2. Desarrollo Experimental
2.4 Diseño de experimentos
2.4.1 Selección de tratamientos: impacto Charpy
Rapid Cyclogenesis = Explosive Cyclogenesis Properties of rapid cyclogenesis:
• a low deepens at least 24 hPa in 24 hours
• often develops in the warm western parts of the Gulf Stream
• causes considerable damage: storm winds, torrential rains and floods, thunder
Rapid cyclogenesis is related to particularly strong ascending motions. In the omega equation the stability parameter is the multiplier of
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When the other factors remain constant, the strength of the ascending motion depends on the stability parameter
. The smaller the stability, the stronger the ascending motion.
The parts of a cyclone where precipitation is strongest are typically: • a broad precipitation area near the center of the low
• precipitation bands accompanying the warm front • convective precipitation accompanying the cold front
(Martin, figure 8.13.)
The development is fastest when an area of strong precipitation forms north and west of the center of an eastbound cyclone. In those conditions the release of latent heat
• increases the energy of the system
• strengthens vertical motions by decreasing static stability locally
• affects the large-scale structure and dynamics so that the cyclogenesis intensifies ("self-development")
Latent heat is released when water vapor condenses into water droplets. Heat is molecular movement, and there is a great deal less of it in water than in vapor. The phase changes of water involve large amounts of energy.
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12.4. Self-development
Consider a cyclone traveling in a westerly flow. The poleward-blowing boundary layer wind on the east side of the cyclone, together with the ascending air, heats the lower troposphere ahead of the warm front.
The heating is partly adiabatic: warm advection, partly diabatic: latent heat release in ascending air. The heating causes the temperature gradient in the lower troposphere to intensify, which strengthens warm advection. The increase in warm advection in turn increases ascending motion.
A stronger ascending motion results in an increase of baroclinic potential energy - a stronger cyclone. The increase in circulation inside the cyclone causes a positive feedback, meaning the disturbance intensifies itself.
a b c
a) The temperature gradient is uniform at first. Heat fluxes heat the lower troposphere in the shaded area.
b) The heating causes the temperature gradient to tighten on the east side of the low.
c) More intense low has stronger winds in the lower troposphere, and they increase warm advection even further.
Something to note about figure B: circulation also increases cold advection west of the low, but this effect is toned down by the absence of diabatic factors.
On the larger scale, the release of latent heat results in a positive thickness anomaly immediately downstream of the trough's axis. This causes the
geopotential to rise in the mid- and upper troposphere, which generates a high pressure ridge over the area where latent heat is released.
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The positive vorticity advection near the front edge of the trough shifts the wave downstream. Because a diabatic ridge is developing there at the same time, the wavelength shortens.
The shortening wavelength causes a significant increase in cyclonic vorticity advection, which further increases ascending motion downstream of the trough's axis.
Increased ascending motion intensifies the low and produces more latent heat ahead of the trough, and this reduces the wavelength of the upper wave. This results in positive feedback.
a) black wave: an upper wave (500 hPa geopotential) X = vorticity maximum
L = center of a surface low
b) The release of latent heat alters the geopotential by forming a small-scale ridge at the front edge of the trough
→
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12.5. Conveyor belts
The quasi-geostrophic model uses Euler co-ordinates, in which the cyclone moves through a fixed coordinate system. The model of conveyor belts
presented by Carlson (1980) examines cyclones with Lagrangian coordinates, where cyclone travels with the coordinate system. The conveyor belt model studies air flows in relation to a low pressure system's center and fronts. Conveyor belts are relatively narrow air flows that travel along isentropic surfaces. These so-called relative flows offer a perspective on the clouds and precipitation associated with cyclones that differs from the quasi-geostrophic model.
There are three kinds of relative flows: • warm conveyor belts
• cold conveyor belts • dry intrusions
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In addition to these three basical flows, conveyor belts have been subdivided further:
Schultz 2001: two cold conveyor belts (CCB)
• Anticyclonic CCB originates in the mid-troposphere and rises toward the peak of the warm sector ahead of the front. Once there, it turns clockwise, separating from the low pressure system in the process.
• Cyclonic CCB is a lower-tropospheric flow, which takes a counterclockwise route around the center of the low.
In a strong, deepening lows there is only cyclonic CCB. Cloudiness and precipitation are mostly related to the humid, rising warm conveyor belt.
There are clear boundaries between the CCBs in lows with well-defined and strong warm fronts. Conversely, if the warm front is weak, the boundary between the CCBs is wider and less definite.
It has also been proposed (by The COMET Program), that warm conveyor belts split into two branches in occluded cyclones:
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There is a further subdivision of warm conveyor belts into forward-sloping and rearward-sloping ones:
A diffluent upper trough forms a forward-sloping warm conveyor belt that generates precipitation (possibly rainbands) into the warm sector:
A confluent upper trough forms a rearward-sloping conveyor belt that generates strong showers inside the cold front and weaker precipitation in the stratiform clouds behind the front:
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12.6. The filling of a low
Cyclolysis: the diminishing of vorticity → disappearance of the low
The vertical axis of geopotential minimum slopes against the flow in developing cyclones. Ascending motion develops downstream of the upper troposphere's vorticity maximum (geopotential minimum), above which one lies divergence. As cyclogenesis progresses, the bottom of the upper trough gradually
approaches the surface low. When the cyclone reaches the occluded stage, the axis of the low rises upright and divergence in the upper troposphere moves downstream of the surface low.
The surface low is at its deepest and surface convergence at its strongest just before the low starts to fill. When there is no more divergence above the surface low, ascending motion ceases and the low begins to fill. Any process that results in lack of divergence above a low pressure system thereby also causes it to fill. This is the case even in the absence of descending motion above the low. In cyclolysis:
• divergence above the low runs out • pressure in the low rises
• the low‟s vorticity becomes more anticyclonic • descending motion often develops above the low