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Air Masses and Fronts C. David Whiteman Atmos 3200/Geog 3280 Mountain Weather and Climate What is an air mass? • • • • A regional-scale volume of air with horizontal layers of uniform temperature and humidity They form during high pressure episodes when weak winds allow air to remain for several days over a flat area with uniform surface characteristics Origin: • • • • • Whiteman (2000) Tropical ‘T’ Polar ‘P’ Arctic ‘A’ Continental ‘c’ Maritime ‘m’ Examples: mT warm, moist, usually unstable; cT hot, dry, unstable air at sfc, stable aloft; mP cool, moist, unstable; cP cold dry stable; cA extremely cold Source regions and trajectories for air masses affecting North America. Fronts • The mid-latitudes are not a good source region for air masses; they are a region where clashing air masses meet • Boundaries between air masses are called fronts or frontal zones. • Fronts are associated with surface low pressure centers • Fronts move counterclockwise around a low as the low moves and evolves Fronts can be visualized by imagining a continuous front separating cold air over the pole with warmer air to the south. Types of fronts • • Surface front – A front that intersects the earth’s surface • Cold front – A front where cold air is advancing • Warm front – A front where cold air is retreating • Occluded front – the surface where cold and warm fronts meet (or zones of warm and cold advection meet) • Back-door cold and warm fronts – fronts that move into an area from an unusual direction (e.g., a cold front from the east) Upper-level front – A front that is located aloft, but does not extend to the surface Whiteman 2000 Typical wintertime weather map with a front Define: warm sector Buys-Ballot Rule Development of a polar front More about fronts • Additional characteristics • • • • Locally high stability (cold air near surface, warm aloft) Wind directions shift across front Change in wind with height (vertical wind shear) Pressure troughing • Frontogenesis – Process of increasing the magnitude of the horizontal temperature gradient (creating a front) • Frontolysis – Process of decreasing the magnitude of the horizontal temperature gradient (destroying a front) • Frontal slopes • • Warm front 1:150 to 1:300, speed 3-7 m/s Cold front 1:50 to 1:150, speed 7-13 m/s Front-cloud associations • Frontal analysis in complex terrain of western U.S. Challenges Errors arise from the reduction of surface pressure to sea level Difficult to determine intensity or even existence of horizontal temperature gradients in regions due to variability in surface station elevation Conventional observations (NWS/FAA/DoD) over western U.S. are of low density and are located primarily in valleys Diabatic effects and boundary layer processes can obscure large-scale airmass changes Surface-based inversions mask temperature changes Terrain-induced flows (thermally or dynamically driven) mask wind changes There can be contrasts in frontal intensity and position between low and high elevation stations • • • • • • • J. Steenburgh