In the extratropical cyclone’s warm sector (the area between the warm front and
ID: 160200 • Letter: I
Question
In the extratropical cyclone’s warm sector (the area between the warm front and the advancing cold front), surface winds are likely to be producing [(cold)(warm)] air advection. As the cyclone progresses across Earth’s surface, the cold and warm fronts rotate about the center of low pressure. The motion of the storm system has similarities to that of a flying Frisbee®, that is, a Frisbee spins as it sails through the air (simultaneously exhibiting rotational and translational motions). Typically, the cold front rotates about the center of the low faster than the warm front. Consequently, the extent of the warm sector occupied by relatively warm and humid air at the surface [(shrinks)(increases)] To the north and west of the center of an extratropical cyclone, surface winds are likely to be producing [(cold)(warm)] air advection. Most cloudiness associated with a warm front develops over a broad area, often hundreds of kilometers wide, [(ahead of)(behind)] the front. From these clouds, light to moderate precipitation may fall for 12 to 24 hours or longer. The expected continuing advancement of the cold front and the Low would likely cause residents of the center portion of the country to see [(rapidly rising)(small changes in)(rapidly falling)] temperatures over the next day or so.
Explanation / Answer
Cyclone Evolution
1) Most cyclones go through a life cycle of formation, growth, weakening, and death over a period of about a week at mid-latitudes. However, lifetimes less than a day and greater than two weeks have been observed.
2) While cyclones exist, they are moved by the jet stream and by other large-scale components of the global circulation.
3) Frontal zones such as stationary fronts are regions of strong baroclinicity.
4) The resulting pressure gradient around the surface low starts to generate lower-tropospheric winds that circulate around the low This is the spin-up stage — so named because vorticity is increasing as the cyclone intensifies. The winds begin to advect the warm air poleward on the east side of the low and cold air equatorward on the west side, causing a kink in the formerly stationary front near the low center.
5) The kinked front is wave shaped, and is called a frontal wave. Parts of the old front advance as a warm front, and other parts advance as a cold front. Also, these winds begin to force some of the warmer air up over the colder air, thereby generating more clouds.
6) The advancing cold front often moves faster than the warm front for three reasons. (1) The SawyerEliassen circulation tends to push near-surface cold air toward warmer air at both fronts. (2) Circulation around the vortex tends to deform the frontal boundaries and shrink the warm-air region to a smaller wedge shape east and equatorward of the low center. This wedge of warm air is called the warm-air sector (3) Evaporating precipitation cools both fronts (enhancing the cold front but diminishing the warm front).
7) These combined effects amplify the frontal wave. As more of the cold front overtakes the warm front, an occluded front forms near the low center . The cool air is often drier, and is visible in satellite images as a dry tongue of relatively cloud-free air that begins to wrap around the low.
8). But the jet stream, still driven by the thermal wind effect, moves east of the low center to remain over the strongest baroclinic zone (over the warm and cold fronts, which are becoming more stationary).
9) Without support from the jet stream to continue removing air molecules from the low center, the low begins to fill with air due to convergence of air in the boundary layer. The central pressure starts to rise and the winds slow as the vorticity spins down. As cyclolysis continues, the low center often continues to slowly move further poleward away from the baroclinic zone .
10) The central pressure continues to rise and winds weaken. The tightly wound spiral of clouds begins to dissipate into scattered clouds, and precipitation diminishes. But meanwhile, along the stationary front to the east, a new cyclone might form if the jet stream is favorable