CE 8231 Atmospheric boundary layer Michele Guala: mguala@umn.edu SAFL: office 382 (612 625 9108) (high probability) CE : office 161 (612 624 7816) class webpge: http://personal.ce.umn.edu/~guala

acknowledge : R. Stull, F. Porte-Agel, A. Liberzon,

Why is it important to study the boundary layer? • We spend most of our lives in the boundary layer • Daily weather forecasts of fog, frost, max and min temperature are boundary layer forecasts • Virtually all water vapor that reaches the free atmosphere is first transported through the boundary layer by turbulent processes • Cloud Condensation Nuclei are lofted into the air from the surface by boundary layer processes (re-suspension, evaporation, clustering, thermal plumes, ...) • Rain and Snow precipitation , flood and tornado hazards

DAY 1: introduction, matlab, syllabus presentation

• Turbulence and gustiness affect architecture and the design of structures: bridge, sky scrapers wind turbines (rotor and gear box). • Wind energy: Wind turbines extract energy from boundary layer winds. • Wind stress on water surface: primary energy source for water currents in lakes & oceans and waves. • Crops are grown in the boundary layer. Pollen is transported by boundary layer processes. • Wind erosion and landscape evolution

Air Pollution • atmospheric transport and diffusion of pollutants • prediction of local, urban, and regional air quality Meso-scale Meteorology • urban boundary layers and the heat island effect • land/sea breezes • drainage and valley flows • development of fronts and cyclones Wind Energy / Alternative Energies Fluid Mechanics & Turbulence in high-Re boundary layers

Agricultural and forest Meteorology / Hydrometeorology • prediction of surface temperatures and frost conditions • dispersion of chemicals, pollution, dust • soil temperature and moisture • evaporation, evapotranspiration and water budget • energy balance of a plant canopy Urban Planning and Management • prediction and abatement of ground fogs • heating and cooling requirements • flow dispersion around buildings • prediction of road surface temperatures and icing

Definition: • The Atmospheric Boundary Layer can be defined as: "that part of the atmosphere that is in direct interaction with the Earth's surface and responds to surface forcing with a time scale of about an hour or less." Scale: • Boundary layer depth is variable, typically between 100-3000 m (Fig. 1.1). • The tropospheric depth is one order of magnitude greater than boundary layer depth • ratio of tropospheric depth to radius of earth: 10 km/6400 km = 0.001 or 0.1% • ratio of boundary layer depth to radius of earth: 1 km/6400 km = 0.0001 or 0.01% !!!

surface forcing: drag, evaporation, transpiration, heat transfer , complex or urban terrain form drag

Variety of scales in the ABL

line of severe thunderstorms evolving along a cold front

dust whirlwind

integral, forcing scale L~ δ Kolmogorov scale η ~ 1mm η / L ~ Re-3/4 where the Reynolds number Re = U L /ν note that Reη ~ η vη / ν ~ 1

River : Re ~ 1m/s * 5m / 10-6 m2/s ~ 5 106

ABL : Re ~ 10m/s * 1000m / (1.5 * 10-5 m2/s)~ 108

Wind tunnel : Re ~ 10m/s * 0.2 m / (1.5 * 10-5 m2/s)~ 3 105

flume: Re ~ 0.5 m/s * 0.2 m / ( 10-6 m2/s)~ 105

heat flux effects: e.g. sea breeze are related to diurnal variation of temperature

NOCTURNAL - DIURNAL VARIABILITY

With density stratification, wind shear creates waves, resembling KH instability. Wave breakings generate peaks of high turbulence in an otherwise smooth velocity field

UTC time > Local time

2PM 12AM 7AM 3PM 11PM 7AM 3PM 11PM 7AM local time

Day Night Day Night

2PM 12AM 7AM 3PM 11PM 7AM 3PM 11PM 7AM

Day Night Day Night

EOLOS field

DAVOS, Switzerland Weissflujoch measuring site

synoptic station

WFJ station

North katabatic winds

Turbulence frictional drag : self organization of turbulent structure

form drag : flow separation around hills, vegetation buildings

An example on the relevance of turbulent fluxes Stoessel et al. WRR 2010

Forcing mechanism: synoptic wind, orographic wind, local convective cells

How do we distinguish between 1) the mean wind variability 2) very large scales of turbulent ?

SPECTRAL GAP

Note that some scientists infer that the spectral gap is the result of nocturnal-diurnal variability

see e.g. convective wind Liberzon et al. 2005

Stop

we have to introduce the concept of virtual potential temperature

ABL by arbitrary definition is set at the cloud base

How to set the interface between clouds and ABL? Important for boundary conditions and pressure gradient conditions. However some situations appear counterintuitive ...

DAY 2: qualitative description of the ABL

Typical day-night structure of the ABL

into the ML Mixing driven by convection and thermals from ground and radiative cooling (from clouds), plus mean shear

stable layer (warm air up) induced by cloud condensation

ML turbulence decays RL is neutrally stratified

or inversion layer

Introduce Virtual Potential Temperature BLACKBOARD

unstable thermal grows

stable capping inversion

LOG

in high pressure region the mixed layer is squeezed, and pollutants get concentrated in a thin layer

Daytime ABL structure

nearly constant virt.pot. T, ML remains neutral

No B.C . at the ground not really a Bound Layer: it is squeezed by SBL but not strongly altered by near surface processes

SBL/NBL: stable air layer (statically) & weak sporadic turbulence (suppressed - occasional burst) 1) occurrence of low level jet/nocturnal jet / supergeostrophic wind (often due to katabatic winds, 1:2 m height > topographic forcing) 2) occurrence of gravity waves

increased shear > TKEP > mixing in the SBL decoupled from synoptic wind

anisotropic, dampened in z, and fluctuating in y (planar meanders or fanning) Note that the SBL dampens mixing between SBL and RL

lofting: smoke released in the RL does not reach the surface

Nocturnal ABL structure

increase SBL

neutral RL decays

growth thermals and ML

both ML and RL are adiabatic (no heat fluxes through the BC and constant VPT)

The Residual layer gets progressively squeezed

Property Free Atmosphere Mostly Laminar. Turbulence in convective clouds, and sporadic clear-air turbulence in thin layers of large horizontal extent Dispersion/dissipation: Small molecular (viscous) diffusion. Often rapid horizontal transport by mean wind. Vertical Transport : Mean wind dominates [slow vertical transport] Winds Winds nearly geostrophic Thickness 8-18 km. Slow time variations.

Property Boundary Layer Turbulence Almost continuously turbulent over its whole depth Dispersion : Rapid turbulent mixing in the vertical and horizontal, strong dissipation due to friction (roughness / separation) Vertical Transport: Turbulence dominates Winds Near logarithmic wind speed profile in the surface layer. Thickness Varies between 100 m to 3 km in time and space. Diurnal oscillations over land