Advanced SynopticM. D. Eastin Surface Pressure Systems Will these Surface Lows Intensify or Weaken? Where will they Move?

Advanced SynopticM. D. Eastin Surface Pressure Systems Formation Climatology Use of QG Theory Effects of Vorticity Advection Effects of Temperature Advection Effects of Diabatic Heating Effects of Topography Common Favorable and Unfavorable Combinations of Effects An Example Case Movement Climatology QG Forcing Aspects of Topography An Example Case

Advanced SynopticM. D. Eastin Where do Surface Cyclones Occur? Genesis  More frequent in the winter than the summer  Occurs further south during the winter  In the lee of the northern Rockies from Alberta to Montana.  In the lee of the southern Rockies from Colorado to New Mexico.  These two are related to flow over topography  Off the east coast from the mid-Atlantic states to New England.  Off the Texas coast in the Gulf of Mexico  These two are related to cold air flowing over relatively warm waters (diabatic) Formation Climatology From Zishka and Smith (1980)

Advanced SynopticM. D. Eastin Where do Surface Cyclones Occur? Decay (Lysis)  Along the Pacific northwest coast  Related to flow over topography  Over New England and eastern Canada  Related to warm air flowing over a relatively cold land surface (diabatic) Many do not decay until well off the east coast over the north-central Atlantic when they become occluded and are cut-off their source of warm moist (tropical) air Formation Climatology From Zishka and Smith (1980)

Advanced SynopticM. D. Eastin Where do Surface Anticyclones Occur? Genesis  Again, more frequent and further south in the winter than in the summer  In the Southern plains from Texas to Kansas and North Dakota  In the Northern Rockies  Related to strong cold air advection to the west of developing Colorado lows and Alberta Clippers (that are moving east) Formation Climatology From Zishka and Smith (1980)

Advanced SynopticM. D. Eastin Where do Surface Anticyclones Occur? Decay (Lysis)  Over the central Rockies  Over the central and southern Appalachians  Related to flow over topography Formation Climatology From Zishka and Smith (1980)

Advanced SynopticM. D. Eastin Hoskins and Hodges (2002) Attributes for tracking of positive vorticity of 850 hPa level Upper panel: Probability Density Function (PDE) of cyclogeneis Lower panel: PDE of cyclolysis

Advanced SynopticM. D. Eastin Goal:  We want to use QG theory to diagnose, understand, and predict the formation and evolution (genesis and lysis) of surface cyclones and surface anticyclones What aspects of QG Theory can be Applied? We can not apply the QG height-tendency equation Lower boundary condition assumes no height tendency at the surface Contrary to what we are trying to infer…  We can use the QG omega equation evaluated above the surface  Then we can use the vorticity equation locally evaluated at the surface Use of QG Theory

Advanced SynopticM. D. Eastin Use of QG Theory Use of the QG Omega Equation above the Surface: Term A Term B Term C Evaluate both Terms B and C using the three-dimensional fields of height, vorticity, and temperature on upper-air pressure surfaces Recall: Rising Motion → An increase in PVA with height → WAA (Warm Air Advection) Sinking Motion → An increase in NVA with height → CAA (Cold Air Advection)  Look for regions:Rising Motion → Surface Cyclone formation / intensify Sinking Motion → Surface Anticyclone formation / intensify How?

Advanced SynopticM. D. Eastin How?... Local application of the QG Vorticity Equation at the Surface: If rising motion (ω < 0) is present above the surface (where ω = 0), then we know: We can then infer from the QG vorticity equation that: Recall (at the surface): Using the relationship between vorticity tendency and height tendency we thus know: Recall: and Finally, using the relation between the height tendency and the pressure tendency: Since: (see your text)  Therefore:Rising motions aloft → Surface pressure decreases Sinking Motions aloft → Surface pressure increases Use of QG Theory Note: From the continuity equation, this relationship is equivalent to convergence at the surface

Advanced SynopticM. D. Eastin For a Typical Synoptic Wave: Areas of PVA are often located east of the trough axis PVA often increases with height in this region (especially below 500mb) An increase in PVA with height results in rising motion aloft, which produces a surface pressure decrease, and the formation of a surface cyclone (or low) Areas of NVA are often located east of a ridge axis NVA often increases with height in this region (especially below 500mb) An increase in NVA with height results in sinking motion aloft, which produces a surface pressure increase, and the formation of a surface anticyclone (or high) Effects of Vorticity Advection From Bluestein (1993) LH PVANVA RisingSinking ConvergenceDivergence Trough Ridge 500 mb

Advanced SynopticM. D. Eastin Effects of Vorticity Advection: Mechanism PVA STEP ONE 1- Cyclonic vorticity advection 2- Divergence = 0 From QG vorticity equation: Cyclonic vertical vorticity advection Decrease in THICKNESS

