In the absence of a global observational data set of evaporation and soil moisture, a simple model of the land-atmosphere interface advances a 1961 theory.
Key Points & Overview
- Authors have developed a simple model to study land-atmosphere interaction and summertime temperature variability
- Negative feedback between evaporation and surface temperature leads to two evaporation regimes
- Important implications for how transitions between wet and dry land surfaces may impact temperature variability as climate warms.
As the climate warms, heat waves and droughts are projected to increase in frequency and severity. However, an unsolved problem in climate science concerns the distribution of temperatures over land; we know mean temperatures will increase with more carbon emissions, but if the underlying temperature probability distribution changes as well it could increase the chances of extreme events with damaging consequences. Evaporation of liquid water is a primary land surface cooling mechanism and is physically dependent on how much liquid water the soil contains. Unfortunately, observing evaporation and soil moisture is extremely difficult, and to date no global observational dataset of both quantities exists. A prevailing theory proposed in 1961 posited a non-linear relationship between soil moisture and evaporation. The theory held that below a certain critical value of soil moisture, evaporation efficiency is a linear function of soil moisture while above this critical value, evaporation is perfectly efficient and only depends on the amount of energy the land surface receives from the sun. Though this critical value has never been observed, the theory became widely accepted, as the observations that became available in the intervening years confirmed it.
In our study, we derive a simple model of the land-atmosphere interface and use it to show that no critical value of soil moisture is required to generate evaporation behavior akin to the original 1961 framework. The simplicity of our model equations allows us to derive closed form solutions for daily evaporative cooling that demonstrate the distinctive non-linearity of the soil moisture-evaporation relationship without invoking a critical value of soil moisture. So where does the non-linear behavior come from? For liquid water to be evaporated by the land surface, it must be demanded by the atmosphere. The amount of demand exerted by the atmosphere is exponentially proportional to temperature, and when evaporation cools the land surface, it lowers the atmospheric demand for water vapor. This negative feedback between evaporative cooling and atmospheric water vapor demand preferentially damps evaporative cooling when soil moisture is high; when soil moisture is low the atmospheric demand is typically so high that this feedback can’t damp evaporation the same way it can when the soil is wet.