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Basic Jet Streak Adjustments & Frontogenesis. MEA 444 January 13, 2005. The Problem of Scale Interaction. Definitions of Scales in the Atmosphere: (i) as deduced from observations (ii) as resulting from sizes of observational networks
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Basic Jet Streak Adjustments & Frontogenesis MEA 444 January 13, 2005
The Problem of Scale Interaction • Definitions of Scales in the Atmosphere: • (i) as deduced from observations • (ii) as resulting from sizes of observational networks • (iii) as defined by theoretical considerations (e.g., Lagrangian Time Scales)
Scale-interactive processes • Synoptic forcing of mesoscale weather phenomena • Generation of internal mesoscale instabilities • Interactions of cloud and precipitation processes with mesoscale dynamics • Influence of orography, boundary layer, and surface properties on mesoscale weather system development and evolution • Feedback contributions of mesoscale systems to larger-scale processes • Energy budgets associated with mesoscale systems • Mechanisms and processes associated with statosphere-troposphere exchange.
Ageostrophic Motions Near Jets Straight Jet Uniform Jet
Stretching Deformation • Confluence and diffluence associated with a jet maximum.
Shearing Deformation • Horizontal Shear in the presence of a positive along-front temperature gradient.
Katafronts • The schematic diagram presented below is that of a katafront. The front-relative flow for a katafront is one in which the ascent core slopes forward, resulting in all the precipitation being prefrontal.
Anafronts • The schematic diagram presented below is that of an anafront. The front-relative flow for an anafront is one in which the ascent core slopes rearwards over the top of the cold frontal surface, resulting in all the precipitation being postfrontal.
Cold Fronts Aloft • A Cold Front Aloft (CFA) can be characterized as "the leading edge of a transition zone above the surface that separates advancing cold air from warmer air" (Locatelli, et al. 1995). • Thus, a CFA is a cold frontal zone that is located in the lower to middle troposphere above the surface, but which has become split from the surface front. • For example, this can happen when a front passes over a mountain range due to the drag from the topography on the surface front, particularly in combination with strong sensible heating over the elevated terrain, which can effectively erase the thermal structure of the low-level cold front.
Cold Fronts Aloft (cont'd.) • A pronounced rainband is usually associated with the CFA, which in some instances is capable of producing severe weather hundreds of kilometers ahead of the surface front. This can create unanticipated consequences for the forecast process, such as: • CFAs and attendant convective weather have been documented during Cold Air Damming events over the cooler air east of the Appalachians, when traditional logic would argue against the possibility of significant convection over such stably stratified air. • CFAs have been shown to be responsible for generating lines of sever thunderstorms to the east of the dryline, and to thereafter support the continual eastward movement of the squall line for hundreds of miles ahead of the surface dryline. • Evidence is accumulating that some CFAs are highly unbalanced phenomena. Thus, they can initiate a geostrophic adjustment process, during which a major gravity-inertia wave event can transpire.
History of CFAs • The causes of "prefrontal squall lines" have been mysteries to meteorologists. • Holzman (1936) and Lichtblau (1936): Most significant winter precipitation events in the Midwest are associated with CFAs. • Crawford (1950): No prefrontal instability lines of any importance over the southeastern states exist without a warm tongue at 850 mb and strong cold advection at 700 mb. • Browning (1985): Suggested that many squall lines in the Midwest might be associated with split CFAs. • Locatelli et al. (1989): Discovered a CFA rainband that developed in the lee of the Rocky Mountains and moved eastward off the Atlantic Coast.
Useful Criteria for Labeling a Cold Front Aloft • Main precipitation band is well ahead of a surface trough. • Pronounce temperature gradient (cold advection) in the mid-troposphere associated with the band. • Cloud band in satellite imagery well ahead of surface trough. • Forecast vertical velocity field shows strong upward motion feature at least 200 km ahead of surface trough.
Useful Criteria for Labeling a Cold Front Aloft (cont'd.) • Vertical cross section of θe and winds indicates the presence of a mid-level front (CFA) ahead of the surface front. • Geostrophic wind along the suspected CFA has a concentrated region of vertical and lateral shear revealed by the field of absolute momentum. • Zero isodop in the radial velocity display from WSR-88D shows mid-level "backward S" pattern above a low-level "S"
CFA Model • Rocky Mountains block eastward progress of cold air at low levels and destroy thermal contrast due to strong sensible heating. Cold air continues to advance at mid levels ahead of surface trough. • Suggested that a thermally direct vertical circulation results from • quasi-geostrophic frontogenesis, and • ageostrophic isallobaric forcing at low levels due to the changing pressure gradient caused by cold advection aloft. • The midlevel zone of frontogenesis well ahead of surface trough is shown to be capable of triggering prefrontal squall lines.
Split Cold Front • When the surface pressure trough takes the form of a cold front with a line of maximum θe running from the surface front to the base of the front aloft, we have a split cold front.
Cold Frontal Rainbands • Strong lifting of the leading edge of the cold front produces a narrow band of heavy rainfall. • The cold-frontal gradient of the surface is very different from the conventional synoptic conceptual model. • Wider bands of precipitation exist behind the surface cold front. These can be associated with local steepening of the cold front. • These wide bands have some similarities to surge rainbands such as their movement through the system and the local enhancement of the baroclinic zone (although wide bands have weaker circulations). • The wide bands are also oriented along the vertical shear vector.
