EAS 486 Lecture Content for Day 9: Organizing Forces in Isolated Storms

The lecture content included:

  1. Behavior of Isolated Storms (Chap. 15) cont.
    1. Physical Mechanisms Controlling Growth and Evolution
      1. Thermodynamic Structure
        1. Key Parameter: CAPE (Convective Available Potential Energy)
          1. 1500-2500 J/kg for moderately unstable convective environment
          2. 4500 J/kg for highly unstable environment
          3. Key parameter related to updraft forcing
            1. Empirical formula: wmax = (2*CAPE)1/2
              1. 2500 J/kg CAPE => 70 m s-1 updraft
              2. Overestimates by 50% due to effects neglected
                1. effects of three-dimensional pressure gradient anomalies generated in convection
                2. water loading (drag on updraft by condensed water droplets)
                3. mixing (entrainment and within cloud)
      2. Moisture Stratification
        1. Need much moisture to sustain convection, but absence above 2-4 km AGL (above ground level) enhances severity
        2. Parameter related to downdraft forcing
          1. Mechanisms for downdraft development/enhancement
            1. Evaporation of water droplets by drier air
              1. Loss of latent heat causes sinking
              2. At middle levels, entrainment of drier air into cloud
                • Sounding characteristic: mid-level dry layer above high moisture PBL
              3. Below the cloud, dry air can evaporate precipitation in the rain shaft
                • Sounding characteristic: dry, deep sub-cloud layer
                • See composite dry downburst sounding in Chap. 15 (small CAPE; deep dry adiabatic layer below LCL)
            2. Drag of falling hydrometeors ("water loading")
          2. Stronger downdraft can result in greater outflow, thus enhancing convergence at the gust front
      3. Vertical Wind Shear
        1. Key role in enabling development of long-lived convection
        2. Association with severe weather long known (Byers and Braham 1949)
        3. Destroys weaker cells, but enhances stronger cells
        4. Possible mechanisms
          1. Ability of gust front to develop new cells
            1. For little shear, downdraft develops when growing droplets/hail nuclei get too heavy to support in updraft.
              • Downdraft forms in the midst of updraft
              • Cold outflow outruns storm so that new cells have to try to form over PBL rain-cooled air
            2. For large shear, new cells move downshear with the mean wind at 5-7 km AGL
              • New cells have access to undisturbed warm, moist PBL air that was the source of the original storm
              • New cells form over gust front with enhanced convergence due to increased relative flow (faster storm propagation speed)
            3. For key value of vertical wind shear, get cell motion and gust front motion at the same speed => continuous updraft redevelopment
          2. Ability of updraft to interact with environmental wind shear
            1. Need stronger wind shear than in i)
            2. Produces quasi-steady storm structure
            3. Key mechanism: development of rotation on the flank of updraft (Rotunno and Klemp, 1985)
              • Result of tilting local vorticity about a horizontal axis into the vertical at the updraft/downdraft boundary (Rotunno 1981; Davies-Jones 1983)
              • Rotating updraft called a mesocyclone, although term actually refers to location of front-like boundaries between undisturbed inflow air and cold outflow from downdrafts
            4. If environmental vertical wind shear extends through the middle levels of the storm (about 4-6 km AGL), rotation induces dynamic pressure deficit which is strongest at mid-levels of the storm
              • Totally non-hydrostatic!
              • Vertical pressure gradient underneath storm accelerates updraft speed
              • Vertical divergence increases PBL horizontal convergence, accelerates inflow of fresh "juicy" air
              • At this point, we have a supercell thunderstorm (long-lived, steady state)
        5. Controlling role of vertical wind shear in influencing gust front interaction with dynamic pressure forcing
          • Unidirectional Shear versus Directional Shear in "moderately unstable environment" (Fig. 15.15)
            1. Unidirectional Shear - Straight line hodograph (not the same as same wind direction.."speed shear")
              1. Weak magnitude
                • Get short-lived cell whose gust front can produce new short-lived convection
              2. Stronger magnitude
                • Get low pressure (strongest at mid-levels) on the left and right flanks (relative to mean vertical shear vector) of the original updraft
              3. Strongest magnitude
                • Dynamic pressure forcing splits updraft into two quasi-steady storms
                • "Left-mover" moves to the left of the mean vertical shear vector and rotates anticyclonically
                • "Right-mover" moves to the right of the mean vertical shear vector and rotates cyclonically
                • => "Mirror-image" supercells!
            2. Clockwise-turning shear - Warm-air advection profile with low-level jet streak
              1. Weak magnitude
                • Short-lived cell regeneration occurs on forward and right flanks of original storm
              2. Strongest magnitude
                • Pressure forcing only on right flank of original updraft, producing only one quasi-steady cyclonically-rotating updaft
                • "Right-mover" moves to the right of the mean vertical shear vector and rotates cyclonically
                • => "Right-moving" supercell only
                • Can get short-lived storms on left flank

Last updated: 05-Mar-2009 10:42 AM

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