Impact of Advection and Entrainment on the Surface Energy Balance in a Valley

Theoretically, the available energy, composed of net radiation and ground heat flux, should equal the sum of sensible and latent heat fluxes. Such a perfect surface energy balance is rarely achieved with field measurements, resulting in a non-zero residual typically on the order of 10-30 % of the available energy, and even larger in mountainous areas. It is now well established that this residual cannot be solely attributed to measurement imperfections. Several effects, including surface heterogeneities and large scale boundary layer eddies have also been reported to cause a non-closure of the surface energy balance. The closure uncertainties become larger over complex, mountainous terrain in the presence of thermally and terrain forced flows. In these situations, advection processes can not be neglected anymore and limited spatial representativeness becomes an issue. Furthermore, entrainment effects by convective thermals can disrupt the sensible and latent heat fluxes at the surface via non-local effects, thus introducing further uncertainties into the causes of the surface energy budget non-closure.

In this poster, we focus our analysis on data obtained during the Terrain-Induced Rotor Experiment in Spring 2006. Measurements were collected in Owens Valley, CA, a long and deep semi-arid valley, with a distinct along-valley gradient in land-use and soil moisture. We analyze 30-min averages of all four energy balance components (sensible heat flux, latent heat flux, ground heat flux, net radiation) collected at 5 m above ground level at the valley floor and at the adjacent western slope. We separate the data set in two categories: fair weather, quiescent conditions, and synoptically forced conditions. We specifically assess the contributions of the long-distance transport of heat and humidity in the along-valley direction and entrainment to the energy balance closure. Using spectral analyses and a network of automated weather stations, we show that the effects of entrainment of dry, warm free-tropospheric air on the surface energy budget non-closure is a function of both the valley flow fetch and location in the valley.