Metr test 3

flooding, tropical cyclones, weather radar, lightning and thunderstorms

flood

The overflowing of the normal

confines of a stream or other body of

water, or the accumulation of water over

areas that are not normally submerged

What can cause floods? Three most

common causes:

heavy rainfall

◦ Tropical cyclones and very strong midlatitude

cyclones, via their storm surge

◦ Snowmelt and ice jams in rivers during winter

◦ Saturation of soil from previous rainfall +

additional rainfall

flood duration and intensity relies on several factors (interactions exist between meteorology and geography)

Intensity of rain events

◦ Duration of rain events

◦ Number of rain events

◦ Size of the rainfall area

◦ Proximity and orientation to a drainage basin

◦ Saturation level of soils

◦ Presence of snow and/or ice

◦ Land use

◦ Presence of dams/levees

◦ Topography along a drainage

flash foods

floods that occur rapidly and

with little warning

◦ Lots of rain + little time

Typically, localized and short-duration

◦ Duration: minutes to hours

o Primarily caused by slow-moving

thunderstorms and high rainfall rates over

a short period of time

Can be associated with more injuries and

fatalities because these floods are

unanticipated

◦ People cannot appropriately prepare for these

floods

Sources: NWS, WFO Pittsburgh

widespread flood

Occur when a large amount of rain falls

over a watershed for many days

◦ Moderate rain + lots of time

Widespread Floods

o Occur when a large amount of rain falls

over a watershed for many days

◦ Moderate rain + lots of time

◦ Watershed: the total area drained by a river

and its tributaries

◦ Water levels along the river rise slowly, and

eventually overflow the natural and artificial

confines of the river

Often happen as a result of training

storms

Main difference from flash floods: long duration

rain event

watershed

the total area drained by a river

and its tributaries

◦ Water levels along the river rise slowly, and

eventually overflow the natural and artificial

confines of the river

training storms

storms all passing over the same

area for an extended period of time

coastal floods

Occur as a result of a rise in the ocean

surface due to storm surge

◦ Storms + high winds + coastal area

storm surge

an abnormal rise in sea level,

above and beyond the rise predicted by

astronomical tides, that is caused by the

winds and low pressure in a TC

Causes of storm surge: tropical cyclones,

strong midlatitude cyclones

conditions that can exacerbate flooding

Proximity to rivers, streams, and/or lakes

◦ Man-made surfaces (blacktop, concrete) and deforestation

◦ Location downslope on mountains/topography

Conditions that are less supportive of flooding

Sand, soil, and plant roots that are porous and can absorb water

x year flood terminology

An “X-Year Flood” means that in any

given year, there is a 1/X chance of a

flood of that magnitude happening

flooding in urban areas

Few natural surfaces; mostly made of

blacktop and concrete

◦ Water pools and cannot soak into the ground

◦ Also, sewers can get overwhelmed by the

sheer amount of water, exacerbating

flooding

flooding in mountain ranges

Surfaces are highly sloped, gravity pulls water

towards the ground

◦ Surfaces are primarily made of rock and are

not porous

◦ Flooding can be a large issue in the valleys

below if significant rain falls on the

mountainside

flood safety

If flooding occurs, get to higher ground and out of areas prone

to flooding.

◦ Avoid areas already flooded, especially if the water is flowing

fast. Do not attempt to cross flowing streams.

◦ Road beds may be washed out under flood waters. NEVER

drive through flooded roadways - you do not know the

condition of the road under the water

tropical cyclone

A low pressure center that forms in the low latitudes

◦ This contrasts with midlatitude (extratropical) cyclones, which form in the middle latitudes

tropical cyclones encompass three subcategories of storms, which are categorized based on

their sustained wind speeds:

Tropical depression: < 39 mph

◦ Minor rotation, no eye has formed yet

◦ Tropical storm: Winds ≥ 39 mph and < 75 mph

◦ Strong rotation, some form of an eye can exist

◦ Hurricane: ≥ 75 mph

◦ Very strong rotation, typically have an eye

Tropical cyclones may form in the low

latitudes, but that doesn’t mean they

stay there

Tropical cyclones regularly move from the low

latitudes toward the midlatitudes

◦ This frequently leads to their demise

◦ They can still cause considerable damage in

the midlatitudes, even as they weaken

what are the six ingredients of TC formation

1. preexisting low pressure area

2. warm sea surface temperature

3. deep layer of warm surface ocean water

4. rapid cooling of the troposphere with heaight

5. Form between 5-30° N or S of the Equator

describe six necessary ingredients to tropical cyclone (TC) formation

A pre-existing low pressure area

◦ Tropical wave/easterly wave or tropical disturbance

◦ Gives a TC a “jump start” by providing a cluster of rising air and thunderstorms to support formation

