Transcript tropical_introductio..

Initial Tropical Training
for Satellite Analysts
AFWA/XOGM
Tropical Section
Originally produced by:TSgt Nast
Revisions by: Paul McCrone
1
Sources
AWS/TR-95/001
AWS
TR240
NAVEDTRA 40970/40971
JTWC Forecasters Handbook
NEPRF TR-85-01
Tropical Weather Course (Keesler)
Tropical TIPS
2
Overview
Primary
Physical Controls of the Tropics
Planetary-Scale Circulation in the Tropics
Non-Severe Weather Systems and Tertiary
Circulation's
Basic Analysis and Circulation Models
Monsoons
Climate Anomalies
Tropical Cyclones
Severe Weather in the Tropics
Tropical Forecasting
3
Primary Physical Controls of the tropics
Earth’s
Energy Budget
Effects of Land-Sea Distribution
Terrain Effects
Diurnal Effects
4
Primary Physical Controls of the tropics
5
AWS/TR-95/001
Page 11
Figure 2-1
6
Earth’s Energy Budget
7
Earth’s Energy Budget
 Vertical
–
–
heat transfer mechanisms
Radiation - not very significant
Sensible heat transfer
» Conduction - molecular boundary layer
» Convection - air warmed by conduction
–
Latent heat transfer - Most important vertical
mechanism in tropics
» Condensation (latent heat becomes
sensible heat)
» Evaporation (Sensible heat becomes
latent heat)
8
Earth’s Energy Budget
9
10
Earth’s Energy Budget
 Horizontal
–
Heat Transfer
Horizontal heat transfer mechanisms
Sensible
heat transfer through warm- or coldair advection
About 40% of earth's total horizontal heat
exchange occurs through ocean currents.
Latent heat transfer.
–
–
Moisture advection.
Latent heat release aloft, carried poleward by the
Hadley cell.
11
Earth’s Energy Budget
12
Effects of Land-Sea Distribution
Determines climate type (monsoon,
maritime, Continental.).
 Max seasonal variations occurs over land.
 Most convection and latent heat-sensible
heat conversion takes place over land.
 Land-sea breeze circulation's are a direct
result.

13
Terrain Effects
High altitude more typical of mid-latitude
weather.
 Rainfall:

–
–
Leeside or windward location usually most
important factor.
In trade winds, heaviest rain usually on
slopes just below trade-wind inversion.
14
Terrain Effects

