Layers of the atmosphere:

Troposphere: the lowest layer of the atmosphere in which temperature decreases with height, on average

• the sphere of change in which most weather occurs and conditions often change significantly from day to day

Boundary Layer: lower part of the troposphere that is significantly affected by exchanges of energy, moisture, and momentum with the ground

• average depth about 1km

Stratosphere: the layer above the troposphere, stretching to ~50km

• on average, temperature is nearly constant in the lower stratosphere and then increases with altitude higher in the stratosphere

• temperature starts increasing with height in the stratosphere

Tropopause: the boundary between the troposphere and the stratosphere

• usually characterized by an abrupt decrease in the environmental lapse rate over an extended depth

Inversion: atmospheric layer in which temperature increases with height

Environmental lapse rate (ELR): the rate at which temperature decreases with height

the higher the latitude the colder the air

primary heating source for the atmosphere is the ground

large bodies of water acts as a moderator for temperature

• water is slow to warm and cool compared to land, making the land around it to have less seasonality

Diurnal Range: difference between the highest and lowest temperature of a day

Surface Station Model: a compact way on a map to show weather data observed at a particular location at a particular time

Wind direction: the direction from which the wind is blowing

• ex: northwest wind is from the northwest

East coast of continents - warm currents (water comes from direction of equator)

west coast of continents - cold currents (water comes from direction of pole)

Average surface winds

• Tropics and subtropics: easterly winds dominate

• mid latitudes: wester winds dominate especially over oceans

average speed tends to be larger over oceans than over land (difference in friction)

Prevailing wind: the wind direction most frequently observed, on average, during a specified period

weather tends to move from west to east over mid latitudes

Air Pressure

P= Force/area or P= Mass x acceleration/area

• air density increases with altitude

average sea level pressure (SLP) = 1000 mb

density = mass/volume

pressures measured at observing sites are “corrected” to sea level :

• this filters out the effect of elevation on pressure

after the correction you are left with centers of high and low SLP, these are the highs and lows that move from day to day and make weather

• difference in pressure is caused by weather

Just the last three digits of SLP is on a station model

^{if the first number is: >5 assume 9, <5 assume 10}

equatorial regions tend to feature relatively low pressure

trough: an elongated zone of pressure

• Winds converge at surface troughs

Ridge: an : zone of high pressure

• winds diverge at surface ridges

Pressure tendency has the most forecasting value (most insight into the future)

• decreasing pressure indicates clouds and precipitation is coming

Low pressure → clouds + precipitation

High pressure → fair weather

Displaying weather data on maps

Isopleth: a line of equal value of something

Gradient of Q = change in value of Q, distance over which change is measured

• the closer that isopleths are together, the larger the gradient

• in general, large gradients in weather variables point us toward interesting meteorological activity

Forecasts

Time scales:

Weather forecasts: a week or two into the future

Subseasonal forecasts: refers to 3 perhaps 8 weeks out

Seasonal forecasts: several months into the future

Techniques

Climatology: use the average as the forecast

Persistence: what has happened will continue to happen

Analog Forecasting: assume weather repeats itself (almost)

Numerical Weather Prediction (computer modeling)(NWP)

computer is programed with mathematical equations that represnt equations derived from the laws of physics

F=ma → used to forecast wind

Different types:

Grid point model: Cover the forecast area with a 3D grid, solve equations as grid point

• interpolate to get the forecast between grid points

• spacing = 3km

Spectral model: because atmospheric variables tend to be wavy build a model using wavy mathematical functions

• these models run faster on computers

• predictions beyond about 4 days is done using spectral models

initial conditions: the values of all the variables that are given to the model at the beginning of a run

Run: a particular computer forecast

Why do they go bad?

