1 / 31

Fronts: Structure and Observations

Fronts: Structure and Observations. Fronts – Structure and Observations. Definition and Characteristics Definition Common Characteristics Frontal Slope Frontal Types Cold Fronts Warm Fronts Occluded Fronts Coastal Fronts Upper-Level Fronts. Definition and Structure.

malha
Download Presentation

Fronts: Structure and Observations

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Fronts: Structure and Observations M. D. Eastin

  2. Fronts – Structure and Observations • Definition and Characteristics • Definition • Common Characteristics • Frontal Slope • Frontal Types • Cold Fronts • Warm Fronts • Occluded Fronts • Coastal Fronts • Upper-Level Fronts M. D. Eastin

  3. Definition and Structure • Definition: • Pronounced sloping transition zone between two air masses of different density • Disagreements and Caveats: • What defines an air mass? What defines a transition zone? • → Are we restricted to the synoptic-scale Bergeron air mass classifications? • → Do baroclinic zones induced by physical geography gradients count? • → Do drylines with minimal temperature gradients count? • → Must a density gradient of certain magnitude be present? Daytime Cloudy Cool Clear-Dry Warm → Do temperature gradients that “disappear” at night (or during the day) count as fronts? Nighttime Cloudy Cool Clear-Dry Cool M. D. Eastin

  4. Definition and Structure • Our Definition: • In this course we will use a less restrictive definition of fronts as air mass boundaries • without certain gradient requirements throughout the diurnal cycle, but we will omit • those baroclinic zones mostly locked in place by topography (e.g., drylines) • Significance of Fronts: • Forecasts must account for frontal type, frontal movement, frontal intensity, • the spatialdistribution of clouds and precipitation, and the precipitation type • Frontal zones are pre-conditioned to support severe weather • Common Characteristics: • Enhanced horizontal gradients of density (temperature and/or moisture) • Relative minimum in pressure (a trough) • Relative maximum in cyclonic vertical vorticity (distinct wind shift) • Strong vertical wind shear (due to thermal wind balance) • Large static stability within the frontal zone • Ascending air with clouds / precipitation (moisture availability) • Greatest intensity near the surface (weaken aloft) • Shallow (1-5 km in depth) • Cross-front scale (~100 km) is much smaller than along-front scale (~1000 km) M. D. Eastin

  5. Definition and Structure Surface Pressure Equivalent Potential Temperature (θe) Vertical Vorticity M. D. Eastin

  6. Frontal Slope • How much does a front “slope” with height? • Let’s derive a simple equation that can describe • the vertical slope of any front • Assumptions • Front is oriented east-west • Only consider variations in “Y-Z space” • Neglect variations in the X direction • Density is discontinuous across the front • Pressure must be continuous so the PGF • remains finite (otherwise very strong winds) • Equation of state (p=ρRT), thus, requires • temperature to be discontinuous • Hydrostatic Balance • Geostrophic Balance • Pressure is steady (no changes in time) y ρc ρw x T ρ p Warm Cold y Front South North M. D. Eastin

  7. Frontal Slope • The differential of pressure is: • (1) • Divide each side by Dy • (2) • Substitute in the hydrostatic equation • (3) • (4) M. D. Eastin

  8. Frontal Slope • Since pressure is continuous across the front: • (5) • (6) • Substitution of (4) into (6) yields: • (7) • We can now solve for (Dz/Dy) • (8) M. D. Eastin

  9. Frontal Slope • Which way can the front slope and still be “stable”? • The front must be able to persist for 1-2 days • (as fronts do in reality) • Thus (9) • And since (10) • Thus (11) • Or (12) • What does this imply about pressure across the front? z Stable ρw ρc y z Unstable ρc ρw y M. D. Eastin

  10. Frontal Slope • What does this imply about pressure across the front? • While pressure is continuous across the front, the • pressure gradient is not continuous • Thus, the isobars must kinkto satisfy this relationship High pressure Or Low pressure High pressure M. D. Eastin

  11. Frontal Slope • What can we say about the winds across the front? • Assume the flow is geostrophic across the front • and does not vary along the front: • (13) • Thus, on the warm and cold sides of the front: • (14) • Substituting (14) into (8) yields: • where (15) • Again, if and then or (16) • What does this imply about the winds across the front? M. D. Eastin

  12. Frontal Slope • What does this imply about the winds across the front? • Recall the definition of geostrophic vertical vorticity • Thus, cyclonic vorticity must exist across the front • Here are more possible examples y ugc ugw x M. D. Eastin

