Amazing grass: a numerical study of atmosphere-grass interactions

The usage of roughness length concept in atmosphere-vegetation modeling is convenient due to its simplicity. However, where the roughness length concept often leads to reasonable results, its empirical nature comes also at the cost of biases and parameter uncertainty as parameters become situation-dependent. For tall canopies, therefore, successful attempts have been made to explicitly model atmosphere-vegetation interaction as to avoid roughness length concepts and replace them by more physical parameterizations. For small canopies, such as grass, studies with that philosophy are very few. Yet, similar problems with roughness length concepts exist. Progress was hampered by our inability to probe the microclimate of grass due to its limited vertical extent. But with the appearance of miniaturized sensors, more and more observations of this thin, intriguing layer have become available. Inspired by this, we aim to numerically simulate grass-atmosphere interaction with respect to momentum and heat and moisture exchange. We use direct numerical simulation (DNS) to explicitly simulate flow-obstacle interactions. Flow over rigid and non-rigid (flexible) grass elements are studied for different idealized configurations. We combine those 3D simulations with simplified 1D surrogates and analytical solutions (parameterizations) with the aim to replace the roughness length concept for grass with simple alternatives. It is shown that indeed such alternatives are possible and that, apart from the Reynolds numbers, there are strong physical parallels between flow over grass and flow of tall canopies. By exploiting those similarities, we think that biases due to usage of roughness length concepts can be avoided in the future.