Meteorology and the Dynamics of
Severe Weather at Sea
(Back to Basics)
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Sea-level pressure (2D of the 3D)
Fronts (temperature boundaries)
Highs and lows (weather systems)
The weather “pattern” (500 mb charts)
Putting it all together
www.weather.gov
www.opc.ncep.noaa.gov
Joe Sienkiewicz
NOAA / NWS Ocean Prediction Center
College Park, MD USA
Why sea-level pressure?
U.S. NOAA
JMA
South Africa
Brazil
ATMOSPHERIC PRESSURE
7 Feb 1861 – First marine storm warning
Adm. Robert Fitzroy
Barometric pressure
• The pressure exerted by the atmosphere as a
consequence of gravitational attraction
exerted upon the "column" of air lying directly
above the point in question. (AMS Glossary)
Average Sea-Level Pressure
1013.25 (mb millibars,
hPa – hecto Pascals)
1018 mb
1022 mb
Barometric pressure
• 2 dimensional reflection of the 3 dimensional
density distribution. (Joe)
Average Sea-Level Pressure
1013.25 (mb millibars,
hPa – hecto Pascals)
1018 mb
1022 mb
1018
1022
1022
1018
1018
1022
• Temperature distribution is the most important
regulator of atmospheric density!
1018
1022
1022
1018
1018
1022
Pressure ~ Density ~ Temperature
Pressure Gradient ~ Wind Dir/Spd
1018
1022
1022
1018
1018
1022
Temperature distribution ~ Wind Dir/Spd
Hi
If earth were not rotating – Highs would fill lows
(Coriolis effect)
Hi
Instead winds blow inward toward lows and
outward from highs
Some thoughts… observing the
weather
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Routine observation of weather and logging
Watch change discussion
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Changes during off going watch
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Anticipated changes oncoming
Barometric trace – relate to wind speed / sea state
Read the water
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Sir Francis Beaufort and the Beaufort Scale
Relate the local conditions to the bigger picture
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Use graphical analyses as a benchmark
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Place your position on the analyses
Wind is THE FORCE!!
Driven by PRESSURE GRADIENT!!
Pressure Gradient due to Temperature Gradient
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Power
Waves (wind wave and swell)
Storm surge
Pressure
Gradient
Wind
Friction
Coriolis
Wind direction
~ 15 to 20 degrees across isobars towards lower pressure
The pressure gradient is the source!
FRONTS
A Front almost invariably separates air masses
of different temperature. (AMS Glossary)
Along with… the common temperature criterion,
many other features may distinguish a front,
such as:
• a pressure trough,
• a change in wind direction,
• a moisture discontinuity,
• characteristic cloud and precipitation forms.
A Front is a zone of temperature difference or
gradient.
Surface Weather Map Symbols
• Pressure Regions:
High
Low
Trough
• Front Symbols
Warm
Cold
Stationary
Occluded
H
L
—————
Sinking air
Rising air
Trough – air density
difference
Boundaries between
air of different character
(temperature,
moisture)
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STORMS
Extratropical cyclones
Frontal lows
Low pressure systems
Mid-latitude cyclones
Shapiro – Keyser Cyclone Model, 1990
I.
Frontal Wave II. Frontal Fracture
III. Bent-back front IV. Warm-core frontal
and T-bone
seclusion
- Evolution 3-5 days (birth to death)
- Can undergo explosive intensification
- Can produce winds of hurricane force
500 hPa pressure surface
• 500 mb pressure surface
– avg. height 5600 m - 18,000 ft.
– range – 4700 m near poles (cold)
to 6000 m near equator (warm)
• Contour Lines - Heights
– Higher the height – the less dense the atmosphere
• Add a 0 to get altitude in meters
– Closer the contours – faster the wind
• Winds
– flow nearly parallel to the contour lines
• wind barbs show direction and speed
– patterns used to predict surface weather
Surface Analysis
Courtesy
NOAA
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North Pacific
500 mb Analysis
http://www.opc.ncep.noaa.gov/shtml/ppaa50s.gif
Courtesy
NOAA
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North Atlantic
500 mb Analysis
Key
25 kt
30 kt
35 kt
Ocean Winds (QuikSCAT)
Courtesy
NOAA
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The 5640 m Contour
• 5640 meter height contour
– the “564 decameter line”
– Usually shown as a thicker line
• Surface storm tracks
– lie 300 to 600 nm north
– parallel the contour line
• Worst weather typically poleward
– Strongest winds and seas
• in the cold air
• western quadrants of mid-latitude storm
systems
• Southern extent of 30 kt summer, 35 kt
winter
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Putting it all together
• Use all available sources for planning/underway
– Gridded forecasts (GRIB)
– Graphical surface and 500 hPa analyses and forecasts
– Plot position on graphics (know what to expect)
• Aboard
– Weather impacts all aspects – make it a discussion for everyone (informed
crew)
• Teaching opportunity for mates and crew
• Visible display of weather information (analyses and forecasts)
• Weather observations
– Know your vessel – tolerances and procedures
– Bad weather – factor in fatigue / seas sickness
• Science
– Forecast capability and observing capabilities have never been better –
take advantage of that!
– And yes there is always room for improvement!