Advanced SynopticM. D. Eastin Effects of Vorticity Advection: Mechanism STEP TWO In the absence of temperature advection in the thermodynamic energy equation: Ascending air is the only way to decrease the temperature of the layer:

Advanced SynopticM. D. Eastin Effects of Vorticity Advection: Mechanism The QG vorticity equation results: Secondary Circulation In the level of a: - + In the level of b: + + Secondary circulation returns the atmosphere to QG equilibrium

Advanced SynopticM. D. Eastin For a Typical Synoptic Wave: Areas of strong WAA aloft are often below 500mb east of trough axes and north of closed lows WAA results in rising motion aloft, which produces a surface pressure decrease, and the formation of a surface cyclone (or low) Areas of strong CAA aloft are often below 500mb west of trough axes and south of closed lows CAA results in to sinking motion aloft, which produces a surface pressure increase, and the formation of a surface anticyclone (or high) Effects of Temperature Advection From Bluestein (1993) L H Rising WAA CAA Sinking

Advanced SynopticM. D. Eastin What effect does diabatic heating or cooling have? Diabatic Heating:Latent heat release due to condensation (Ex: Cumulus convection) Strong surfaces fluxes (e.g. Cold air advection over the Gulf Stream) (e.g. Intense solar heating in the desert) Diabatic heating always leads to temperature increases → thickness increases Consider the three-layer model with a deep cumulus cloud The maintenance of geostrophic balance requires rising motion through the layer Identical to the physical processes induced by WAA Therefore, diabatic heating aloft induces rising motion → surface cyclone formation Effects of Diabatic Heating ΔZ increasesΔZΔZ Surface Rose Surface Fell Z-400mb Z-700mb Z-bottom Z-top

Advanced SynopticM. D. Eastin What effect does diabatic heating or cooling have? Diabatic Cooling:Evaporation (e.g. Precipitation falling through sub-saturated air) Radiation (e.g. Large temperature decreases on clear nights) Strong surface fluxes (e.g. Warm air flowing over a cold surface) Diabatic cooling always leads to temperature decreases → thickness decreases Consider the three-layer model with evaporational cooling aloft The maintenance of geostrophic balance requires sinking motion through the layer Identical to the physical processes induced by CAA Therefore, diabatic cooling aloft induces sinking motion → surface anticyclone formation Effects of Diabatic Heating ΔZ decreases ΔZΔZ Surface Rose Surface Fell Z-400mb Z-700mb Z-bottom Z-top

Advanced SynopticM. D. Eastin What effect does flow over topography have? Downslope Motions: Flow away from the Rockies Mountains Flow away from the Appalachian Mountains Subsiding air always adiabatically warms Subsidence leads to temperature increases → thickness increases Consider the three-layer model with downslope motion at mid-levels The maintenance of geostrophic balance requires rising motion through the layer Identical to the physical processes induced by WAA and diabatic heating Therefore, downslope flow induces rising motion → surface cyclone formation Recall: Lee-side cyclogenesis locations that were often observed Effects of Topography ΔZ increasesΔZΔZ Surface Rose Surface Fell Z-400mb Z-700mb Z-bottom Z-top

Advanced SynopticM. D. Eastin What effect does flow over topography have? Upslope Motions: Flow toward the Rockies Mountains Flow toward the Appalachian Mountains Rising air always adiabatically cools Ascent leads to temperature decreases → thickness decreases Consider the three-layer model with upslope motion at mid-levels The maintenance of geostrophic balance requires sinking motion through the layer Identical to the physical processes induced by CAA and diabatic cooling Therefore, upslope flow induces sinking motion → surface anticyclone formation Effects of Topography ΔZ decreases ΔZΔZ Surface Rose Surface Fell Z-400mb Z-700mb Z-bottom Z-top

Advanced SynopticM. D. Eastin Use of QG Theory The Modified QG Omega Equation for Surface System Evolution: + Diabatic + Topographic Forcing Forcing Vertical Motion Temperature Advection Change in Vorticity Advection with Height

Advanced SynopticM. D. Eastin Combined Effects of Forcing  You must consider the combined effects from each forcing type in order to infer the expected total surface pressure change Sometimes one forcing will “precondition” or “ripen” the atmosphere for another so as to enhance surface cyclogenesis or anticyclogenesis Other times forcing types will oppose each other, thereby inhibiting surface cyclogenesis or anticyclogenesis Common Favorable Combinations Vorticity advection with temperature advection Temperature advection with diabatic heating Vorticity advection with temperature advection and diabatic heating Downslope motions and vorticity advection Common Unfavorable Combinations Vorticity advection with temperature advection Temperature advection with diabatic heating Note: Nature continuously provides us with a wide spectrum of favorable and unfavorable combinations

