OVERVIEW:

We will discuss constant-pressure sections of the upper atmosphere; the equivalence of pressure gradient and contour gradient, vertical profiles of mid-latitude pressure systems; and the types and characteristics of fronts and troughs.

OUTLINE:

1. Constant pressure sections of the upper atmosphere.

1.1. The troposphere and lower stratosphere are routinely measured by radiosondes (instrumented balloons) on a world-wide basis. Measurements are taken at 0000 and 1200 GMT. These data are collected at a central location and processed into a three-dimensional snapshot of the atmosphere.

1.2. Horizontal cross-sections of the upper atmosphere (UA) are taken at constant pressures rather than constant heights. (A surface isobaric chart, with observed pressures extrapolated to sea level, is a constant-height chart.) The pressure gradient force (PGF) and the contour gradient force (CGF) are equivalent. In the upper atmosphere, the CGF is balanced by the HDF. There are five constant-pressure UA charts routinely used in meteorology:

1.2.1. 850 millibar (hPA): Approximately 1500 meters (5500 feet) AGL. This chart is especially useful for identifying airmass types, fronts, and pressure centers free of local (surface) effects.

1.2.2. 700 millibar (hPA): Approximately 3000 meters (10,000 feet) AGL. This chart is useful for locating areas of significant moisture and vertical velocity in the middle troposphere. It is also helpful in forecasting the peak wind speeds possible during thunderstorm-related downdraft events.

1.2.3. 500 millibar (hPA): Approximately 5500 meters (18,000 feet) AGL. This chart is useful for locating middle-tropospheric baroclinic troughs and ridges, i.e. sources of chimney exhaust for dynamic lows and dampening for dynamic highs. A related form of analysis involves identifying areas of positive and negative vorticity or spin in the middle troposhere. The Arctic jet stream is often visible on this chart.

1.2.4. 300 millibar (hPa): Approximately 9200 meters (30,000 feet) AGL. This chart is useful for locating the Polar jet stream, and upper-level pressure systems (such as cutoff lows and highs). The long wave pattern of barotropic troughs and ridges is clearly visible on the hemispheric version of the 300 mb chart.

1.2.5. 200 millibar (hPa): Approximately 12000 meters (40,000 feet) AGL. This chart is often useful for locating the Subtropical jet and the Polar jet (in summer). Areas of warm air at 200 millibars usually correspond to areas of cold air at 300 millibars, and indicate regions of sinking motions in the stratosphere.

1.3. UA charts are routinely contoured with lines of equal height, as well as isotherms. The 300 and 200 hPa charts are also analyzed for isotachs. Plots also contain information about height change over the previous 12 hours.

2.1. Warm core -- Central, vertical column of warm air:

2.1.1. High - The Subtropical Ridges. Strong closed cell of high pressure on the surface; grows progressively stronger with height.

2.1.2. High - "Cut-Off" High. Open ridge (anticyclonic curvature in open flow) to weak closed cell of high pressure on the surface; grows progressively stronger with height.

2.1.3. Low - The Equatorial Trough (a.k.a. The Intertropical Convergence Zone, or ITCZ). Complex of multiple closed cyclonic (and cyclostrophic) cells and weak ridges on the surface; deep trof aloft.

2.1.4. Low - Thermal Lows (e.g. The "Taco" Low). Open trof of cyclonic curvature to moderately deep closed low on the surface; growing progressively weaker above 850 hPa; often capped by strong ridge or closed warm-core high at 500 hPa and above.

2.2. Cold core -- Central, vertical column of cold air:

2.2.1. High - The Polar Highs. Very strong but relatively shallow dome of high pressure (closed cell) on the surface; grows progressively weaker aloft and is replaced in the upper troposphere and lower stratosphere by "the polar vortex."

2.2.2. High - The Continental Highs. Shallow dome of high pressure (closed cell) on the surface, often quite strong (esp. in winter months); grows progressively weaker with height, and is replaced at 500 hPa and above by a deep cold-core trof or closed low pressure (heights) cell. "Arctic" airmass highs are much shallower than "polar" airmass highs.

2.2.3. Low - The Subpolar Lows (e.g. The Icelandic Low). Moderate to very strong closed low pressure on the surface, growing progressively stronger with height.

2.2.4. Low - "Cut-Off" or Distal Lows. Open trof (cyclonic curvature in open flow) to weak closed cell of low pressure on the surface; grows progressively stronger with height.

2.2.5. Low - Deeply Occluded Lows. Weak to moderate closed low pressure cell on the surface; grows progressively stronger with height.

