences, University of Tsukuba, Tsukuba, Ibaraki, Japan 2 Pacific Consultants Co., Ltd, Tokyo, Japan 3 Faculty of Agriculture, Shizuoka University, Shizuoka, Japan 4 Public Works Research Institute, Tsukuba, Ibaraki, Japan Received 29 August 2014; Revised 16 June 2015; Accepted 18 June 2015 *Correspondence to: Kenichi Ueno, Life and Environmental Sciences, University of Tsukuba, Tsukuba, Ibaraki, Japan. E-mail: ueno.kenichi.fw@u.tsukuba.ac.jp ABSTRACT: Freezing and thawing processes play an important role for the gravitational transport of surface materials on steep mountain slopes in Japan. The effects of deforestation on frost heave activity were observed through the 2012/2013 winter sea- son in Ikawa University Forest, a southern mountainous area in central Japan (1180–1310 m above sea level). During periods without snow cover, needle ice development prevailed at a clear-cut site, and the downslope sediment movement of upper soil was 10 to 15 cm through the winter season. At a non-cut site, rise and fall in the ground surface level prevailed on a weekly scale, with no evident downslope movements at the surface; ice lens formation in the soil layer is assumed. Abrupt changes in the radiation budget, such as the strengthening of nighttime radiative cooling and increases in daytime direct insolation, in- duced frequent development/deformation of needle ice at the clear-cut site. In snow-free periods, the day-to-day variability in needle ice growth length and in nighttime averaged net radiation showed significant correlations; cloudy weather with warmer and moist air intrusion associated with synoptic disturbances prevented the occurrence of needle ice. Namely, day- to-day weather changes directly affected the mass movement of the upper soil after deforestation. Shallow snow cover occurred discontinuously through the winter and is likely an important factor in keeping the soil moisture sufficiently high in the upper soil layer for initiating needle ice during snow-free periods. We also discuss contributions of coastal extratropical cyclone ac- tivities providing both snow cover and cloudy weather in the southern mountain areas of central Japan to the intra-seasonal variability in frost heave and its indirect effect on soil creep and landslides on the deforested steep slopes. Copyright © 2015 John Wiley & Sons, Ltd. KEYWORDS: diurnal frost heave; deforestation; micrometeorology; snow cover; central Japan Introduction Freezing and thawing processes contribute strongly to the activation of surface soil layers on steep slopes (Matsuoka, 2001; Imaizumi et al., 2006). They play an important role in the development of peri-glacial landscapes as they con- stantly cause gravitational mass movement, such as dry ravel, the gravitational transport of surface materials by rolling, sliding, and bouncing across the surface (Gabet and Dunne, 2002; Imaizumi et al., 2015), and soil creep that sometimes accounts for more than 40% of the sediment transport rate on hillslopes. Freezing and thawing processes activate soil surface erosion (Roberts and Church, 1986; Benda, 1990; Kværnø and Øygarden, 2006). Since soil creep and dry ravel are considered important infilling processes for landslide scars and debris-flow torrents, a clear understand- ing of freezing and thawing processes is required to achieve better sediment disaster mitigation (Benda, 1990; Sasaki et al., 2000). Most studies of frost heaving have been conducted in high latitudes in permafrost areas (after Williams and Smith, 1989) causing diurnal to seasonal-scale deformation of land surface (Smith, 1988; Matsuoka, 1998a). Segregation ice, such as nee- dle ice and ice lenses, results from heaving in the upper soil layers (e.g. Higashi and Corte, 1971; Anderson, 2002). Needle ice forms on the soil surface below 0 °C and is characterized by diurnal development stages. The necessary conditions include: (1) sufficient radiative/evaporative cooling; (2) sufficient soil moisture; (3) stable water flux by capillary force (after Outcalt, 1971). Ice lenses are generally oriented normal to transient freezing, with a banded structure that changes depending on the soil type/structure, applied load, and freezing conditions (Konrad and Duquennoi, 1993). Environmental controls, such as ground cover (i.e. snow cover and vegetation) and weather (i.e. diurnal temperature variation, precipitation, surface radia- tion balance), limit the frequency of effective frost days consid- erably (Boelhouwers, 1998; Boelhouwers et al., 2003; Haussmann et al., 2009). Nel et al. (2009) noted that EARTH SURFACE PROCESSES AND LANDFORMS Earth Surf. Process. Landforms (2015) Copyright © 2015 John Wiley & Sons, Ltd. Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/esp.3776