EAS 486 Lecture Content for Day 19: Dry Line

1. The dry line or drytrough (STORM model)
   1. Known to play a key role in organizing supercell convection in Tornado Alley
   2. Present about 45% of April-June days
   3. Convection develops within 400 km of dryline on 70% of days when it is present
   4. Not a true front
      1. Gradient of T or theta reverses from day to night
      2. Warm dry air at surface to west of dry line
      3. Warm dry air aloft to east of dry line with warm, moist air underneath
      4. Diurnal forcing
         1. Moves eastward during the day as surface-based inversion broken
            1. High surface dew points drop as surface moisture mixed through a deeper layer
         2. Moves westward at night as new surface-based inversions form
            1. Surface dew points rise as ground moisture stays within smaller boundary layer
         3. Easiest to find during mid-afternoon
            1. Keep southeast winds in moist sector
            2. In dry sector, deep mixed layer brings westerly momentum to surface
               1. Tend to get gusty southwest winds and blowing dust
      5. Pressure trough, dew point gradient, and wind shift not always co-located (sometimes true of dry cold fronts as well)
         1. Texas "rule of thumb:" 55°F isodrosotherm or 9 g/kg isohume as "first guess"
      6. Inversion provides "lid"
         1. Broken with sufficient synoptic-scale forcing in spots in spring during severe weather outbreaks
            1. Prevents too many random Cbs from developing so each one can tap more CAPE
            2. Synoptic-scale systems can "drag" dry line away from Plateau, producing severe weather further to the east
            3. Synoptic-scale systems can also pull warm dry air further along northeast flank of system (dry line bulge, Bluestein, Fig. 2.40, p.285)
               1. Indicates stronger synoptic-scale dynamics, better chance of severe weather
            4. Also, mesoscale waves along dry line (not well-understood; see E)
         2. Remains as "cap" when Plateau heats further during the late spring and summer
   5. Convergence is greater than what one would be expected from frontogenesis
      1. Benjamin and Carlson (Feb. 1986, MWR) showed diabatic heating lowered the dry line pressure, forcing an isallobaric wind component towards the dry line (intelligent modelling study)
   6. Why are some spots along dry line favored to trigger convection?
      1. Major subject of current research
      2. Atkins et al. (1999)
      3. VORTEX (May 1999) experiment day in which nothing happened
         1. Convective signal did not mask small scale circulations in area
      4. Fine scale data showed that the dry line was not colocated with trough
      5. Radar sections showed dry line, plus horizontal convective rolls (HCRs)
         1. Develop in capped boundary layer as waves travel along top of inversion
         2. Not amplified as in Schneider (1990), Bosart and Seimon (1986)
         3. Updraft portion of circulation associated with what Atkins et al calls cloud streets (not parallel to flow like in lake effect band) on moist side of dry line
         4. Thermally-direct circulation also develops between moist (sinking) side and dry (rising) side
         5. Ascent reaches local maximum where HCRs intersect dry line.

Last updated: April 14, 2009

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