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Understanding Frontogenesis and Its Application to Winter Weather Forecasting

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Understanding Frontogenesis and its Application to Winter Weather Forecasting Ted Funk WFO Louisville, KY January 2004 with contributing graphics from Peter Banacos Storm Prediction Center Ron Przybylinski WFO St. Louis, MO General Outline ¾ Frontogenesis defined ¾ Components of frontogenesis ¾ What causes frontogenesis?: ¾ Horizontal divergence/difluence ¾ Horizontal deformation ¾ Vorticity ¾ Tilting effects ¾ Diabatic heating ¾ Development of the frontogenetical circulation ¾ Use of frontogenesis in forecasting ¾ Common synoptic patterns: strong sfc low; weak or no sfc low ¾ Strong low case: Jan 6, 2002 ¾ Weak low case: Oct 15, 2001 ¾ Weak low case: Jan 7, 1999 ¾ Role of deep-layered wind shear ¾ Deep-layered wind shear examples ¾ Non-banded, multiple banded, and warm season banded precipitation ¾ Spectrum of mesoscale instabilities; use of EPV and CSI ¾ Stability versus banding comparison ¾ Numerical model considerations: Feb 7, 2003 ¾ Suggested snow band checklist Frontogenesis Defined Frontogenesis (in general terms) refers to the change in the magnitude and orientation of the temperature gradient at a level or in a layer (e.g., 850-700 mb) due to directional and speed changes in the wind field. Frontogenesis (in specific terms) refers to an increase in the horizontal thermal gradient with time. Frontolysis refers to a decrease in the horizontal thermal gradient with time. QG frontogenesis (using geostrophic winds) allows for diagnosis of forcing and vertical motion on the synoptic-scale (e.g., extratropical cyclones/large troughs and ridges) which may or may not support mesoscale processes. Petterssen’s 2-D frontogenesis uses the total wind which can help diagnose features on the mesoscale (100-500 km) such as banded precipitation structure. Frontogenesis Defined (cont.) D F = ∇pθ (Petterssen 1936) Dt The 2-D scalar frontogenesis function (F ) (i.e., F vector) quantifies the change in the horizontal (potential) temper- ature gradient following air parcel motion, where: F > 0 is frontogenesis, F < 0 is frontolysis Conceptually, frontogenesis is the local change in the horizontal temperature gradient near an existing front, baroclinic zone, or feature as it moves. Components of the Frontogenesis Function (F Vector) (Keyser et al. 1988, 1992) The F vector is the full wind version of the quasi- F = Fn n + Fss geostrophic Q vector. It can be broken down into 2 components (natural coordinates): Fn and Fs.Fs D Fn: Fn = − ∇pθ -Frontogenetical (dominant) component of F Dt -Directed perpendicular to temperature lines D -Refers to changes in magnitude of thermal gradient Fs = n⋅(k × ∇pθ ) -Corresponds to vertical motion on the frontal scale Dt (mesoscale bands) Fs: -Rotational component of F -Directed parallel to isotherms/thicknesses -Refers to changes in direction (orientation) of thermal gradient with no magnitude change -Corresponds to synoptic-scale vertical motion on the scale of the baroclinic wave itself Fn and Fs Vectors When F vectors point from cold-to-warm (warm-to-cold) air in the low-to-mid levels of the atmosphere, frontogenesis (frontolysis) is occurring. Fn vectors can be very important (the dominant term), and force vertical motion on the mesoscale/frontal scale. Fn describes how the magnitude of the thermal gradient is changing, i.e., the gradient is becoming stronger (frontogenesis) via confluence or deformation or weaker (frontolysis) via difluence. Fn vectors are longest where the thermal gradient is changing the most, not necessarily where the tightest thermal gradient exists. Fn vectors are available in AWIPS. Fs vectors describe temperature advection patterns, and force vertical motion on the synoptic scale. Fs describes how the orientation of the isotherms/thicknesses is changing with time due to horizontal changes in the wind. Fs vectors are most pronounced in areas where the wind is tending to rotate isotherms significantly, i.e., in areas of strong warm and cold advection. The longer the Fs vectors, the greater the temperature advection pattern and forcing for synoptic scale vertical motion. What Causes Frontogenesis? The geometry of the horizontal flow has a strong influence on frontogenesis in most situations. Two main processes (parameters) make significant separate contributions to the field of frontogenesis: Divergence Deformation The focus here is exclusively on Petterssen’s 2-D scalar frontogenesis (Fn) Horizontal Divergence/Difluence Regardless of isotherm orientation, divergence (convergence) acts frontolytically (frontogenetically). Frontogenesis (F>0; right) due to convergence oriented nearly perpendicular to a thermal gradient. F<0 Frontolysis F>0 (F<0; left) due to divergence oriented nearly perpendicular to a