ATMS 316- Mesoscale Meteorology Packet#6 Interesting things happen at the boundaries, or at the interface Cold air, warm air http://www.ucar.edu/communications/factsheets/Tornadoes.html ATMS 316- Mesoscale Meteorology Outline Background

Synoptic Fronts ATMS 316- Background Air masses and fronts, a review Main features of the Norwegian cyclone model (early 1900s) Fronts are transition zones separating air masses having different origins Cross-front scale is mesoscale, along-front scale can be synoptic scale Form as a result of large-scale

geostrophic deformation (a.k.a. confluence) http://www.bsmo.co.uk/newsfeatures/beginnersguides/guides/coldfrontsimulation/1.htm Baroclinic zone A region of strong thermal contrast Significant horizontal temperature gradient N E Baroclinic zone T1 T2

T3 T4 T5 Observed Structure [Carlson 1991] A zone of stronger Temperature, moisture, and vertical motion gradients normal to the frontal boundary on the cold side of the front Frontal gradients that appear discontinuous from

those of the synoptic-scale background Relative minimum in pressure Relative maximum of vorticity along the front Zone of confluence along the front Strong vertical and lateral (cyclonic) wind shear Rapid changes in cloud cover and precipitation Frontal Surface Transition (Frontal) Zone ATMS 316- Synoptic Fronts Chapter 5, p. 115 133

General characteristics Types of fronts Motion of fronts Slope of fronts Kinematics of frontogenesis Dynamics of frontogenesis and the vertical circulation at fronts Frontal rainbands Focus is on air mass boundaries observed at the surface.

ATMS 316- Synoptic Fronts General characteristics Fronts elongated zones of strong horizontal temperature gradient, where the gradient magnitude is at least an order of magnitude larger than the mean meridional temperature gradient

Average air temperature near sea level in January (F). ATMS 316- Synoptic Fronts General characteristics Fronts, also corridors of enhanced cyclonic vorticity vertical wind shear static stability (if the frontal zone is tilted over the cold air mass) ATMS 316- Synoptic Fronts

General characteristics Fronts, Rossby number (Ro) relatively small (Ro << 1) in the along-front direction relatively large (Ro 1) in the cross-front direction quasi-geostrophic theory is inadequate ATMS 316- Synoptic Fronts General characteristics Fronts; inadequacy of quasigeostrophic theory semigeostrophic theory

Advection of momentum and temperature by the ageostrophic wind is allowed, in addition to the advections accomplished by the geostrophic wind ATMS 316- Synoptic Fronts Types of fronts cold front (a, b) cold air mass advances relative to contours = potential temperature the warm air mass

intense cold front (b) Near-discontinuity in potential temperature similar characteristics to density currents ATMS 316- Synoptic Fronts Types of fronts warm front (c) warm air advances relative to the cold air mass stationary front little or no motion of air masses

contours = potential temperature ATMS 316- Synoptic Fronts Types of fronts slopes of fronts are impacted by surface drag steepens cold fronts shallows out warm fronts surface fluxes on the cold side of the front Stronger positive surface heat flux in

the cold frontal zone stronger deeper vertical mixing isentropes more vertical increasing cold frontal slope ATMS 316- Synoptic Fronts Types of fronts slopes of fronts are impacted by cloud cover and precipitation greater insolation and associated low-level destabilization generally might

be favored behind cold fronts ATMS 316- Synoptic Fronts Types of fronts chinook fronts warm fronts that form when air crosses a mtn range, and the downsloping adiabatically warming air advances in the lee of the mtn range (common east of Rocky Mountains) ATMS 316- Synoptic Fronts

