What are the characteristics of an anafront?
There is a sudden, large drop in temperature with frontal passage, there is a high, slight decrease in relative humidity with frontal passage, the clouds clear slowly with frontal passage, the precipitation is moderate to heavy rain with frontal passage; steady postfrontal rain, and the wind veers sharply, followed by decrease in speed behind front.
What are the characteristics of a katafront?
There is a slight, gradual drop in temperature with frontal passage, there is a moderate, sharp decrease in relative humidity with frontal passage, there is rapid clearing of clouds with frontal passage, there is very light rain, centered in a narrow band along front; possible convection ahead of front, and there is gradual veer of the wind, with slight speed changes behind front.
What type of flow and moisture profile does an anafront have?
An upslope flow that warms moist adiabatically and is moist throughout the sounding.
What type of flow and moisture profile does a katafront have?
A downslope flow that warms dry adiabatically and is moist at lower levels then dries out in the sounding.
What are some facts about the forward-sloping warm conveyor belt: katatfront type cold front?
The surface front is typically poorly defined- mostly through dew points and relative humidity. Many times there is an upper-level, so-called "split front" or "cold front aloft", which represents the leading edge of overrunning dry air. The CFA can be hundreds of km downstream from the surface front. Since dry, mid-level air is overrunning moist, low-level air convective instability is typically associated with the CFA- leading to convective precipitation along the leading edge of the upper-level front. There is a shallow moist zone upstream from the CFA. Many katafronts are associated with a "dry" cold frontal passage or just a narrow line of precipitation at or near the surface front.
What are some facts about the rearward-sloping warm conveyor belt: anafront type cold front?
The surface front is often found downstream from mid-level confluent troughs. The surface front is often sharply defined by temperature. There is an area of abrupt ascent, approximately 2-3 km deep at the surface front- this is a zone of intense vertical motion and narrow band of heavy, often convective, precipitation. Upstream from the surface front there is typically a broad band of lighter, but persistent precipitation, associated with stratiform clouds and weaker ascent. In many cases this stratiform precipitation can be hundreds of km into the cold side of the front. In many cases, anafront type cold fronts can lead to heavy rainfall (in warm season) or even mixed freezing precipitation (in the cold season).
What are some real life examples of anafront and katafront characteristics?
A cold front can change its characteristics with time- it is not always an ana- or kata- front. Many times the northern part of the cold front moves east and has strong front-normal winds aloft leading to katafront characteristics. However, the southern tail of the cold front moves southeast with weaker front-normal winds aloft, thereby leading to anafront type conditions (with "overrunning" and stratiform precipitation in the cold air).
How does a cold front aloft (CFA) work?
As the CFA moves east it occludes with the upward-sloping region of high equivalent potential temperature air ahead of the drytrough. CFA is associated with frontogenesis at the "nose" of the colder (and drier) air aloft. Also, beneath CFA nose the rate at which the pressure falls decreases due to the advection of cold air aloft. This produces an ageostrophic wind, convergence and upward vertical motion. Ahead of the CFA there is convectively unstable air.
Does an anafront or katafront produce more instability?
Katafront has more CAPE, therefore more instability
What is frontogenesis (FGEN)?
The generation of fronts can be calculated, a high value indicates a front is forming or, if already present, is getting stronger- and intensification of the horizontal temp gradient.
What is frontolysis (FLYS)?
The dissolution of fronts due to 3 things: horizontal deformation of the flow (shear and confluence terms), vertical deformation of the flow (tilting), and unequal (differential) diabatic heating.
What is the physical interpretation of frontogenetical circulation?
Represents the rate of increase (decrease) of the magnitude of the 3-D temperature gradient with time following a parcel (Lagrangian).
What is the frontogenesis function dependent on?
The changes in the parcel perceived temperature gradient, which is not necessarily the changes in the frontal temperature gradient. Really need to know the front relative flow to determine which and how parcels are interacting with the imposed thermal gradient- difficult to measure.
What is the units for frontogenesis?
K 100 km^-1 3 hr^-1, this will give values that range from -2.5 to +2.5, so we often scale these values to give magnitudes of -25 to 25. The change in the thermal gradient per unit time.
Where can we look for FGEN?
At all different levels. At the surface- surface frontogenesis/ frontolysis, aloft at mid-levels- confluence of conveyor belts (airstreams relative to the moving cyclone), and aloft at upper-levels- related to a tropopause undulation or tropopause fold.
