KEYWORDS: surface hoar, bulk transfer coefficient, surface roughness ROUGHNESS EFFECT ON VAPOR TRANSFER FOR SURFACE HOAR GROWTH Akihiro Hachikubo* Kitami Institute of Technology, Kitami, Japan hoar periods from a calculation of latent exchanges with the observed surface h periods. Hachikubo (2001) simulated the ti eVOlution of the heat balance when surface h formed and the validity of the turbulent tran coefficient in Crocus was discussed. T numerical simulation showed promising res and further detailed examination has b expected by researchers. . This paper is intended as investigation of the bulk transfer coefficient water vapor Ceo In a practical snow model, C e needed for calculating a latent heat flux which . one of the heat balance components at the sn surface. When the surface hoar crystals up to several millimeters in height, it see reasonable to suppose that they disturb the flow more or less and increase the turbulen This effect may contribute to the increase in transfer coefficient of water vapor. Hachiku and Akitaya (1997) reported that C e increased the surface hoar grew to several millimeters height. In this study, field observations surface hoar growth were carried out to cia" the roughness effect on C e quantitatively. 2. OBSERVATION SITE AND METHODS Observations were carried out selected clear and humid nights in the th seasons of January to March 1994, Decem 1994 to March 1995 and December 1995 March 1996. The site is located on a mounta" ridge near the avalanche-research station (240 above sea level, 45°N, 142°E) of the Institute Surface hoar is one of the most frequently occurring types of weak layer and has long been of interest to avalanche researchers (e.g. Perla and Martinelli, 1976). Surface hoar crystals are caused by sublimation (solid condensation) of water vapor in the atmosphere onto a snow surface, and often form under nocturnal, clear and humid conditions with perceptible wind. These meteorological conditions suitable for surface hoar growth have been discussed by Lang et al. (1985), Breyfogle (1987), Colbeck (1988) and Holler (1998). Recently, a transfer coefficient of water vapor was obtained from field observations (Hachikubo et a/., 1994; Hachikubo and Akitaya, 1997) and the sublimation rate of surface hoar can be estimated from meteorological conditions. Assessing the formation of the surface hoar layers remains an additional challenge for operational avalanche forecasting. A numerical model, named Crocus, has been developed to simulate the energy and mass balance at the snow surface and the evolution of snow-cover stratigraphy as a function of meteorological conditions (Brun and others, 1989, 1992). Mingo (1995) and Mingo and McClung (1998) applied Crocus and compared potential surface 128 1. INTRODUCTION ABSTRACT: In order to examine the roughness effect of surface hoar crystals on their growth obtain a relation between surface roughness and the amount of sublimation, field observation surface hoar formation were carried out in the three seasons of 1993 to 1996 in Hokkaido, no Japan. The sublimation rate increased linearly with the product of the vapor pressure gradient the wind speed. This relation made it possible to estimate the sublimation rate of surface hoar b bulk method from the meteorological data. The bulk transfer coefficient of water vapor was rou constant (2.4 x 10- 3 , dimensionless) when the surface hoar crystals were small, whereas it sho some increase as the hoar crystals. grew to several millimeters in height. The coefficient expressed as (2.0+9S) x 10- 3 , where S (kg m- 2 ) is the total amount of sublimation, or (2.0+0.4d) x1 where d (mm) is the major axis of surface hoar crystal. These relation indicate that a pas' feedback system exists on the mechanism of surface hoar formation. * Corresponding author address: Akihiro Hachikubo, Kitami Institute of Technology, 165 Koen-cho, Kitami 090-8507, Japan; tel: +81-157-26-9522; fax: +81-157-25- 8772; email: hachi@snow2.civil.kitami-it.ac.jp