Transcript Undertanding Weather and Climate Ch 3

Understanding Weather
and Climate
3rd Edition
Edward Aguado and James E. Burt
Anthony J. Vega
Part 1. Energy and Mass
Chapter 3.
Energy Balance and Temperature
Introduction
Solar radiation is the primary heat source for the
atmosphere
Most gases are transparent to solar radiation and, instead,
absorb terrestrial radiation
Gases are also responsible for scattering incident energy
The balance between incoming solar radiation, the
absorption of terrestrial radiation, and outgoing terrestrial
radiation describes the global energy budget
Atmospheric Influences on Insolation
Radiant energy incident upon the Earth-atmosphere system is
either absorbed, reflected, or transmitted by atmospheric gases
and/or the Earth’s surface
Energy reflected and/or transmitted (scattered) does not contribute
to heating
Absorbed energy encourages direct heating
Absorption
Particular gases, liquids, and solids in the atmosphere absorb
radiant energy
Heat increases in the absorber while less energy is transferred to
the surface
Although atmospheric gases are rather selective in the wavelengths
they absorb, they are overall poor absorbers of energy
Reflection
Energy is effectively redirected by objects through reflection
Process does not increase heat in the reflector as energy is not
absorbed
In most instances, only a portion of incident energy is reflected
Albedo = the percentage of reflected energy
Specular reflection is reflection of energy as an equally intense
energy beam
Energy reflected in such a way as to disperse energy into many
weaker wavelengths is diffuse reflection or scattering
Scattering of energy
Scattering
Gases in the atmosphere effectively scatter radiation
Energy that reaches the surface is diffuse radiation and different in
intensity from direct radiation
Characteristics of scattering are dependent upon the size of the
scattering agents
Rayleigh Scattering
Involves gases, or other scattering agents that are smaller than the
energy wavelengths
Scatter energy forward and backward
Partial to shorter wavelength energy, such as those which inhabit
the shorter portion of the visible spectrum
A blue sky results
Mie Scattering
Larger scattering agents, such as suspended aerosols, scatter
energy only in a forward manner
Larger particles interact with wavelengths across the visible
spectrum
Produces hazy or grayish skies
Enhances longer wavelengths during sunrises and sunsets,
indicative of a rather aerosol laden atmosphere
Longer radiation
path lengths lead to
an increase in Mie
Scattering and reddish
skies
Nonselective Scattering
Water droplets, typically larger than energy wavelengths, equally
scatter wavelengths along the visible portion of the spectrum
Produces a white or gray appearance
No wavelength is especially affected
Transmission
The percentage of energy transmitted through the atmosphere to
the surface
Dependent upon the ability of the atmosphere to absorb, scatter,
and reflect
Transmission of energy varies diurnally from place to place
The Fate of Solar Radiation
Insolation annually varies by 7%
Useful to think of a constant supply of radiation at the top of the
atmosphere
Need to account for the relative amount of radiation that is
transmitted through the atmosphere, absorbed by the atmosphere
and surface, and scattered back to space
A global energy budget
Assume global annual insolation of 100 units of energy
Atmosphere directly absorbs 25 units with 7 absorbed by
stratospheric ozone, the rest by gases such as water vapor (absorbs
near-infrared wavelengths)
Atmospheric reflection averages 25 units, 19 of which are reflected
to space by clouds and 6 units which are back-scattered to space
from atmospheric gases
Remaining 50 units are available for surface absorption
50 units of energy overall exist at the surface
5 units reflected back to space
These 5 units combined with the 25 scattered to space from the
atmosphere equates to a total planetary albedo of 30%
Remaining 45 units of energy at the Earth’s surface are absorbed
This warms the surface
Earth processes eventually transfer this energy from the Earth
system back to space
Energy Transfer between the Surface and Atmosphere
Surface-Atmosphere Radiation Exchange
Due to Earth’s low temperatures, terrestrial radiation emitted is
primarily longwave
Longwave energy transfer is more complex than solar energy
because longwave energy has no obvious beginning or end
Longwave radiation emitted from the surface is largely absorbed
by the atmosphere
This increases the temperature of the atmosphere which causes it to
radiate more energy outward
Energy is transferred in all direction, including downward
This causes additional surface heating, and the cycle repeats
To describe longwave energy, we begin with 104 units of radiation
100 units are absorbed by the atmosphere
Water vapor and CO2 are the primary absorbers
These, along with other gases, are known as greenhouse gases
A portion of the longwave spectrum can pass through the
atmosphere unimpeded
This range of wavelengths, 8-15μm, match those radiated with
greatest intensity by the Earth’s surface
This range of wavelengths not absorbed is called the atmospheric
window
The atmospheric window
Although gases do not effectively absorb wavelengths in the
