Better models of turbulence to improve weather forecasting
The planetary boundary layer (PBL) is the strongly turbulent atmospheric layer linking the Earth's surface with the free-flowing atmosphere above. Turbulent and PBL mechanisms control features of climate change and extreme weather events such as heat waves, droughts, extreme cold and air pollution episodes. These phenomena have been poorly reproduced by existing atmospheric models. The PBL-PMES (Atmospheric planetary boundary layers: Physics, modelling and role in Earth system) project used a new conceptual model for PBLs. Turbulence can be seen as having three components: regular flow, chaotic turbulence and self-organised structures. Self-organized structures, like convection cells, are the components that have most often been poorly handled. New energy- and flux-budget (EFB) turbulence-closure theory was developed and prepared for implementation into weather prediction, climate and air quality models. The theory shows that geophysical turbulence does not degenerate even in supercritically stable stratifications (when smaller-scale flows become laminar) due to two mechanisms. The buoyancy flux is self-regulated by the counter-gradient heat transfer driven by turbulent potential energy, and there is efficient exchange between kinetic and potential turbulent energies. The concept of convective PBL based on the three-fold decomposition has been developed and verified against available data and topical large eddy simulations. Advanced models of stably and neutrally stratified PBLs, including the recently recognised conventionally neutral PBL (typical over oceans) and long-lived stable PBL (typical over continents at high latitudes), have been derived and employed to develop new surface-flux algorithms that account for interactions between the surface layer and the PBL core. A number of real-time studies parallel to field measurements were performed and used to enrich the outputs from the observations, including for turbulent fluxes over sea ice in the Arctic and for urban air quality hazards under very stable PBL. A new concept of PBL-climate feedback, which accounts for strength of PBL thermal sensitivity, was developed and employed to explain up to 70 % of the observed temperature trends and variability of climate change at high latitudes. The EFB closure and PBL parameterisations are being prepared for implementation into weather, air quality and climate models in several EU countries and the United States.
Keywords
Atmospheric turbulence, convective cells, planetary boundary layer, climate change, PBL-PMES