2.3. Baroclinic systems: -- System center lies in the zone between the cold core and the warm core:

2.3.1. Low - The Dynamic Low. Vertical profile is highly dependent on the stage in its life cycle. During the mature stage, it is marked by strong, closed circulation throughout the lower 500 hPa. Closed cyclonic circulation may reach as high as 300 hPa. Unlike the barotropic systems, the baroclinic lows tilt back toward cold air with increasing height.

2.3.2. High - The Dynamic High. Vertical profile is highly dependent on the stage in its life cycle. Marked by strong, closed circulation throughout the lower 500 hPa. Closed anticyclonic circulation may reach as high as 700 hPa. Unlike the barotropic systems, the baroclinic highs tilt forward toward warm air with increasing height.

3.1. Definition: Fronts are regions of low pressure (i.e. troughs) that form the boundaries between airmasses of different densities.

3.2. Related terminology:

3.2.1. Transition zone: Region of rapid density (temperature) change between airmasses.

3.2.2. Front: Edge of the transition zone, next to the warm air.

3.2.3. Frontal surface: The edge of the transition zone (on the warm side) in three dimensions.

3.2.4. Surface front: Where the frontal surface intersects the ground.

3.2.5. Upper front: Line of intersection of the frontal surface with an upper-atmospheric isobaric surface.

3.2.6. Slope: Change in the vertical height of the frontal surface with horizontal distance from the surface front, expressed mathematically as (Y_f - Y_i)/(X_f - X_i).

3.3. Semi-continuous Polar Front: The Polar Front represents the boundary between the Earth's polar and tropical airmasses. (It is dynamically linked to the Polar Front Jet.) In areas where the transition zone is highly diffuse (such as within a high pressure area), the front is impossible to identify.

3.4. Types of fronts: Frontal types are determined by the instantaneous movement of the airmasses.

3.4.1. The stationary front - The airmasses show no discernable movement.

3.4.2. The cold front - A colder (denser) airmass is advancing, undercutting, and displacing a warmer (lighter) airmass.

3.4.3. The warm front - A warmer (lighter) airmass is advancing and replacing a retreating colder (denser) airmass.

3.4.4. The quasi-stationary front - Airmass movement is less than 5 knots, or 1 degree of latitude or less in 12 hours.

3.4.5. The occluded front - Three airmasses air involved: Cold, warm, and cool.

3.4.5.1. The cold occlusion - The coldest air is behind the cold front (e.g. cP), and the cool airmass is ahead of the warm front (e.g. mP). Warm air (e.g. mT) is south of the transition zone. The occlusion occurs where the advancing cold airmass undercuts the retreating cool airmass, lifting the surface warm front aloft. Common on the east coast of continents.

3.4.5.2. The warm occlusion - The coldest air is ahead the warm front (e.g. cP), and the cool airmass is behind of the cold front (e.g. mP). Warm air (e.g. mT) is south of the transition zone. The occlusion occurs where the advancing cool airmass overrides the retreating cold airmass, lifting the surface cold front aloft. Common on the west coast of continents; has a characteristic "T" shape.

3.5. Surface trofs (a.k.a. troughs): Roughly linear regions of low pressure (i.e. fronts are trofs in addition to being airmass boundaries), often extending from closed low centers. They are created by a number of different processes. Old occlusions (where the airmass discontinuities have all washed out) are one cause. Trofs are often the only surface expression of a powerful upper-tropospheric distal low. The dry-line trof separates the southwestern U.S. cT airmass from the southeastern U.S mT airmass, and is a common trigger for thunderstorms and tornadoes during the summer months.

3.6. UA trofs: Identified as roughly linear regions of low heights on constant-pressure charts. Range from weak disturbances to strong, highly baroclinic systems. At 850 hPa, trofs reflect the surface front, but are stacked toward the cold air. At 500 hPa, trofs associated with surface fronts are known as "major shortwaves" (and are stacked toward the cold air) and large-scale barotropic trofs associated with the jet stream are known as "longwaves." (700 hPa trofs are similar to 500 hPa; but may also reflect 850 hPa features.) 300 and 200 hPa trofs are generally longwaves.

LAB:

Divide class up into groups of three. Group divides up the following tasks:

1. Analyze isotherms on 850 hPa chart -- Identify fronts and airmasses.

2. Analyze height contours on 500 hPa chart -- Identify highs, lows, troughs, and ridges.

3. Analyze appropriate streamlines and isotachs on 300 hPa chart -- Identify jet axes and speed maxima.

HOMEWORK:

1. Read Lutgens and Tarbuck chapter 11.

2. Analyze another upper air chart (850, 500, or 300 hPa) -- try one that you did NOT do while in lab. Download the pre-plotted chart from the SUNY Albany Weather Page.