EAS 486 Lecture Content for Day 18-19: Needs for Deep Convection

The lecture content included:

Johns, R., and C. W. Doswell, 1992: Severe local storm forecasting. Preprints Symposium on Forecasting. American Meteorological Society, 225-236.

1. Needs for Deep Convection
   1. Moist layer of sufficient depth in the lower and/or middle troposphere
   • Forecast clue: Average RH from NGM >= 40-45%
     • Comments:
       1. NGM (the no-good model) was "state-of-the-art" forecast model of the time
          • Eta had greater resolution, but wasn't accepted yet
       2. Note that RH threshold is lower than "green shading"
          • 70% RH "cloud threshold"; 90% RH precipitation threshold for cold season
          • Some web sites don't even provide show values of RH < 70%
   2. Steep enough laps rate above the moist layer to allow for a "substantial positive area"
   • Forecast Clue: LI values and Surface-based LI values <= 0°C
     • Parameter derived from LFM or NGM
       • LFM: 1970's forecast model (resolution of about 220 km; 6 vertical layers)
• 700 mb temperature from NGM forecast < 12°C
• Would probably have to be higher from the Eta, since it tends to overheat the higher terrain
• 1000-500 mb thickness <= 5790 meters
  • Last two criteria "rules of thumb" for capped soundings
  • Last two criteria don't apply in cases of
    • Elevated stations
    • Strong mid-level lift
3. Sufficient lifting of parcel from the moist layer to allow it to reach its level of free convection (LFC)
• Forecast Clue: NGM vertical motion field either in
  1. Neutral range (-3 ub s-1 < omega < 3 ub s-1)
  2. Ascent range (omega < -3 ub s-1)
  • NGM cannot regularly generate lift of < -5 ub s -1 like Eta and current GFS since resolution is far worse
  • Convection can fire off with little help from synoptic-scale
  • Carr and Millard (1987) composite of a comma cloud said that severe weather is most likely
    1. In comma tail from position of upper-level jet and equatorward (gets out of left front quadrant of jet)
    2. Just to east of comma cloud if heating cycle in phase with presence in dry slot
    • Dry slot air parcels are often rising, but have a history of descent from upper levels and/or downslope down Rockies
    • Atmosphere can remoisten by evaporation from wet ground
    • Cold air aloft can cause steep lapse rates
2. Large Hail
   1. Key requirement: strong updraft
• Strong updraft results in hail embryo having longer time period available to grow to large enough size
• Idea of hailstone making multiple trips through updraft has been discredited
• Forecast clue: Large CAPE
• Secondary forecast clue: large vertical wind shear
• Empirical requirment since large hail frequently produced by supercells that require strong vertical wind shear to form
2. Time available for melting on the way down
• For large hail, it has to survive to ground (longer melting time means smaller hail at ground)
• Forecast clue: Wet-Bulb Zero (WBZ) level
  • Point at which hailstone would actually begin melting
• Other factors: mean temperature in downdraft below freezing level (higher means faster melting)
• Other factors: size of hailstone (smaller stone has larger percentage of volume exposed to melting longer)
3. Updraft is the most important factor
• Can be awfully warm near ground in conditions favorable to supercell development
• Raleigh, NC tornado outbreak of 28 November 1988
  • Several F2, F3 tornadoes, no significant hail
  • Supercells were a high wind-shear, low CAPE example

4. Synoptic and climatological patterns (besides supercell-generating patterns)
   1. "Cold Low" (Miller 1972 - Type D)
   • Upper-level cold low with large amounts of surface moisture can produce moderate to strong instability near closed low center if in phase with diurnal heating pattern
   • Low WBZ height plays a big role
   2. High Plains - frequent outbreaks in late spring and summer
   • Possible in any strong storm situation when moderate to severe instability is present, even if vertical wind shear is weak
   3. Front Range of Rockies - Most hail occurrence in US
   • Get hail accumulation in Denver area about once every 1-2 years
   • Strong lift due to upslope producing frequent convection
   • Extra instability will produce hail
   • Most pronounced to north of surface front (winds turn from downslope in warm sector to upslope) when there is moisture pooling (relatively high dew points) on the cool side of the front.
   • Note that 50°F, due to dewpoint lapse rate, is a high dew point in the lee of the Rockies.