• computer models are imperfect

• observational limits

• the atmosphere is a nonlinear system

Ensemble Forecasting

Ensemble forecasting: a set of computer model forecasts rather than just one

US Ensemble systems

SREF: short range ensemble forecast (26 members)

GEFS: global ensemble forecast system (31 members)

Plume diagrams: in general the lines separate as the forecast goes farther out, indicating that the forecast is becoming more uncertain with time

MOS

MOS: model output system

a statistical forecasting technique that turns output from NWP models into local weather forecasts

NBM

NBM: national blend of models

NBM is forecast guidance based on a blend of the output from various NWP models

Atmospheric Moisture

Humidity: generic word used widely to describe moisture in atmosphere

• does not relate to human comfort

on average, airs water vapor content

• decreases toward poles

• higher in warm season

the higher you go, the less water vapor in the air

Energy Staircase

melting and evaporation require energy to go up the staircase

freezing and condensation release energy as you go down the staircase

Saturation: condition at which evaporation rate = condensation rate. when condensation rate is less than the evaporation rate, condensation nuclei grow and make clouds

Calorie: amount of energy required to raise temperature of one gram of water by 1ºC

Latent Heat: the amount of energy that is released or absorbed during a phase change at a constant temperature

• “hidden heat” hidden in molecules, to get back reverse the process

Condensation Nuclei: microscopic particles of dust, dirt, soot, salt and other particles in the air onto which water vapor condenses to form cloud drops

• to make clouds the atmosphere need to have condensation nuclei, these act as cloud nuclei

• called ice nuclei if water vapor deposits onto them to form ice crystals

Consider rain falling into cloud-free air… some drops will evaporate

• evaporation is a cooling process so the temperature decreases

• but the amount of water vapor in the air will increase (ie. dew point increases)

Wet-bulb Temperature: lowest temperature to which air can be cooled by evaporating water into it

• to quantify this we need to know how much water vapor is in the are and how much vapor is needed to saturate the air (water vapor in air/ amount needed to saturate)

• in most air, this ratio is between 0 and 1 (ie. the air is not cloudy)

• the closer the ratio is to 1 the closer the air is to being saturated (Cloudy)

You need more water vapor at higher temps to saturate the air

• as temperature goes up, air molecules move faster (higher energy) so water vapor molecules are less likely to condense at higher temps

Relative Humidity

Vapor pressure/ equilibrium vapor pressure x 100% = relative humidity

• the closer relative humidity is to 100% the closer the air is to saturation

• even a relative humidity of 70% is enough for clouds

  • Usually shaded in green in computer model forecast maps

The relative humidity changes if the vapor pressure changes or if the temperature changes

• thus you can change the relative humidity even if the amount of water vapor in the air does not change

How to quantify human comfort

Dew point: temperature to which air must be cooled at constant pressure for saturation to occur

• the higher the dew point, the more water vapor in the air

Dew point

below 55º

55-60º

60-65º

65-70º

70-75º

above 75º

Human comfort

dry (pleasant)

hint of humidity (still comfortable)

moist (becoming unpleasant)

sticky (unpleasant)

muggy (gross)

sultry (oppressive and unbearable)

When air rises, it expands and cools (EVP decreases)

• RH increases → can get clouds

when air sinks, it compresses and warms (EVP increases)

• RH decreases → not conducive to cloud formation

Orographic lifting

Orographic Lifting: let sloping terrain do the work

• wind blows into the windward side (air rises on this side) favoring clouds and precipitation

• air sinks on the leeward side, unfavorable for clouds and precipitation

a precipitation minimum on the leeward side is called a rain shadow

Measuring liquid precipitation

Rain + melted solid precipitation

If the atmosphere had no water vapor and no carbon dioxide it would be 60ºF colder on average than it is now

Radiation Basics

Planck’s law: all matter emits radiation constantly and at all wavelengths

Wien’s Law: matter does not emit radiation at all wavelengths equally. the hotter the object, the shorter the wavelength of maximum emission

Stefan-Boltzman Law: the total energy emitted per unit area is proportional to the fourth power of temperature

• valid for only blackbody objects

E=𝜎T⁴

Kirchoff’s Law: an object that absorbs radiation efficiently at a particular wavelength will also emit radiation efficiently at that wavelength

Balancing the earth’s energy budget

Energy in = energy out

Tropics and subtropics: receive more energy from the sun than they emit

Rest of the globe: emit more energy than they receive from the sun

• the atmosphere and oceans move energy around in a never ending yet futile attempt to alleviate the impalence

most of heating of atmosphere comes from the ground not directly from the sun

• atmosphere gets heated mainly by the surface

  • the sun heats the surface, the surface heats the air

Conduction: Molecular collisions transfer energy

Convection: blobs (parcels) of rising air transfer energy

• less dense air rises when submersed in more dense air

• often refers to the types of clouds that form from this (convective clouds)

