Multi-scale interactions in the atmosphere. Application to the prediction of heat waves and urban heat islands
The climate system is described by approaches focusing on Zonal jets, Waves and Eddies, hypothesizing that they act at separate space/time scales. However, recent studies have shown that, for the atmosphere and oceans, wavelike features interact with turbulent cascades and give rise to distinct regimes that are crucial for mixing and dissipation.
In these planetary fluids, turbulent statistics at smaller scales appear to be correlated with the long-time and large-scale field of the same quantity. For example, local, strong fluctuations of temperature are affected by daily, seasonal, and even interdecadal phenomena. The aim of this contribution is to provide physical arguments for this conditioning of the smaller scales by the largest ones. To achieve this aim, we investigate turbulent statistics at each one of a range of scales, for several phases of the large-scale, long-time, phenomena.
Our methodology consists in the use of transport equations for second- and fourth-order moments of the temperature field, at one-and two-points (i.e., filtered at different space scales). The emphasis is on the interaction between the temperature gradient’s large-scale dynamics, which acts as a forcing, and the second- and fourth-order moments – namely the energy and the flatness, or kurtosis – of the temperature field at smaller scales. Terms in the equations are assessed against ERA5 reanalysis data for the wind velocity and temperature fields over the Euro-Atlantic region, at the 500 hPa pressure level. We highlight the nature of different phenomena such as diffusion production, and advection, conditioned by the season and the local behaviour of large-scales, such as the jet stream.
Applications concern prediction of heat waves (e.g., summer 2003) and urban heat islands.
Contact Nathanael Machicoane for more information or to schedule a discussion with the seminar speaker.