2. Warm sea surface temperatures (SSTs)

◦ Rule of thumb: SSTs ≥ 80 °F

◦ Creates warm, moist air parcels that give a TC its energy

◦ Parcels are low density and rise rapidly, reaching saturation easily

3. A deep layer of warm surface ocean water

◦ Rule of thumb: 200 ft deep

◦ This keeps SSTs warm, even when strong TC winds stir up the ocean and cause upwelling

◦ Upwelling: colder water rises up from the deep ocean, cooling the SSTs

4. Rapid cooling of the troposphere with height

◦ Creates an unstable environment where warm, moist parcels can rise rapidly

◦ Leads to rapid/intense release of latent energy within parcels from condensation of water

5. Calm winds through the depth of the troposphere

◦ Also required (and related to these calm winds): weak vertical wind shear

◦ Allows the updraft to remain upright!

◦ Air parcels rise rapidly, and energy stays concentrated around the center of the cyclone

◦ Therefore: TCs form in the absence of jet streams!

6. Form between 5-30° N or S of the Equator

◦ Need Coriolis ≠ 0, so that wind rotates!

◦ Never form on the Equator, because Coriolis = 0

◦ Formation in this zone means Coriolis ≠ 0 and conditions 1-5 are still met

how do tropical cyclones form

he warm sea surface heats the air

above it by conduction.

• Additionally, water from the sea

surface evaporates into the warm air.

• Result: warm, moist air mass next to

ocean surface.

• This warm, moist air mass is the main

source of energy for the cyclone

Warm, moist air is the least dense type

of air

• Air parcels in this type of air mass are

less dense than the colder tropospheric

air above

• This leads the air to rise rapidly through

cool air (it is unstable)

• Weak vertical wind shear exists through

the depth of the troposphere. This helps

the updraft stay upright

• Parcels rise as rapidly as possible

Rapidly rising air parcels create a low

pressure center...

• Next to the ocean’s surface

• At the base of the updraft

• With enough time and storm

presence (may be multiple “pulses”

of storm activity).

• Once the low pressure center forms, the

classification changes to a Tropical

Depression (TD).

Pressure gradient force causes surface

winds to strengthen and converge into

the low pressure center

• Pulls in more warm, moist air

• The updraft strengthens from the

increased energy input!

• Strong surface winds also cause choppy

waves and sea spray

• Enhances evaporation of warm

ocean water into the air

• Further strengthens the TD

As air is lofted and reaches the top of

the troposphere (at the tropopause), the

air cannot move upward any more.

• The air instead fans out horizontally.

Therefore, the updraft diverges aloft

above the low pressure center (i.e. at

the top of the troposphere).

• Due to the stability of the

stratosphere

• Creates a high pressure center aloftDiverging air aloft cools as it

radiates heat to space

• Gradually becomes cooler

(more dense) than the

surrounding air

• This cooler, denser air sinks

downward

• This completes the primary

circulation of the TD!

Condensation releases latent heat,

further strengthening the updraft

• Little vertical wind shear =

latent heat stays closer to the

center of the storm

• Latent heat is the other main

source of energy for the

storm, besides the warm,

moist air mass

Condensation releases latent heat,

which acts to further strengthen

the updraft

• Little vertical wind shear =

latent heat remains closer to

the center of the storm. This

energy accumulates, and

strengthens the updrafts.

• Therefore, latent heat is the

other main source of energy

for the storm (besides the

warm, moist air mass).

Tropical waves/easterly waves

elongated areas of low pressure that

persist for at least a day

◦ Are advected westward by the easterly Trade

Winds

◦ Low pressure creates a cluster of weak

thunderstorms

◦ The start of a TC

In Atlantic, tropical waves form off

Africa’s western coast

o In Pacific, tropical waves form off the

western coast of Central and South

America

Tropical depression (TD)

when a low

pressure center develops within a

tropical wave

◦ Only about 10% of all tropical waves become

TDs

◦ TDs are numbered, not named

◦ Getting all the ingredients together at once to

get a TD is difficult to do!

Sources: NBC, Fort Myers Affiliate

Vertical wind shear

change in

wind speed and/or direction

with height above the ground

• Speed shear: winds

increasing/decreasing with speed

• Directional shear: winds changing

direction

• Vertical wind shear can tilt

updrafts, which is detrimental to

(bad for) TC development.