Water vapor and carbon dioxide are the best absorbers of infrared

Clouds are made of water and are therefore better absorbers of infrared radiation

• they keep the atmosphere warmer than if there were no clouds

• insulating the earth

Cloud classification

low = below 6500 ft

middle = between 6500 and 20000 ft (alto)

high = above 20000 ft (cirro)

Cumulus: puffy clouds

Stratus: low light or dark gray clouds that cover most of the sky

Cirrus: wispy, feathery (tufts of hair)

altocumulus: white or gray patches that look like sheep’s wool

Nimbus: clouds that produce precipitation

Weather observing networks

In situ observations: instrument in direct contact with the medium it is measuring

remote observations: instrument not in direct contact with the medium it is measuring

In situ weather observing networks

primary federal government run surface network: ASOS/AWOS

• automated surface (weather) observing system

Mesanetworks

• some state have their own fine scale surface observing networks

NWP model data ingest:

• Satellites = 89%

• surface observations = 6%

• aircraft observations = 5%

• radiosondes = 0.1%

Satellites

Remote: Passive, the sensor merely senses information coming from the source (satellite imagery)

Sensing: active, sensor must send out a single in order for the remote sensing to work (radar)

Satellite imagery can provide us with surface temperature, high-altitude winds, vertical profiles of temperature and dew point, lightning in real time, and precipitable water

A Radiometer onboard the satellite measures radiation coming from earth… either Reflected visible light from the sun or radiation that the earth emits (infrared radiation)

Types of weather satellites

GPS: Global positioning satellites

Geostationary: earth stationary satellite (GOES)

• 22500 miles up over equator, provides best views of tropics and mid-latitude

• orbits at the same speed as earth

• ideal for making movies

Polar Orbiting: orbit pole to pole, 500 miles up, image in swaths (so must piece together

• sees each point twice a day and provides the vast majority of satellite data for NWP models

• Higher resolution than geostationary models

Types of satellite imagery

Visible Imagery: radiometer measures back-scattered visible radiation (ie. albedo)

• thick clouds , high albedo → bright white

• thin clouds, low albedo → not as bright

• if you can see outlines of unfrozen bodies of water that means there is snow and not clouds

Infrared: radiometer turned to infrared wavelengths emitted by clouds and earth.. the warmer the emitter, the more radiation emitted

• the higher you go the colder it gets

  • bright white = high cold clouds

  • dull white = warmer clouds closer to the ground

  • dark = surface

Water Vapor imagery: a special type of infrared imagery (so it senses the temperature of the emitters)

• water and water vapor are the dominant atmospheric emitter of radiation

• cant give information about low in the atmosphere

Radar

radar: Radio detection and ranging

• active remote sensor

• uses microwaves

• emitted radiation strikes targets, back scattered radiation is colleded, interpreted, and displayed

Hydrometer: any product of condensation or deposition of atmopsheric water vapor

Terminal doppler Radar (TDWR): network of higher resolution doppler radars used for detection of hazardous thunderstorm related phenomena (shifting low level winds)

• at 45 major airports

WSR-88D

WSR-88D = Weather Surveillance Radar-1988 Doppler

radar dish rotates, sends a pulse of microwave radiation, then measures the amount of radiation back scattered by targets

• radiation focused into canonical beam 1º wide

Operating states - Clear air mode and precipitation mode

Clear air mode: antena scans 5 elevation angles in 10 minutes. typically used on days with no precipitation or very light precipitation. Most sensitive mode

Precipitation mode: antena scans 9-14 elevation angles in 5-6 minutes. used on days with precipitation. can miss lighter precipitation due to faster scanning

reflectivity is the amount of reflected radiation, it shows precipitation (bigger values = more precipitation)

Radar issues

• Radar beam can overshoot shallow clouds that are far from the radar so nothing shows up

• snow is much less reflective so it will have lower values of dBZ

• evaporating precipitation- radar indicates precipitation but it evaporates before hitting the ground

