Relating nocturnal mesoscale fluxes to scalar vertical profiles

In stable boundary layers, mesoscale fluxes may constitute an important portion of total scalar exchange. Often, they show erratic behavior, with no clear dependence on typical scaling parameters. Furthermore, they may be of either sign, even in cases when there is a preferential direction for overall turbulent exchange. Their magnitude may greatly exceed the total turbulent flux. In many cases, the mesoscale fluxes tend to average out over a large number of records, so that their effect on total scalar exchange is reduced. However, there are cases when systematic mesoscale fluxes have an important effect on the total flux.

The present study consists of two parts. In the first, data collected from a deforested site in the Amazon region from the LBA project, is analyzed. Consistently, large mesoscale fluxes of both sensible heat and carbon dioxide were found. These fluxes do not average out over a large number of records. During the wet season they have the same sign as does the turbulent flux, while they are opposed to the turbulent exchange during the dry season. The vertical carbon dioxide profile has a strong correlation with the direction of the turbulent portion of the fluxes. We observed negative CO2 fluxes when the gas concentration increases with height, but this finding applies only during the dry season.

In the second part of the study, mesoscale fluxes and vertical profiles measured at a 30-m tower, from the FLOSSII project are compared. This grass-covered site is located in northwest Colorado. Seven levels of eddy correlation are available, so that the vertical structure of the mesoscale events can be inferred and the vertical scales that control the mesoscale exchange can be identified.