Spatial and temporal variability of nocturnal summer frost in elevated complex terrain

Abstract

This paper presents an investigation of summer night temperatures as influenced by a sharply undulating terrain, with a view towards the consequences of summer frost on forest regeneration in thermally marginal areas. The focus is on the presentation of the spatial distribution of monthly mean frost (MMF) in an elevated frost prone area, located approximately 100 m below the timber-line in the southern Swedish Scandes. The MMF is defined as the lowest temperature over a 4 h period at each location on each frost occasion in the study area which is then summed up and averaged for each month of the growing season. Geostatistics (kriging) is used for modelling the MMF distribution in June, July, August and September, 1994. Kriging is essentially a weighted moving average technique which aims to estimate the temperature at a given site based on observations at neighboring sites. The results from modelling using two different sampling strategies are compared over a 90,000 m2 clear-felled area. A lower density of 18 observations is used for sampling on a fixed grid and a higher density of 29 observations aims to cover the spacing of distinct terrain features in the area. The investigated region is characterized by a local topography with sharp vertical variations, i.e. 'basin and knob' terrain. The estimated frost patterns are evaluated in relation to a set of typical pre-defined forms in such terrain. It is found that the spatial distribution of low temperatures corresponded well with local terrain forms and that frost is experienced during each month of the vegetative period. However, there are obvious differences in frost intensity and duration. The single most important factor for explaining the frost patterns during the peak of the growing season is the difference in radiative cooling due to the surrounding local terrain. Variations in the cooling rate of up to 6°C h-1 are recorded between sites that are partially screened from solar radiation and sites that are well exposed. Wind shelter provided by the immediate surroundings of a site is also found to have a significant influence on the near-surface temperature. Maximum differences of 3-5°C are maintained between sites with various degrees of shelter at a mean wind speed of 1.5 m s-1. In June, August and September, drainage of cold air is identified as another important process affecting the distribution of frost. It is suggested that the drainage flow is a shallow gravitational cold air flow of a strictly localized nature.