8 Atmospheric Boundary Layer

Anandakumar Karipot

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1. Learning outcomes
2. Introduction
2.1. Definition and features
2.2. Significance of ABL
3. Vertical structure and diurnal evolution
3.1. Vertical structure
3.2. Diurnal evolution of ABL
3.3. Convective boundary layer (CBL)
3.3.1. Convective surface layer
3.3.2. Mixed layer
3.3.3. The entrainment zone
3.4. Stable boundary layer
4. Internal boundary layer
4.1. Urban boundary layer
5. Surface energy balance (SEB)
6. Summary
  1. Learning outcomes
  • After studying this module, you shall be able to:
  • Distinguish Atmospheric Boundary Layer (ABL) from rest of the troposphere Understand diurnal evolution and structure of the ABL
  • Identify important characteristics of the ABL
  • Learn internal boundary layer formation due to heterogeneous surface Get familiar with surface energy balance
  1. Introduction

2.1.  Definition and features

 

The Atmospheric Boundary Layer (ABL) is the lowest part of the troposphere which got adapted to the underlying surface through turbulent exchange between the surface and atmosphere. The ABL responds to the surface forcings with a timescale of 1-2 hours through highly efficient turbulent mixing processes. The ABL undergo large variability in depth, from few tens of meters to more than 3000 m, depending on the diurnal period, atmospheric conditions, season, geographical location, land/ ocean and synoptic influences. The ABL is also known as (Planetary Boundary Layer, PBL). The deep tropospheric layer above the boundary layer (BL) is called free atmosphere. The ABL height depends on surface heating rate, strength of winds, surface roughness, free atmospheric characteristics, presence and amount of clouds, presence of moisture, topography, etc. In ABL, the flow field is strongly influenced directly by interaction with the earth’s surface.

 

Typical ABLs over the ocean are shallow with small diurnal variations compared to that over land. This is due to the near-constancy of SST during the diurnal period due to large heat capacity of water, transmission of solar radiation to larger depths and mixing and transport of ocean water from one location to other. However, daily average ABL depth over ocean may exceed that over land due to shallowBLs over land during nighttime. The ABLs over high pressure regions are with well-defined vertical structure and shallow, due to dominant descending motions. The BLs tend to be deeper over low pressure regions attributed to convergence and rigorous ascending motions. However, formation of clouds in moist conditions complicates the vertical structure.

 

The most important characteristic of ABL is that it is almost always turbulent, in contrast to rest of the atmosphere. The turbulence in the ABL is generated mainly by means of surface forcing, and the two most important forcing mechanisms are:

  • Surface radiative heating or cooling leading to vertical and horizontal temperature gradients and buoyant motions. The turbulence thus generated is categorized as convectively induced turbulence.
  • Frictional drag close to earth’s surface introduces shear in the horizontal wind, causing formation of mechanically induced turbulence.

The turbulence contributes to vertical exchange of momentum, heat, water vapour and trace gases between Earth’s surface and atmosphere, though the mean vertical velocity is zero. Comparison between ABL and Free Atmosphere are presented in the table below.

 

Table 1. Comparison between Atmospheric Boundary Layer and free atmospheric characteristics

 

2.2.Significance of ABL

 

Some of the important aspects of ABL are:

  • ABL is the lowest layer of the troposphere which connects earth’s surface to rest of the atmosphere.
  • Nearly all the energy and moisture that drives weather and general circulation enters the atmosphere through the boundary layer.
  • Boundary layer conditions influences air pollutant transport and dispersion and thus the air quality.
  • Aerosols vital for cloud formation are produced and transported in the BL, and often clouds form in the BL layer itself.
  • Sources and sinks of major greenhouse gases responsible for global warming resides in the BL and underlying surface.
  • Human beings and other living organisms on earth spend most of their life within the BL.
  1. Vertical structure and diurnal evolution

3.1. Vertical structure

 

