Monin-Obukhov Surface Layer Similarity Theory (MOST) is a powerful tool for understanding and quantifying turbulent surface fluxes and vertical variations of mean atmospheric parameters near the surface. It is based on the concepts that there is a layer near the surface where turbulent fluxes are virtually constant and the height above the surface is a critical length scale. If MOST is valid, surface fluxes can be determined from a measurement anywhere in the surface layer. MOST forms the basis for the use of drag and transfer coefficients for estimating surface fluxes.
During the field phase of the Surface Heat Budget of the Arctic (SHEBA) project, the authors' group deployed and maintained a 20 meter tower with 5 levels of turbulent and mean wind vector, temperature and humidity sensors. The information from these sensors, and other related measurements, provided an opportunity to examine MOST and its applicability in the central Arctic.
The tower measurements revealed many situations when the heat fluxes at different tower levels had significantly different magnitudes or even opposite signs. One situation was when the lower atmosphere was very stable and the mean temperature and wind vector vertical gradients were relatively large. In these cases, the thickness of the surface layer of the atmosphere collapsed to just a few meters or centimeters. Intermittent or zero turbulent mixing prevented vertical interactions, and therefore heat fluxes at the upper tower levels were not well-correlated with the lower levels. Another situation was when there was a rapid change in surface temperature due to a cloud cover change or diurnal solar heating changes (the latter in spring). In these cases, the effects of the surface change took some time O(1 hour) to reach the upper tower level and while this was happening considerable vertical variations in sensible heat flux existed. A third situation was when the immediate upwind surface temperature conditions were very heterogeneous, in particular, when leads formed upwind of the tower. The leads would create internal boundary layers that sometimes intersected the tower levels and resulted in different fluxes across the internal boundary layer interface.
This study will quantify how common these situations were during SHEBA and what conditions produced them. Typically, estimations of surface fluxes are based on mean or turbulent measurements at just one level. A goal of this study will be to quantify the errors in one-level estimates of turbulent fluxes which may occur due to violations of MOST assumptions in the central Arctic.