Andi Syahid Muttaqin (22412004)Ashneel Chand (20212701)

ATMOSPHERIC BOUNDARY

LAYER

ATMOSPHERIC SCIENCES (SB5224)FACULTY OF EARTH SCIENCES

BANDUNG INSTITUTE OF TECHNOLOGY2013

OUTLINES

o Definitiono ABL Structureo Laminar and Turbulent Flowo Processes in ABLo Importance and Applicationso Summary/Synopsis

DEFINITION

o Atmospheric Boundary Layer (ABL) / is the Layer bottom of the planetary atmosphere formed by the interaction between the atmosphere and the surface (land and ocean) with a time scale of one day or less.

o Usually around 1 km deep.o Temperatures vary diurnally.o The surface influences the ABL by friction and

by heat fluxes.o Characterized by turbulence, which is

generated by wind shear.o All momentum exchange between the

atmosphere and the surface take place through boundary layer.

STRUCTURE OF ATMOSPHERIC

BOUNDARY LAYER

Depth of ABLo Over oceans: varies more slowly in space & time

because sea surface temperature varies slowly respectively due to large specific heat of water

o Over land: varies more rapidly in space & time because surface condition vary more rapidly respectively

o In case of high pressure: the boundary layer tends to be shallower near the center of high pressure regions. This is due to the associated subsidence and divergence.

o In case of low pressure: there is an updrafts and often it is difficult to find the top of boundary layer.

Boundary Layer Stability Condition

o Is determined by Richardson number (Ri).o (is a convenient means of categorizing

atmospheric stability in the boundary layer)

o Ri: > 0 stable= 0 neutral < 0 unstable

o Boundary layer crucially upon its density structure. i.e. whether layer is being heated or cooled from beneath and upon distribution of water vapour.

o Unstable boundary layer-situation arise because of heating from below, boundary layer is vigorously stirred & its properties to be quite well mixed.

o Stable boundary layer- situation arise usually caused by cooling from below, turbulence is suppressed & there is little mixing except in a layer close to the surface.

LAMINAR AND TURBULENT FLOWS

Laminar Flowo Where the fluid moves slowly in the layers,

without much mixing among the layers. (Typically occurs when the velocity is low or the fluid is very viscous).

Turbulent Flowo Turbulent – the apparent chaotic nature of

many flows, which is manifested in the form of irregular almost random fluctuations.

Reynolds Number

o Reynolds number can be used to characterize laminar and turbulent flows:

o NR< 2000 – laminar flow

o NR> 4000 – turbulent flow

o 2000 < NR < 4000 – transition region or critical region.

PROCESSES IN THE ATMOSPHERIC

BOUNDARY LAYER

Processes in ABL

o Factors Influencing the ABL

o Weather Processeso Pollutant Removal

Mechanismo Evapotranspirationo Exchange in ABL

Factors that Influence the ABL

o Energy Budgetso Moistureo Diurnal

Variationso Buoyancyo Shearo Roughness

Layer

Weather Processes in ABL

o Temperature and pressure gradients caused by differential heating force the winds that drive air masses together producing warm, cold and fronts.

o The lifting mechanisms, produce the upward motion which causes the cooling necessary for cloud development to occur and precipitation to form.

o Though, each of these processes are important in the role they play in the production of various weather events.

Orographic Lifting

Frontal System

Source: http://www.shodor.org/metweb/session6/

Pollutant Removal Mechanisms

o Wet deposition: acid rain, acid fog, fog, haze, and smog

o Dry deposition: aerodynamic, sub-layer, and surface

o Chemical reactions: Example reaction of acid rain: SO2 + moisture

H2SO4

Source: http://www.physicalgeography.net/fundamentals/8h.html

Evapotranspiration

o It is used to describe the exhange of water vapor from the surface to the air via water reservoirs, soils, and plant life.

o Evapotranspiration is, therefore, an important process within the atmospheric boundary layer.

o The amount of water vapor in the air varies from 0 to 4 percent by evapotranspiration.

o Evapotranspiration is the combined process of evaporation and transpiration.

Source: http://static.skynetblogs.be

Exchange in ABLoWhat are exchanged

in ABL? Heat Momentum Masses (water

vapour, CO2, biogenic gasses, pollutant, dust, spores, pollen, seeds, smoked).

oWhy does this process occur? Characteristics of the surface change with time. The surface characteristics are different Change the temperature of the air. Change the water content.

http://www.eoearth.org/article/Eddy_covariance_method?topic=49537

Main Causes of the Exchange

o Exchange momentum in ABL is mainly caused by turbulence processes.

o Turbulence causes of highly efficient mixing: 106 more efficient than molecular diffusion.

Source: http://apollo.lsc.vsc.edu/classes/met455/notes/section2/1.html

Example

Unit of Exchangeo The transfer of a quantity per unit area per

unit time is called a flux: Moisture flux Heat flux Momentum flux Eddies also transport

heat, moisture, momentum, pollutant, etc.

o It’s not the mean flow that transport heat, moisture, etc. from near surface up to the boundary layer, this is the role of turbulence.

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Eddy Flux and Stability

Example:Eddy flux is defined by (turbulent heat flux).When w‘ is turbulent part of vertical velocity and θ’ is turbulent part of potential temperature.

''w

Mixing Height/Deptho Because turbulent fluxes vary based on surface

heating and other factors, the height of the ABL also varies.

o The height of the ABL is called the mixing height. o The mixing height is very important to air quality

experts.