Tropical disturbances

clusters of

thunderstorms that form along the ITCZ

◦ Form due to enhanced convergence present

◦ As they strengthen, they become closed low

pressure centers, and Coriolis Effect causes

them to rotate

◦ Once the closed low develops, classified as a TD

Factors that inhibit tropical cyclone

formation

1. The lack of a tropical wave can prevent this whole process from starting at all!

2. Cool SSTs or a shallow warm ocean surface layer inhibit the formation of warm,

moist air parcels that can rise freely

3. Dry air prevents rising air parcels from reaching saturation

4. An inversion (warming of the troposphere with height) creates a stable

environment that prevents air parcels from rising

5. Strong vertical wind shear, especially in a jet stream, prevent the central updraft

from remaining upright

◦ Parcels rise less rapidly, weakening the storm

◦ Latent heat is dispersed over a greater area, weakening the storm

Ingredients for TC formation are consistently found in tropical cyclone basins

Basins are all in the low latitudes

◦ Located far enough from the Equator that Coriolis effect is sufficiently strong to create rotation

◦ Dominated by the calm Trade Winds and weak subtropical jet (wind shear is minimized)

◦ Warm/very warm SSTs exist here

Sources: NWS

What we call the strongest TCs

Hurricanes: North Atlantic and Eastern Pacific basins

Typhoons: Western Pacific basin

Cyclone: All other basins

TCs form in the warm season (summer and fall) in each hemisphere

Hurricane forecasts are issued by the National

Hurricane Center (NHC)

NHC forecasts include a “cone of uncertainty”

◦ 60-70% chance the center of the cyclone will stay in

the cone for...

◦ 1-3 days in the future (solid white part of cone)

◦ 4-5 days in the future (stippled white part of cone)

◦ The graphic also plots where the center of the

cyclone would be if it stayed in the middle of the

cone

◦ But, as we know, the center has a 60-70% chance of being

anywhere in the cone! And a 30-40% chance of being outside of

the cone!

o Takeaway: cone tells you where TC is mostly likely

to go

Tropical Storms: The Eye

Although the eye is calm, the eyewall (the

boundary surrounding the eye) is where the

most severe thunderstorms, heaviest rain,

and strongest winds in the entire cyclone

form

◦ Heavy rainfall can cause flooding

◦ Strong winds: damage structures, hurl debris, blow

people over, wreak havoc

how does the eye form

Vertical PGF drives a thin area of

sinking air from the high pressure

center aloft to the low pressure

center at the surface

• Sinking air warms and dries, so this

area is unsaturated

• The eye

• Inside the eye, the weather is

completely calm

• Sinking motions calm the

weather in the eye.

At what point are tropical systems named

Once the TD becomes a TS, it is given a

name

◦ The WMO has lists of names for TSs that form in

various regions around the world

◦ Lists repeat every 6 years, except for any names

retired during the season

what affects the intensity of a storm surge

caused by the winds and low pressure in a TC

o Storm surges are enhanced in narrow places

like inlets, bays, and rivers

◦ Large volumes of water in tiny spaces

◦ Sea level rises even higher than elsewhere along the

coast

Storm surge is enhanced along coasts

with flat topography

◦ Water can travel further inland

o Also enhanced on coasts that are at or

below sea level

◦ Water has to rise less to have serious impact

Sources: NASA

Light blue shading = areas at

or below 10 m above sea level

spiral rainbands

encircle the eyewall

◦ Areas of moderate to heavy thunderstorms

◦ Narrow: tens of miles wide

◦ Can extend 50-300 miles outward

◦ Tornadoes, if they form, most often form here

◦ Air parcels rise within the spiral rainbands and sink in the

downdrafts around them

◦ Creates alternating secondary areas of rising and sinking motion

within the cyclone

RADAR

RAdio Detection And Ranging

o “A device that transmits and receives pulses of

electromagnetic (EM) energy to determine the

location of an object.