Whether a location is on the leeside or
windward side is a very important factor
Rainfall
15
Diurnal Effects
 Temperature
Range
 Clouds
 Rainfall
16
Diurnal Effects
 Temperature
–
–
–
Range
On small islands and coastlines 3-5°C
with prevailing onshore flow.
On inland locations or coasts 5-10°C
with prevailing land breeze.
In the interior in the dry season >
10°C.
17
Diurnal Effects
 Clouds
– Oceans
Maximum
(0400-0700L),
minimum (1400L-1900L)
– Land
Daytime
maximum,
nocturnal minimum
18
Diurnal Effects
 Rainfall
Nocturnal max over oceans, and
small islands.
– Shower maximum over land in
afternoon.
– Monsoon areas and areas in
disturbances have night to early
morning maximum.
–
19
Planetary-Scale Circulation in
the Tropics
Definitions
and General Characteristics
Primary Weather Zone
Tropical Wind Profiles
Upper Tropospheric Features
20
Definitions and General
Characteristics
 Classical
–
Definitions.
Ancient Greeks - Tropics of Cancer to
Capricorn (23.5N to 23.5S)
–
–
Alexander Supan - Average Annual Temp >
68 F (20°C)
Early 1900s - W.P. Koppen
 World
is divided into climatic Zones A to F.
 A= tropical rainy, Af = rain forest, Aw = savanna,
Am = monsoon.
21
Definitions and General
Characteristics
 Dynamic
Definitions - allow for
seasonal, longitudinal variations.
– Dividing line
H
H
between midTropospheric
H
H
easterlies and
westerlies (axes
of STRs).
H
H
22
Definitions and General
Characteristics
 Tropical
weather characteristics
– Cumulus clouds predominate.
– Conditionally unstable.
– Air temperatures comparable to
ocean surface temperatures.
– Predominately easterly low-level
flow.
23
Definitions and General
Characteristics
 Climatology
is more important in
the tropics than it is in the midlatitudes
–
Weather dominated by semipermanent, slowly migrating systems
Subtropical
Ridge (STR)
Equatorial Trough (ET)
24
Primary Weather Zones and
Systems
 Subtropical
–
Ridge (STR)
Surface
Centered
on five oceanic anticyclones.
Over the oceans:
– shrinks and retreats equatorward in winter,
grows and builds poleward in summer.
(inducing TUTT formation)
25
Primary Weather Zones and
Systems
 Subtropical
–
Ridge (STR)
Surface
Axis
tilts ENE-WSW in Northern
Hemisphere, ESE-WNW in Southern
Hemisphere.
Ridge axis normally 23-35 degrees
latitude North and South
26
Primary Weather Zones and
Systems
27
Primary Weather
Zones and Systems
 Subtropical
Ridge (STR)
H
Aloft
H
»Slopes to equator
H
L
with height.
H
»Position of axis at 200 H
mb is 15-20° latitude.
H
»More elongated east-west
than the surface ridge.
–
28
Primary Weather
Zones and Systems
 Trade-wind
–
–
–
region
Region of low-level prevailing easterlies
between STR and ET.
NE trades in Northern Hemisphere, SE
trades in Southern Hemisphere.
General subsidence with capping tradewind inversion.
29
Equatorial Trough (ET)
 Other
–
–
–
–
–
terms
Near equatorial trough (NET).
ITCZ or ITC.
Doldrums.
Meteorological or thermal equator.
Near-equatorial Tradewind Convergence
(NETWC).
30
Equatorial Trough (ET)
 Location,
–
–
–
strength, circulations vary
Generally found where trade winds from
each hemisphere converge
Low pressure from heating and upward
motion
May be axis of convergence or series of
cyclones
31
Equatorial Trough (ET)
 Several
–
–
–
–
streamline configurations:
Near Equatorial Convergence Zone (or
trade-wind trough).
Monsoon trough
Confluent westerlies.
Equatorial Buffer Zone
32
(ET) Streamline
Configurations
 Near
Equatorial Convergence
Zone (or trade-wind trough).
Confluent zone between trade
winds of each hemisphere.
– Usually over oceans, but can
extend over adjacent land areas.
–
33
Trade Wind Trough
34
(ET) Streamline Configurations
 Monsoon trough
– Often
a series of cyclonic centers:
Heat lows (over land primarily).
Thunderstorm clusters, non
developing.
Monsoon depressions.
Tropical disturbances.

35
(ET) Streamline Configurations
 Monsoon
trough
– Locations
Africa
- Sub-Sahara in July and
Kalahari-Madagascar in January.
Central America - East Pacific:
south of Central America all year
and occasionally crosses into
Caribbean.
36
Africa - Sub-Sahara in July
37
(ET) Streamline Configurations
 Monsoon
trough
– Locations
South Asia
- West Pacific: Iraq to
northern India through SE Asia to
Guam.
Australia - along the northern coast
from December through February.
38
39
40
(ET) Streamline Configurations
• Confluent westerlies.
Locations:
West of Central America in fall.
NW of Australia in Southern
Hemisphere summer.
Winds generally light, speed
convergence increasing
downstream.
41
(ET) Streamline Configurations
H
H
C
Monsoon
Trough
C
Confluent
Westerlies
Tradewind
Trough
H
H
H
(ET) Streamline Configurations

Equatorial Buffer Zone
–

July:
Southern Hemisphere SE winds become
SW entering monsoon trough.
Equatorial Buffer Zone
– January:
 Northern
Hemisphere NE winds become NW
entering Southern Hemisphere monsoon trough.
43
(ET) Streamline Configurations
 Equatorial
–
Buffer Zone
B
Buffer Cells
Weak,
closed circulations near
equator where curving winds "cut
off".
No net inflow or outflow.
Labeled with a "B" on streamline
analysis.
44
Tropical Vertical Wind
Profiles: Deep Easterlies