Advanced SynopticM. D. Eastin Favorable Combinations of Forcing Vorticity Advection with Temperature Advection: Scenario: A region of increasing PVA with height (located downstream from a trough) is collocated with a region of strong warm air advection PVA Max Vort WAA Upper Levels Lower Levels

Advanced SynopticM. D. Eastin Favorable Combinations of Forcing Temperature Advection with Diabatic Heating: Scenario: A region of strong warm advection collocated with deep convection Commonly observed near warm fronts and in the warm sector WAA

Advanced SynopticM. D. Eastin Favorable Combinations of Forcing Vorticity Advection with Temperature Advection and Diabatic Heating: Scenario: A region of increasing PVA with height (located downstream from a trough) is collocated with a region of warm air advection and deep convection Max Vort WAA Upper Levels Lower Levels PVA

Advanced SynopticM. D. Eastin Favorable Combinations of Forcing Vorticity Advection with Downslope Motions: Scenario: A region of increasing PVA with height (located downstream from a trough) is located over the leeside of a mountain range PVA Max Vort Downslope Motions Upper Levels Lower Levels

Advanced SynopticM. D. Eastin Unfavorable Combinations of Forcing Vorticity Advection with Temperature Advection: Scenario: A region of increasing PVA with height (located downstream from a trough) is collocated with a region of strong cold air advection PVA Max Vort CAA Upper Levels Lower Levels

Advanced SynopticM. D. Eastin Unfavorable Combinations of Forcing Temperature Advection with Diabatic Heating: Scenario: Strong cold air advection over a warm surface Commonly observed during the winter months off the east coast as cold continental air flows over the warm Gulf of Mexico Warm Ocean H CAA If the cold air advection effects are greater than the diabatic heating effects, the net result will be a surface pressure increase and anticyclone formation Surface Fluxes (Diabatic Heating) Note: The complete opposite scenario could happen, whereby the CAA is weaker than the surface fluxes This will produce a surface cyclone Often occurs off the east coast: “Bomb” Cyclones Noreasters

Advanced SynopticM. D. Eastin Example Case: Formation Will these Surface Lows Intensify or Weaken?

Advanced SynopticM. D. Eastin Vorticity Advection: L L L Example Case: Formation

Advanced SynopticM. D. Eastin Vorticity Advection: L L PVA Assume NVA below Expect Pressure Decreases NVA Assume PVA below Expect Pressure Increases L Example Case: Formation

Advanced SynopticM. D. Eastin Temperature Advection: L L L Example Case: Formation

Advanced SynopticM. D. Eastin Temperature Advection: L L L WAA Expect Pressure Decreases CAA Expect Pressure Increases Example Case: Formation

Advanced SynopticM. D. Eastin Diabatic Heating: L L L Example Case: Formation

Advanced SynopticM. D. Eastin Diabatic Heating: L L L Diabatic Cooling Expect Pressure Increases Diabatic Heating Expect Pressure Decreases Note the snow and cloud cover Note: Time is 12Z or 5:00-7:00 am (before sunrise) Note the clear skies Example Case: Formation

Advanced SynopticM. D. Eastin Flow over Topography: L L L Note direction of surface winds from the previous slide Example Case: Formation

Advanced SynopticM. D. Eastin Flow over Topography: L L L Downslope Flow Expect Pressure Decreases Note direction of surface winds from the previous slide Example Case: Formation

Advanced SynopticM. D. Eastin Moderate NVAR Weak CAAR Diabatic CoolingR Downslope FlowF Net Pressure DecreaseR Z: Pressure rose 2 mb Moderate NVAR Weak WAAF Diabatic CoolingR Downslope FlowF Net Pressure IncreaseR Z: Pressure rose 3 mb Weak PVAF Moderate CAAR Diabatic HeatingF Downslope FlowF Net Pressure DecreaseF Z: Pressure fell 1 mb Example Case: Formation

Advanced SynopticM. D. Eastin Surface Pressure System Motion Will this Surface Low Move?

Advanced SynopticM. D. Eastin Where do Surface Cyclones Move? Cyclones move from their genesis regions to their decay (or lysis) regions  Most surface cyclones move to the northeast  Related to motion toward maximum surface pressure decreases WAA maximum is often to the northeast An upper-level PVA maximum is often to the northeast The warm front and its associated convection (or diabatic heating) is often to the northeast Motion Climatology From Zishka and Smith (1980)

Advanced SynopticM. D. Eastin Where do Surface Anticyclones Move? Anticyclones move from their genesis regions to their decay (or lysis) regions  Most surface anticyclones move to the southeast  Related to motion toward maximum surface pressure increases CAA maximum is often to the southeast An upper-level NVA maximum is often to the southeast Motion Climatology From Zishka and Smith (1980)

