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Final Activity Report Summary - PBL-TMRES (Planetary boundary layers - theory, modelling and role in Earth systems)

Further improvement of General circulation models (GCMs) used in climate, weather and air quality forecasting is impeded until parameterization schemes for the turbulent fluxes at the Earth's surface, specifying the boundary conditions for GCMs, remain insufficiently advanced. The fluxes are controlled by physical processes in comparatively shallow, stratified, rotating, and turbulent flows called Planetary boundary layers (PBLs). During the last decades, GCMs have experienced very fast progress inspired by close attention of the society to human impacts on the environment in the conditions of climate change. However, GCMs still employ PBL parameterisation schemes based on traditional theories assuming the steady state, horizontally homogeneous regimes with not very strong stratifications. These schemes were designed for low-resolution GCMs with grid sizes > 100 km incapable of resolving any extremes, and became the weak point in modern very high resolution (~1 km) GCMs. Poor PBL schemes almost kill potential capabilities of GCMs to reproduce fine features of microclimate and extreme weather events or air pollution episodes. Presentation of PBL physics in current university courses limits to traditional theories and does not show ways to improve PBL schemes. The EXC project PBL-TMRES makes up for these deficiencies by advancing the PBL theory and parameterization updating the course of PBL physics, and dissemination of new knowledge to as wide audience as possible.

For the stable stratification, the classical theory applies only to short-lived, nocturnal PBLs typical of mid-latitudes. To cover long-lived, high-latitudinal and polar PBLs, a new non-local theory is developed, empirically verified, and used in the new PBL scheme. Representation of the turbulence energetics in operational Reynolds averaging Navier-Stokes equation (RANS) modelling, traditionally limited to the sole use of the Turbulent kinetic energy (TKE) budget equation, is corrected by involving the turbulent potential and total energies (TPE and TTE = TKE + TPE). This approach eliminates the turbulence cut-off problem (solved only heuristically in the TKE closures) and provides a hierarchy of energetically consistent closures applicable to any stable stratification.

Perturbation and spectral analyses of turbulent convection are performed to clarify the nature of large-scale coherent structures. The latter are recognised as principally regular flow patterns caused by the inverse energy cascade, in contrast to the direct cascade in usual turbulence. On this basis, new heat and mass transfer laws are derived and empirically validated, which allowed improving the convective PBL scheme.

Essential stability dependence of the roughness length is obtained theoretically, confirmed experimentally and recommended for use in modelling applications. A state of the art radiation scheme is further advanced and used in high-resolution modelling of the PBL type meso-scale flows over complex coastal and ice-covered surfaces - with the final goal to parameterise the integral effect of unresolved circulations in lower-resolution GCMs.

Educational activities include:
(i) development and presentation of a new lecture course on PBL-TMRES by Chair holder (CH) - short version is give in 2005 and extended version, in 2006;
(ii) modernisation of the meso meteorology course by Scientist-in-charge (SiC) - during 2005-2006;
(iii) half a hundred invited lectures and six short courses by CH and SiC in Europe and beyond;
(iv) two international summer schools: 'PBL over complex and vegetated land surfaces' (4-9.06.05, Sodankyla, Finland) and 'Air-sea interaction' (28-31 August 2006, Helsinki, Finland) including series of lectures by CH and SiC;
(v) NATO Advanced Research Workshop involving world experts in boundary-layer meteorology (57 researchers and students from 21 countries) and covering all aspects of PBL-TMRES (18-22 April 2006, Croatia; selected lectures and presentations are published in a special issue of international journal 'Boundary-layer Meteorology');
(v) Final, international 31-hour course on PBL-TMRES (29 Mary - 1 June 2007, Finland) involving 15 lecturers and 33 students: 7 from Finland and 26 from 10 European countries, Russia and United States;
(vi) new textbook on PBL-TMRES by CH and SiC focusing on the aspects required in modelling applications (to be published by Cambridge University Press, United Kingdom).

International cooperation on PBL-TMRES gave rise to new ongoing projects: educational EU TEMPUS 26005 and networking NORDPLUS neighbour 177039/VII; and initiated seven research proposals to Finnish, Danish and Norwegian national foundations.

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