EAS 486 Lecture Notes for Day 16: Thunderstorm Climatology and Nocturnal Convection

1. Typical Convective activity
   1. Florida - highest frequency of thunderstorms
      1. Sea breeze convergence during summer
      2. Atmosphere convects so often that it's typically moist adiabatic
   2. High Plains - highest frequency of tornadoes
      1. Normal during spring - strongest mean jet stream of year
      2. Ingredients
         1. Upper-level jet streak induced low level jet streak
         2. Possibility of LLJ carrying true Gulf air
         3. Drytrough - frequent surface focus for convection
         4. Sounding type - "loaded gun"
            1. Very moist unstable boundary layer to east of drytrough - presence of low cumulus
            2. Restraining inversion as downsloped air rides over warm, moist boundary layer
            3. Elevated mixed layer (EMT) - dry air aloft with dry adiabatic lapse rate
               1. Restrains convection to allow maximum heating
               2. Steep lapse rate allows surface-based parcel to generate high CAPE
               3. Very dry - allows for good downdraft conditions
   3. Front Range of Rockies - maximum hail incidence
      1. Summer thunderstorms generated on cool side of stalled front in Plains
      2. Lower tropospheric easterly flow
      3. Lingering moisture in boundary layer
      4. Lift up steepest slope of Rockies
      5. Denver Vorticity Convergence Zone - cyclonic terrain-forced circulation around bowl marked by Continental Divide to west, Cheyenne Ridge (near CO-WY border) to north, and Palmer Lake Divide (between Denver and Colorado Springs to south)
      6. Can't usually convect in warm sector - southwest wind is downslope
   4. Severe Weather Maximum
      1. Along immediate Gulf Coast in Jan, Feb
      2. Moves to "Tornado Alley" in March-May
         1. Strong dynamic systems early in season
         2. Slow-moving cut-off lows late in spring
      3. Summer maximum to northern and central Plains
         1. Weakest jet stream strength due to lack of Canada-Mexico temperature contrast
         2. High heat, moisture air rarely cleaned out to south of Omaha or northern Missouri
         3. High CAPE, low shear storms
         4. Often on the northern side of west-east front
            1. Provides extra vertical shear between surface air and high theta-E air aloft
   5. Corresponds to maximum seen by Wallace (1975) diurnal time of thunderstorm activity
      1. Mountain areas in late afternoon
      2. Progression seen from east slopes of Rockies across the Plains
      3. Des Moines severe weather
         1. Tornadoes late afternoon
         2. Hail early evening
         3. Floods most likely near midnight
      4. Riley et al (1987, MWR) shows more detail
         1. Progression stops at midnight in eastern SD, NE, and KS
            1. Possible source of new nocturnal convection
      5. Zajac and Rutledge (2001) uses lightning data to show the same thing
         1. Also done here by Thomas (450 project)
         2. Maximum lightning more smeared out overnight than in mountain areas.
2. Mesoscale convective complexes (MCCs)
   1. Discovered by Maddox et al (BAMS, 1980)
   2. Composite study in development area (Maddox 1983)
   3. Large storm complexes that peak at night
      1. Can be associated with tornadoes in formation stage in early evening.
   4. Size criteria (see Maddox) defined to allow for composite study using radiosonde data.
   5. Dynamics
      1. Main synoptic-scale mechanism: weak warm-air advection in lower troposphere
      2. Require very high moisture feed
      3. Low-level jet precursor, but upper-air dynamics often extremely weak.
      4. Composites show jet may be created by flow around complex.
      5. Reach a state of balance nocturnally, keeping most warm, humid air converging into it
         1. Air underneath complex stays warmer than surrounding air with clear skies, so convergence is enhanced under the MCC
   6. Estimated that 25% of growing season rainfall in Plains caused by MCCs
   7. Areas of frequent tracks include northwestern SD north of Black Hills.
   8. Tend to weaken during mid-morning daylight hours.
   9. Some regenerate (Bosart 1980 - Johnstown, PA flood)
   10. Some produce Mesoscale Convective Vorticies
       1. Comma-like pattern of low cloud seen in visible imagery once high cloud erodes
       2. Can spark afternoon convection along convergence lines

Last updated: April 7, 2009 10:29 AM

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