Types of fronts occluded fronts formation first postulated as part of the Norwegian cyclone model their formation mechanisms have been controversial and remains a topic of ongoing research ATMS 316- Synoptic Fronts Types of fronts occluded fronts cold occlusion Schultz and Mass (1993) were

unable to find a single case of a cold occlusion documented in the literature in their review of occluded fronts warm occlusion [shown in Panel (d) to the right] ATMS 316- Synoptic Fronts Types of fronts occluded fronts formation examples a low deepens on the cold side of a

front, independent of a narrowing of the warm sector by a cold front outrunning a warm front a low initially located at the warm & cold front junction often propagates into the cold air as it deepens, thereby separating itself from the junction- occluded front joins low center with warm & cold front junction ATMS 316- Synoptic Fronts Types of fronts

occluded fronts, formation recent research; type of occlusion is diagnosed by the difference in static stability within the cold air masses straddling the narrowing warm sector (Stoelinga et al. 2002) warm occlusion; statically more stable air lies ahead of the warm front (example above)* *static stability differences do not cause occlusions to form ATMS 316- Synoptic Fronts Types of fronts

occluded fronts, examples whose static stability structures resemble a warm occlusion Pacific cold front overtakes a dryline (Great Plains) modified maritime polar air mass overtakes a moist air mass results in a cold front aloft (CFA) ATMS 316- Synoptic Fronts Types of fronts occluded fronts, examples whose static stability structures

resemble a warm occlusion* forward-tilting cold fronts cold fronts that encounter a stable prefrontal layer not associated with a warm front or dryline (Schultz and Steenburgh 1999) *forward-tilting frontal boundaries involve processes more complicated than simply the result of interactions between surface fronts separating air masses with different stratifications ATMS 316- Synoptic Fronts Motion of fronts

surface front propagation is strongly dependent on changing pressure on either side the front (front-normal isallobaric gradient) movement highly correlated with temperature advection (and wind direction) on cold side of the front temperature gradient is usually largest on the cool side of a front ATMS 316- Synoptic Fronts Motion of fronts

a comparison cold fronts tend to move faster than warm fronts ageostrophic cold air advection* works with the forward speed contributed by geostrophic temperature advection for a cold front (acceleration) against the forward speed contributed by geostrophic temperature advection for a warm front (deceleration)

*to be examined in greater detail in Dynamics of ATMS 316- Synoptic Fronts Motion of fronts fronts having a relatively small along-front length scale (Ro ~ 1) motion is a function of the temperature differential across the front horizontal pressure gradient drives the motion motion can also be a function of

latent cooling surface fluxes which can promote frontal jumps ATMS 316- Synoptic Fronts Slope of fronts simplest model zero-order temperature (density) discontinuity [a] Eq (5.2) here where dz/dy is the slope of the front. The slope must be + in order for the density distribution to be statically stable (less dense

air overlies relatively dense air) ATMS 316- Synoptic Fronts Slope of fronts simplest model To have a positive slope, Eq. (5.2) requires that the front lies within a pressure trough Eq (5.3) here ATMS 316- Synoptic Fronts Slope of fronts simplest model, after

applying hydrostatic and geostrophic balance assumptions Eq (5.4) here where Tw Tc is the temperature difference across the front, ugw = ugc is a measure of the cyclonic wind shear across a front, and T is some representative temperature [Margules formula] ATMS 316- Synoptic Fronts Slope of fronts Margules formula provides estimates that are the

right order of magnitude for the slope of warm fronts underestimates the slope of cold fronts surface drag, surface heat fluxes, nonhydrostatic effects cannot assume that strong fronts are more gently sloped than weak fronts (Tw Tc) T

ATMS 316- Synoptic Fronts Slope of fronts more realistic model first-order temperature (density) discontinuity [b, c] Eq (5.5) here where the subscripts c and w indicate that the derivatives are to be taken on the immediate cold and warm sides of the front, respectively (front is on the warm side of the frontal zone) ATMS 316- Synoptic Fronts Slope of fronts

more realistic model Eq (5.5) here numerator on the RHS is always negative slope of front is determined by the static stability change across the front ATMS 316- Synoptic Fronts Slope of fronts more realistic model Eq. (5.5) implies that a frontal

zone can tilt over the warm air while remaining statically stable [Panel (c)] forward-tilting frontal zones are characterized by a relative minimum in static stability Margules formula cannot predict warm occlusions nor forward-tilting cold fronts ATMS 316- Synoptic Fronts Kinematics of frontogenesis frontogenesis

