Transcript cold front
How do the blizzards from? Review of last lecture Tropical climate: Mean state: The two basic regions of SST? Which region has stronger rainfall? What is the Walker circulation? Mean state: Two types of ocean upwelling, ocean-atmosphere feedback El Nino and La Nina: Which region has warm SST anomaly during El Nino? 4-year period. Land-sea contrasts: seasonal monsoon Extratropical climate: Mean state: westerly winds, polar vortex What is the primary way El Nino affect extratropics? (PNA) The oscillations associated with strengthening/weakening of polar vortex: AO, AAO Video: Weather fronts http://www.youtube.com/watch?v=tkK4 _F0VKhM Air masses An airmass is a large (usually thousands of km across) volume of air that has horizontally uniform properties of temperature and moisture. Airmasses acquire their properties from spending days to weeks over the same part of the Earth. “Polar” airmasses are colder than “tropical” airmasses “Maritime” airmasses are wetter than "continental" airmass Other specific airmass types include "arctic", "equatorial", and “monsoon” Bergeron classification of air masses 3 letters: e.g. mTk, cPw 1st letter for moisture properties: c - continental, m - maritime 2nd letter for thermal characteristics: T - tropical, P -polar, A Artitic/Antarctic, M - monsoon, E - equatorial, S -superior air(dry air formed by significant downward motion in the atmosphere) 3rd letter for stability: k/w - air colder/warmer than ground Source regions The areas where air masses form are called source regions. Fronts A weather front is a boundary separating two air masses Types: cold front, warm front, stationary front, occluded front, dry line, squall line Cold Fronts • A cold front is a mass of cold air advancing towards warm air. • Typically associated with heavy precipitation, rain or snow, combined with rapid temperature drops. • Since friction decreases with height, winds move faster at higher altitude. Then the surface of cold front becomes more steeper through time, leading to a narrow belt of precipitation. • Moving speed up to 30mph Satellite and radar images of cold fronts (narrow belt of clouds/precipitation) Warm Fronts • Warm fronts are warm air moving towards cold air. • Friction decreases with height, so winds move faster at higher altitude. This causes the surface of the front to become less steep through time. Then clouds will be spread to a wider region. • Shallow stratus clouds dominate and bring light precipitation. Frontal fogs may occur as rain evaporates in the colder air near the surface. • Moving speed about 12 mph Satellite and radar images of warm fronts (wide region of clouds/precipitation) Stationary Fronts • Stationary fronts do not move. They do not advance. They are two unlike air masses side by side. • They may slowly migrate and warmer air is displaced above colder. From Environment Canada Video: Science of a Blizzard http://www.history.com/videos/science-of-ablizzard#science-of-a-blizzard What is a mid-latitude cyclone? The mid-latitude cyclone is a synoptic scale low pressure system that has cyclonic (counterclockwise in northern hemisphere) flow that is found in the middle latitudes (30N-55N, 30S55S). It has a larger size than a tropical cyclone Midlatitude cyclones often form near the fronts and jet streams Jet streams are caused by steep temperature gradients between cold and warm air masses Polar front - marks area of contact, steep pressure gradient polar jet stream Low latitudes subtropical jet stream Stronger in winter, affect daily weather patterns How does a mid-latitude cyclone form? In mid-latitude there is a boundary between northern cold air and southern warm air In the boundary a initial cyclone can advect warm air northward and cold air southward Mature stage. Cold air begins to catch up with warm air (occluded). If the upper level low is to the west of surface low, the cyclone will amplify and precipitation will form. Cold air cools down the cyclone. Dissipation. Why do some frontal waves develop into huge cyclonic storms, but others don’t? Complex challenge to forecasting; Atmospheric conditions at the surface and aloft affect cyclogenesis. Key is to look at the upper level winds (longwaves, shortwaves). Longwave disturbances (Rossby waves) Earth's poles are encircled by 3 to 6 longwaves, or Rosby waves, directing upper level winds around lows at the 500 mb surface. Small disturbances in these waves can trigger storms. Shortwave Disturbances Shortwave ripples within the Rossby waves move faster, and propagate downwind into the Rossby troughs and cause them to deepen. Barotropic conditions, where isobars and isotherms are parallel, then degenerate into a baroclinic state where the lines cross and cold or warm air is advected downwind. Regions of cyclogenesis and typical tracks • Gulf of Mexico, east coast • Alberta Clipper from eastern side of Canadian Rockies • Colorado Low from eastern slope of American Rockies Lee-side lows, lee cyclogenesis Summary 1. Definition of airmasses. Bergeron classification of air masses (3 letters). 2. Fronts: 6 types (cold, warm, stationary, occluded, dry line, squall line) 3. Cold front (narrow, fast, heavy precipitation), Warm front (wide, slow, light precipitation) 4. The developmental stages and vertical structure of middle latitude cyclones (boundary between northern cold air and southern warm air, upper level low to the west of surface low) 5. How upper level longwaves and shortwaves may enhance cyclonic development at the surface (upper level low to the west of surface low) 6. The three regions of cyclogenesis and typical tracks