CHAPTER 5 ENERGY, MATTER, AND MOMENTUM EXCHANGES NEAR THE SURFACE
Properties of the Troposphere
 The troposphere is characterized by temperatures that normally decrease with height both
because of the decreasing compression of atmospheric gases with increasing distance from
the surface and because of the increasing distance from the (indirect) heat source – the
surface
 The Near-Surface Troposphere
o The laminar layer supports smooth atmospheric flow and is within a few
millimeters at most) of the surface or elements on the surface
o The roughness layer is above the laminar layer, and is characterized by a large
component of vertical motion compared to horizontal motion caused by
mechanical turbulence generated by the friction, irregular flow, and vertical
gradients of momentum associated with the roughness elements
o In the roughness layer, energy and matter are transported upward via the
turbulent flux of sensible heat (also known as the sensible heat flux (QH)) and
the turbulent flux of latent heat, or latent heat flux (QE), and is used to drive
atmospheric processes between the surface and the atmosphere above this layer.
o Unlike energy and moisture, the vertical flux of horizontal momentum or
shearing stress (τ – “tau”) is usually transported downward, from the atmosphere
to the surface, because winds usually strengthen with increasing height
o The laminar layer and the roughness layer combined are sometimes referred to as
the surface boundary layer (SBL), where the turbulent fluxes and momentum
flux are generally considered to be constant with height, but not over time
o The transition layer, where vertical transfer and friction remain important
properties, extends from the top of the SBL to approximately 500 – 1000 m above
the surface
o Collectively, the laminar, roughness, and transition layers comprise the planetary
boundary layer (PBL)
o Above the PBL is the free atmosphere, where friction is generally assumed to be
negligible and flow is governed by the pressure gradient force, the apparent
horizontal deflective force known as the Coriolis effect, centrifugal force, and
geostrophic balance
Energy in the Climate System
 The Sun as Energy Source
o Without the sun’s unequal heating of the earth, there would be no need for the
atmosphere or oceans to circulate, because no differences in energy would be
received from one place to another on the earth’s surface
o The position of the sun in the sky can be plotted on a sun path diagram and
largely governs the receipt of shortwave radiation input on a local surface
o Solar time is the “true” time at a location based on the location’s position with
respect to the sun and can be calculated based on longitude and time of year
 Measuring Radiant Energy
o Any instrument that measures the flux of radiation over a unit area of the surface
(in Watts per square meter) is termed a radiometer
o Pyranometers measure only shortwave (i.e., emitted by the sun) radiation, while
pyrgeometers measure only longwave (i.e., emitted by the earth or atmosphere)
radiation
 The Radiation Balance
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o The net radiation (Q*, pronounced “Q-star”) is the difference between the radiant
energy (shortwave (K) and longwave (L)) incident upon a surface and that which
has been emitted by that surface, and it varies tremendously over space and time
o Solar radiation that enters the atmosphere is involved in various processes,
including atmospheric absorption (retention by a particle and conversion to
internal energy), scattering in all directions by the atmosphere, and reflection
from the tops of clouds or the surface
The Turbulent Fluxes
o Convection is the mechanism by which energy is transferred vertically from the
surface to the atmosphere
o The sensible heat flux is manifested in the atmosphere by the familiar observation
that warmer air rises and cooler air sinks
o For the earth as a whole, QH transfers only a relatively small percentage of the
excess energy from the surface upward because relatively little of the earth is
covered by land, particularly at tropical latitudes where abundant surface heating
occurs
o In QE, excess radiant energy at the surface is used to evaporate water or melt or
sublimate (convert from solid directly to vapor) ice; later, when the evaporated
water condenses (changes phase from vapor to liquid form) or when the liquid
water freezes or even deposits (changes phase from water vapor directly to ice,
bypassing the liquid stage altogether; the opposite of sublimates), the same
amount of energy that was required to evaporate, melt, or sublimate the water is
then released
The Substrate Heat Flux
o Conduction is the transfer of energy by one molecule to another molecule
touching it, to the next molecule, etc., and the flux of energy by conduction is
known as the substrate heat flux (QG)
o Conduction is negligible in the atmosphere, but QG can sometimes play an
important role locally (particularly at short time scales) in transferring energy from
the surface downward (when the surface is being warmed), or from beneath the
surface upward (when the surface is colder than the ground beneath it)
