Upslope flow of warm air is associated with surface heating and significant turbulent mixing. This mixing transport stronger momentum downward toward the surface which can eliminate the upslope flow. As a result, daytime upslope flows are self-limiting and are less persistent than downslope drainage flows. Nonetheless, upslope flows can be well defined in some circumstances. Upslope flows are strongest on slopes directly facing the sun. Morning upslope flows on east facing slopes are more common than afternoon upslope flows on west facing slopes when the boundary layer is deeper with more downward mixing of larger-scale flow. However, like all simple rules in micrometeorology, exceptions are numerous.
For simplicity, we continue with the example of a south facing slope at midday. Steeper south-facing slopes are generally hotter and drier in summer compared to weaker slopes, not only because they absorb more solar energy, but also because they often contain thinner soils that dry out more quickly. Less evapotranspiration leads to greater surface heating compared to deeper moister soils.
The temperatures on south facing slopes do not increase unabated but are constrained by motions which mix heat away from the surface.
1. larger scale (ambient) air flow on a scale larger than individual valleys and ridges, and associated vertical mixing
2. upslope flow generated by heating on the south-facing slope
3. direct vertical convection of heat out of the valley is thought to be less important but less efficient.
Isolated hills will be better ventilated even with steep slopes because they are more exposed to the larger scale flow. More narrow valleys are less ventilated (more sheltered) from the large-scale flow and the south-facing slopes are expected to be hotter compared to that on isolated south-facing slopes. Air temperatures farther up the valley may be warmer than at the mouth of the valley because of less ventilation.
The microclimates of sloped terrain have been examined mainly for large amplitude very well defined terrain features and much less understood for small less organized hills.The aspect of the slope and slope magnitude can strongly influence the diurnal variation of the surface temperature with clear skies and weak large scale flow.
As an example, a 10 degree south facing slope at 45 degrees N (e.g., Corvallis, Or, USA) receives the same solar radiation (energy) as a flat land surface at 35 degrees N (e.g., Central California). Assessing the microclimate and influence of increased surface temperature on evapotranspiration requires information on the soil type, soil water distribution, stomatal behavior of the vegetation and meteorological variables such as cloud cover, wind speed and direction, and relatively humidity.
Upslope flows are generated with a variety of scenarios. This sketch shows upslope flow beginning on the south facing slope above a cold pool on the valley floor that has not yet been entirely eliminated. The valley floor remains cooler during the entire day because of less solar radiation and generally deeper more moist soils with greater vegetation. The upslope flow eventually encounters northerly flow (in this scenario) near the top of the slope. The microfront between the upslope flow and ambient flow shifts up and down the slope over periods of minutes or tens of minutes, probably due to variations in the northerly flow. As times the upslope flow can be temporarily eliminated.
With partial tree cover, surface temperatures may actually be hotter in spite of partial shading. The tree canopy reduces upslope circulations and mixing with the ambient flow that exports heat away from the slope. With more complete canopy cover, the shading dominates and the surface is cooler.
Understanding upslope flows and associated daytime microclimates on sloped surfaces are helpful for restoration and maintenance of oak savanna.