atmospheric window, clouds readily absorb virtually all longwave
radiation
• This is the reason cloudy nights are typically not as cool as clear nights
The result of the reabsorption of energy is that a total of 154 units
of energy are reradiated by the surface
Net longwave radiation, the difference between absorbed (100) and
emitted (154) longwave radiation equals 54 units
The surface receives 88 units of longwave radiation
• Amount is exceeded by the 104 units that are radiation for a net
longwave radiation deficit of 16 units
Shortwave and longwave radiation undergo different amounts of
absorption and reflection
• They are not separate entities relative to heating
• When either is absorbed, the absorber is warmed
Net all-wave radiation or simply net radiation equals the
difference between absorbed and emitted radiation or the net
energy gained or lost by radiation
The atmosphere absorbs 25 units of solar radiation but undergoes a
net loss of 54 units for a net deficit of 29 units
The surface absorbs 45 units of solar radiation but has a longwave
deficit of 16 resulting in a net surplus of 29 units
So, the atmosphere has a net deficit of radiation energy exactly
equal to the surplus attained by the surface
If radiation were the only means of exchanging energy, the surplus
obtained by the surface would result in a perpetual warming while
the atmospheric deficit would lead to continual cooling
• Our feet would be scorched while our bodies froze
This does not happen because energy is transferred from the
surface to the atmosphere and within the atmosphere by
conduction and convection
The surplus and deficits offset as a result
Conduction
As the surface warms, a temperature gradient (rate of change of
temperature over distance) develops in the upper few cm of the
ground
• Temperatures are greater at the surface than below
This transfers energy downward
Surface warming also causes a temperature gradient within a very
thin sliver of adjacent air called the laminar boundary layer
Net radiation
Surface/atmosphere
offset surplus and
deficits
Convection
The temperature gradients in the laminar boundary layer induce
energy transfer upward through convection
This occurs any time the surface temperature exceeds the air
temperature
Normally, this occurs during the middle of the day
At night, the surface typically cools more rapidly than air and
energy is transferred downward
Convection can be generated by two processes in fluids
Free Convection
• Mixing related to buoyancy
• Warmer, less dense fluids rise
Forced Convection
• Initiated by eddies and other disruptions to smooth, uniform flow
Free Convection
Forced Convection
Sensible Heat
Heat energy which is readily detected
Magnitude is related to an object’s specific heat
• The amount of energy needed to change the temperature of an object
a particular amount in J/kg/K
Related to mass
• Higher mass requires more energy for heating
Globally, 8 units of energy are transferred from the surface to the
atmosphere as sensible heat
Latent Heat
Energy required to induce changes of state in a substance
In atmospheric processes, invariably involves water
When water is present, latent heat of evaporation redirects some
energy which would be used for sensible heat
• Wet environments are cooler relative to their insolation amounts
Latent heat of evaporation is stored in water vapor
Released as latent heat of condensation when that change of state
is induced
Globally, 21 units of energy are transferred to the atmosphere as
latent heat
Heat content
of substances
Net Radiation and Temperature
Earth’s radiation balance is a function of an incoming and outgoing
radiation equilibrium
If parameters were changed, a new equilibrium would be achieved
Balances occur on an annual global scale and diurnally over local
spatial scales
Best exemplified by examining radiation and temperature over a
cloudless day
• Coolest temperatures just after dawn as outgoing radiation is
maximized and incoming radiation is weak
• As solar declination improves, temperatures increase but maximum
solar angle (noon) and max temperatures (2-4 pm) are offset
Diurnal temperature lag caused by afternoon to evening energy
surpluses and slow energy transfer mechanisms of conduction,
convection, and latent heat
• Similar to larger-scale, hemispheric seasonal temperature lags
Latitudinal Variations
Radiation equilibrium varies by latitude
Areas between 38oN and S run net energy surpluses
Areas poleward of 38o experience net energy deficits
The margin between net gains and net deficits migrates seasonally
Summer hemispheres
• Net energy gains occur poleward of about 15o
Winter hemispheres
• Net energy deficits occur poleward of about 15o
Latitudinal imbalances are neutralized through mean horizontal
mass advection
This occurs as energy balances create pressure inequalities which
result in winds and currents which transport energy latitudinally
Annual average net radiation
Ocean circulation
The Greenhouse Effect
Trapping terrestrial radiation by certain atmospheric gases
Moniker stems from greenhouses which allow solar radiation to
enter through glass panes but trap outgoing heat energy
Earth is different from a true greenhouse as greenhouses simply
stem heat loss by preventing convection
Without atmospheric gases (namely H2O, CO2, and CH4) trapping
