Radiation Balance and Heat Transfer Studies at Okstindsjøen, Nordland, Norway

ABSTRACT

This paper examines the radiation balance and heat transfers over undisturbed subarctic tundra. The data varied considerably with the three synoptic periods observed at the site; viz. anticyclonic, changeable/transitional and cyclonic. For example, the incoming shortwave radiation averaged 25.8 Megajoules per square metre per day (MJm^-2 day^-1) with clear anticyclonic weather compared with 19.3 MJm^-2 day^-1 in the changeable period (with alternating cloudy and sunny spells) and 11.3 MJm^-2 day^-1 in the cloudy cyclonic period. The remaining shortwave and longwave components displayed similar decreases. Albedo changes were related to variations in surface moisture, ranging from 22.5 percent at the end of the dry spell to 15.5 percent when standing water occupied all surface depressions.

The Bowen ratio and heat transfer data emphasise the dominance of latent heat fluxes over the freely evaporating and transpiring terrain. However, the transfers were controlled by the synoptic variations which influenced the supply of net radiation and surface moisture levels. Obviously, the latent heat flux was at its greatest when snow ablation maintained the field capacity of the soil and especially in wet cyclonic weather, when surface water was evident. During this rainy period, the lowest Bowen ratio values were recorded (0.22) despite the limited supply of net radiation. Conversely, when the soil was at its driest, the Bowen ratio averaged 0.51 with frequent surges over 1.00 when sensible heat fluxes become temporarily dominant.

Latent heat fluxes exceeded the available net radiation for most of the period and the supplementary energy was provided by ground heat flux, which was characterised by large negative values. The exception was the period 13–16 July, when the driest soil conditions recorded were associated with minimum latent heat flux demands. At this time, the available net radiation was adequate for all heat transfers and a perfect energy balance resulted. The paper concludes with a discussion of Bowen ratio and heat transfers at the study area vis-a-vis other snow-free tundra environments.