Transcript ATOC 1050 - NCAR Research Applications Laboratory | RAL

Weather and the
Atmosphere
NSAP Short Course
for SEs and SAs
About the course
• Selected topics and lectures from a CU
course – ATOC 1050 (45, 50-min.
classes).
• Because of varied backgrounds – no
significant memory of calculus or
differential equations is assumed
• Please interrupt with short questions when
necessary.
• Course content goes beyond what is
applicable for any individual RAL project,
but most is related to some project.
• Terminology is important as well as
concepts.
• Course web site with notes at
http://www.rap.ucar.edu/general/events.ht
ml
Content
SESSION 1
• Part 1 – Composition, mean structure, variables,
•
diurnal and annual cycles
Part 2 – Water in the atmosphere, cloud and
precip formation
SESSION 2
• Part 3 – Winds and their cause; air-masses,
•
fronts and cyclones
Part 4 – Mesoscale processes, severe weather
Part 1
Composition of the
Atmosphere
The atmosphere – A molecular cocktail
COMPOSITION OF THE ATMOSPHERE
NEAR EARTH’S SURFACE
“Permanent” gases (percent is about the same
everywhere)
• Nitrogen – 78 %
• Oxygen – 21 %
• Hydrogen - .00006 %
• Others (less than 1 %) – ozone, methane, helium, etc.
Variable gases (amount depends on place and time)
• Water vapor – 0-4 %
• Carbon dioxide – about .037 %
Water vapor – dark-dry, light-humid
CO2 varies with time
Upward trend from 1958-2000, plus seasonal
variational
Not
zero
Content of the atmosphere
in addition to gases
• Mineral particles – dust from ground,
man-made pollution
• Water droplets – clouds, fog
• Water drops – rain
• Ice – small crystals, snow flakes, hail
Structure of the atmosphere
• Varies depending on the meteorological
conditions
• But we can talk about average conditions
- horizontal structure (as shown on a
weather map)
- vertical structure
Vertical structure of the
atmosphere
• First – how deep is it?
White line is the thickness
of the atmosphere (99%
of molecules)
Earth
Within this thin atmosphere,
there are different ways of
defining layers
• How temperature changes with height
(increase versus decrease with height)
• Composition (mixture of gases)
• Electrical structure
• Turbulence intensity
Different Layers of the Atmosphere
Planetary
boundary layer –
more later
A Glossary of Weather
Variables
• Air temperature
• Air pressure
• Humidity – amount of water vapor
• Clouds
• Precipitation – rate, type, distribution
• Wind – direction and speed, turbulence
intensity
• Visibility
Air Temperature
Temperature
• Related to rate of motion of molecules: The
•
warmer the air, the more rapidly the molecules
move.
Primary temperature scales
– Fahrenheit (F): freezing point of water = 32 F,
boiling point = 212 F
– Celsius (C): freezing = 0 C, boiling =100 C
– Absolute (A) or Kelvin (K): absolute zero = 0 A =
-273 C (all molecular motion stops)
• Temperature observations near the surface (2
m) are reported in degrees F, and above that
are reported in degrees C.
Vertical Temperature Structure
of the Atmosphere
Commercial
Aircraft
All the
Weather
Denver temperature profile at 5:00 AM
28 Feb 03
Stratosphere
Tropopause
Up
Troposphere
Dry adiabatic
lapse rate
reference
lines
Air Pressure
Why do we care about
pressure?
Air Pressure - Definition
• Pressure = force/area (e.g., pounds per
square inch)
1) Hydrostatic pressure – weight of the
atmosphere above the surface
2) Dynamic pressure – force of the wind
(e.g., against a door)
Air Pressure - Units
• In this course, pressure is expressed in millibars
•
•
•
(mb) – average sea-level pressure = 1013 mb
In the science literature, it is expresses in hecta
Pascals (hPa = 1 mb)
Sometimes (e.g., in public weather forecasts) it
is given in inches of mercury – average = 29.92
in. mercury
Surface pressure values are “adjusted” to sea
level, so that values show high and low pressure
patterns rather than the elevation of the
observation
How is sea-level pressure estimated
for plotting on weather maps?