Why is it useful to look at FGEN over several different levels?
Because looking at FGEN as an average over a chosen layer will often "wash out" the strongest FGEN present within the layer.
What are the assumptions that we make to simplify the three-dimensional frontogenesis equation?
y' axis is set normal to the frontal zone, with y' increasing towards the cold air (note: y' might not always be normal to the isentropes- i.e., the frontal zone might be oriented at an angle to the isentropes, as we'll later see). x' axis is parallel to the frontal zone and neglect vertical and horizontal diffusion effects.
What are the four terms in the simplified form of the frontogenesis equation?
The shear term, confluence term, tilting term, and the diabatic heating/ cooling term. Neglecting vertical deformation and vertical divergence terms and any change in a wind component with respect to the x axis is neglected, as we are only examining the change in potential temp with respect to the y axis.
What does the shear term do?
Shearing advection changes orientation of isotherms and it is important around fronts, troughs, and outflow boundaries. ∂u/∂y'<0, ∂v/∂y'=0 (v=0), ∂θ/∂x'<0, (∂θ/∂x') (∂u/∂y')=S>0
What does the confluence term do?
There is cold advection to the north and warm advection to the south. It is important around fronts, troughs, outflow boundaries. ∂v/∂y'<0, ∂θ/∂y'<0, (∂θ/∂y') (∂v/∂y')=C>0
How does shear and confluence relate to each other near a cold front?
Shear and confluence terms tend to work together near cold fronts. u winds decrease (u>0 warm air; u~0 cold air), θ decreases towards low, v winds decrease (more northerly), θ decreases, S>0, and C>0.
How does shear and confluence relate to each other near a warm front?
Shear and confluence terms oppose one another near warm fronts. u winds decrease (more easterly), θ increases from low, v winds decrease, and θ decreases, S < or = 0, and C>0.
What does the tilting term do?
Adiabatic cooling to north (cooling off cold side) and warming to south (warming up warm side) increases horizontal thermal gradient. Important in upper levels- vertical circulation near jet streak (ITC in exit region)- causes tropopause to sink several km and form a frontal layer. Also important near mountains- change in slope of terrain or change in wind speed along the terrain. ∂w/∂y'>0, ∂θ/∂z>0, (∂θ/∂z) (∂w/∂y')=F>0
Why is FGEN smaller for a warm front?
Because the thermal gradient isn't as strong as it is in a cold front.
What does the diabatic heating/cooling term do?
Can be latent heating, sensible heating, radiational cooling/warming and some examples are in winter there are large differences between the temp of snow surfaces and adjacent water bodies (ocean, large lakes). Local fronts set up due to heating/cooling from below. Cloud cover prevents parts of earth's surface from heating during the day. Front generated between heated clear air and cold air under the cloud cover. Different parts of a front can experience frontogenesis/frontolysis due to each of the terms.
What is the only difference between stretching deformation and shearing deformation?
The rotation of the axis of dilatation by 45°.
How is frontogenesis related to total deformation?
The wind compacts isotherms, as cos(<90) is positive.
How is frontolysis related to total deformation?
The wind separates isotherms as cos(>90) is negative.
What is kinematic frontogenesis?
There is no relationship between the isotherm deformation and the v component of the wind responsible for compressing the isotherms- in reality there is a feedback between the two. Fronts can double their intensity in a matter of several hours; kinematic frontogenesis suggests that it takes on the order of a day.
What does kinematic frontogenesis not account for?
The changes in the divergence of momentum fields (accelerations); values of divergence and vorticity in frontal zones are on scales < or= 100 km, suggesting highly ageostrophic flow.
Why does kinematic frontogenesis fail?
Because temperature is treated as a passive scalar. As the thermal gradient changes the thermal wind balance is upset,therefore there is a continual readjustment of the winds in the vertical in an attempt to re-establish geostrophic balance.
What is frontogentical circulations and what do they do?
As the thermal gradient stretgthens the geostrophic wind aloft and below must respond to maintain balance with the thermal wind. Winds aloft increase and "cut" to the north while winds below decrease and "cut" to the south, thereby creating regions of div/con. By mass continuity upward motion develops to the south and downward motion to the north-a direct thermal circulation. This direct thermal circulation acts to weaken the frontal zone with time and works against the original geostrophic frontogenesis. But the v-component of the wind (ageostrophic) in the low levels will enhance frontogenesis.
What are the frontogenetical circulation factors?
