Hands-On: What Controls Surface Evaporation?

In this chapter, we systematically study the coupling between the surface and atmosphere. We select evaporation as a key process in linking the coupling between surface and atmosphere. By analyzing the sensitivity of the latent heat flux to other variables and parameters, we can thus learn the dependence of evaporation on, for instance, soil moisture availability, warm and dry air entrainment, and associated feedbacks. These variations have a direct impact on boundary layer growth and on the lifting condensation level, key factors in the onset of clouds. In turn, all these dynamic processes control concentration levels in air quality by influencing the dilution capacity of the ABL and the exchange between the ABL and free atmosphere.

Since there are several processes involved that depend on surface properties and atmosphere conditions, it is convenient first to study separately the effect of each process. We propose the following sequence of numerical experiments specifically addressed to understanding the different relationships and effects in the land-atmosphere system. The following aspects are investigated:

1. Surface properties influencing the atmosphere: sensitivity to soil moisture and dew

2. Atmosphere conditions influencing the surface: sensitivity to mixed-layer thermodynamic conditions and to dry-air entrainment into the ABL

3. Onset of clouds.

In order to answer these questions, we design a set of numerical experiments. We take as a reference the typical characteristics of a mid-latitude site at the end of the summer. In doing so, we support our experiments with previous studies that have analyzed in depth the interaction between land and atmosphere (de Bruin, 1983, McNaughton and Spriggs, 1986, van Heerwaarden et al., 2009, van Heerwaarden et al., 2011). For vegetation, we provide typical values for the following vegetation characteristics: short grass, broadleaf trees (for instance, oak or chestnut), and needle-leaf trees (for instance, conifers). For soil we assume typical values of a sandy loam soil, but the user can modify these settings according to his or her interests to determine the sensitivity of evaporation to other soil types such as clay or sand.