an increase in the magnitude of the horizontal density (temperature) gradient frontolysis a decrease in the magnitude of the horizontal density (temperature) gradient ATMS 316- Synoptic Fronts Kinematics of frontogenesis expression for the time rate of change of the strength of the

baroclinity along a front Eq. (5.7) here where F is the scalar frontogenetical function or accumulation of the potential temperature gradient. Note that the +y direction must be pointing normal to the front, toward the cold air. ATMS 316- Synoptic Fronts Kinematics of frontogenesis scalar frontogenetical function (5.7) terms on RHS

horizontal shear (a) confluence (b) tilting (c) diabatic heating (d) ATMS 316- Synoptic Fronts Kinematics of frontogenesis the form of Eq. (5.7) implies that F is the change of the horizontal potential temperature gradient

following a parcels motion it is shown [see Eqs. (5.8) and (5.9)] that changes in frontal strength can be evaluated accurately using the terms on the RHS of Eq. (5.7) by following the frontal zone ATMS 316- Synoptic Fronts Kinematics of frontogenesis For adiabatic, horizontal motion, Eq. (5.7) simplifies to Eq. (5.10) here

Note, Eq. (5.10) is evaluated using x and y axes that are unrotated where b is the angle between the isentropes and the axis of dilatation, d is the horizontal divergence, and D is the resultant or total (stretching + shearing) deformation [see Eq. (5.11)] ATMS 316- Synoptic Fronts Kinematics of frontogenesis Simplified frontogenetical function [ Eq. (5.10) ] axis of dilatation is indicated

by the dashed lines in the figure axis along which a fluid element is stretched in a flow containing deformation ATMS 316- Synoptic Fronts Kinematics of frontogenesis Simplified frontogenetical function [ Eq. (5.10) ] frontogenesis whenever the total deformation field acts upon isentropes that are oriented

within 45o of the axis of dilatation convergence (d < 0) ATMS 316- Synoptic Fronts Kinematics of frontogenesis Simplified frontogenetical function [ Eq. (5.10) ] frontolysis whenever the total deformation field acts upon isentropes that are oriented between 45o and 90o of the axis of dilatation divergence (d > 0)

vorticity does not contribute directly but can affect frontogenesis or frontolysis by rotating the isentropes ATMS 316- Synoptic Fronts Dynamics of frontogenesis and the vertical circulation at fronts KEY POINT!! positive feedback ageostrophic wind accelerates

frontogenesis once the process has started through geostrophic deformation also applies to frontolysis ATMS 316- Synoptic Fronts Dynamics of frontogenesis and the vertical circulation at fronts positive feedback ageostrophic flow contributes through increasing confluence (v/y) defined in Eq. (5.7) as the horizontal temperature

gradient increases, contributing to additional frontogenesis ATMS 316- Synoptic Fronts Dynamics of frontogenesis and the vertical circulation at fronts feedback loop thermal wind balance hydrostatic and geostrophic balance ageostrophic winds are a response that maintain thermal wind balance

ATMS 316- Synoptic Fronts Dynamics of frontogenesis and the vertical circulation at fronts initial time [Panel (a)] winds are in geostrophic balance frontogenetical geostrophic deformation is advecting warmer air into the warm air and colder air into the cold air

ATMS 316- Synoptic Fronts Dynamics of frontogenesis and the vertical circulation at fronts later time [Panel (b)] winds are out of geostrophic balance force imbalances yield ageostrophic winds that accelerate the horizontal winds back into geostrophic balance* force vertical motion (Continuity Equation)

*see Eqs. (4.77) and (4.78) ATMS 316- Synoptic Fronts Dynamics of frontogenesis and the vertical circulation at fronts later time [Panel (b)] Coriolis force acts on the ageostrophic winds, increasing the vertical shear necessary to maintain thermal wind balance in the presence of an increasing temperature

gradient ATMS 316- Synoptic Fronts Dynamics of frontogenesis and the vertical circulation at fronts later time [Panel (b)] vertical motion and attendant adiabatic cooling (rising) and warming (sinking) oppose the effects of the horizontal temperature advection by the geostrophic wind