Low-level east winds remain easterly with
height
Found within 15° of equator
May extend to 30° in summer hemisphere
Not in all seasons or regions
Narrowest north-south extent at 200 mb
45
Tropical Vertical Wind
Profiles: Shallow Easterlies
 Shallow
–
–
–
Easterlies
Low-level easterlies become westerly with
height
Average 15-30° latitude, closer to equator in
winter hemisphere
Example - Hawaiian islands (20°N) in
shallow easterlies all year
46
Tropical Vertical Wind
Profiles: Shallow Westerlies
 Low-level
westerlies become easterly
with height.
 Found in monsoon regions in
summer.
 Examples: western Pacific, Indian
Ocean, western Africa (shallower in
Africa).
47
Upper-Tropospheric Features
 Subtropical
Jet (STJ)
Found 20-35°N and 25-32°S
(average).
– Causes
–
Upward
branch of Hadley cell
circulation.
Conservation of angular momentum
accelerates air to east as it moves north.
48
Sub Tropical Jet
49
Upper-Tropospheric Features
 Tropical
–
–
–
–
Easterly Jet (TEJ)
Feature of northern hemisphere (NH) summer
monsoon in Asia
Very persistent in NH summer months
Location: central Africa to SE Asia (from 5
degrees to 20 degrees North latitude)
Strongest winds (80-100 kts) over Arabian Sea
between 40,000-55,000 ft (200-100 mb).
Highest observed winds: 152 kts
50
Upper-Tropospheric Features
51
Upper-Tropospheric Features
 Tropical
–
Easterly Jet (TEJ)
Causes
 Uniquely
high temperatures and heights of the
isobaric surfaces of the Tibetan Plateau during
summer (See Southwest Monsoon later)
 The above causes an upper-level high to develop
over Tibetan Plateau.
 Flow from high conserves angular momentum as it
moves south, accelerates toward the west.
52
Upper-Tropospheric Features
 Tropical
–
Easterly Jet (TEJ)
Causes
 Creates
upper level divergence over India, and
speed convergence over eastern Africa
 This circulation contributes to the widespread
lifting and convection north of the jet over southern
Asia, and strong subsidence and aridity (dryness)
over North Africa and Middle East.
53
54
Upper-Tropospheric Features
 Tropical
Upper-Tropospheric Trough
(TUTT)

Forms in summer hemisphere over mid-oceans.
Subtropical ridge - part that moves
poleward over land masses (North America,
eastern Asia).
Subequatorial ridge - part that stays near
equator over the ET.
55
Upper-Tropospheric Features
 Tropical
Upper-Tropospheric Trough
(TUTT) - Continued Level:
Most intense - 200 mb.
Orientation:
ENE-WSW
Season:
Late April - Mid November (most
intense in August
56
Upper-Tropospheric Features
 Tropical
Upper-Tropospheric Trough
(TUTT) - Continued Position:
South of Surface Subtropical
Ridge over trade winds (convergent
weather associated with TUTT occurs in
tradewinds)
Major Convergent Weather: Few degrees
southeast of upper level center
Weather in axis or Cyclonic Cell Center:
Few clouds, general sinking motion
57
Upper-Tropospheric Features
 Tropical
Upper-Tropospheric Trough
(TUTT) - Continued Temperature:
Cold trough (3-5 degrees
colder than environment, Isold TCU and
TSTMs in TUTT, and center of cells
Axis Windshift: Often an abrupt 180 degree
turn at trough axis
Cirrus Tracers: Indicates > 50 kts either
side of TUTT or embedded cell.
58
59
Non-severe Weather Systems
and Tertiary Circulations
•Lines
•Tropical Waves
•Vortices
•Land and Sea Breezes
•Valley and Mountain Breezes
60
Non-severe Lines
 Lines
- synoptic scale convergence with
length much greater than width.
– Squall lines
– Cold fronts
– Shear lines
– Surge Lines
– Near Equatorial Convergence Zones
61
Shear Line
62
63
Non-Severe Tropical Waves
National Hurricane Center (NHC)
definition - a trough or cyclonic curvature
maximum in the trade-wind easterlies.
 Sometimes called “Tropical Easterly
Waves” in the Atlantic.
 Originally, this term was based on older
research on the tropics that was based on
sparse surface and upper air data.

64
Non-Severe Tropical Waves
When NHC started using METSAT
imagery in 1967, they saw new aspects of
the so-called “wave” that didn’t fit the
theoretical model.
 Not as common as previously thought;
many systems called waves were actually
vortices
 This model is falling out of favor with
many in the tropical community.

65
Non-Severe Tropical Waves

AFWA/XOGM policy is to not use this
term (“wave”) to describe tropical
phenomena of any kind, despite the fact
that NHC and other DoD units use it - we
believe many so-called “waves” are
actually vortices, and that the term is
overused. Further, it has been shown that
these “waves” very seldom develop into
cyclones.
66
Non-Severe Tropical Waves

If you call it anything, call it a “tropical
disturbance”.

