Understanding the ``Atmospheric Boundary Layer'' · 2018-09-02 · Understanding the “Atmospheric...

Understanding the “Atmospheric Boundary Layer”

D. BALA SUBRAHAMANYAM

Boundary Layer Physics and Atmospheric Modelling (BLPAM) Branch

Space Physics Laboratory, Vikram Sarabhai Space Centre

Thiruvananthapuram - 695 022

SPL - Orientation Course for New Research Fellows

March 2013

D. Bala Subrahamanyam (SPL, VSSC) Orientation Course for New Research Fellows March 2013 1 / 22

Introduction to the “Boundary Layer” “Boundary Layer” to a layman

Introduction to the “Boundary Layer”

Let us understand - what a “boundary layer” is ... ???

Clearly Visible Boundaries ...

(1) Solid-Solid; (2) Solid-Liquid; (3) Solid-Gas; (4) Liquid-Liquid; (5) Liquid-Gas and (6) Gas-Gas

Whenever a Fluid interacts with a solid or another fluid, it leads to the

formation of a boundary-layer


Introduction to the “Boundary Layer” Definition of the BL

A precise definition of the “Boundary Layer”

A Boundary Layer is defined as the layer of a fluid in the immediate vicinity of a material surface in

which significant exchange of momentum, heat or mass takes place between the surface and the fluid.

The “Atmospheric Boundary Layer (ABL)”:

The ABL is formed as a consequence of the interactions between the

atmosphere (fluid) and the underlying surface (land or water).


The Atmospheric Boundary Layer Text-Book definitions of the ABL

Text-Book definitions of the “ABL”

... ... ... “ the layer of air directly above the Earth’s surface in which the effects

of the surface (friction, heating and cooling) are felt directly on time scales less

than a day, and in which significant fluxes of momentum, heat or matter are

carried by turbulent motions on a scale of the order of the depth of the

boundary layer or less. ” ... ... ...

... ... ... “ that part of the troposphere that is directly influenced by the presence

of the earth’s surface, and responds to surface forcings with a timescale of about

an hour or less. These forcings include frictional drag, evaporation and

transpiration, heat transfer, pollutant emission, and terrain induced flow

modification. ” ... ... ...


The Atmospheric Boundary Layer Motivation for the ABL Studies

What’s the Motivation for ABL Studies ???

Why should I study this “ABL” ???


The Atmospheric Boundary Layer Significance of the ABL Studies

Why to study this “ABL” ???

Significance of the “ABL” Studies

1. Atmospheric Energetics

The primary energy source for the whole atmosphere is solar radiation,

which for most part is absorbed at the ground and transmitted to the rest

of the atmosphere through boundary-layer processes.

About 90% of the net radiation absorbed by oceans cause evaporation,

amounting to the evaporation of about 1 metre of water per year over the

earth’s ocean area.

About 50% of the atmospheric kinetic energy is dissipated in the ABL.

The latent heat stored in water vapour accounts for about 80% of the fuel

that drives atmospheric motion.



Why to study this “ABL” ... ???


2. Weather and Safety

Daily weather forecasts of dew, frost, and other meteorological features

are actually the boundary-layer forecasts

Aviation, shipping, and other navigation activities are directly linked

with the boundary-layer processes

Thunderstorm and hurricane evolution are directly tied to the inflow of

moist boundary-layer air

Turbulence and gustiness affects architecture in the design of structures

Pollution is trapped in the boundary-layer

Fog occurs within the boundary-layer



Why to study this “ABL” ... ???


3. ABL as an Energy Hub

Wind turbines extract energy from the boundary-layer winds

Wind stress on the sea surface is the primary energy source for ocean

currents

Turbulent transport and advection in the boundary-layer move water and

oxygen to and from immobile life forms like plants

4. Relevance and Importance of ABL processes in NWP

5. ABL Processes and their impact on the Clouds

6. Climate Change and its final impacts on the ABL

... ... ... ... ...


Vertical Structure of the Atmosphere Various Layers of the Atmosphere

Vertical Structure of the Atmosphere

Various Layers of the Atmosphere in vertical

Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere ...


Classification of the Troposphere ABL and Free Atmosphere

Vertical Structure of the ABL

Atmospheric Boundary Layer and Free Atmosphere

Image Source: T. R. Oke (1987) Image Source: Roland B. Stull (1988)


Classification of the Troposphere Characteristics of the ABL and FA

Characteristics of ABL and FA

Atmospheric Boundary Layer (ABL) and Free Atmosphere (FA)

TURBULENCE:

The ABL is almost continuously turbulent over its whole depth, while turbulence is not

significant in FA. (Turbulence = any irregular flow that produces gusts and eddies).

