Final Exam Concepts Flashcards
Fujita Scale connects what two scales?
Beaufort and Mach scales
Scale that’s derived from Fujita (F) scale
Enhanced Fujita (EF) scale
EF Scale Winds?
Estimated by the damage, not direct measurements. Three second gusts are at different levels, not the same as surface obs.
Scales of Rotation
Mesocyclone: 2-7 km, often detectable on Doppler radar
Tornado: 100-1000 m, often not detectable on Doppler radar (TVS)
Suction vortices: 1-50 m, recently observed in high-resolution Doppler radar
Tornado Characteristics
Rotate counterclockwise (some clockwise) Typical diameter: 300-2000 feet Usually move SW to NE Average path length: 4 miles Wind speeds range: 75-250 mph One vortex or multiple
Georgia Tornado Peaks
March-May
U.S. Tornado Peaks
May-June
Tornado measuring device
TOTO with different sensors
Small portable instrument recorders
Turtles (developed at OU in 1987)
Hardened In-situ Tornado Pressure Recorder (HITPR) (Low profile, handles winds up to 80 m/s)
Theories of supercell tornadogenesis
- Circulations on gust front (serve as incipient circulation)
- Tornadogenesis often occurs just after the formation of an occlusion downdraft.
- Rear-flank downdraft (RFD)
Non-supercell tornadoes
Tornadoes can often form in association with convective storms. Circulations (misocyclones or mesovorticies) will form on a boundary (gust front, trough line, etc.)
Maximum hail storms
Wyoming, Colorado, and New Mexico
Why don’t we see hail storms in the winter?
- Hail storms need convective clouds (warmer months)
2. Strong updrafts associated with convective clouds
How does hail grow?
Collision-coalescence process
Hail recipe
- Cumulonimbus cloud
- High liquid water content
- Strong and sustained updraft
- Hailstone embryo (“seed”)
Types of hail growth
- Wet growth (warmer cloud temperatures)
2. Dry growth (colder cloud temperatures)
Most rapid hailstone growth
Between -25°C and -10°C
Where are the embryos of larger hailstones?
SW side of the updraft in a typical Midwestern multicellular storm.
Largest hailstones?
Supercells.
Updrafts required to hold a hailstone up?
- 3 cm diameter = 25 m/s
- 8 cm diameter = 55 m/s
- 10 cm diameter = 83 m/s
Thunderstorm charges?
- Positive charges in the upper regions
- Negative charges lower down, just above 0°C isotherm
- Positive charges just below the melting layer
Three theories of charge generation
- Ice particles collide with hailstones
- Supercooled droplets collide with hailstones
- Precip and cloud particles polarized by downward-directed electric field
Thermoelectric effect in charge generation
- Under the conditions of a steady temperature difference between two ends of a rod of ice
- Some of the water molecules in ice are always dissociated into positive and negative ions and the number of these ions is greater at higher temperatures
- Temperature difference of 2°C = voltage difference of 4 mV
Largest hailstone in U.S.
Vivian, South Dakota, 2010. 8 inches in diameter and 18.5 inches in circumference
Dryline
Near-surface convergence zone between moist air flowing off of the Gulf of Mexico and dry air flowing off of semi-arid high plateau regions of Mexico and the SW U.S.
When is the dryline mostly observed?
40% of the time from April-June
Where is the dryline observed?
Great Plains up into the Dakotas
Isohume
Lines of equal humidity (not just RH)
What isohume/isodrosotherm is used for dryline position?
9 g/kg and 55°F
How big is the Td gradient in drylines?
15°C per 100 km or larger
Two theories why the dryline moves west in mid-late afternoon?
- It is advected back to the west by an enhanced ageostrophic flow produced by a deepening lee trough. The lee trough deepens as a response to strong solar heating of the higher terrain.
- It’s moving back to the west like a density current.
How is the dryline a very sharp boundary when it is within a broad confluence zone between two synoptic-scale air masses?
“Inland” sea breeze effect: Enhanced convergence when inland sea breeze is superimposed on broad confluence.
Sequence of events for gust front passage
- Pressure rises and winds decrease
- Wind speed/direction change
- Temperature decreases
- Precipitation
High pressure on both sides of gust front?
Hydrostatic pressure rise behind gust front, non-hydrostatic pressure rise in front of gust front
Coastal Fronts
- Boundary between easterly maritime flow off of Atlantic coast and cold, northerly outflow from an anticyclone
- Late fall, early winter
- From northern New England down to the Carolinas
- Boundary between rain and freezing rain/snow
- deltaT = 5-10°C over 5-10km
How do coastal fronts form?
- Surface friction
- Orography
- Coastal configuration
- Land-sea thermal contrast
Denver Convergence Zone (DCZ)
Forms when the ambient flow is S or SE (low Froude number/small upstream velocity)
Five NWP Model “Primitive” Equations
- Three Equations of Motion (Newton’s Second Law)
- First Law of Thermodynamics
- Conservation of mass
- Perfect Gas Law
- Conservation of water
Examples of Physics Parameterizations and why
Radiation, cumulus convection, cloud microphysics, boundary layer physics, etc. The primitive equations lack the necessary physics or sufficient resolution to solve key processes.
Why was MOS added?
To help solve local processes that can’t otherwise be solved by the parameterizations.
Which model product is depended on by humans to generate?
NWS Interactive Forecast Preparation System (IFPS)
Short-range models
More suitable for specific features such as fronts, temperatures, and convection. Non-hydrostatic. 1 hour to 3.5 days.
Long-range models
Hemispheric or global models and are highly skilled at wave patterns within the jet stream. Also have skill with synoptic features and may outperform short-range. Hydrostatic and isentropic. 15 days max.
Rapid Update Cycle (RUC)
NOAA-made very short-range model, with forecasts as short as 1 hour. Ran at NCEP, local weather offices, and even colleges. Run every 3 hours, forecasts are valid for the next 12 hours. Used for “analysis” data such as SPC mesoanalysis.
Rapid Refresh (RAP)
NCEP model that covers North America, run every hour. Uses numerical forecast model and data assimilation to initialize it.
HRRR
NOAA model with 3-km resolution, hourly updated, cloud resolving, convection-allowing atmospheric model. Radar data is assimilated every 15 minutes into the model.
North American Mesoscale (NAM)
NCEP model that operates at a short range (non-hydrostatic). Model itself is called the Weather Research and Forecasting (WRF) model. Non-hydrostatic mesoscale model (WRF-NMM). Run many ways in many places. Four times daily (0Z, 06Z, 12Z, 18Z). Forecasts every 6 hours, good for 84 hours. 12km resolution, 60 layers of atmosphere.
Global Forecast System (GFS)
Short, medium, and long-range model with forecasts up to 384 hours. Updates four times daily (0Z, 06Z, 12Z, 18Z). At 180 hours, resolution decreases. Through 180 hours, it has 30km resolution and 64 layers of atmospheric depth.
Three types of temperature products in models
- Basic air temperature (T)
- Wet-bulb temperature (Tw) used for precip type
- Potential temperature (Theta-e) a measure of total heat