Vertical Pressure Variation in the
Atmosphere
Air Density
• General definition of density - Mass per
unit volume
• Air density – Mass (e.g., kilograms) of air
molecules per unit volume (cubic meter)
More dense
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Less Dense
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Density Variation With Height
Humidity
• A measure of the amount of water vapor in the
•
air
There are many different measures of humidity
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Relative humidity
Vapor pressure
Dew point temperature
Specific humidity
Absolute humidity
Clouds
• Percent of sky covered by clouds
• Types of clouds
– Cumulus
– Cirrus
– Stratus, etc.
Wind
• Speed – miles per hour or knots (nautical miles
•
•
•
per hour), meters per second
Direction - Given in terms of the direction from
which the wind is blowing. A northwesterly wind
is blowing from the northwest.
Gustiness – turbulent component of wind
Horizontal wind is measured, but vertical wind is
also very important
Turbulence near the ground
Visibility
• The maximum distance at which an
observer can distinguish an object against
the sky as a background
• May be limited by fog, air pollution, etc.
Precipitation
• Type of precipitation
- rain, snow, sleet, hail, etc.
• Rate at which it is falling
(inches/hour)
Scales of atmospheric motion Storms of all sizes
• Mid-latitude cyclones – 1000+ mile
• Hurricanes – 500+ miles
• Thunderstorms – 1-100 miles
• Tornadoes – few hundred yards
• Turbulence – centimeters-meters
Measuring the atmosphere
• Weather balloons, or radiosondes make
“upper-air” measurements of horizontal
wind, temperature and humidity.
Radiosonde –
helium
balloon with
measurement
instruments
Measuring the atmosphere
• Weather balloons, or radiosondes make
“upper-air” measurements of horizontal
wind, temperature and humidity.
• Surface weather stations (land and seas)
measure horizontal wind, temperature,
humidity, cloud cover
• Satellites can estimate winds, temperature
and humidity
Radiosonde launch locations
“Surface” observations - NWS
Zooming in to see even more
Northern Utah
Other data sources
• Cloud-track winds
• Water-vapor-track winds
• NEXRAD radar winds and reflectivity
• Doppler lidar winds
• Wind profilers
• Commercial aircraft – TAMDAR, AMDAR
The Planetary Boundary Layer –
more later, but here’s a preview
• BL - the region of the troposphere immediately
above Earth’s surface where vertical turbulent
transfers of heat, moisture and momentum are
large compared to the troposphere above.
• Daytime temperature lapse rate is nearly dry
adiabatic, or neutral (9.8 C/km).
• Nighttime temperature lapse rate is typically
stable (inversion).
Why care about boundary layers
• We live there.
• Transport and diffusion of plumes takes place
•
primarily in the boundary layer.
Some of our meteorological conditions at the
surface originate higher in the troposphere
(precip, large scale T changes, etc.), but many
develop within the BL.
– Air pollution (public health) –most originates at
surface and stays within BL
– Diurnal temperature fluctuations
– Mesoscale wind circulations
Jaxonville,
Florida
sounding
7 PM local
1.5 km
Jaxonville,
Florida
sounding
7 AM local
Part 2
Warming Earth and its
Atmosphere – The Diurnal
and Seasonal Cycles
Or, what happens to the
energy received from the sun?