The ABL can be broadly divided into two regions as shown in Fig. 1: i) the surface layer or inner layer and ii) the outer layer. The surface layer extends from the surface to about 10% of the depth of the ABL. The lowest few millimeters of surface layer in which molecular diffusion dominates over turbulence is known as the micro layer or interfacial layer. Above the interfacial layer, the exchange of momentum and scalars in the vertical is governed by turbulent eddies. Other features of surface layer are: flow features are governed by surface friction, Coriolis force is very small and cross-isobaric flows are common. The turbulence characteristics in the lower part of surface layer are directly influenced by roughness elements such as grass, plants, trees or buildings. This layer is known as roughness sub-layer, which usually extends upto two to three times the average height of roughness elements.

 

 

The outer layer, also referred as Ekman layer in neutral stability conditions, is the layer extending from the top of the surface layer to the top of the ABL. The size of dominant turbulent motions can be of the order of the depth of the BL. At the top of the outer layer, the BL is capped by an entrainment zone in convective conditions and capping zone in stable conditions. The neutral Ekman layer is characterized by turning of wind with height as the effect of friction diminishes and the wind approaches its geostrophic value.

 

3.2.Diurnal evolution of ABL

 

The evolution of BL over a land surface (Fig. 2) can be briefly explained as follows: Shortly after sunrise, surface heating triggers development of turbulent eddies near the surface, causing formation of Convective Boundary Layer (CBL). The CBL grows in depth and usually reaches maximum in early afternoon hours. The CBL consists of a convective surface layer, Mixed Layer (ML) above and an Entrainment Zone (EZ) at the top. In the evening hours, the surface cooling creates a Stable (nocturnal) Boundary Layer (SBL/ NBL) close to the surface, depth of which may grow as night progresses. Above SBL lies the residual layer (RL), basically a leftover part of the daytime mixed layer. A capping Inversion overlies RL. The SBL also has a thin surface layer.

 

 

During a typical diurnal cycle, the ABL evolves through a 4-phase process:

 

i)  Formation of a shallow ML after sunrise, followed by slow growth in the initial morning hours due to weak surface forcing and presence of stable layer above.

ii)   Rapid ML growth during later part of morning once the top of ML reaches the base of residual layer. Energetic turbulent eddies, known as thermals, enhances entrainment of free atmospheric air into ML.

iii)  Deep ML of nearly constant depth by the early afternoon hours followed by decrease in depth.

iv)  Formation of a stable boundary layer (SBL) at the surface close to sunset due to radiative cooling of the surface and decay of turbulence. The weak turbulence generated by wind shear help SBL to grow during the course of night, but the depth remains much smaller than the daytime BL. A residual layer forms above the stable boundary layer.

 

The convective and stable boundary layers are discussed in sections 3.3 and 3.4 respectively.

 

3.3.Convective boundary layer (CBL)

 

Convective boundary layer (CBL) forms over land during daytime when surface heating is strong, and over oceans when the air near the sea surface is colder than the sea surface temperature. The CBL consists of a convective SL (~10 % of BL), followed by mixed Layer (ML, ~ 35 to 80 % of BL) and an Entrainment Zone (EZ, ~ 10 to 60 % of BL).

 

•  The depth of the ML and EZ are highly variable.

•  EZ is usually a stable layer at the top of the ML and acts as a lid to rising thermals. It inhibits free mixing of BL air with FA, thus confines BL air and pollutants within.

 

The CBL growth is through the process of entrainment. When a rising large turbulent eddy (thermal) hits the inversion at the BL top, it becomes negatively buoyant, but will continue to rise for a short distance into the free atmosphere due to its vertical momentum, which is called overshooting. This thermal generally returns to the mixed layer without much mixing with the (non-turbulent) free atmosphere above. The subsiding thermal drags free atmospheric air into the ML, which gets mixed there due to the turbulence within ML. The net effect is the entrainment of FA air into the ML, resulting in the growth of ML.