IMPORTANCE AND APPLICATIONS

ABL and Urban Climatologyo Urban climate is a mutual relationship of the

urban surface and atmospheric effects that occur on it.

o Conceptual scheme is very important in the study of urban climatology in particular the study of the Boundary Layer.

o The structure of the atmosphere boundary layer is well understood over homogenous rural areas.

o On the other hand, the urban boundary layer requires special treatment.

o This is especially important for nocturnal periods when the atmosphere is stable and leads to the trapping of pollutants near ground level.

Urban Boundary Layer

Diurnal Conditionso During the day, the pollution is releases into the

mixed layer. o The convection within the mixed layer quickly

disperses the pollutants downwind.

o This is caused by rising thermals in some areas and subsidence in others.

o The rapid mixing of particles in the mixed layer is beneficial in that it prevents the build-up of pollution into dangerous concentrations in any one area.

Source: http://www.shodor.org/metweb/session7/focus7.html

Nocturnal Conditionso Pollutant released during the night from a tall stack would spread out

evenly, a process called coning. o If a plume were released from a short stack within the stable layer,

the plume would not disperse very quickly and behave in the process called fanning.

o Coning disperses particles more effectively than fanning. Fanning disperses pollution in the horizontal, but not very effectively in the vertical.

Source: http://www.shodor.org/metweb/session7/focus7.html

Disasters

NOAA-ARL Research and Development

o The Air Resources Laboratory (ARL) uses state-of-the-art methods and techniques, and develops new ones as necessary, to better understand and model the atmospheric boundary layer and air-surface exchange processes.

o Few examples of ARL’s Atmospheric Boundary Layer and Surface-Exchange Research and Development are: DCNet (www.atdd.noaa.gov) Extreme Turbulence Probe (

www.noaa.inel.gov/capabilities/etprobe.htm)

DCNeto The network currently has 10 stations, most of

them on building rooftops, which collect not only the standard meteorological parameters but also measure characteristics of atmospheric turbulence.

o The data have allowed researchers to determine the spatial and temporal fluctuations of horizontal winds throughout the District, as well as to characterize the atmospheric layer immediately above the urban canopy where winds are poorly predicted by meteorological models.

o The main goal of DCNet is to refine understanding of how hazardous trace gases and particles are dispersed across the kind of area where people work and live.

Extreme Turbulence Probeo The ET Probe is designed to measure winds,

turbulence, and air-sea exchanges in conditions with heavy rain and high winds, such as those encountered in hurricanes.

Clear Air Turbulence (CAT)

Source: http://www.youtube.com/watch?v=lxlSZ-SB1XQ

Summary/Synopsis Definition ABL Structure Depth of ABL Boundary Layer

Stability Condition Laminar flow Turbulent flow Reynolds Number Processes in ABL Factors that influence

the ABL Weather Processes in

ABL Pollutant Removal

Mechanisms Evapotranspiration

Exchange in ABL Main causes of the

Exchange Unit of Exchange Eddy and Stability Mixing height/depth Importance and

Applications ABL and Urban

Climatology ARL Research and

Development DCNet Extreme Turbulence

Probe Clear Air Turbulence

(CAT)

Theodore Von Karman (1881-1963)

“There are two great unexplained mysteries in our understanding of the universe. One is the nature of unified generalized theory to explain both gravity and electromagnetism. The other is an understanding of the nature of turbulence. After I die, I expect to God to clarify the general field theory to me. I have no such hope for turbulence.” – Theodore Von Karman

.: Quotations :.

ReferencesAir Resources Laboratory – Atmospheric Boundary Layer and

Surface Exchange Research and Development (www.arl.noaa.gov)

Atmospheric Boundary Layer Structure: (http://lidar.ssec.wisc.edu/papers/akp_thes/node6.htm)

Baklanov A., Grisogono B. 2007. Atmospheric Boundary Layers. Nature, Theory and Application to Environmental Modelling and Security. Springer Science, New York.

Basic Meteorological Process: (http://www.eng.utoledo.edu/~akumar/IAP1/NEWMET.htm)

DCNet: (http://www.atdd.noaa.gov/?q=node/15)Extreme Turbulence Probe: (

www.noaa.inel.gov/capabilities/etprobe.htm)Garrat J.R. 1992. The Atmospheric Boundary Layer.

Cambridge University Press, Cambridge.General Boundary Layer Characteristics and Evolution: (

http://apollo.lsc.vsc.edu/classes/met455/notes/section2/index.html)

Kaimal J.C., Finningan J.J. 1994. Atmospheric Boundary Layer Flows, The Structure and Measurement. Oxford University Press, New York.

Planetary Boundary Layer: (http://kadarsah.wordpress.com/2011/07/01/planetary-boundary-layer-pbl/)

Stewart R.W. 1997. The Atmospheric Boundary Layer. WMO No.523, World Meteorological Organization.

Stull R.B. 1988. An Introduction to Boundary Layer Meteorology. Springer.

Surface Energy Budget Network (SEBN): (http://www.atdd.noaa.gov/?q=node/23)

The Planetary Boundary Layer: (http://www.cmmap.org/learn/climate/energy8.html)

The Planetary Boundary Layer: (http://www.shodor.org/metweb/session7/session7.html)

Urban-Rural Campaign: http://www.engr.ucr.edu/~marko/urban_rural_field_measurments.htm

Washington D.C. Mixing Height Study: (http://www.atdd.noaa.gov/?q=node/79)

Appendices

Fig. Wind velocity profile

CAT – Invisible Trouble

Turbulence Forecast