The next slide shows examples of
phenomena that used to be called “waves”
67
The late, great
“Tropical Easterly Wave”
68
Non-Severe Tropical Waves

Characteristics
–
–
Many stem from upper-level cyclones (cold
lows).
Characteristics of Atlantic (easterly) waves.
 Form
over Ethiopian Highlands June to October.
 Move across baroclinic zone south of Sahara often form squall lines.
 Dampen under STR axis in eastern Atlantic and
strengthen near Lesser Antilles.
69
Inverted “Vee” or Screaming Eagle - really a circulation
70
Screaming Eagle
71
Non-Severe Vortices

Fair weather vortices - (Equatorial
anticyclone, heat low, and TUTT)…

Bad weather vortices - (Tropical cyclones,
monsoon depression, west African cyclones,
and mid Tropospheric cyclones).
72
Fair Weather Vortices

Equatorial anticyclones
–
–
–
Found where monsoon trough is more than
10° from equator
Surge from opposite hemisphere crosses
equator and turns anticyclonically
Curved band of cloud seen at leading edge
of anticyclone
73
Fair Weather Vortices

Heat lows/ heat troughs
–
–
–
Low-level air rises over hot land with
subsidence aloft.
Lowest pressure coincides with highest
temperature.
Examples
 Sahara and SW Asia in summer (may be part of
monsoon trough).
 Southern South America, southern and eastern
Africa all year.
74
Bad Weather Vortices

Mid-tropospheric cyclones.
–
Subtropical cyclones.
 Cut-off
portion of deep mid-latitude trough in
the tropics in winter.
 Example: "Kona" storms in Hawaii, cause SW
winds and heavy rains.
–
Arabian Sea cyclones
 Major
rainmaker on west coast of India in SW
monsoon.
 Cloud pattern resembles a typhoon.
75
Tertiary Circulations

Land and sea breezes
–
Sea-breeze characteristics.
Sets up a few hours after sunrise.
 Moves inland until late afternoon to evening.
 Strong breezes may extend 30 - 50 miles inland.

–
Land-breeze characteristics.
Generally shallower with weaker winds than sea breeze.
 Normally does not penetrate as far offshore as sea breeze
penetrates onshore.
 Land breeze front often triggers convection, especially
converging land breezes

76
Rainfall - Special Cases
Prevailing flow onshore:
Prevailing flow offshore
Daytime max on inland
slopes
Afternoon max on
coastline
Day
Sea Breeze
Nighttime max on coastline
Night
Land Breeze
77
Sea Breeze
78
Sea Breeze
79
Land Breeze
80
Tertiary Circulations

Orographically-induced winds.
–
Valley breeze.
 Slopes
warm during day.
 Upslope wind.
 Clouds and convection over peaks during daytime.
–
Mountain breeze.
 Slopes
cool at night.
 Downslope wind.
 Clouds and convection in valleys at night.
81
Tertiary Circulations

Orographically-induced winds.
–
Mountain gap winds.
 Funneling
through passes when surface gradient
across mountains is strong.
 Example: Tehuantepecer winds off Mexico.
82
Tehuantepecers
83
Tehuantepecers
84
Tehuantepecers
85
Basic Analysis and Circulation
Models
•Definitions
•General Techniques
•Draw Asymptotes
•Streamlining to satellite Images
•Recommended Procedures
•Auxiliary Analysis Techniques
86
Definitions

Streamline - solid lines which are tangent to the
instantaneous wind direction

Isotachs - lines which connect points of equal wind
speed. Not necessarily parallel to the flow

Isogons - lines which connect points of equal wind
direction

Asymptotes - special streamlines onto which other
streamlines intersect (confluent) or emanate from
(diffluent)
87
Definitions

Singular point - a point into which or
from which more than one streamline
can be drawn. Examples are:
–
–
–
–
Vortices
Neutral points (Cols)
Cusp
Buffer Cell
88
General Techniques
Streamline spacing should not necessarily
correspond to wind speed.
 Try to find neutral points and vortices first.
 Consider all wind reports carefully before
ruling them out.
 Draw arrowheads and tails at edges of
charts and at singularities.