1. Damaging straight-line winds
   1. Need strong downdraft
   • Often occurring as upper, middle, and possibly lower portion of storm shows weakening dBz trend as core of maximum reflectivity descends through the storm
   • Downdraft produced by:

1. Negative buoyancy produced by evaporational cooling
   1. Dry air at mid-levels entrained into thunderstorm
      • Dry intrusion at 3-6 km above ground where most entrainment occurs
      • See "wet microburst" sounding typical of eastern cities
   2. Evaporation of falling raindrops in sub-cloud layer
      • High LCL (frequently in High Plains and lee of Rockies, sub-cloud layer can be 200-400 mb deep)
      • See "dry microburst" sounding typical of eastern cities
   3. Enhanced by small droplet size (easier to evaporate)
   4. Steep lapse rate
      • Downdraft CAPE: As parcels fall, surroundings warm up as fast as possible if lapse rate dry adiabatic
2. Drag force produced by falling hydrometeors (precipitation loading)
3. Downward transfer of strong flow aloft once the downdraft is established
4. Strong wind shear
   • Again empirical, since supercells frequently produce both tornado damage and straight-line wind damage
   • LEWP (line echo wave pattern) in favorable tornado pattern (crossing LLJ and ULJ)
   • Portions of supercells will bend outward, but not really a bow echo pattern

2. Downburst
• Production of straight-line wind damage reaching severe weather criteria
• Aerial surveys by Fujita and Wakimoto, among others, noted difference between tornado damage and straight line wind damage
• When not associated with supercells, frequently produced by multicellular squall lines
  • "Classic bow echo"
  • Bowing caused by mid-level, dry jet eroding echo
  • See Case D from Weisman and Klemp model
  • Structure (from 1982 PRE-STORM)
    • Front-to-rear jet carries updraft air to back of storm
    • Updraft begins to rise in front of storm at or near roll or shelf cloud
    • Rear-to-front dry air fuels downdraft as air entrains into storm
    • Result is evaporation of much of the rain in the back of the cluster
    • Tend to get worst weather at front of the storm, unlike supercell
    • After gust front passes, damaging wind, followed by heaviest rain and hail, then rainfall rate slowly eases after strongest lead echoes pass
    • Possibility of gustnado (tornado along gust front: more on non-supercell tornadoes later)
3. Derecho (day-RAY-cho)
   • Defined by Johns and Hirt (WAF, March 1987)
   • From Spanish meaning "straight"
   • Long-lived series of straight-line damaging wind reports
   • Progressive pattern shows extremely high instability and capped warm-sector
   • Relatively little wind shear
   • LLJ focused at surface front
   • Most unstable low-level air has to "underrun" (Farrell and Carlson 1989 MWR) warm, mid-level air to find where cooler upper-level conditions
     • Accomplished to north of surface front where warm-air advection can produce last bit of lift necessary to overcome weak CIN
   • Frequently start in afternoon and retain intensity overnight
     • Inflow air sufficiently unstable without daylight (Surface Td's in 70's F or higher)
     • LLJ increases and veers overnight (follows movement of storm during the night)
     • Example: 1 July 1997 - derecho starting in Monticello and Big Lake with damage all the way to Michigan
   • Can be triggered late night
     • Example: 4 July 1999 BWCA blowdown event began at 12Z with 90 MPH winds at KFAR and damage equivalent to 120 MPH winds in Bemidji. Storm hit BWCA during midday
   • LLJ not coupled to upper-level jet (little if any forcing in middle levels)
   • Maximum derecho in US begins in the southern third of Minnesota and stretches eastward to Ohio and Pennsylvania (found by Johns and Hirt, confirmed by Bentley and Mote BAMS November 1998)
4. Microburst

• Defined by Fujita (1977) due to investigation of Kennedy Airport crash in 1975
• Small (order of 1 km) and fast-occuring (2-4 min danger period) downdraft
• Horizontal wind shear of 25 m s-1 across landing/take-off area (produced by 7 MPH downdraft) can produce crashes
• Scenario
  • Plane hits gust front, gets maximum head wind (increases lift to aircraft)
  • Next will experience maximum downdraft, followed by increasing tail wind (reduces lift to aircraft)
  • If pilot backs off on power when getting head wind, plane can stall when power applied later
  • NYC case: wet microburst
  • CINDE study near Denver's Stapleton Airport (about 2 dry microbursts per day during summer season)
    • Cloud often in dissipating stage and possibly no visible cloud
    • New airport site suspected to be worse
  • Now part of commercial aircraft training simulations
  • Occasional problem for less experienced pilots, smaller airports

Last updated: 8:30 PM 13-April-2004

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