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.html
Content SESSION 1 • Part 1 – Composition, mean structure, variables, diurnal and annual cycles • Part 2 – Water in the atmosphere, cloud and precip formation • Part 3 – Winds and their cause; air-masses, fronts and cyclones • Part 4 – Mesoscale processes, severe weather SESSION 2
COMPOSITION OF THE ATMOSPHERENEAR 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 %
CO2 varies with timeUpward 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
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 Temp Dry adiabatic lapse rate reference lines Troposphere
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
General definition of density - Mass per unit volume • Air density – Mass (e.g., kilograms) of air molecules per unit volume (cubic meter) More dense Less Dense ******************************************** * * * * * * * * * * * * * * * * * * * *
Humidity • A measure of the amount of water vapor in the air • There are many different measures of humidity • 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
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.
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
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