thermal gradient. Horizontal Divergence/Difluence Left 2 diagrams: Confluent wind field applied to the thermal gradient. At a later time (T+1), the wind acts to increase the thermal gradient, thus a frontogenetical situation. Right 2 diagrams: Difluent wind field applied to the thermal gradient. At a later time (T+1), the wind acts to decrease the thermal gradient, thus a frontolytical situation. Horizontal Divergence/Difluence 850-700 mb winds (kts) and temps, and 1000-500 mb RH image from AWIPS on Feb 15, 2003. A rough qualitative assessment of 2-D frontogenesis can be made by viewing level or layer- average temperatures and isotherms. In the image above, frontogenesis in the 850-700 mb layer is implied over central and southern IN and OH, northern KY, and central IL where the winds indicate convergence superimposed on and directed nearly perpendicular to an existing tight thermal gradient. The frontogenesis can be quantified by viewing 2-D frontogenesis and Fn vectors in AWIPS (see next slide) . The convergence of the Fn vectors represents forcing for ascent. Horizontal Divergence/Difluence 850-700 mb Petterssen's 2-D frontogenesis (contours), Fn vectors (blue arrows), and 1000-500 mb RH image (purple=highest RH) from AWIPS on Feb 15, 2003. The contoured frontogenesis above quantifies the implied frontogenesis from observing winds and isotherms from the previous slide. Fn vectors are longest where frontogenesis is greatest. The length of a vector is valid at its origin point (not its arrowhead). The vectors point from cold-to-warm air, thus a frontogenetical situation is shown over the Ohio Valley. Horizontal Deformation F>0 Flow fields involving deformation acting frontogenetically are prominent in the majority of banded precipitation cases. Axis of dilatation: axis along which flow is being stretched (deformed) due to conver- gence in the flow; oriented parallel to the stretching due to the deformation field Axis of contraction: axis along which flow is being compacted within the deformation field; oriented perpendicular to the dilatation axis Horizontal Deformation Deformation of the wind can act frontogenetically or frontolytically. One needs to consider the orientation of the isotherms relative to the axis of dilatation. In the case at left, the axis is oriented perpendicular to isotherms, thus convergence F<0 and stretching along the axis actually results in frontolysis. The key is the angle between isotherms and the axis of dilatation. If the angle is < 45º (far left), the wind field is acting frontogenetically (increase in thermal gradient). If the angle is > 45º (near left), the wind field is acting frontolytically (decrease in thermal gradient). Streamline analysis is a good tool for detecting deformation zones. Normally, the axis of dilatation is associated with a frontogenetical situation. Other Contributing Factors to Frontogenesis Deformation and divergence fields play the most prominent role in 2-D frontogenesis aloft. However, other processes can contribute to frontogenesis, including: Vorticity Tilting effects Diabatic heating Vorticity/Shear Pure vorticity acts to rotate isotherms, it cannot tighten or weaken the gradient. Fn=0 If a sheared wind field (containing vorticity) is applied to an initially uniform thermal gradient (warm – left side; cold – right side), the wind field will act to rotate the thermal gradient at a later time. Tilting Effects In these cases, the temperature gradient is in the vertical as opposed to the horizontal in the preceding examples. The temperature gradient is being tilting by the vertical motion field. Above (left 2 images), differential vertical motion acts to increase the thermal gradient, i.e., frontogenetically. Above (right 2 images), differential vertical motion acts to decrease the thermal gradient, i.e., frontolytically. Diabatic Heating Diabatic Heating can act frontogenetically (top image) or frontolytically (bottom image). Top (cold air on left; warm air on right): cloud cover is limiting radiational warming on colder left side, while cloud-free warm area on right heats up. Thus, thermal gradient strengthens (frontogenetical). Bottom (cold air on left; warm air on right): the sun warms the cloud-free cold air while cloud cover limits radiational warming in the warm air mass. Thus, thermal gradient weakens (frontolytic). Small-scale low-level frontogenesis due to diabatic heating can be important in unstable environments, where the resulting small-scale frontal lift causes convective development. Development of the Frontogenetical Circulation -When the temperature gradient strengthens, geostrophic and hydrostatic balance are disturbed. -Thus, QG theory states that the atmosphere responds to the disturbance (frontogenetical forcing) through ageostrophic vertical circulations which attempt to restore thermal wind balance
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