Final Report Summary - PBL-PMES (Atmospheric planetary boundary layers: physics, modelling and role in Earth system)
The planetary boundary layer (PBL) is defined as the lower strongly turbulent atmospheric layer linking the Earth surface with the free atmosphere. The latter is only weakly turbulent, which is why thermal influences, emission of pollutants and other impacts from the surface are blocked within PBL over hours or even days. This is precisely the reason why weather and climate are much more variable in the PBL (our habitat) than in upper layers of the atmosphere. For example, diurnal temperature variations caused by the solar irradiation warming the surface are limited to PBL. Similarly, dust, aerosols, gases, etc. released from ground sources immediately enter the PBL but only gradually penetrate to the free atmosphere through the PBL upper boundary. The PBL heights vary from dozens meters to a few kilometres depending on the intensity of turbulence, which, in turn, depends on the wind speed (the stronger wind, the more intensive turbulence and the deeper PBL) and the static stability. The latter enhances turbulence in unstable stratification, when the surface is warmer than air, and diminishes turbulence in the opposite case of stable stratification. Turbulent and PBL mechanisms control fine features of climate change and extreme weather events, e.g. heat waves, droughts, extreme colds and heavy air-pollution episodes. These phenomena are still poorly reproduced in atmospheric models, which call for advancing the physics and methods of modelling of stratified turbulence and PBLs. Our project responds to this challenge. Its major results are summarised below.
New Energy- and Flux-Budget” (EFB) turbulence-closure theory is developed; and its optimal version is being prepared for implementation into weather-prediction, climate and air-quality models. The theory resolves the problem of the “energetics critical Richardson number” and demonstrates that geophysical turbulence does not degenerate even in supercritically stable stratifications (when smaller-scale flows become laminar) due to the two mechanisms: self-regulation of the buoyancy flux by the counter-gradient heat transfer driven by turbulent potential energy, and efficient exchange between kinetic and potential turbulent energies.
Advanced concept of convective PBL based on the 3-fold decomposition “regular mean flow + chaotic turbulence + self-organised structures” is developed and verified against available data and topical large-eddy simulations. The concept includes (i) non-local heat/mass transfer laws accounting for the enhancing effect of the near-surface structural motions; (ii) advanced PBL-height and turbulent entrainment equations; and (ii) analytical treatment of organised structures. This high-risk/high-gain research effort provides advanced framework for modelling geophysical convection.
Advanced models of stably and neutrally stratified PBLs, including recently recognised conventionally-neutral PBL (archetypal over the ocean) and long-lived stable PBL (archetypal over continents at high latitudes), are derived and employed to develop new surface-flux algorithms accounting for interactions between the surface layer and PBL core. This analysis reveals limits of applicability of the familiar Monin-Obukhov similarity theory. Essential stability dependences of the aerodynamic roughness length and displacement height are established and verified against data from observations over forest- and urban-canopies. A number of real-time LES studies parallel to field measurement are performed and used to enrich outputs from observations, e.g. for turbulent fluxes over sea ice in the Arctic, and for urban air-quality hazards under very stable PBL. Advanced methods of retrieving the PBL height from the ceilometer and sodar observations are developed and employed in the FMI air-quality models CAR-FMI and SILAM. New “Helsinki Urban Boundary Layer Network” has been set up. A new concept of the PBL-climate feedback accounting for strong the PBL thermal sensitivity is 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 parameterizations are being prepared for implementation into weather, air-quality and climate models in the EU: HARMONIE and AROME (>10 EU countries), Enviro-HIRLAM (Denmark), and prospectively in USA: WRF.
New Energy- and Flux-Budget” (EFB) turbulence-closure theory is developed; and its optimal version is being prepared for implementation into weather-prediction, climate and air-quality models. The theory resolves the problem of the “energetics critical Richardson number” and demonstrates that geophysical turbulence does not degenerate even in supercritically stable stratifications (when smaller-scale flows become laminar) due to the two mechanisms: self-regulation of the buoyancy flux by the counter-gradient heat transfer driven by turbulent potential energy, and efficient exchange between kinetic and potential turbulent energies.
Advanced concept of convective PBL based on the 3-fold decomposition “regular mean flow + chaotic turbulence + self-organised structures” is developed and verified against available data and topical large-eddy simulations. The concept includes (i) non-local heat/mass transfer laws accounting for the enhancing effect of the near-surface structural motions; (ii) advanced PBL-height and turbulent entrainment equations; and (ii) analytical treatment of organised structures. This high-risk/high-gain research effort provides advanced framework for modelling geophysical convection.
Advanced models of stably and neutrally stratified PBLs, including recently recognised conventionally-neutral PBL (archetypal over the ocean) and long-lived stable PBL (archetypal over continents at high latitudes), are derived and employed to develop new surface-flux algorithms accounting for interactions between the surface layer and PBL core. This analysis reveals limits of applicability of the familiar Monin-Obukhov similarity theory. Essential stability dependences of the aerodynamic roughness length and displacement height are established and verified against data from observations over forest- and urban-canopies. A number of real-time LES studies parallel to field measurement are performed and used to enrich outputs from observations, e.g. for turbulent fluxes over sea ice in the Arctic, and for urban air-quality hazards under very stable PBL. Advanced methods of retrieving the PBL height from the ceilometer and sodar observations are developed and employed in the FMI air-quality models CAR-FMI and SILAM. New “Helsinki Urban Boundary Layer Network” has been set up. A new concept of the PBL-climate feedback accounting for strong the PBL thermal sensitivity is 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 parameterizations are being prepared for implementation into weather, air-quality and climate models in the EU: HARMONIE and AROME (>10 EU countries), Enviro-HIRLAM (Denmark), and prospectively in USA: WRF.