89
Drawing Asymptotes
 In
general
– Convergers
begin at neutral
points and flow into cyclones
– Divergers begin at anticyclones
and end at neutral points
(Cols)
90
Rules for Drawing Asymptotes
 Rules
for confluent asymptotes.
 Flow
should converge smoothly
from either side.
 They should line up with
convective cloud bands on lowlevel charts and clear areas on
upper-level charts.
91
Rules for Drawing Asymptotes
 Rules
for diffluent asymptotes.

Flow should diverge smoothly from a
single axis
 In low-levels, they should be in clear areas
(on satellite pictures) or in areas of closedcell stratocumulus.
 In upper-levels, they should overlay areas
of upper-level clouds
92
Rules for Drawing Asymptotes
 Rules
for linked vortices.
– Two like-vortices must have a neutral
points in between.
 A series of cyclones is bounded by
diffluent asymptotes. A series of
anticyclones is bounded by confluent
asymptotes.
93
Recommended procedures
 Overlay
current chart on 24-hour
continuity and mark important
features in yellow. Keep previous
chart for reference.
 Overlay current chart on satellite
image, or draw the significant
features from the image on your
chart.
96
Recommended procedures

Tentatively mark major features (large
cyclones and anticyclones, hurricanes, etc.).

Start streamlining near dominant features like
the STR and undisturbed trade-wind areas.
Work in toward complicated areas.

Light winds occur with cols, high centers,
buffer cells, and areas of sharp curvature.

Strong winds occur with cyclones and broad
areas of little curvature.
97
Auxiliary Analysis Techniques

Vertical Time Cross-section
–
–

An analysis of changes in the atmosphere with
time over a station
Useful in the tropics because small changes in the
vertical over a station become readily apparent
Vertical Space Cross-section
–
–
An analysis of a cross section of the atmosphere
along a line or route at a particular time.
Selected where reporting stations are available
reasonably close to the line of interest.
98
Auxiliary Analysis Techniques
 Checkerboard
Can
Diagram
show current weather over an area
Diurnal
weather pattern recognition and
monitoring
Is used to represent climatology
Identifying small changes in weather that
may be masked by normal diurnal patterns
99
Auxiliary Analysis Techniques
 Skew-T
–
/ Rawinsonde data.
Determine thermodynamic variables
Stability
- determine indices particular to
station.
Moisture availability - LCL, CCL, LFC,
etc.
Height rises/falls.
Analysis same as in mid-latitudes.
100
Monsoons
*** Monsoon rains flooding Bangladesh
(AP report from 22 July 1999)
DHAKA, Bangladesh (AP) - Two days of torrential
rains flooded another 10 villages in northern
Bangladesh, marooning at least 20,000 people,
authorities said Tuesday. Rescue teams were
dispatched with food and medicine after heavy
rains swelled the Padma and Jamuna rivers in an
101
Monsoons
*** Monsoon rains flooding Bangladesh (con’t)
area about 65 miles north of Dhaka, the
Bangladeshi capital. Last week, 55,000 people
were washed out of their homes when the Gumti
River overflowed its banks and submerged 200
villages in Comilla district, 55 miles east of
Dhaka. Most of those people are now camped
on the muddy river embankment.
102
Monsoons

Definitions
–
–
Glossary of Meteorology, Huschke, 1959: "A
name for a seasonal wind. Derived from Arabic
'Mausim', a season":
Monsoon is one of those overused words in
meteorology, often used to describe any
regularly occurring summertime rainy period.
103
Monsoons

Definitions
–
Colin Ramage, 1971, defined a monsoon in
terms of four criteria:
 Prevailing
wind directions shifts by at least 120°
between January and July.
 Average frequency of occurrence of the prevailing
wind in both January and July exceeds 40%.
 Mean resultant winds in at least 1 of these 2 months
exceeds 3 m/s (6 kts).
 Definite lack of migratory systems.
104
Monsoons

General characteristics of monsoons
–
Summer
 Surface
pressure falls over continents due to heating.
 Increased pressure gradient between continent and
nearby ocean causes onshore flow.
 Circulation forms.
–
–
–
Rising air over continent.
Offshore return flow aloft.
Sinking air over oceans.
105
Monsoons

General characteristics of monsoons
–
Winter
 Surface
pressure rises over continents due to cooling.
 Pressure gradient reverses from summer situation,
causing offshore flow.
 Circulation forms:
–
–
–
Rising air over ocean.
Onshore return flow aloft.
Sinking air over continent.
106
107
Monsoons
Asian Monsoon
 African Monsoons
 Australian Monsoons
 Central American Monsoons

