ON SURFACE-ENERGY BALANCE OVER THE NORTHERN WETLANDS .2. THE VARIABILITY OF THE BOWEN-RATIO

An energy balance model for land surfaces with small-scale temperature and wetness heterogeneity [Guo and Schuepp, this issue] was used to study the variability of the Bowen ratio over northern wetlands. It was shown that the variation of the Bowen ratio can be caused by three factors: change in primary partition of surface available energy, local heat advection, and changes in atmosphere-surface conditions. The first factor is the most sensitive one because it is directly proportional to the change in the ratio of dry to wet patch size. Due to its scale-dependent, nonuniform reallocation of a portion of surface available energy between patches, the second factor adjusts the spatial variation of Bowen ratio in two aspects. On the one hand, it reduces the Bowen ratio variation caused by the change in the ratio of dry to wet patch size. On the other hand, it introduces the variation of the Bowen ratio caused by changes in absolute patch dimension, when the ratio of dry to wet patch size is constant in a given area. In model simulations for the case of equally sized dry and wet patches the latter influence was shown to generate variation in the Bowen ratio between 0.6 and 1.2, when patch dimensions varied up to 100 m. This helps explain the variability of the regionally averaged Bowen ratio observed by aircraft over different parts of the northern wetlands in the NOWES project. The third factor may affect both the absolute values and the range of the Bowen ratio by modifying directly the primary energy budgets and, indirectly, the local heat advection.