Lesson 20 - Tropical Cyclones

what is tropical weather like temperature wise? why?

• () daily and seasonal () variations; much () than in middle latitudes

• () summer v. winter differences in insolation

• () influence on air temperatures

• mild daily and seasonal temperature variations; much smaller than in middle latitudes

• smaller summer v. winter differences in insolation

• maritime influence on air temperatures

what is tropical weather like pressure wise?

• pressure variations only a () mb during a typical day, except near () cyclones

• pressure variations only a few mb during a typical day, except near tropical cyclones

what is the tropical weather like wind wise?

• () trade winds prevail ( () in NH, () in SH )

• () winds and () conditions in ITCZ

• easterly trade winds prevail (NE in NH, SE in SH)

• light winds and humid conditions in ITCZ

what about away from the equator?

• rainfall maximum is generally in ()

• convection in late () over (), and during early () over ()

• winter precipitation maxima can be created by interaction of trade () currents with () ranges

• rainfall maximum is generally in summer

• convection in late afternoon over land, and during early morning over ocean

• winter precipitation maxima can be created by interaction of trade wind currents with mountain ranges

what are characteristics of hurricanes?

• intense () of tropical origin with maximum sustained surface wind at least () mph (64 kt)

• called () in NW pacific; () cyclones in the southern hemisphere; () otherwise

• intense cyclones of tropical origin with maximum sustained surface wind at least 74 mph (64 kt)

• called typhoons in NW pacific; tropical cyclones in the southern hemisphere; hurricanes otherwise

what is the structure of a typical hurricane?

• symmetric () structure

• symmetric circular structure

where do hurricanes typically form?

• over () oceans between () degrees and () degrees latitude (need the () effect to establish strong cyclonic circulation)

• over tropical oceans between 10 degrees and 20 degrees latitude (need the coriolis effect to establish strong cyclonic circulation)

when is hurricane season?

• ()-() (max in () ) but recent () in pre and post season storms

• june-november (max in september) but recent increase in pre and post season storms

what is the structure of a hurricane?

• () (the center of the hurricane)

• () (around the eye of the hurricane)

• () bands (around the eyewall that curve outwards)

• motion often (), though steered by (…); intensity fluctuates in ()

• eye (the center of the hurricane)

• eyewall (around the eye of the hurricane)

• spiral bands (around the eyewall that curve outwards)

• motion often erratic, though steered by ambient wind; intensity fluctuates in time

what are characteristics of a hurricane structure?

• ()

• (), (), ()less eye ()-() km wide

• eye surrounded by () convection (strongest winds and precip in () )

• ()-level winds spiral in toward () and ascend in () (spiral out () at about () mb)

• sea level () surround the structure

• symmetric

• warm, calm, cloudless eye 20-50 km wide

• eye surrounded by eyewall convection (strongest winds and precip in eyewall)

• low-level winds spiral in toward center and ascend in eyewall (spiral out (diverge) at about 150 mb)

• sea level isobars surround the structure

what is the circulation of a hurricane?

• () circulation ( () pressure) below about 400 mb, () outflow ( () pressure) above 400 mb

• cyclonic circulation (low pressure) below about 400 mb, anticyclonic outflow (high pressure) above 400 mb

what makes hurricanes work?

• () and (…) release in tropical thunderstorms () the troposphere, producing () pressure aloft and () surface pressure

• as surface pressure (), surface winds ()

• stronger surface wind picks up more () (more (…) from the ocean)

• more water vapor fuels more () and more (…) release, further () the pressure

• a () feedback cycle

• convection and latent heat release in tropical thunderstorms warm the troposphere, producing high pressure aloft and low surface pressure

• as surface pressure decreases, surface winds strengthen

• stronger surface wind picks up more evaporation (more water vapor from the ocean)

• more water vapor fuels more convection and more latent heat release, further decreasing the pressure

• a positive feedback cycle

what is the hurricane formation feedback cycle?

• () cyclone

• 1. () pressure, 2. () surface wind, 3. high () rates, 4. more (…), then cycle repeats back to #()

• intensifies cyclone

• 1. decreasing pressure, 2. stronger surface wind, 3. high evaporation rates, 4. more latent heat, then cycle repeats back to #1

how is a tropical cyclone typically gauged by?

• the strength of the maximum sustained wind

what are the four development stages and their intensity?

1. tropical () - organized ()

2. tropical () - < () mph

3. tropical () - () to () mph

4. () (typhoon; tropical cyclone) - > () mph

1. tropical disturbance - organized convection

2. tropical depression - < 39 mph

3. tropical storm - 39 to 73 mph

4. hurricane (typhoon; tropical cyclone) - > 74 mph

what is the saffir-simpson scale of intensity in terms of categories?