108
Monsoon Seasonal Locations
110
111
112
113
114
The Asian Monsoons

Division of Asia into 3 monsoonal regions
–
East of Tibet
 Damp
–
cold winters, wet summers.
West of Tibet
 Mostly
–
Desert.
South of Tibet
 Dry
mild winters, hot wet summers.
115
The Asian Monsoons

Summer (Southwest) Monsoon (Jul-Aug)
–
–
–
The Himalayan Mountain range prevents CAA
Monsoon trough extends from NE Africa to near
Guam
Tropical Cyclones
 Rare
–
–
–
in South Asia in summer.
Vertical shear associated with TEJ.
Subsidence over oceans.
Monsoon trough anchored over land.
 Typhoons
increase over western Pacific in summer.
116
The Asian Monsoons

Summer (Southwest) Monsoon (Con’t)
– Onset: Main Driving Force - extreme differential heating
between land and water areas, which results in:
–
–
–
–
–
Strong Subtropical highs in the Southern Hemisphere (SH)
Strong heat lows develop over Northern Africa, Saudi
Arabia, Pakistan, Northern India, and SE Asia.
Strong surface heating that causes the land mass to become
warmer then the equatorial region
Over land, heights are lower in the low levels and higher in
the upper levels (compared to equatorial region) creating a
strong pressure gradient force
Low level flow is directed out of the SH Subtropical Ridge,
across the equator and into monsoon trough.
117
The Asian Monsoons

Summer (Southwest) Monsoon (Con’t)
– Moisture:
Air travels a great distance over water and becomes
high in mixing ratio (moisture content).
 Upward vertical motion within the trough produces
condensation and release of large amounts of latent
heat.
 Latent Heat warms the atmosphere which results in
stronger pressure gradient force (PGF).

118
The Asian Monsoons

Summer (Southwest) Monsoon (Con’t)
– Coriolis:
As southeasterly winds cross the equator, the
coriolis force turns the winds to southwesterly,
resulting in the Indian Ocean/Westpac buffer zone.
 Southwesterlies tend to produce upward vertical
motion, resulting in another mechanism for the
release of latent heat and stronger PGF.

119
The Asian Monsoons

Summer (Southwest) Monsoon (Con’t)
– Key Features:


TEJ - Tropical Easterly Jet (mentioned previously)
Somalian Jet
– A low level jet that extends from the east coast of Africa
across the North Arabian Sea (NAS)
– Core of Jet: between 3000 and 5000 ft (850 mb)
– Max winds: 55 kts
120
The Asian Monsoons

Southwest Monsoon Key Features (con’t)
– Somalian Jet Dynamics:
Winds Cross the equator near Africa and turn to southwesterlies
 Terrain over Eastern Africa creates a barrier and wind speeds
increase due to a channeling effect
 Winds parallel the coast of Somalia while heat lows inland and
friction create cross isobaric flow over land This results in low
level diffluence and upwelling of cold water off the Somali
coast.
 Cycle then feeds off itself - clear skies bring more heating,
stronger diffluence, cooler sea surface temps, and stronger wind
speeds.

121
The Asian Monsoons
122
The Asian Monsoons

Southwest Monsoon Key Features (con’t)
– Monsoon Depressions
 Bad
weather vortices that form in the monsoon trough.
 Most common in northern Bay of Bengal, occasionally over
land or the Arabian Sea.
 Resemble Tropical Cyclones on satellite images. Seldom
become Tropical Cyclones, but they can develop as such
 Characteristics.
–
–
–
–
Horizontal diameter: 100 km
Can persist a week or more, usually 3 - 4 days.
Locally very heavy rain (mainly in southwest quadrant ), winds
commonly 40 kts sustained.
Can be cold core.
123
The Asian Monsoons

Southwest Monsoon Key Features (con’t)
– Mid-Tropospheric Disturbances
 North-South
wind shear causes these systems to develop
 Most common in Northern Arabian Sea.
 Occur between the low level westerlies and the upper level
easterlies (600 mb)
 Characteristics.
–
–
–
Horizontal diameter: 500 km
Unlike monsoon depressions, they can persist for many days.
Also produce locally very heavy rain
124
The Asian Monsoons