◦ Pulsed Doppler Radar

◦ Energy is in the “microwave” spectrum

◦ Sent out in a specific direction (focused by an

antenna)

o Energy is absorbed and then scattered by

objects it encounters; some energy is scattered

back to the radar

◦ Gives us information about the size, shape,

composition, orientation, and velocity of the particles

how radar works reflectivity

1. The radar dish emits a beam of

microwave energy (radar beam) in

pulses (not continuously). The beam

encounters precipitation as it travels

2. Precipitation scatters a portion of the

energy from the radar beam in all

directions

• Some is reflected back toward

the radar

• This reflected energy is called

the “radar echo” or simply the

“echo”

3. The energy in the echo returns to

the radar, and the radar antenna

collects and measures the reflected

energy

4. The radar will then emit another

pulse, and the whole process starts

again!

radial velocity

the component of precipitation’s

motion toward or away from the radar dish

winter weather precipitation and temperature

Rain

◦ Temperature is above freezing at the surface, much of profile

◦ Ice melts into rain

Snow

◦ Temperature is below freezing for the entire depth of the profile

◦ Ice never melts, falls as snow

Sleet

◦ Temperature is above freezing for a short period of time, where ice

partially melts

◦ Temperature is then below freezing again, where any liquid water freezes

into ice on the surface

Freezing Rain

◦ Temperature is above freezing for a long period of time, where ice fully

melts into rain

◦ Temperature is then below freezing again, but rain doesn’t have enough

time to freeze back into ice

◦ Rain freezes into ice when it hits the below-freezing surface.

Rain Snow

Sleet Freezing Rain

Extratropical cyclone ETC

large, swirling storm systems that form along the polar jet

◦ Size: several hundred to a thousand miles across

◦ Lifespan: days to well over a week

◦ Essentially, this is a very strong midlatitude cyclone

how are ETCs One major way of balancing Earth’s temperature!

◦ As the Equator warms and poles cool, ETCs transfer warm air northward and cold air

southward

◦ Typically occur in late fall to spring, when temperature gradient is the greatest

lake effect ingredients

2 main ingredients:

◦ Warm Lake

◦ Cold/very cold airmass flowing across

the lake

wind chill

A calculation of the “feels like” temperature based on:

◦ Actual temperature

◦ Wind speed

How are low pressure systems and ETCs related?

low pressure systems (which influence winter storm

formation/lifecycles)

Oftentimes, ETCs form from deep troughs

◦ These deep troughs are associated with intense polar jet streams

o Large “reservoir” of cold air is often associated with these systems

◦ Large, cP or cA airmasses move into the region in their wake

◦ This very cold air helps to fuel the system, intensifying the trough and associated midlatitude

cyclone.

◦ This produces heavier precipitation, as the stronger low pressure -> greater vertical motion

blizzard

A severe weather condition characterized by high winds and reduced

visibility

What is required for a winter storm to be considered a blizzard by the

NWS?

Sustained wind or frequent gusts of 16 m/s (30 kt or 35 mph) or greater

◦ Falling and/or blowing snow

◦ Visibility reduced to less than 400 m (0.25 mi) for 3 hours or more

◦ No temperature requirements

lake effect

localized,

convective (instability-based) snow

bands that occur on the Lee side

(mainly east side) of lakes when

relatively cold air flows over

warmer waters. (AMS 2018)

◦ Can result in huge snow accumulations

over short time periods

describe lake effect formation

:

1. Cold air flows from the upwind side of the lake. As air moves from the shore out over the open

water, it accelerates due to the decreased friction. This creates surface divergence on this

shoreline, and fosters the development of sinking motions/localized high pressure

development.

2. This air moves over the lake’s surface, and heat/moisture is transferred to the air as it does so.

3. This heat/moisture transfer creates upward motions due to the unstable environment. The

longer the air’s residence time, the greater the instability will be.

4. As parcels of air travel upward, moisture within quickly condenses and forms clouds,

precipitation. The clouds and precipitation travel along the lake with the wind flow towards

the shore.

5. As the flow encounters the shoreline, friction increases once again, causing winds to converge

which further encourages upward motions and precipitation generating processes to occur.

This upward motion/convergence also triggers localized low pressure development.

6. Snow falls on the Lee (downwind) side of the lake. The high/low pressure further reinforces the

wind flow pattern across the lake, and the cycle repeats

what are the six steps of lake effect formation

Cold air flows from the upwind side of the lake.

This air moves over the lake’s surface, and heat/moisture is transferred to the air as it does so.