  13. Frontal Slope • How much does a front slope with height? • Returning to the frontal slope equation: • (15) • Using the Equation of State, (15) can be written as: • Margules Equation • for Frontal Slope • If we substitute in typical values: This is similar to observations! Surface fronts are shallow! M. D. Eastin

  14. Frontal Slope • Similar conclusions can be reached for a front • oriented north-south using similar assumptions • Margules Equation • Again, frontal stability requires: • Thus, it can be shown: • The pressure gradient is discontinuous and the • isobars must kink across the front • The geostrophic wind must contain cyclonic • vorticity across the front y ρc ρw x T ρ p Cold Warm x Front West East M. D. Eastin

  15. Frontal Slope • Synoptic-scale Vertical Motion: • The vertical motion immediately adjacent to a given • frontal slope can also be estimated: • where: v = cross-front velocity • c = the speed of the front • Example: • Dz/Dy ~ 1/300 • v ~ 5 m/s • c ~ 2 m/s • w ~ 0.01 m/s z ρw w v c ρc y This is similar to observations! Synoptic-scale vertical motions are weak! M. D. Eastin

  16. Cold Fronts • Observational Aspects: • Cold air advances into a warmer air mass • Stereotypical passage includes: • Thunderstorms • Rapid (gusty) wind shift • Rapid temperature drop • Tremendous variability in weather ranging • from dry, cloud-free frontal passages to • heavy downpours with severe storms • Variability related to the cold front’s spatial • orientation relative to the warm-conveyor • belt ahead of the cold front • Katafront → Precipitation ahead of the • surface front • Anafront → Precipitation along / behind • the surface front M. D. Eastin

  17. Cold Fronts • Observational Aspects: Katafronts • Warm conveyor belt parallel to surface front • Limited lift along the surface front • Most lift associated with an elevated surge • of cold-dry air above the surface front, often • called a cold front aloft (CFA) • Occur later in the parent cyclone’s lifecycle • (when the cold front has a N-S orientation) B A A B • Warm front precipitation • Convection cells ahead of CFA • Precipitation from CFA falling • in warm conveyor belt • Shallow warm-moist zone • Surface front (light precipitation) M. D. Eastin

  18. Cold Fronts • Observational Aspects: Anafronts • Warm conveyor belt crosses • the surface front at some angle • Significant lift along surface front • Often accompanied by a southerly • low-level jet just ahead of the • surface frontal zone • Increased risk of winter precipitation • during the cold season • Tend to occur earlier in the parent • cyclone’s lifecycle (when the cold • front has greater E-W orientation) M. D. Eastin

  19. Cold Fronts • Observational Aspects: Arctic Cold Fronts • Second surge of cold air • Very shallow • Strong temperature gradient • Often lack precipitation • Behind primary cold front • Behind false warm sector Arctic Cold Front Primary Cold Front M. D. Eastin

  20. Cold Fronts • Observational Aspects: Back-door Cold Fronts • Caused by differential • cross-front advection • along a pre-existing • warm/stationary front • Surge of near-surface • cold air originating • over a cold surface • moves south/southeast • Most common along the • U.S. East coast • Don’t assume all cold • fronts move southeast!!! Back-door Cold Front M. D. Eastin

  21. Warm Fronts • Observational Aspects: • Warm air advances into a colder air mass • Motion is slow than cold fronts → dependent upon turbulent mixing along stable boundary • Warm fronts often have shallow slopes → the pressure trough is weaker • (makes warm fronts difficult to analyze) • Low clouds / stratiform precipitation common • Deep convection less common FFC M. D. Eastin

  22. Warm Fronts • Observational Aspects: Back-door Warm Fronts • Warm air advances into a colder air mass • Importance of source region → maritime polar air is warmer than continental polar air • Don’t assume warm fronts always move north!!! M. D. Eastin

  23. Occluded Fronts • Observational Aspects: • When “a fast-moving cold front overtakes a slow-moving warm front from the west” • Cyclone become cut-off from the warm sector → baroclinic instability ends • Marks the mature stage of a midlatitude cyclone → dissipation ensues • Rising motion above the frontal zone is weak as warm air lifted over cool/cold air • Stratiform precipitation is the norm M. D. Eastin

