Methods and limitations in validation of footprint models

This paper assesses the progress made over the last decade in validating footprint models, and highlights shortcomings which need addressing. This paper also presents alternative methods of validating footprint models. The vigorous expansion of footprint models was spurred by the development and subsequent widespread application of primarily analytical solutions to the advection–diffusion equation and Lagrangian simulations. In this paper, we draw attention to the range of experimental conditions supporting both a judicious use and application of these methods. We discuss present limitations restricting footprint model applicability, mostly for sites which depart from ideal conditions and model conditions which deviate from their original assumptions. Intercomparisons of footprint models reveal differences amongst the different footprint models, prompting subsequent recent validations of footprint models. Despite the few existing studies, the issue of validation remains an outstanding problem in micrometeorology. In this paper, three concepts are discussed and compared: (1) the use of artificial tracer gases; (2) the use of natural sources of scalars; (3) the presence of obstacles in the flowfield and their influence on the footprints. Finally, the conditions of applicability of each method are discussed.