Advanced SynopticM. D. Eastin Goal: We want to use QG theory to diagnose the motion of surface pressure systems General Motion Characteristics There is no mean flow at the surface (a boundary condition) to advect the systems  Surface cyclones (anticyclones) always move away from regions of pressure increases (decreases) toward regions of pressure decreases (increases) Application of QG Theory and the QG Omega Equation: Surface pressure increases (decreases) result from sinking (rising) motion CycloneRegions of sinking motion→ Regions or rising motion MotionRegions of NVA aloft → Regions of PVA aloft (From → To)Regions of CAA→ Regions of WAA Regions of diabatic cooling → Regions of diabatic heating Regions of upslope flow→ Regions of downslope flow AnticycloneRegions of rising motion→ Regions of sinking motion MotionRegions of PVA aloft→ Regions of NVA aloft (From → To) Regions of WAA→ Regions of CAA Regions of diabatic heating→ Regions of diabatic cooling Regions of downslope flow→ Regions of upslope flow QG Forcing

Advanced SynopticM. D. Eastin Topography can have significant effects on system motion: Consider a cyclone (low pressure system) east of a mountain range: Motion will be to the south along the range Consider an anticyclone east of a mountain range Motion will be to the south along the range Aspects of Topography L Upslope Flow → Pressure Increase Downslope Flow → Pressure Decrease H Upslope Flow → Pressure Increase Downslope Flow → Pressure Decrease

Advanced SynopticM. D. Eastin Topography and Temperature Advection: Consider a low pressure system initially just east of a mountain range: Motion will be to the southeast Consider the low at a later time southeast of the mountain range Motion will now be to the east-southeast  As the low moves further away from the mountain range, it begins to feel less topographic effects and more temperature advection effects → acquires a more northeastward motion Aspects of Topography L Upslope Flow → Pressure Increase Downslope Flow → Pressure Decrease WAA → Pressure Decrease T T-ΔT T-2ΔT L Weaker Upslope Flow → Pressure Increase Weaker Downslope Flow → Pressure Decrease WAA → Pressure Decrease T T-ΔT T-2ΔT

Advanced SynopticM. D. Eastin Example Case: Motion Where will this Surface Low Move?

Advanced SynopticM. D. Eastin Vorticity Advection: L Example Case: Motion Maximum PVA Assume NVA below Expect motion toward the south

Advanced SynopticM. D. Eastin Temperature Advection: L Maximum WAA Expect motion toward the southeast Example Case: Motion

Advanced SynopticM. D. Eastin Diabatic Heating: L Maximum Heating Expect motion toward the northwest Example Case: Motion

Advanced SynopticM. D. Eastin Flow over Orography: L Maximum Downslope Flow Expect motion toward the southwest Example Case: Motion

Advanced SynopticM. D. Eastin Motion Summary LL WAA PVA Diabatic Downslope Expected Motion Initial Location Later Location Example Case: Motion

Advanced SynopticM. D. Eastin Surface Pressure Systems Summary: Formation (Cyclones and Anticyclones) Climatology (primary genesis, occurrence, lysis locations) Application of QG Omega Equation Effects of Temperature and Vorticity Advection Effects of Diabatic Heating Effects of Topography Common Favorable and Unfavorable Combinations of Effects Movement (Cyclones and Anticyclones) Climatology Application of QG Omega Equation Effects of Temperature and Vorticity Advection Effects of Topography

Advanced SynopticM. D. Eastin References Bluestein, H. B, 1993: Synoptic-Dynamic Meteorology in Midlatitudes. Volume I: Principles of Kinematics and Dynamics. Oxford University Press, New York, 431 pp. Bluestein, H. B, 1993: Synoptic-Dynamic Meteorology in Midlatitudes. Volume II: Observations and Theory of Weather Systems. Oxford University Press, New York, 594 pp. Boyle, J. S., and L. F. Bosart: 1983: A cyclone / anticyclone couplet over North America: An example of anticylone evolution. Mon. Wea. Rev., 111, Boyle, J. S., and L. F. Bosart: 1986: Cyclone / anticyclone couplets over North America. Part II: Analysis of major cyclone event over the eastern United States. Mon. Wea. Rev., 114, Charney, J. G., and A. Eliassen, 1964: On the growth of the hurricane depression. J. Atmos. Sci., 21, Pearce, R. P., 1974: The design and interpretation of diagnostic studies of synoptic scale atmospheric systems. Quart. J. Roy. Meteor. Soc., 100, Petterssen, S., and S. J. Smebye, 1971: On the development of extratropical storms. Quart. J. Roy. Meteor. Soc., 97, Tracton, M. S., 1973: The role of cumulus convection in the development of extratropical cyclones. Mon. Wea. Rev., 114, Zishka, K. M., and P. J. Smith, 1980: the climatology of cyclones and anticyclones over North America and surrounding oceans environs for January and July, Mon. Wea. Rev., 108,