ATMS 316- Synoptic Fronts Dynamics of frontogenesis and the vertical circulation at fronts later, later time ageostrophic horizontal winds further enhance (q/y), which then requires even stronger vertical motions and ageostrophic winds, leading to a continued enhancement of (v/y) and (q/y)

ATMS 316- Synoptic Fronts Dynamics of frontogenesis and the vertical circulation at fronts effects of ageostrophic wind cannot be represented using the quasigeostrophic (QG) approximation advections are only considered by the geostrophic wind semigeostrophic momentum

equations ATMS 316- Synoptic Fronts Dynamics of frontogenesis and the vertical circulation at fronts semigeostrophic equations [Eqs. (5.12) and (5.13), rotated such that the +y axis points toward the cold air] ATMS 316- Synoptic Fronts Dynamics of frontogenesis

and the vertical circulation at fronts Fig (5.7) - large ATMS 316- Synoptic Fronts Dynamics of frontogenesis and the vertical circulation at fronts without the deformation, divergence, and advections by the ageostrophic winds (QG), frontogenesis proceeds too slowly

frontal zone tilt is unrealistic air on warm side of front is unrealistically statically unstable ATMS 316- Synoptic Fronts Dynamics of frontogenesis and the vertical circulation at fronts Sawyer-Eliassen equation The Sawyer-Eliassen equation gives the structure of the vertical circulation associated with frontogenesis or frontolysis under given

atmospheric conditions ATMS 316- Synoptic Fronts Dynamics of frontogenesis and the vertical circulation at fronts Sawyer-Eliassen equation LHS static stability inertial stability horizontal temperature gradient RHS

frontogenetical forcing by the geostrophic wind differential diabatic heating ATMS 316- Synoptic Fronts Dynamics of frontogenesis and the vertical circulation at fronts Sawyer-Eliassen equation linear, second-order partial differential equation if elliptical, frontal circulation is entirely determined by the RHS

terms [see Eq. (5.19) for ellipticity condition] if hyperbolic, non-unique solutions arise equation is hyperbolic if the atmosphere is statically, inertially, or symmetrically unstable ATMS 316- Synoptic Fronts Dynamics of frontogenesis and the vertical circulation at

fronts Sawyer-Eliassen equation RHS [Eq. (5.17)] (forcing) thermally direct circulation (RHS +) thermally indirect circulation (RHS -) intensity of the circulation is proportional to the magnitude of the RHS forcing ATMS 316- Synoptic Fronts Dynamics of frontogenesis and

the vertical circulation at fronts Sawyer-Eliassen equation cold fronts (a, b) stronger vertical circulations sharper temperature gradients both deformation terms in the frontogenetical forcing by the geostrophic wind term on the RHS of Eq. (5.17) are usually frontogenetic ATMS 316- Synoptic Fronts Dynamics of frontogenesis and the vertical circulation at fronts

Sawyer-Eliassen equation warm fronts (c, d) weaker vertical circulations weaker temperature gradients first term of the frontogenetical forcing by the geostrophic wind sum on the RHS of Eq. (5.17) often opposes frontogenesis (is negative) ATMS 316- Synoptic Fronts Dynamics of frontogenesis and the vertical circulation at fronts

eccentricity of the ageostrophic circulation, controlled by the relative strengths of static stability inertial stability ATMS 316- Synoptic Fronts Dynamics of frontogenesis and the vertical circulation at fronts eccentricity of the ageostrophic circulation static stability >> inertial stability squashed in the vertical ( b & d )squashed in the vertical ( b & d )

