Transcript lecture01

Weather Systems
Part 1: Overview
Goal: provide a ‘big picture’ overview of:
a.
why weather systems exist
b.
different types of weather systems, including
interactions
c.
why should we care
The General Circulation
The role of the general circulation is to redistribute energy from
the tropics (surplus) to the poles (deficit)
The General Circulation - Tropics
The Hadley circulation
describes a large (almost half
the surface of the Earth)
thermal circulation
DLA Fig. 10.27
The General Circulation – Subtropical Jet
DLA Fig. 10.28
The conservation of angular momentum requires a steadily
increasing eastward component to the flow as the air
progresses poleward in the upper branch of the Hadley
circulation
AM = mvr = constant
Global Scale – Middle and High Latitudes
General features of note
• pressure: subtropical high and
polar high separated by the
polar front
• horizontal flow: prevailing
midlatitude westerlies and polar
easterlies (again, polar front)
• vertical motion: Ferrel cell
DLA Fig. 10.30
there are many discrepancies between the cartoon and
observed weather conditions because: (i) large day-to-day
variability in position of H and L, (ii) large seasonal variability,
and (iii) land sea contrasts
Ocean-Atmosphere Interactions – Ocean Currents
Surface winds exert frictional force on ocean surface
Ocean circulation transports large amounts of heat poleward
The General Circulation
August 31, 2015 (03 UTC)
in reality, unsurprisingly, the atmosphere ‘appears’ much more
chaotic than the idealized, time mean pictures shown in the
previous slides
Weather Systems
Part 1: Overview
Goal: provide a ‘big picture’ overview of:
a.
why weather systems exist
b.
different types of weather systems, including
interactions
• El Niño-Southern Oscillation (ENSO)
• Tropical cyclones
• Madden Julian Oscillation
• Extratropical transition
• Downstream development
• Rossby wave breaking
• Atmsopheric blocking
c.
why should we care
Scales of Atmospheric Motion
DLA Fig.10.2
The space and time scales of motion for a particular type of system are
the characteristic distances and times traveled by air parcels in the
system (or by molecules for molecular scales).
El Niño – Southern Oscillation (ENSO)
observational ‘representation’ of the current El Niño from NOAA
El Niño – Southern Oscillation (ENSO)
Change in the tropical pressure,
wind, precipitation and ocean
temperature (with global
repercussions)
normal
El Niño
El Niño – Southern Oscillation (ENSO)
Weekly Sea Surface Temperature Anomalies
Source: Australian Bureau of Meteorology
(www.bom.gov.au)
El Niño – Southern
Oscillation (ENSO)
5-day sub surface
temperatures
anomaly couplet =
shift of warm waters to the east
El Niño – Southern Oscillation (ENSO)
5-day Trade Winds
Westerly anomaly =
‘weaker’ trade winds
El Niño – Southern Oscillation (ENSO)
Outlook
El Niño – Southern Oscillation (ENSO)
Global Impacts
El Niño – Southern Oscillation (ENSO)
1997
2015
Tropical Cyclone
Typhoon Maysak
(2015 ) as seen
from the
international space
station
Large rotating storms forming over warm oceans in tropical regions.
Growth mechanism is typically attributed to a positive feedback
between surface processes (fluxes) and deep moist convection
Tropical Cyclone
Tropical Cyclone
Necessary (but not sufficient) Conditions:
1. Sea surface temperatures greater than 26 C
2. Potentially unstable atmosphere allowing deep moist
convection
3. Moist mid-troposphere
4. Latitudes poleward of 5 degrees providing strong enough
Coriolis
5. Low vertical wind shear
6. Pre-Existing low-level disturbance providing sufficient vorticity
Tropical Cyclone
Capable of inflicting significant damage to property and life
(roughly 26 billion USD / year worldwide)
1.
2.
3.
4.
High winds
Storm surge
Heavy precipitation
Tornado genesis
Tropical Cyclone - Structure
Tropical Cyclone
August 31, 2015
0730 UTC
Major Hurricane
Ignacio:
200 km/hr
Major Hurricane
Kilo:
200 km/hr
Major Hurricane
Jimena:
240 km/hr
Invest:
80% of
formation
Tropical Cyclone
Source: Joint Typhoon Weather Center
http://www.usno.navy.mil/JTWC/
Source: National Hurricane Center
http://www.nhc.noaa.gov/
Hurricane Fred
130 km/hr
Tropical Cyclone – Formation Mechanisms
Tropical Cyclone – Formation Mechanisms
Nearly 60% Atlantic TCs
exhibit baroclinic
features
McTaggart-Cowan
etal. 2008
McTaggart-Cowan
etal. 2008
The Madden Julian
Oscillation
• Large-scale coupled patterns in
deep convection and atmospheric
circulation
• Zonal extension of roughly 1220,000 km
• Eastward propagation of about ~
5 m/s
• Average lifecycle of 48 days
• Dominant intraseasonal
variability in the tropics
Importance of the MJO
• The MJO modulates:
–
–
–
–
active/break cycles of monsoons
tropical cyclone numbers
air-sea interaction
tropical-extratropical interaction: latent heat
release forces planetary scale Rossby wave
trains that can propagate over the globe
Madden Julian Oscillation (MJO)
Madden Julian Oscillation (MJO)
Madden Julian Oscillation (MJO)
review paper attempts to connect the MJO to sensible
weather outside of the tropics
Madden Julian Oscillation (MJO)
Precipitation
Madden Julian Oscillation (MJO)
Tropical Cyclones
Madden Julian Oscillation (MJO)
Surface Temp.
MJO Supplement