The major and minor axis of the elliptical circulation are determined by the relative magnitudes of the static stability and the absolute geostrophic vorticity; the vertical slope is a function of the baroclinicity. High static stability compresses and weakens the circulation cells.
What doesn't the frontogenesis equation take into account?
Diabatic heating and the balance between the thermal gradient and the wind.
When are cyclones and anticyclones more numerous?
January. In general, cyclones and anticyclones are more numerous, more intense, and displaced further south in January than in July.
Where is there greater weather activity in the lower 48 states of the US?
East of the Rocky Mountains. Cyclogenesis occurs most frequently along the East coast of the U.S. and in the lee of the Rocky Mountains, while anticyclones tend to originate in northern Canada and in the Great Plains region of the U.S. in January and in southwestern Canada in July.
Where do cyclones and anticyclones tend to propagate?
Cyclones generally propagate to the east and northeast from genetic regions, while anticyclones propagate to the east and southeast after forming.
What usually blocks the cyclones and anticyclones from propagating any further on their treck?
The Rocky Mountains are a barrier to cyclones penetrating eastward from the Pacific, while anticyclones tend to dissipate along the eastern coast of the U.S.
Where do cyclones become stronger in January and why does this occur?
The East coast of the U.S. because east coast storms undergo more deepening than Rocky Mountain storms. This underlies the importance of the East coast baroclinic zone aided by the warm Gulf Stream waters and the influence of the strong oceanic heat fluxes in winter.
Why are the cyclones in July similar throughout the country?
This is likely due to the weakening of the East coast baroclinic zone. Not as strong temperature difference between land and water in the summer.
Where do cold anticyclones form in January?
They form over the snow/ice fields of Montana and the Northwest Territories.
Where do warm anticyclones form in January?
They form over the region from South Dakota to Texas on the south side of the jet stream.
What are the tracks of cyclones and anticyclones dictated by?
The long term radiation balance, land/water distributions, and orographic features.
What is the Norwegian Cyclone Model?
It is based on surface reports and cloud observations, no upper air sounding data and many aspects still accepted today but not all.
What are the major aspects of the Norwegian Cyclone Model?
Cyclone is perturbation on frontal boundary, advancing cold air overtakes retreating warm air, occluded front formed—warm air aloft, ascent in warm air, descent in cold air, precipitation occurs on cold sides of fronts and broadest area of precipitation ahead of warm front
What are many old occluded systems that finally merge with the great semi-permanent low pressure systems on the weather map called in the North Atlantic and in the Pacific?
Icelandic low in the North Atlantic and Alention low in the Pacific.
What is cyclone structure?
Wave cyclones have broad scale flow and superimposed concentrated flow. The combination of these determines distribution of clouds and precipitation. Use isentropic coordinate to determine flows. Air travels on these surfaces if adiabatic. Isobaric surfaces intersect the flows. Use coordinate system traveling with system of interest (get system relative flow). Will depict a while different world.
What is the conceptual model?
See area of clouds (comma cloud) and three major air currents. Low and middle clouds to south along cold front. Cirrus along northeastern edge and stratocumulus along northwestern edge. This model is found to be wrong.
What is the warm conveyor belt (WCB)?
Warm, moist air originates to south and rises as it heads north, major ascent along warm front. It exits at high levels toward northeast. A major transporter of moisture—the main cloud producer and usually sharp western cloud boundary—limiting streamline. Southern reaches often corresponds to low level jet with speeds of 50-60 knots.
What is the cold conveyor belt (CCB)?
Actually as wide as WCB, just shown narrower. Flow just north of warm front (in cold air), and it travels underneath the WCB and rises and loops back around to east. Moisture causing comma head (mostly scattered) is from precipitation associated with WCB. Mixing aids in cloud development. Cold belt may merge with WCB in upper levels. Contrasting cloud tops near warm front seen in satellite imagery (especially in IR images).
What is cold, dry flow?
Most cold air entering system comes from the west (at G&H)—a deeper layer. Air is dry—history of descent, perhaps in a tropopause fold—might be rising as it approaches the cold front. May have a "split front".
What are sloping warm conveyor belts?
Main WCB flow is parallel to cold front. Small component (often ageostrophic) is perpendicular to cold front (system relative) and the nature of this flow affects frontal structure. Rearward sloping ascent is relative to moving front, some flow to rear and forward sloping ascent is relative to moving front, some flow forward.