Southwest Monsoon Key Features (con’t)
– Onset Vortex:
 Cyclonic
Circulation off the southern coast of India (in the
NAS) prior to the onset of the SW Monsoon
 Caused when the southwesterlies are deflected around the Ghats
Moutains (Southern India)
 Characteristics.
–
–
–
–
Vorticies are small and last 2-3 days
Also produce locally very heavy rain (they dumped 10 cm of rain
in 24 hours!)
Existence is not always noticeable from year to year
Rarely, can become tropical cyclones
125
The Asian Monsoons

Southwest Monsoon Key Features (con’t)
– Breaks in the Monsoon:
 A decrease
in rainfall in central India
 Caused
by an extreme northward shift in the Tropical Easterly
Jet and Monsoon trough
 Causes
an increase in rainfall over the foothills of the
Himalayas and extreme southerm tip of India
126
The Asian Monsoons

Winter (Northeast) Monsoon (Nov.-April)
–
–
Siberian High sets-up, CAA kills Indian Ocean
cyclones
Crachin" can form in coastal Southern China
northern Indochina.
 Persistent
stratus regime.
 Rain, fog, drizzle, low visibilities.
 Sets in about January, may last till April.
127
The Asian Monsoons

Winter (Northeast - NE) Monsoon (Con’t).
–
–
Mainly southern Asian region
During NE monsoons, strong winds flow from
the extremely cold continental High centered
near Lake Baykal in southern Siberia (near
Mongolia) in a series of surges often greater
than gale force speeds
128
The Asian Monsoons

Winter (Northeast - NE) Monsoon (Con’t).
–
Onset:
Sun
position migrates southward (decrease in solar
declination), thus decreasing solar insolation
(heating) over Asia. This will, in turn, fill the heat
lows south of the Tibetan Plateau
The Tibetan Plateau cools rapidly, resulting in a
temperature/pressure reversal.
 Polar outbreaks begin to occur north of the
Himalayas as the Siberian High continues to
intensify.
129
The Asian Monsoons

Winter (Northeast - NE) Monsoon (Con’t).
–
Onset (con’t):
Surface
winds over Asia become northeasterly.
 As this takes place, the upper level anticyclone
which feeds the Tropical Easterly Jet weakens. This
allows the more conventional subtropical westerly jet
to become predominant.
 By the last week in November, the NE monsoon is
normally established. This seems to be a fairly firm
time, and corresponds to a dramatic decrease in
tropical cyclone activity in the South China Sea.
130
The Asian Monsoons

Fall/Spring Transition
– Spring.
 Mei-yu
(Baiu) front is most active from Japan to
Taiwan.
 Peak formation time for tropical cyclones
–
Fall.
 Cold
surges start in Southeast Asia
 Tropical cyclones increase.
131
132
133
134
The African Monsoons

West African Monsoons
–
July: North African summer monsoon, South
African winter monsoon.
 Heat
low forms over Sahara.
 High over southern Africa and South. Atlantic.
 Pressure gradient sets up strong southerly flow.
 Circulation shallower than Asian monsoon (onshore
flow up to 850 mb).
135
137
1000 mb Winds
138
925 mb Winds
139
850 mb Winds
140
700 mb Winds
141
500 mb Winds
142
400 mb Winds
143
300 mb Winds
144
250 mb Winds
145
200 mb Winds
146
150 mb Winds
147
The African Monsoons

West African Monsoons
–
January: North African winter monsoon, South
African summer monsoon
 Cooling
produces high pressure over Sahara.
 Heat low forms over Kalahari, not as intense or dry
as over Sahara.
 Precipitation
–
–
–
Drought over North Africa.
Scattered Thunderstorms over southern Africa.
Precipitation less organized and persistent than Asian or
North Africa monsoon.
148
The Australian Monsoons

Summer (Northwest) monsoon
–
–
Low pressure over Australia, high over Asia.
Monsoon trough
 Anchored
from low near 13°S, 170°E
 Extends across northern Australian interior to
southern Africa.
 Monsoon strongest N - NW of Australia due to
land/sea temperature contrast and the NW flow from
buffer zone.
149
The Australian Monsoons

Winter (Southeast) Monsoon
–
–
–
Easterlies over most of Southern Hemisphere
tropics.
Continuous STR northern Australia.
STJ strongest (> 100 kts), near 30°S over
central Australia.
150
The Central American Monsoons

Regional Effects

Region normally dominated by NE trade winds.
– Channeling causes 3 quasi stationary cyclones
on Pacific side of Central America which anchor
monsoon trough.
– Northern Hemisphere easterlies to north and
Southern Hemisphere westerlies to south
maintain cyclones.
151
The Central American Monsoons

Panama Bay cyclone
Strongest
–
–
–
anchoring cyclone of the three.
Reinforced by warm ocean currents.
Southwesterly flow channeled cyclonically by
Andes year-round.
Persistent feature May through January.
152
The Central American Monsoons

Lake Nicaragua cyclone
 Anchored
between Nicaragua and Costa Rica.
 Weaker than Panama bay cyclone, persists May November.

