Download chapter 5 energy, matter, and momentum exchanges near the surface

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
 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
 Radiation Balance
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
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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
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
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)
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
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
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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 upward-directed
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
Putting It All Together: Thermal and Mechanical Turbulence and the Richardson Number
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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
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