Impacts of the representation of orographic drag on the climate of an atmospheric model

Many current state-of-the-art climate models show well-known and long-standing biases in representing the atmospheric circulation of Northern Hemisphere midlatitudes. In particular, models typically underestimate atmospheric blocking frequency and have problems in representing the observed properties of the storm tracks. These errors are usually accompanied by, and possibly related to, the models’ inability in reproducing the amplitude of the stationary waves, simulating an overly zonal circulation. Since their introduction in numerical models, it is known that orographic drag parameterizations plays a key role in improving midlatitude circulation. However, they are still considered an important area of uncertainty, and possibly of residual errors, mainly due to the complexity of the air-orography interactions and to the lack of adequate large-scale observational data relative to such processes. In this work, I studied the effects of a new version of the parameterizations of both orographic form drag and gravity-wave drag in the Community Atmosphere Model. I designed a set of experiments to study their physical effects, their interactions, and to test the model response to adjustments of two fundamentals, but unconstrained, parameters of the schemes. It is found that the two parameterizations, while showing a strong mutual interaction, improve the representation of blocking frequency, storm tracks structure, cold and heat waves over Europe, and that the precise values of the parameters modulate the beneficial effects of the schemes. These findings underline the importance of orographic parameterizations to improve climate models adequacy to represent reality, but also the need for a better characterization and a unified representation of drag processes in climate models.