Gulf of Tehuantepec (Guatemalan) cyclone.
 Weakest
of the three.
 Maximum strength in October
 Merges with NECZ to west, which is anchored over
SST thermal maximum.
153
The Central American Monsoons

American Monsoons don't meet Ramage's
Criteria because

Semi-annually reversing North-South pressure gradients do
not form.
–
Instead, Central America has a "transitional"
monsoon.
 Oscillates
north - south semiannually.
 Location determined by strengths of NE trades and
Southern Hemisphere southwesterlies.
154
The Central American Monsoons

Monsoon Surges
–
Often caused by low-latitude (10 - 20°N)
tropical cyclones.
 Acceleration
of southwesterlies from south of
monsoon trough.
 NE
flow aloft back into Southern Hemisphere.
155
The Central American Monsoons

Monsoon Surges (Con’t)
Atemporalado
Index
–
“Atemporalado”: term used to describe a winter
rain event in Central America due to a vogorous
cold front or shear line that crosses far enough
south.
–
Often results in Tehuanapecers.
156
The Central American Monsoons
Monsoon Surges (Con’t)
 Rule of Thumb:

Take
SLP difference (DP) between Merida
Mexico and Houston TX.
•
•
•
•
If DP < 12 MB, no SURGE
If DP 12-14 MB, marginal SURGE
If DP 15-19 MB, Nominal SURGE
If DP >20 MB, STRONG SURGE
157
Surge
158
Tropical Anomalies
El Nino / Southern Oscillation (ENSO)
- Warming
of equatorial Pacific waters
~
- Occurs at irregular intervals of 2 to 12 years
 30 - 50 Day Oscillation
- Observed cycles in tropical storm frequency

worldwide

Quasi-biennial Oscillation (QBO)
- Periodic wind direction change in stratosphere
- 2 year cycle of increased occurrence of Atlantic
Hurricanes
159
160
Tropical Cyclone Weather

Precipitation
–
–

Tornadoes
–

Reaches a maximum over oceans in the right rear
quadrant
Reaches a maximum after landfall in the right front
quadrant
Most likely near time of landfall due to strong shearing
Storm surge
–
Storm surge is responsible for most of the deaths along
the coast.
161
Severe Weather In The Tropics
•Severe Thunderstorms
•Non-Convective Winds
162
Thunderstorms
 More
common in tropics than in high
latitudes
 82 % of thunderstorms are over South
America, Africa, and Indonesia
 18% of thunderstorms are over water
 Most are not severe by mid-latitude
(midwest) standards
163
Severe Thunderstorms

Hail
–

Tornadoes and waterspouts
–

Rare in tropics
The typical WBZ in tropics above 12,000 ft
and is usually over 15,000 ft
Most common in the U.S. and Australia.
Location
–
Most severe storms occur when continental
polar air penetrates the tropics and squall lines
or shear lines develop
164
Severe Thunderstorms
165
Severe Winds
 Severe
Winds (winds greater than 30 kts)
– Nonconvective - usually due to
orographic channeling
Surges
through Taiwan Strait (up to 50 kt
winds)
Shearlines moving through Central
America (Northers, Tehuantepecers)
166
Tehuantepecers
167
Tropical Forecasting
 Short-Range
Forecasting.
 Medium-Range and Long-Range
168
Schools of Thought in Tropical
Forecasting
 Climatological
Method.
Day-to-day weather varies little from
climatology.
– The best guide to forecasting is
detailed knowledge of climatology.
–
169
Short-Range Forecasting
Techniques
 Persistence
and Extrapolation.
Works 80% - 85% of time (up to 95%
in monsoon or wet/dry climate).
– Satellite data is great aid in
extrapolation.
–
170
Medium-Range and Long Range
Forecasting Techniques
 Climatology
is best forecast beyond
two to three days

Numerical Weather Prediction
–
–
Current models barely beat persistence over 24
hours
Models work best when weather is linked to
mid-latitude systems
171
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172