 

3.3.1.    Convective surface layer

 

Some of the important features of convective SL are:

  • Gradients of temperature, moisture, pollutants, other scalars and wind can be very large in the surface layer.
  • The lapse rate within the convective surface layer tends to be super-adiabatic.
  • Vertical fluxes of heat, moisture and momentum are approximately constant (varies < 10 % with height) during the daytime hours.
  • Evapotranspiration is a dominant exchange mechanism if the surface is wet.
  • Organized turbulent motions in the surface layer are known as plumes or coherent structures having diameters of the order of the surface layer depth, i.e., ~ 100 m.

3.3.2.   Mixed layer

  • Turbulence in ML is generated mechanically by wind shear or convectively by buoyancy. Convectively generated turbulence dominates ML.
  •  Large column of rising buoyant air in the mixed layer are known as thermals. Convective circulation including both thermal updraft (narrow regions) and associated downdraft (broad regions) have horizontal scales of the order of 1.5 zi, where zi is the mixed layer depth.
  • Mean vertical velocities up to 5 m s-1 are often observed in rising thermals.
  • Wind speeds are nearly constant with height and sub-geostrophic in the ML.
  •  Virtual potential temperature shows a constant profile in the middle part of the mixed layer, nearly adiabatic in this region.
  • Mixing ratios tend to decrease gradually with height, even within the center portion of the ML.
  •  Gradients of atmospheric variables are small in ML attributed to convective mixing by large eddies.
  •  Mixed layer top zi is often defined as the level of most negative heat flux, mostly the middle of the entrainment zone where inversion is strongest. This is the altitude where 50 % of the air has free atmospheric characteristics on a horizontal average.
  • Convective time scale, the time taken for air to circulate between the surface and top of ML, is approximately 15-20 minutes.

3.3.3.    The entrainment zone

  •  Mixed layer grows by entrainment of FA air through EZ, with the help of overshooting thermals.
  •  Entrainment brings down potentially warmer and usually drier, less turbulent air into the mixed layer from above. This air is then well-mixed with the ML air.
  •  Velocity with which FA entrains into ML is called entrainment velocity, which is governed by turbulence intensity and strength of capping inversion.
  •  EZ is thinner when stronger temperature inversion caps the BL (morning hours, after sunrise) and thicker when turbulence and thermals are more vigorous.
  • Top of the entrainment zone is defined as the top of the highest thermal within a horizontal region containing several thermals.
  • Bottom of EZ is not well-defined. Often taken as a horizontal plane at which 5 to 10 % of the air has FA characteristics. EZ transforms to capping inversion at night.

3.5.  Stable boundary layer

 

The SBL forms when surface is cooler than the overlying air, which usually occurs at night. The advection of warmer air over a cooler surface can also generate stable boundary layers. Surface temperature inversion (temperature increasing with height) is a prominent feature of the SBL. The important features of SBL are:

  •  Stable layer can be turbulent and well-mixed, non-turbulent or intermittently turbulent (see examples in Fig. 3) depending on the balance between buoyancy damping and mechanical generation. If mechanical generation (by wind shear) exceeds buoyancy damping, SBL can become turbulent. Turbulence decreases with height in the SBL.
  • Decoupling of the flow from surface may occur in the highly stable SBL and such SBL is called z-less boundary layer (with no clear height dependency in BL characteristics).
  • Pollutants emitted into the stable atmosphere are found to disperse very little in the vertical, instead travel far downwind horizontally.
  • Nocturnal LLJ, occurrence of a maximum in the wind speed profile with speeds in the range 10 to 20 m s-1, are often found in SBL. The jet nose areusually located between 100 to 500 m above the surface. A night with LLJ formation and decay are depicted in Fig. 4.
  • LLJs may reach super geostropic wind speeds, but not necessarily super geostropic always. Important causes for the LLJ formation reported are: Baroclinicty, land and sea breezes, mountain and valley winds and inertial oscillation.
  • Enhanced wind shear in presence of LLJs helpgenerate turbulence in the SBL. In some situations, higher levels in the SBL may be more turbulent than the layers close to surface and such BLs are referred as upside-down boundary layer.