The Energy Balance
o After including the effects of the radiant, convective, and conductive fluxes, we can
express the energy balance as Q *  QH  QE  QG
o In other words, the sum of the net shortwave and net longwave radiation receipt at
the surface must be balanced by the convective loss of energy from the surface
upward through QH and QE and the conductive loss downward through QG
The Local Flux of Matter: Moisture in the Local Atmosphere
 Atmospheric Moisture
o Can be represented by relative humidity, the ratio of vapor pressure (e) to
saturation vapor pressure (es)
o The Clausius-Clapeyron Equation shows that es can be considered solely a
function of temperature at the range of temperatures and pressures experienced
on earth
o The temperature at which saturation occurs at a given place and time is known as
the dewpoint temperature (Td)
o If the air dewpoint temperature is reached, condensation or deposition begins,
releasing latent energy during the phase change
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o Absolute humidity is another quantity that may be used to represent the amount
of vapor in the air, but it isn’t very convenient to use because it changes with
elevation
o Specific humidity (q) is the ratio of the mass of water vapor to the mass of the
air, and is used frequently in climatological applications
Moisture in the Surface Boundary Layer
o Highly precise and accurate thermoelectric sensors can derive q from surrogate
variables, and q is used in estimating the flux of water from the surface to the
atmosphere through the combined processes of evaporation and transpiration
o The transpiration process is driven by the sun, in the same way that the sun
provides radiant energy to drive the evaporation process. Transpiration rates are
generally greatest on a dry, sunny day when abundant water is available in the soil
Measuring Evapotranspiration (ET)
o The simplest (but perhaps only marginally accurate) method used by the National
Weather Service for determining ET is through the use of a Class A Evaporation
Pan
o A lysimeter measures ET by weighing the moist soil and vegetation on top of it;
changes in weight are attributed to changes in soil water content
o Far more sophisticated methods, such as eddy correlation, exist for measuring
ET, but because these require precise and expensive equipment
Atmospheric Statics, the Hydrostatic Equation, and Stability
 Statics and the Hydrostatic Equation
o For vertical motion in the atmosphere, the relationship between the upwarddirected buoyancy force and the downward-directed acceleration caused by
gravity can be expressed by the hydrostatic equation
 Atmospheric Stability
o Stability refers to the likelihood of a “parcel” (a hypothetical blob) of air in the local
atmosphere to rise or sink spontaneously
o In a stable atmosphere, if a parcel is forced to rise for any reason, it will sink
once the lifting force is removed, because it is cooler than the air adjacent to it at a
given level
o Positive buoyancy, which occurs in an unstable atmosphere, refers to a case in
which a rising parcel continues to rise, even as it cools, because it remains
warmer than the air adjacent to it at a given level
o To determine the stability conditions at any height, compare the parcel’s
temperature (using the unsaturated adiabatic lapse rate (UALR or Γu) (or the
saturated adiabatic lapse rate (SALR, or Γs) if the atmosphere is saturated)) to
the temperature of the surrounding environment (represented by the
environmental lapse rate (ELR or γ))
o The UALR and SALR are known theoretically, and the ELR can be measured by
sending up radiosondes on weather balloons
 Assessing Stability in the Local Atmosphere
o The stability conditions of a local atmosphere are identified easily using a
thermodynamic diagram, which plots the various lapse rates for easy
comparison
o Potential temperature (θ) – the temperature that a parcel of air would have if
moved dry adiabatically from its height (z) in the atmosphere to the 1000-mb level
– can be used to simplify the calculations that would be necessary to determine
local stability
o A stable atmosphere (or layer of the atmosphere) is one in which θ increases with
height, while unstable conditions are characterized by θ that decreases with
height, and a neutral atmosphere is one in which θ is constant with height
The Momentum Flux
 When wind speed is much greater at (say) 2 meters above the surface than at 0.5 meters
above the surface, momentum transport is enhanced because the extra momentum
associated with the faster winds must be transported downward to satisfy Fick’s Law
Tying It All Together: Thermal and Mechanical Turbulence and the Richardson Number
 The Gradient Richardson Number (Ri) provides a convenient index for describing the
relative importance of thermal and mechanical turbulence in the local near-surface
atmosphere and provides a useful index for assessing stability conditions locally
 The logarithmic wind profile allows for the estimation of wind speed at any height if wind
speed at one height is known, and the atmosphere can be assumed to have neutral