Site-specific frost warning in mountainous regions by estimating geographic potential for cold-air accumulation

Nocturnal cold air pools, which frequently form in complex terrain under anticyclonic systems, are a hazard to flowering buds of temperate fruit trees, but the spatial resolution to anticipate them exceeds the current weather forecast scale. To supplement the insufficient spatial resolution of official minimum temperature forecasts, a spatial interpolation scheme incorporating local geographic potential for cold air accumulation was developed. Air temperature was measured at 10-minute intervals at eight locations within a 2.1 by 2.1 km hilly orchard area in South Korea from September 2002 to August 2003. Minimum temperature data for suspected radiative cooling nights were collected, and deviations from estimations by a conventional spatial interpolation were calculated. A digital elevation model with a 10m cell size was used to calculate the cold air accumulation at 8 locations. Zonal averages of the cold air accumulation were computed for each location by increasing the cell radius from 1 to 10. Temperature deviations were regressed to a common logarithm of the smoothed averages of cold air accumulation to derive a linear relationship between the local temperature deviation and the site topography. The highest coefficient of determination was found at a cell radius of 5, which corresponds to an approximately 1 ha boundary surrounding the point of interest. Nocturnal temperature profiles were observed by using a tethered balloon system and the inversion cap height was determined. Empirical equations describing the potential effects of cold air accumulation and of the inversion profile on minimum temperature were combined with a conventional lapse rate-corrected inverse distance weighting interpolation scheme. This new interpolation scheme was successfully validated with an independent data set, showing a strong feasibility for development of a site-specific frost warning system for mountainous areas.