The link below takes you to the NWS Louisville’s page on squall lines and bow ec
ID: 290550 • Letter: T
Question
The link below takes you to the NWS Louisville’s page on squall lines and bow echoes. Read the material and answer the questions which follow. (http://www.weather.gov/lmk/squallbow)
Describe the pre-storm environment of warm and cold season events with regards to the following:
1a. Characteristics of the surface frontal boundary and orientation of squall line relative to it.
1b. CAPE
1c. 850mb wind and moisture characteristics
1d. What does ‘RIJ’ refer to? Where does it descend relative to the leading edge off the squall line in a weakly sheared environment? In a moderate to strongly sheared environment? Which shear pattern is associated with the greatest damage?
Explanation / Answer
1a. The warm season surface pattern is frontal boundary. Generally it is east-west boundary.Length of squall line/bow echo relatively short and curved, and oriented perpendicular to mean environmental wind. Line bulges/bows downwind which is associated with downburst activity. Warm season squall line/ bow echoes associated with high instability and an east-west surface front often exhibit a progressive pattern.
1b. In warm season Average maximum convective available potential energy (CAPE) values in genesis area roughly 2400 J/kg with even greater instability downwind where average maximum CAPE is about 3500-4000 J/kg (range of 2500-6000 J/kg). In cold season CAPE values vary from less than 500 J/kg to over 2000 J/kg.
1c. In warm season surface patterns showing High dewpoints pooled near front/genesis area; maximum values just south of front , upper level patterns showing 850 mb moisture very high and pooled just south of bow echo track; drier air can be present at 700 and 500 mb enhancing damaging wind potential and thermodynamic and vertical shear wind profile pattern showing Winds at 850 and 700 mb show good directional shear (veering) near genesis area, and often mainly speed shear parallel to storm track downwind.In cold season upper level pattern showing moderate/strong wind fields throughout atmosphere; 850 mb wind speeds 30-60 kts common with upper-level jet stream axis aloft nearby and thermodynamic and vertical shear wind profile pattern showing Cool season squall lines/bow echoes associated with moderate/strong wind shear within the lowest 2.5 km layer (surface to 850 or 700 mb).
1d. RIJ refers to Rare Inflow Jets.If the ambient wind shear is moderate-to-strong, the RIJ tends to remain elevated up to near the leading edge of the squall line/ bow echo, then rapidly descends at the updraft/downdraft interface causing significant wind damage. Systems with elevated RIJs tend to be long-lived with rapid multicell growth along the leading edge of the system.If the ambient shear is weak, the RIJ tends to descend and spread out along and behind the leading line, still with possible wind damage but less intense/shorter-lived than for stronger sheared MCSs.
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