If low level CAA is a sinking mechanism, then why does a cold front produce a lifting of air? The answer is because each is an independent dynamic process. A cold front is the immediate divide between synoptic scale air masses where colder air is separated from warmer air and the colder air is advancing. The density discontinuity created at the boundary can lead to synoptic scale uplift. Usually this uplift occurs along and ahead of the cold front boundary. Low level CAA is a condition in which temperatures decrease over time at a point location due to the bringing in of colder air about a horizontal plane. This not only occurs just behind a cold front boundary but also extends to hundreds of miles behind cold front passage. In an ideal case, precipitation occurs along the cold front boundary. After the cold front boundary passes a location, the temperature continues to cool and skies begin to clear or the precipitation comes to an end. Several hours after cold frontal passage the skies clear. The front itself helps contribute to synoptic uplift along the frontal boundary. As this lifting mechanism moves away over time, the precipitation ends and skies clear. The CAA pattern behind the cold front helps contribute to the sinking of air and clearing skies. Drier air behind the cold front can also contribute to the decrease in precipitation and clouds. Additional lifting and sinking mechanisms such as DPVA, DNVA, differential thermal advection, jet streak divergence or convergence, etc. can contaminate this ideal case. On a weather map, the lifting from a cold front will tend to cover a linear region while the sinking from low level CAA will tend to cover a large area behind the cold front. Lifting combined with greater moisture and low-level WAA out ahead of a cold front contributes to most precipitation being in the warm sector ahead of the cold front. |