Local flux-profile relationships of wind speed and temperature in a canopy layer in atmospheric stable conditions

Abstract

The particularities of the physics of the canopy layer pose challenges to the determination and use of traditional universal functions so helpful in the atmospheric surface layer. Progress toward "universal-like functions" such as those provided by Monin-Obukhov similarity theory for the canopy layer has been modest. One of the challenges lies in that the assumptions underlying Monin-Obukhov similarity theory do not hold within a canopy layer. This paper thus examines the local flux-profile relations for wind (<i>&Phi;</i>_<i>m</i>) and for temperature (<i>&Phi;</i>_<i>h</i>). It uses three different stability parameters, i.e., <i>h/L(h)</i> at tree top, local <i>z/L(z)</i>, and the local bulk Richardson number (<i>Ri</i>), within a tall forest canopy in nighttime stable (indicated by <i>h/L(h)</i> > 0) conditions. Results suggest that the in-canopy <i>&Phi;</i>_<i>m</i> can be described using the local Richardson number <i>Ri</i>. Furthermore, <i>&Phi;</i>_<i>m</i> is found to increase linearly with <i>Ri</i> in the upper canopy layer for |<i>Ri</i>| < 1. When local |<i>Ri</i>| > 1, |&Phi;_<i>m</i>| decreases with |<i>Ri</i>| in a power function, a result consistent for all levels of measurements within the canopy. When both local <i>&Phi;</i>_<i>h</i> and local <i>Ri</i> are positive, i.e., the local downward turbulent heat flux is consistent with the local temperature gradient, the local <i>&Phi;</i>_<i>h</i> increases with the local <i>Ri</i> when <i>Ri</i> < 1. However, <i>&Phi;</i>_<i>h</i> does not change with <i>Ri</i> (or much more scattered) when <i>Ri</i> > 1. The relationship between local <i>&Phi;</i>_<i>h</i> and <i>Ri</i> disappears when counter-gradient heat transfer occurs in strongly stable conditions. A self-correlation analysis is used to examine the influence of self-correlation and the physical meaning of these relationships.