• category 1: (wind = ()-() mph), (storm surge = ()-() ft)

• category 2: (wind = ()-() mph), (storm surge = ()-() ft)

• category 3: (wind = ()-() mph), (storm surge = ()-() ft)

• category 4: (wind = ()-() mph), (storm surge = ()-() ft)

• category 5: (wind = > () mph), (storm surge = > () ft)

• category 1: (wind = 74-95 mph), (storm surge = 4-5 ft)

• category 2: (wind = 96-110 mph), (storm surge = 6-8 ft)

• category 3: (wind = 111-130 mph), (storm surge = 9-12 ft)

• category 4: (wind = 131-155 mph), (storm surge = 13-18 ft)

• category 5: (wind = > 155 mph), (storm surge = > 18 ft)

what are formation conditions of a hurricane?

• sea surface () greater than () degrees C

• form in () and () (warmest SSTs) - ( ()-() in NH, peak () ) & ( ()-() in SH, peak () )

• typically form in () to () degree latitude (need coriolis force)

• require a pre-existing (), or () wave, often within the () (sometimes beneath an upper-level (), atlantic hurricanes commonly form on the () side of an easterly wave west of africa)

• () winds in environment required for development

• sea surface temperatures greater than 26 degrees C

• form in summer and fall (warmest SSTs) - (june-nov in NH, peak sep) & (dec-apr in SH, peak feb)

• typically form in 10 to 20 degree latitude (need coriolis force)

• require a pre-existing disturbance, or easterly wave, often within the ITCZ (sometimes beneath an upper-level trough, atlantic hurricanes commonly form on the eastern side of an easterly wave west of africa)

• weak winds in environment required for development

what are the usual movements of hurricanes in the NH?

• move toward the west at first then curve toward the northeast

• often erratic

what are the usual movements of hurricanes in the SH?

• move toward the west then recurve toward the southeast

• often erratic

what is the path dictated by?

• () structure and interaction with surrounding () fields, () level winds, () streams, and sea surface ()

• storm structure and interaction with surrounding wind fields, upper level winds, jet streams, and sea surface temperature

what are hurricane hazards?

1. () surge ( () cause of damage/death) - rise in sea level up to () ft or more above normal levels due to () and () pressure

2. strong () (up to about () mph sustained)

3. () (several feet of rainfall possible with excessive runoff)

4. () are also favored in the strong thunderstorms that develop around hurricanes

1. storm surge (main cause of damage/death) - rise in sea level up to 20 ft or more above normal levels due to wind and low pressure

2. strong wind (up to about 200 mph sustained)

3. flooding (several feet of rainfall possible with excessive runoff)

4. tornadoes are also favored in the strong thunderstorms that develop around hurricanes

what is a hurricane watch?

• indicates that hurricane force winds are possible within the next 36 hr (long stretch of coastline)

what is a hurricane warning?

• when hurricane force winds are expected to strike a shorter stretch of coastline within 24 hr

what are the hurricane watches/warnings issued by?

• () hurricane center (coral gables, florida), or () hurricane center (honolulu, hawaii)

• national hurricane center (coral gables, florida), or pacific hurricane center (honolulu, hawaii)

are hurricanes more common in recent decades?

• there are () cycles of hurricane frequency - (there is about a () yr cycle in the north atlantic hurricane season, the last maximum was during the 1960s-1970s, another active phase occurred during 1995-2006)

• () warming may take hurricanes more intense, while the overall numbers might () change significantly

• () and () impact increasing as more development occurs along coastline

• there are natural cycles of hurricane frequency - (there is about a 30 yr cycle in the north atlantic hurricane season, the last maximum was during the 1960s-1970s, another active phase occurred during 1995-2006)

• global warming may take hurricanes more intense, while the overall numbers might not change significantly

• economic and societal impact increasing as more development occurs along coastline

how do TC and ETC form and derive their energy?

• TC: () heat (from () ocean)

• ETC: () gradients

• TC: latent heat (from warm ocean)

• ETC: temperature gradients

propagation characteristics of TC & ETC?

• TC: ()ward in the () winds

• ETC: ()ward in the ()-latitude ()erlies

• TC: westward in the trade winds

• ETC: eastward in the mid-latitude westerlies

structure of TC & ETC?

• TC: () core with () bands; (s); () pressure aloft

• ETC: () shaped w/ () bands; () in jet stream upstream (aloft)

• TC: circular core with spiral bands; symmetric; high pressure aloft

• ETC: comma shaped w/ frontal bands; trough in jet stream upstream (aloft)

the weather TC & ETC produce?

• TC: () surge, extreme (), torrential ()

• ETC: () winds, (r )/(s)/(i), (b), () lines

• TC: storm surge, extreme winds, torrential rainfall

• ETC: weaker winds, rain/snow/ice, blizzards, squall lines

how are TC & ETC similar?

• () precip

• c…

• severe () possible

• () impacts

• heavy precip

• cyclones

• severe TS possible

• societal impacts