This heat/moisture transfer creates upward motions due to the unstable environment.

as the flow encounters the shoreline, friction increases once again, causing winds to converge

which further encourages upward motions and precipitation generating processes to occur

Snow falls on the Lee (downwind) side of the lake

long lake axis parallel snow bands

Long residence time over lake (winds travel across parallel the longer

axis) usually yields strong

heatwave

several days of higher than normal temperature

Defined relative to the normal (average) air temperature at a given

location

◦ Heat wave in August = way hotter than heat wave in November

5 causes of heatwaves

Deep ridges

◦ Clear skies

◦ Calm winds

◦ Dry ground

◦ High pressure

heatwave impacts

The number one cause of weather-related deaths in the world

o Health risks

◦ Rash, edema, cramps, dehydration,

◦ In worst cases: low blood pressure, heat stroke, death

o Psychological strain

◦ Productivity decreases

◦ Psychological strain and crime increase

o Infrastructure

◦ Damage caused by expansion and contraction of the ground in high heat

◦ Cement cracks, roads buckle, water lines burst, metal kinks

o Rising power and fuel costs

◦ Cooling needs for occupied buildings

how do heatwaves form

Ridges are regions of calm weather—clear skies and calm winds

◦ Clear skies = more sunlight = more solar heating

◦ Calm winds = little temperature advection = warm air persists

◦ Mor/e solar heating = ground dries out via evaporation, and dry ground heats up even faster

than wet ground

◦ Dry, hot ground = clouds are even more unlikely

o A heat wave produces a positive feedback that sustains itself

What upper air pattern is observed with heat waves?

Ridge of High Pressure: A ridge is an elongated area of relatively high atmospheric pressure. In the upper atmosphere, this ridge causes air to sink, which compresses and warms the air, leading to higher temperatures at the surface

How does moisture affect heatwaves?

Surface high pressure centers typically

form on the eastern side of a ridge

◦ Circulation around that surface high can advect

warm, moist air into the region experiencing the

heat wave

o Moisture exacerbates health effects of the

heat wave

◦ People and animals can’t cool off as easily,

because it is harder for their sweat to evaporate

o Also, air sinks over a high pressure center

◦ Sinking = drying and warming occurs (parcels)

apparent heat

takes into account the air temperature, relative humidity, radiant energy

(sunlight), and wind speed

heat index

same as apparent temperature, but assuming some constant radiant

energy and wind speed

◦ Heat index is solely a factor of temperature and relative humidity

higher temperatures = higher heat index

Higher relative humidity = higher heat index

urban heat island

During heat waves, the heat is often worse in cities, especially in

downtown areas

◦ Up to 10 °F higher

o This phenomenon happens ESPECIALLY at night when compared

to countryside

◦ More solar radiation absorbed by dark, man-made materials during the day, so

more released at night

◦ Less evapotranspiration off of plants at night, so less evaporative cooling

◦ Heat from air conditioners and vehicles warms up the area more

◦ Buildings radiate heat both laterally and upwards; lateral radiation retains the

heat

◦ Winds overall are light at night, so no advection of cooler countryside air into

the city

drought

A deficiency (lack) of precipitation in an area over an extended

period of time.

◦ Consideration is commonly given to the local climate of an area when determining whether a

drought is occurring or not

relationship between droughts and heatwaves

Often times, droughts occur hand-in-hand with heatwaves, as interactions

exist between the two.

◦ Think of how the positive feedback mechanism between dry conditions and heatwaves.

These things exacerbate one another

Many of the same mechanisms that trigger heatwaves also help to contribute

to droughts.

o Ex.) Large scale atmospheric circulation (ridges) remain in place for long

periods of time.

o Ridges are locations of very warm air

◦ Due to relationship with Atmospheric Thickness

Does radar send out energy continuously?

The radar does not emit energy

continuously, but in discrete pulses

• Allows the radar to “listen” for

returns

• Set time between pulses, called

the pulse repetition time (PRT)

• Typically 1 ms

• Duration of the pulse: 𝜏

• Typically 1 us

• Time between pulses (dashed

line) is when the radar listens

for echoes from precipitation

• Time spent listening for

returns >> time spent

emitting pulses

what affects magnitude of reflectivity

The magnitude of reflectivity depends on

precipitation size, type, and amount

What do various reflectivity values/colors mean

Blues: typically only for snow and edges of rain

echoes

◦ Greens: light precipitation

◦ Yellows: moderate rain

◦ Reds: heavy rain/small hail

◦ Pinks, purples, and whites: intense rain, possibly

severe hail

What does reflectivity tell us about our scatterers?

eflectivity reveals the size, material, density, and surface characteristics of scatterers by measuring how effectively they reflect incident waves

What colors are used to show velocity on a radar display?

Green colors indicate that the wind is

moving toward the radar

o Red colors indicate that the wind is

moving away from the radar