• beam blocked by mountains and unable to see other side

• biological targets

• subrefraction - radar beam bend into the ground

Doppler radar

Doppler radar can tell if winds are blowing toward or away from the radar (velocity mode)

• positive = blowing away

• negative = blowing towards

can detect rotation inside a thunderstorm (hook echo)

Doppler effect: the pitch of sound is different depending on how the object making the sound is moving relative to the listener

Pressure gradient force (PGF)

horizontal pressure differences set air in motion, from higher toward lower pressure

• the larger the pressure gradient, the faster the wind

• isobars close together = faster winds

if PGF was the only force acting on air, the wind would blow away from higher pressure toward lower pressure

Coriolis effect (CF): apparent deflection imparted to moving objects that results from earths rotation, if no rotation then no coriolis effect.

• the effect acts to the right of the intented motion in northern hemisphere (left in southern hemisphere)

when PGF and CF are only forces acting on air, they balance to produce winds that are parallel to isobars, lower pressure to left

Geostrophic (earth turning) wind: hypothetical flow that results from a balance between the pressure gradiant force and the coriolis force

• an unaccelerated wind that is a good approximation direction of the real wind when friction is negligable

geostrophic wind is parallel to isobars, lower pressure to the left of win (in Northern Hemisphere)

Friction slows the wind and impacts direction

Magnitude of the coriolis force decreases as you approach equator

depends on three factors:

• rotation rate: faster → greater coriolis

• latitude: 0 at equator, increases with latitude, largest at poles

• wind speed: faster → great apparent deflection

Because of friction the wind does not blow as fast as the PGF would dictate. the CF depends on windspeed so with friction in the mix, the magnitude of the CF is weaker than what is needed to balance the PGF

• with friction PGF wins a little so air crosses isobars a bit toward lower presure (angles ~30º)

  • the more friction the greater the crossing angle

Friction depends on the underlying surface

• water is smooth so there wont be as much friction

• air is rougher

Air at a surface low: blow inward (converge) while circulating counterclockwise

Air at a surface high: blow outward (diverge) while circulating clockwise

Dual Polarization (dual pol)

conventional doppler radar: radar wave only in horizontal plane so captures only one dimension of targets

dual polarization: radio waves in both horizontal and vertical directions so it captures two dimensions of targets

• improved estimates of rainfall rates

• improved ability to identify different types of precipitation

• better detection of airborne tornado debris

• better able to differentiate meteorological and biological targets

New products from dual pol:

differential reactivity (ZDR): difference between the horizontal and vertical reflectivity, helps with target shape

• spherical (symmetric)= ZDR ~ 0

• more wide than tall = ZDR > 0

• more tall than wide = ZDR < 0

Correlation coefficient (CC): good for discriminating meteorological from non meteorological targets

• when nearby targets are very similar to each other, CC is close to 1

• when nearby targets are not that similar, CC is lower

Non meteorological: cc<0.8

meteorological: cc>0.9

the patterns of wind around surface highs and lows are linked to vertical motions

• convergence at surface tends to be associated with rising motion (clouds and precipitation)

• divergence at surface tends to be associated with sinking motion (fair, dry, lack of clouds)

Convection: the vertical transport of energy by rising parcels of air

Advection: the process of transporting some quality or characteristic by the movement of air

• can be horizontal and vertical advection

• controlled by the speed of the wind, the gradient of x, and the angle at which wind is crossing the isopleths of x

Bergeron-Findeisen (ice-crystal) Process

a typical raindrop is very large compared to a typical cloud drop… net condensation does not work fast enough to grow raindrops from cloud drops

most raindrops outside the tropics begin as snowflakes and just melt on the way down

for water to freeze, it typically requires an impurity to begin the freezing process

Cold cloud: in cold clouds, ice, water, and water vapor can coexist (also called mixed phase clouds)

• when water and ice coexist in a cloud, vapor migrates away from the water to the ice.. in essence the ice grows at the expense of the water

  • water gets smaller, ice gets bigger

The bergeron findeison process (ice-crystal process) is the dominant precipitation producer in mixed phase clouds

in warm clouds (ie not much ice) the warm rain process is more important: falling or suspended drops bump into each other and stick together in a “collision-coalescene” process

• important in the tropics