  24. Occluded Fronts Observational Aspects: Two Conceptual Models • Norwegian Cyclone Model • Initial cyclone development from a stationary front • Cold front advances and “overtakes” warm front • Cyclone near peak intensity as “occlusion” starts • Extension of the occluded front is southward • Shapiro-Keyser Cyclone Model • Initial cyclone development from a stationary front • Fast-moving cold front “fractures” • A “bent back” warm front (develops) • As cold front surge continues, warm air becomes • “secluded” (or occluded) from cyclone center M. D. Eastin

  25. Occluded Fronts • Observational Aspects: Two Occlusion Types • Depend on the relative temperature of • the pre- and post-frontal air masses • Cold occlusions should be much more • common in the eastern US → Why? • Warm occlusions are much more • common in western Europe → Why? • (and have been studied more) • Completion of your homework will provide • a new perspective to all this “conventional • wisdom” regarding occluded fronts! M. D. Eastin

  26. Coastal Fronts • Observational Aspects: • Strong temperature contrast caused • by warm-moist maritime air adjacent • to cold-dry continental air • Temperature differences of 5°-10°C often • occur over distances of 5-10 km • Shallow (less than 1 km deep) • Occur during the cold season (Nov-Mar) • Form along concave coastlines • (New England, Carolinas, Texas) • Cross-front structure similar to warm front • Pressure field often an “inverted trough” • Heaviest precipitation on “cold side” • Often the boundary between rain and • frozen precipitation types • Can serve as a primary or secondary site • for cyclogenesis M. D. Eastin

  27. Coastal Fronts • Observational Aspects: Formation • Cold anticyclone north or northeast • of frontal location → onshore flow • Onshore flow acquires heat / moisture • via strong surface fluxes from relatively • warm offshore waters (Gulf Stream) • Differential friction at coastline causes • distinct wind shift that favors frontal • formation along the coastline • Can be enhanced by cold-air damming • events along the Appalachians • Can be enhanced by a land breeze M. D. Eastin

  28. Coastal Fronts Observational Aspects: Motion Onshore migration → anticyclonic shifts eastward → geostrophic wind intensifies or primarily onshore Offshore migration → anticyclonic shifts northward → geostrophic wind weakens or primarily along-shore M. D. Eastin

  29. Upper-Level Fronts Observational Aspects: • Sharp thermal gradients in the upper/middle troposphere → don’t extend to the surface • Associated with “tropopause folds” whereby stratospheric air is drawn down into the troposphere → subsidence due to ageostrophic flow near jet streaks (right-exit region) → subsidence produces adiabatic warming (thermal front) → subsidence leads to vortex stretching (pocket of high PV) Isentropes (solid) Isotachs (dashed) Potential Vorticity (solid) Jet Core Jet Core Subsidence Tropopause Upper-level Front M. D. Eastin

  30. Upper-Level Fronts Observational Aspects: Significance • Have little to no impact on synoptic or mesoscale weather • Regions of strong clear air turbulence → significant hazard to aircraft • Regions of mixing between the troposphere and stratosphere • Transport → Radioactivity downward • → Ozone downward • → CFCs upward B A A B M. D. Eastin

  31. References Bluestein, H. B, 1993: Synoptic-Dynamic Meteorology in Midlatitudes. Volume II: Observations and Theory of Weather Systems. Oxford University Press, New York, 594 pp. Bosart, L. F., 1985: Mid-tropospheric frontogenesis. Quart. J. Roy. Meteor. Soc., 96, 442-471. Lackmann, G., 2011: Mid-latitude Synoptic Meteorology – Dynamics, Analysis and Forecasting, AMS, 343 pp. Miller, J. E., 1948: On the concept of frontogenesis. J. Meteor., 5, 169-171. Newton, C. W., 1954: Frontogenesis and frontolysis as a three-dimensional process. J. Meteor., 11, 449-461. Petterssen, S., 1936: A contribution to the theory of frontogenesis. Geopys. Publ., 11, 1-27. Sanders, F., 1955: An investigation of the structure and dynamics of an intense surface frontal zone. J. Meteor, 12, 542-552. Schultz, D. M., and C. F. Mass, 1993: The occlusion process in a midlatitude cyclone over land, Mon. Wea. Rev., 121, 918-940. Shapiro, M. A., 1980: Turbulent mixing within tropopause folds as mechanisms for the exchange of chemical constituents between the stratosphere and troposphere. J. Atmos. Sci., 37, 995-1004. Shapiro, M. A., 1984: Meteorological tower measurements of a surface cold front. Mon. Wea. Rev., 112, 1634-1639. M. D. Eastin

More Related