outgoing terrestrial radiation, average Earth temperatures would be
about -18oC (0oF)
Increases in greenhouse gas concentrations through human
activities may lead to future climatic changes
A greenhouse stems
convection
Global Temperature Distributions
There is a general decline of temperatures with increases in latitude
There is a stronger (weaker) latitudinal thermal contrast in the
winter (summer) hemisphere
Summertime poleward and wintertime equatorward shift in
isotherms over continents
Documents the quick heating and cooling of land surfaces as
opposed to water surfaces
Northern hemisphere thermal gradients are more pronounced than
in the southern hemisphere due to greater landmass
Influences on Temperature
Latitude
Mean annual temperatures decrease with increasing latitude
Due to lower solar angles, and axial tilt influences
Low latitudes enjoy high solar angles and similar lengths of day all
year
With increasingly high latitude comes exacerbated insolation
extremes as day length shifts seasonally
Altitude
Tropospheric temperatures typically decrease with altitude
Energy enters atmosphere from the surface
Temperatures at high altitudes remain fairly constant
Air at high elevations (but near a surface) undergo more rapid
diurnal temperature fluxes than air at lower elevations
Atmospheric Circulation
Latitudinal temperature, and pressure, differences cause large-scale
horizontal energy transport through advection
• Also influences latitudinal moisture regimes and cloud cover which
then impact temperatures
Contrasts between Land and Water
Surface composition influences atmospheric heating
Water bodies heat slower than land given similar insolation
Continentality is the exacerbation of seasonal temperature
extremes experienced by continental interiors
Maritime locations experience more moderate seasonal
temperature extremes due to presence of water bodies which
change temperature very slowly
• Water heats less due to a higher specific heat, transparency,
evaporative cooling, and horizontal and vertical mixing factors
Warm and Cold Ocean Currents
Due to ocean-atmospheric circulation coupling, western ocean
basins have warm ocean currents while eastern basins maintain
cold currents
Coastal air temperatures are affected accordingly
• West coast mid-latitude locations are more moderate than east coast
locations
Local Impacts on Temperature
Small spatial scale features impact temperatures
• Equatorward facing slopes heat more quickly than poleward slopes,
forest regions reduce surface insolation during the day and trap
radiation at night leading to cooler daytime and warmer nighttime
temperatures
South-facing slopes are
typically more vegetated than
north-facing slopes
Vegetation reduces surface
radiation during the day and
traps it at night
Measurement of Temperature
Either mercury or alcohol based thermometers are used
A maximum thermometer is used to record daily temperature
maximums while a minimum thermometer records minimums
Thermistors are fast response temperature recording devices based
on resistance to electrical current
• Used mainly in radiosondes
Instrument Shelters
Vented Weather shelters are necessary to accurately gauge
temperatures
• Painted white to reduce direct insolation gains through higher albedo
• Must be 5 ft from a vegetated surface to reduce laminar layer bias
Temperature Means and Ranges
Standard averaging procedures used to obtain means
• Daily means = average of daily maximum and minimum temperature
• Surface heating occurs rapidly in late afternoon, the method induces a
bias towards higher daily temperatures over averaging 24 hourly
observations
• Daily temperature range = daily maximum - daily minimum
• Monthly mean = sum of all daily means/number of days
• Annual mean = sum of all monthly means/number of months
Continuous temperature plot.
Note, short period of high temperatures
which skew daily averages toward
temperatures higher than actually
experienced through most of the day
Global Extremes
Due to continentality, greatest extreme temperatures occur at
continental interiors
• World record high = 57oC (137oF) at Azizia, Libya, 1913
• World record low = -89oC (-129oF) Antarctica, 1960
Temperature and Human Comfort
Human discomfort due to temperature may be compounded by
other weather factors
Wind during cold conditions causes a body to lose heat more
quickly
The Wind Chill Temperature Index indicates how cold a particular
temperature feels given a certain wind speed
High humidity values cause warm days to feel oppressively hot
due to reductions in evaporative power
A Heat Index incorporates the effect of high atmospheric moisture
at high temperatures
Thermodynamic Diagrams
• Thermodynamic diagrams depict the vertical profile of temperature
and humidity with height
• Extremely important information for forecasting
• Enable forecasters to determine the height and thickness of existing
clouds and the ease with which air can be mixed vertically
• Data obtained from radiosondes carried aloft by weather balloons
twice a day at weather stations across the globe
• Stuve diagrams plot temperatures as a function of pressure levels
through the vertical
• Plots are extremely useful for forecasting applications
End of Chapter 3
Understanding Weather and
Climate
3rd Edition
Edward Aguado and James E. Burt