First – We Need to Understand The
Ways in Which Heat Can be
Transferred in the Atmosphere
•Convection
•Latent heat
•Conduction
•Advection
•Radiation
1. Convection
Heat transfer in the vertical
through buoyant motion
Atmospheric convection
Cumulus
Cloud
Upward
Heat
Transfer
2. Latent heat – the
concept
Personal Experiences –
Latent Heat Consumption by
Evaporation
• Chill stepping out of a shower –
evaporation consumes heat
• Use of “swamp coolers” instead of air
conditioners in dry climates
• It is cooler over green grass than over a
stone surface – evaporation from leaves
• Cool breeze with thunderstorms in vicinity
– evaporation of rain below the cloud
Personal Experiences –
Latent Heat Release by
Condensation or Freezing
• Spraying fruit trees with water when frost
damage is possible – freezing of water
releases heat
• The “bubbly” appearance of cumulus
clouds – released latent heat in cloud
makes them buoyant
The Transport of Latent Heat
Condensation
Water Vapor
Air Moves
Vapor to
Cloud
Droplets
*****
*****
Heat Released
In Cloud
Evaporation –
Heat Taken
From Ocean
3. Conduction
• Transfer of heat from molecule to
molecule
• Think of heat in terms of how fast the
molecules are moving
• Fast moving molecules where something is
hot collide with adjacent slow moving
molecules where it is cooler, causing the
slower molecules to speed (heat) up.
Heat conductivity – ability of
material to conduct heat
• Air • Wood • Water • Wet soil • Stone • Iron -
.02 (Watts per meter per oC)
.08
.60
2.1
2.7
80.
Thus, Air is a Poor
Conductor of Heat
(ever think about why
thermopane windows work?)
4. Advection
Warm
air
Wind direction
X
YOU ARE HERE
(Temperature
will increase)
Cold
air
5. Radiation
• electromagnetic radiation
• Different names are used for different
wavelengths of electromagnetic radiation.
Everything with a Temperature
above absolute zero emits EMR
Intensity of Light Versus Wavelength
(µm)
Temperature of the sun = 6000° K (10,500° F)
The Sun’s Emission Spectrum
Versus Earth’s
What Interferes with
Transmission of Light
Through the Atmosphere?
• Some gases – called optically active gases.
Different gases absorb different
wavelengths
• Particles of dust – natural and man-made
• Clouds and fog
Now That We Know How Heat
is Transferred in the
Atmosphere!
---------------------Putting it All Together
The Atmospheric Energy Checkbook – Balance
Gains and Losses (On the Average)
Putting the Balance Under a
Microscope
How the Warm Ground
Heats the Air In Contact With It
Convection
Ground
Conduction in
lowest millimeter
The Diurnal and Seasonal
Cycles of Heating –
Exist Because of the EarthSun Geometry
The Cycles
• Diurnal – Rotation of Earth on axis
every 24 h
• Annual or seasonal – Revolution of
Earth around the sun every 365.25
days
Equinoxes: Points in Earth’s orbit around the sun where the sun is
directly over the equator (12 hrs light & 12 hrs of darkness).
Solstice: Points in orbit where the sun is “displaced” farthest N or S.
Seasonal Cycle
-Causes-
• The sun heats the ground less
intensely when it is at a low angle in
the sky (winter), and more intensely
when it is at a high angle above the
horizon (summer).
• The suns heat the ground less when
the daylight is shorter (winter), etc.
Factors That Cause Temperature
Variation From Place to Place
•Latitude – warmer to south
•Land and water distribution
winter → warmer near water
summer → cooler near water
•Ocean currents
Warm currents generally on east side of
continents, cold currents on the west side
•Elevation of the surface above sea level
Temperature decreases with elevation
• Surface type – e.g., urban versus
rural
• Surface wetness
Cooler of
Higher Elev.
Warmer
Over Ocean
in Winter
Warmer
to South More
Radiation
Two cities
with the
same mean
annual
temperature
but with very
different
climates
Daily Temperature Cycle
Balance of Incoming Solar and
Outgoing Infrared
Max
Temperature Profile Near The Ground
DAY
profile
Note: Temperatures almost always decrease with height
NIGHT
FROST
Inversion: Temperatures increase with height
What Can Effect The DayNight Temperature Change?
• Cloud cover reduces diurnal variability
• Terrain: surface type, mountains,
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•
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canyons,…
Wind: causes turbulent mixing of the
atmosphere which reduces diurnal
variability
Approaching weather systems: cold
fronts, warm fronts, squall lines,
hurricanes
Humidity
Topographic Effects on
Nighttime Temperature
Cold air pooling