EAS 485 Content: The Trowal

1. Map Discussion
1. Trowal: Trough of Warm Air Aloft
   1. Formal name for "backlash" precipitation from a mature or occluding low pressure system.
   2. Result of warm-air advection "pulled behind" cyclone.
   3. Can only happen when the system retains tilt with height so that rising warm-air parcels encounter southeast or east winds aloft.
   4. Precipitation often appears over coldest air at 850 mb and/or 700 mb.
   5. See isotherm ridge on the northern and northwestern flanks of 850 mb and 700 mb charts
   6. Soundings show veering from northerly to northeasterly winds.
   7. Trowal reflects "valley" of warm air lifting to higher elevations over the cold dome.
   8. This air is already saturated, so any further lift will lead to precipitation.
   9. Don't always have occluded front at surface.
   10. Best evidence: look for same theta-E from lower troposphere at higher elevations to north and northwest.
   11. References: Martin, Jan. 1999, MWR

Last updated: February 27, 2009

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