ATMS 316- Synoptic Fronts Dynamics of frontogenesis and the vertical circulation at fronts eccentricity of the ageostrophic circulation static stability << inertial stability squashed in the vertical ( b & d )squashed in the horizontal ( a & c ) ATMS 316- Synoptic Fronts Dynamics of frontogenesis and the vertical circulation at fronts

eccentricity of the ageostrophic circulation tilt of the major axis of the ageostrophic circulation is equal to the slope of local geostrophic absolute momentum (Mg) surfaces Mg = ug f y [see Section 3.3] ATMS 316- Synoptic Fronts Dynamics of frontogenesis and the vertical circulation at

fronts slope of local geostrophic absolute momentum (Mg) surface For a given horizontal shear of the geostrophic wind, the slope of an Mg surface [front*] decreases as the horizontal temperature gradient increases *similar conclusion reached using Margules formula (5.2) ATMS 316- Synoptic Fronts Dynamics of frontogenesis and

the vertical circulation at fronts frontogenetical geostrophic deformation acting on a preexisting temperature gradient jump starts frontogenesis intensity of a front is limited by vertical and horizontal mixing prevents the magnitude of the horizontal temperature gradient from becoming infinitely large ATMS 316- Synoptic Fronts

Frontal rainbands mesoscale narrow cold-frontal rainbands; convective wide cold-frontal rainbands; enhanced stratiform precipitation warm-frontal rainbands; enhanced stratiform precipitation warm-sector rainbands; convective or stratiform ATMS 316- Synoptic Fronts Frontal rainbands mesoscale narrow cold-frontal rainbands

cross-band dimension as small as 12 km [Panel (a)] along surface front, at the location of the wind shift [Panel (b)] intense updrafts (10 m s-1) at altitudes as low as 1 km can spawn tornadoes ATMS 316- Synoptic Fronts Frontal rainbands mesoscale narrow cold-frontal rainbands form of forced convection driven principally by vertical

perturbation pressure gradients associated with the vertical circulation along the front neutrally stratified [Panel (b)] little work needed to sustain updrafts laminar rope cloud in strongly stratified environments ATMS 316- Synoptic Fronts Frontal rainbands mesoscale narrow cold-frontal rainbands

maintenance depends on a balance between horizontal vorticity in prefrontal air mass vertical circulation along the leading portion of the cold air mass kinks and breaks in the rainband [Panel (a)], core-and-gap structure ATMS 316- Synoptic Fronts Frontal rainbands mesoscale

narrow cold-frontal rainbands core-and-gap structure meso-g-scale vortices occasionally amplify to tornado strength likely arise from the instability of horizontal wind shear (Section 3.5) fracture frontal updraft can also be the result of a front interacting with prefrontal boundary layer

convective structures ATMS 316- Synoptic Fronts Frontal rainbands mesoscale wide cold-frontal rainbands only indirectly tied to processes near the surface motion independent of the motion of the front move with the winds in the cloud layer move ahead of cold front

may overtake and move ahead of narrow cold-frontal rainband ATMS 316- Synoptic Fronts Frontal rainbands mesoscale wide cold-frontal rainbands multiple wide cold-frontal rainbands can be associated with a single cold front associated with a local steepening of the frontal surface May be attributable to the release of

conditional symmetric instability (CSI) ATMS 316- Synoptic Fronts Frontal rainbands mesoscale warm-frontal rainbands occur within and above the warm-frontal zone, on the poleward side of the surface warm front can be attributed to the release of CSI

lifting of a potentially unstable layer over the warm-frontal surface ducted gravity waves Regions of CSI are also often regions of frontogenesis, and the ageostrophic vertical circulation that accompanies frontogenesis is believed to be a mechanism that can release CSI. ATMS 316- Synoptic Fronts

Frontal rainbands mesoscale warm-sector rainbands likely associated with a CFA or a split front upward velocity near a CFA is likely the result of frontal vertical circulations associated with the CFA midtropospheric potential instability associated with the relatively dry air behind the split front

ATMS 316- Synoptic Fronts Which scenario? Scenario#1; synoptic scale forcing alone Scenario#2; synoptic scale dominates mesoscale forcing Scenario#3; weak synoptic scale forcing 17 February 2006 Cold front passage

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