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Atmospheric Pressure Meteorology and the Dynamics of Severe Weather at Sea (Back to Basics) • 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 • Routine observation of weather and logging • Watch change discussion • Changes during off going watch • Anticipated changes oncoming • Barometric trace – relate to wind speed / sea state • Read the water • Sir Francis Beaufort and the Beaufort Scale • Relate the local conditions to the bigger picture • Use graphical analyses as a benchmark • Place your position on the analyses Wind is THE FORCE!! Driven by PRESSURE GRADIENT!! Pressure Gradient due to Temperature Gradient • 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 H Sinking air Low L Rising air Trough ————— Trough – air density difference • Front Symbols Warm Boundaries between air of different character Cold (temperature, Stationary moisture) Occluded 21 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 39 North Pacific 500 mb Analysis http://www.opc.ncep.noaa.gov/shtml/ppaa50s.gif Courtesy NOAA 40 North Atlantic 500 mb Analysis Ocean Winds (QuikSCAT) Key 25 kt 30 kt 35 kt Courtesy NOAA 41 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 42 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! .
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