Small-scale albedo-temperature relationship contrast with large-scale relations in Alaskan acidic tussock tundra

摘要

Arctic tundra vegetation is characterized by an extreme heterogeneity at a small spatial scale. Optimizing the parameterization of tundra ecosystems in climate models requires detailed knowledge and understanding of soil-vegetation-atmosphere feedback mechanisms at different spatial scales. We used a mobile multi-sensor platform for observing variable spectral and thermal responses of different representative vegetation communities within two 50 m long transects in August 2010. The observations sites are located on the North Slope of Alaska. The sensor platform was attached to a cable set up at a height of ~1 m above ground. The data were aggregated to distance increments of 45 cm along the transects and standardized (mean-centered) to account for observation date-specific offsets in measurements that were related to specific light and weather conditions but not to the local vegetation surface. A relative increase in the albedo of 0.01 (1%) was related to an increase in radiometric surface temperatures of 0.1 to 1 K, which is the inverse of the generally accepted surface temperature-albedo relationship observed at larger spatial scales. We explain this finding with cooling effects of the albedo-influencing surface wetness which primarily results from moss and soil evaporation. This cooling effect dominates over other more general heating effects that can be expected over surfaces with lower albedo under absence or near-absence of evaporation. Our findings are also supported by NDVI measurements. These locally inverted temperature-albedo feedbacks need to be considered in climate models that resolve Arctic environments with a high abundance of moss covers. Our results show that frequent observations of different tundra ecosystems using mobile multi-sensor platforms can provide data critical for understanding the land-atmosphere-interactions for the Arctic and the global system.