4. Internal boundary layer

 

Perfectly horizontally homogeneous surface conditions in the nature can be expected only

 

over ocean, desert, extensive forests, grass lands and vast agricultural fields. Over such surfaces, the BL will also be horizontally homogeneous and in equilibrium with underlying surface characteristics. However, very often, different types of surface in homogeneities exist in the nature, for e. g.,

  •  Discontinuities between two types of surfaces (e.g. land and water bodies).
  •  Transition from rural to urban or urban to rural.
  •  Forest – agricultural field interface.
  •  Bare soil and forest, and hills and valley.
  •  Complex terrain (Hills/Mountains/ Valleys)

 

The changes in surface characteristics may be sudden or gradual and may be patchy or large. The in homogeneities may be in just one surface characteristic such as roughness, temperature, wetness/moisture content or a there may exist in homogeneities in combination of the above characteristics.

 

When air flows across a discontinuity in surface property (heat, roughness, etc. or their combinations), the BL gradually gets adjusted to the new surface property downwind from the discontinuity. This modified layer of the boundary layer is known as the Internal Boundary Layer (IBL). It is called ‘internal BL’, because it forms within an existing boundary layer. There are two major IBL categories based on surface properties: i) IBL forming due to differences in surface roughness and ii) thermal internal boundary layer due to differences in thermal characteristics and associated modification in heat flux.

 

The depth of IBL increases gradually as the air moves from one surface to the other. The IBL growth (vertical) will depend on the surface characteristics and atmospheric conditions (e.g. wind speed, stability). The distance downwind from a change in surface characteristics over which IBL forms is called fetch. In order to make meaningful measurements representative of the surface of interest, surface layer micrometeorological measurements should be performed within IBL. A measurement height to fetch ratio of 1:100 is often considered while conducting measurements.

4.1.  Urban boundary layer

 

The urban boundary layer (UBL) is an internal boundary layer which develops over urban areas due to both roughness and thermal contrast with the upwind rural areas. As flow from rural approach urban areas, air is progressively warmed and depth of the urban IBL increases downwind. The thermal effects dominate during calm wind conditions, whereas roughness effects dominate during windy conditions.

 

Important UBL features are:

 

• Rising motion at the city center and subsidence over the surrounding areas. This is associated with radially inward flow toward the city center at lower levels and outward at the upper levels.

•  Daytime BL is more convectively turbulent, warm, dry, polluted and with enhanced depth.

•  The winds are at reduced speed due to increased roughness introduced by buildings.

•    At night, thermal IBL may develop over urban areas as stable cool air from rural areas move over warm urban areas. The IBL over city center can be much thicker than the nocturnal boundary layer over rural areas upwind.

•  The urban heat island effect can destroy nocturnal inversion over urban areas. Diurnal variations in stability over urban areas can be small and BL may remain mostly well mixed even at night.

•   Mean wind drifts excess of temperature, pollutants and deficit of humidity over urban areas to several kilometers downwind – known as urban plume. The plume can be as wide as the city.

• Turbulence is more intense in the urban boundary layer as both thermal and mechanical contribution is large.

 

5. Surface energy balance (SEB)

 

The surface energy fluxes drive diurnal variations in the atmospheric boundary layer, and it is a common practice to assess the magnitude and variation of energy fluxes at the surface while studying ABL characteristics. Primarily, surface energy balance analysis provides information on the partitioning of available energy, i.e. net radiation, into other forms of energy, i.e., sensible heat, latent heat and ground heat flux components. Large spatial and temporal variability from surface to surface, geographical locations, seasons, etc. are common for energy fluxes.

