The Diurnal Cycle of Turbulent Fluxes in an East-West Oriented Valley

The diurnal cycles of turbulent fluxes of heat and moisture are strongly related to the diurnal cycle of solar radiation under synoptically undisturbed, clear-sky conditions. In mountainous terrain, the diurnal cycle of solar radiation can deviate significantly from a symmetric curve and vary strongly from site to site. These deviations are caused by differences in the exposure towards the sun resulting from variations in the slope angle and orientation and shading by the surrounding topography. As a first hypothesis, it might therefore be assumed that the daily cycle of sensible heat flux closely follows the strongly site-dependent diurnal cycle of net radiation. In addition, the availability of soil moisture will impact the partitioning of the fluxes into sensible and latent heat.

In this presentation, we will show data from the i-Box measurement sites in the approximately east-west oriented Inn Valley, Austria. The six sites are located within an approximately 6.5-km long section of the 2-3-km wide valley. One of the sites is located on the almost flat valley floor, one site at a mountain top about 1500 m above the valley floor, two sites on the south-facing sidewall, and two sites on the north-facing sidewall.

We will compare the diurnal cycles of the turbulent fluxes at the six sites under synoptically undisturbed, clear-sky conditions to identify differences in the magnitude and the timing of the diurnal cycles. It will be shown that on average differences in the magnitude of the sensible heat flux are indeed strongly related to the location of the sites within the north-south cross section through the valley and thus to differences in solar radiation. A strong day-to-day and site-to-site variability, however, occurs, particularly with respect to the timing of the maximum. The sensible heat flux can thus peak up to a few hours before or after solar noon on otherwise comparable days. It will be shown that these differences are related to the valley-wind circulation.