DIURNAL VARIATION:

Diurnal variations of different meteorological parameters is one of the characteristics of

the ABL over land, while FA shows very little diurnal variation.

THICKNESS:

The thickness of the ABL varies from few meters to few kms in time and space, whereas

the FA is less variable, typically ≈ 8 to ≈ 18 kms.

DISPERSION:

Rapid turbulent mixing in the vertical and horizontal directions is found in the ABL,

whereas in FA, very small molecular diffusion is found.


Vertical Structure of the ABL Various Layers within the ABL

Various Layers within the ABL

Zooming inside the “ABL” [Image Source: J. R. Garratt (1992)]

Outer (Ekman) Layer:

Flow shows very little dependence on the surface, and the Coriolis force is important

Inner (Surface) Layer:

Mainly dependent on the surface characteristics and is little affected by rotation

(a) Inertial sublayer: velocity profiles in neutral conditions remain logarithmic

(b) Interfacial sublayer: mean profiles are strongly influenced by the roughness elements



Various Layers within the ABL

Zooming inside the “ABL” ... ... ...



Surface Roughness Length (z0)

Roughness length: (A measure of surface roughness)

Determination of Roughness Length (z0)

It is defined as the height closest to the Earth’s surface where the wind speed becomes zero.



Typical Values of Surface Roughness Length (z0)

Table adopted from: S. Pal Arya

Terrain description z0 (m)

Open sea 0.0002

Mud flats 0.005

flat terrain 0.03

bushes, numerous obstacles 0.5

city, low buildings, large forests > 2

Similar to z0 (also referred to as z0m), roughness lengths for heat and moisture

(z0h and z0q) are determined through extrapolation of temperature and

humidity profiles towards the surface.


Turbulence Introduction to the Turbulence

Introduction to the Turbulence

Nobel Laureate Richard Feynman described turbulence as “the most

important unsolved problem of classical physics.”

In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and

stochastic property changes. This includes low momentum diffusion, high momentum

convection, and rapid variation of pressure and velocity in space and time.


Turbulence Characteristics of the Turbulence

Characteristics of the Turbulence

Turbulence is highly characterized by the following features:

Irregularity: Turbulent flows are always highly irregular and therefore treated

statistically rather than deterministically. Turbulent flow is always chaotic but not all

chaotic flows are turbulent. The fluid motions are unpredictable in detail.

Diffusivity: The readily available supply of energy in turbulent flows tends to accelerate

the homogenization (mixing) of fluid mixtures. The diffusivity of the flow is responsible

for the enhanced mixing and increased rates of mass, momentum and energy transports.

Rotationality: The flows are rotational and three dimensional (vorticity fluctuations are

therefore important). Turbulent flows have non-zero vorticity and are characterized by a

strong three-dimensional vortex generation mechanism known as vortex stretching.

Dissipation: To sustain turbulent flow, a persistent source of energy supply is required

because turbulence dissipates rapidly as the kinetic energy is converted into internal

energy by viscous shear stress.


Turbulence Energy Cascading in a Turbulent Flow

Energy Cascading in a Turbulent Flow

Energy Cascading in a Turbulent Flow:

Turbulent flows are viewed as made of an entire hierarchy of eddies* over a wide range of length scales.

Integral length scales (largest scales)These eddies obtain energy from the mean flow and also from each other. Thus these are the

energy production eddies which contain the most of the energy. They have the large velocity

fluctuation and are low in frequency and are highly anisotropic.

Kolmogorov length scales (smallest scales)In this range, the energy input from nonlinear interactions and the energy drain from viscous

dissipation are in exact balance. The small scales are in high frequency which is why

turbulence is locally isotropic and homogeneous.

Taylor microscales (inter-mediate scales)These scales are not dissipative scale but passes down the energy from the largest to the

smallest without dissipation.

*eddies: they are loosely defined as coherent patterns of velocity, vorticity and pressure.


Turbulence Energy Cascading in a Turbulent Flow

Energy Cascading in a Turbulent Flow

Energy Cascading in a Turbulent Flow:

Turbulent flows are viewed as made of an entire hierarchy of eddies* over a wide range of length scales.

*eddies: they are loosely defined as coherent patterns of velocity, vorticity and pressure.


Turbulence Quantification of Turbulence

Quantification of Turbulence

Back to ABL and FA


Turbulence Diurnal Variation

Diurnal Variation

What is the “Diurnal Evolution” ?

Back to ABL and FA


Understanding the ``Atmospheric Boundary Layer'' · 2018-09-02 · Understanding the “Atmospheric...

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