 

The surface energy balance for a homogeneous bare ground surface can be expressed as (with all terms in W m-2):

RN = H + LE + G

 

where, RN is the net radiation (short wave + long wave), G is the ground heat flux density, H is sensible heat flux density and LE is the latent heat flux density. LE = E, where E is the moisture flux (evaporation rate) has units kg m-2 s-1 and =2.5 106 J kg-1 is the latent heat of evaporation. Certain sign conventions are followed while assessing the energy balance: Radiation is taken as positive if directed towards the surface, whereas all other fluxes are taken positive when directed away from the surface.For a vegetated surface, E is evapotranspiration = evaporation + transpiration

 

Based on the above sign conventions, all the surface energy budget components are positive during daytime and negative during nighttime, in general.

 

 

The net radiation consists of solar and terrestrial radiation. The solar radiation varies largely diurnally and seasonally, the later depending on the geographical location. In addition, presence and characteristics of clouds and aerosols also have large influence on solar radiation. The surface albedo is decided by surface type, soil cover such as vegetation, manmade surfaces, ice, snow, etc. The terrestrial radiation depends on clouds and its characteristics, green house gases and atmospheric temperature. Sensible heat transport arises from vertical temperature differences in the ABL and the heat transport between surface and the boundary layer takes place through turbulent mixing. The LE is related to the vertical gradients in the humidity, and G results from surface and sub-surface temperature gradients. G tends to be the smallest term in the surface energy balance; often neglected because over the course of a day, it can average to zero. The H will be more than LE over dry surfaces and vice versa over wet surfaces.

 

Energy balance closure is expected under ideal surface and atmospheric conditions with accurate measurements of flux components. However, the SEB closure is difficult to attain experimentally owing to various errors in the measurement, such as instrumental errors, not meeting underlying principles of measurements, assumptions involved in flux estimations and in homogeneity of surfaces.

 

The relative contribution of H and LE in surface energy balance is often expressed through a measure known as Bowen ratio (β), which is defined as the ratio of sensible to latent heat fluxes at the surface. It is smaller over moist surfaces where most of the energy is utilized for evaporation, and larger over dry surfaces where most of the energy is converted as sensible heat flux. Some typical values of β are: β>> 1 for deserts and semi-arid regions, ~ 0.5 for grass lands, 0.2 for irrigated lands & 0.1 for ocean.

  1. Summary
  • The Atmospheric Boundary Layer (ABL)is the lowest part of the troposphere with large spatio-temporal depth variations in the range ~ 100 m – 3000 m.
  • Presence of turbulence is the most important characteristic of ABL.
  • Typically, the ABL has three layers: the surface layer, outer layer and entrainment zone (daytime) and capping inversion (nighttime). Vertical fluxes of heat, moisture and momentum are approximately constant in the surface layer during the daytime hours.
  • The boundary layer evolution during a diurnal period includes formation of convective boundary layer during daytime and stable boundary layer during nighttime.
  • The convective boundary layer grows through entrainment of free-atmospheric air with the help of overshooting energetic turbulent eddies (thermals).
  • Weak and intermittent turbulence, flow decoupling and occurrence of low level jets are salient features of stable boundary layer.
  • Heterogeneities in surface characteristics, roughness or thermal, lead to formation of internal boundary layer downwind of discontinuities.
  • Surface energy budget provides information on the partitioning of radiation energy into sensible, latent heat and ground heat fluxes.
  • ABL height undergoes large variations over the Indian subcontinent in relation to monsoon.
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Reference books

  • Introduction to Boundary Layer Meteorology. R. B. Stull. 1988, Kluwer Academic Publishers/ Springer, ISBN-10: 9027727694, ISBN-13: 978-9027727695, 670 pp.
  • Atmospheric Boundary Layer Flows: Their Structure and Measurement. 1994. J. C. Kaimal and J. J. Finnigan. Oxford University Press, ISBN: 9780195062397, 304 pp.
  • The Atmospheric Boundary Layer. 1992. J. R. Garrat, Cambridge University Press, 316 pp.
  • Boundary Layer Climate. 1987. T. R. Oke, Routledge publishers, ISBN 0-415-04319-0, 464 pp.
  • Introduction to Micrometeorology. 2001. S. Pal Arya. Academic Press, 420 pp.