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Severe Weather Part II: The Synoptic Scale Sets
the Table
Lab 11
Anthony R. Lupo
Atms 4310 / 7310
Severe Weather Part II: The Synoptic Scale Sets the Table
Severe Weather Part II: The Synoptic Scale Sets the Table
-or-
A Synoptic Cookbook Approach
First let’s review stability – the atmospheric resistivity to lift.
Severe Weather Part II: The Synoptic Scale Sets the Table
Red – e (unstable) purple – e (conditional) blue – e (stable) Black - d and green m
Profiles:
Severe Weather Part II: The Synoptic Scale Sets the Table
Static Stability: S = d – e =
Hydrostatic:
when ’ greater (smaller) than , air parcels will descend (ascend).
pT
Severe Weather Part II: The Synoptic Scale Sets the Table
Stability:
We also have examined SI, LI, and CAPE.
Now we’ll look at more empirical information.
These indicies were drawn up by looking at many case studies and stating something general about the stability conditions.
Severe Weather Part II: The Synoptic Scale Sets the Table Usually we look at the 500 hPa 850 hPa lapse
rate. Some of these take moisture conditions, or attempt to include moisture empirically also.
Some indicies used:
1) Vertical Totals (VT): T850 – T500
this is based on the 850 hPa 500 hPa lapse rate, or stability. We are looking for values in excess of 26 as a good indicator of severe weather.
Severe Weather Part II: The Synoptic Scale Sets the Table 2) Cross Totals (CT): Td 850 – T 500
cross totals because we are mixing dewpoint temperatures with ambient temperatures. This is a crude measure of bouyancy.
3) Total Totals (TT) = CT + VT = T850 + Td 850 – (2T500)
This is an index that is very popular to use. It’s very simple to calculate.
Empirically includes stability and bouyancy. This index was popularized by the Air Force.
Severe Weather Part II: The Synoptic Scale Sets the Table
The “K” index:
In 1971 The K index was incorporated into the NMC’s (now NCEP’s) regular data stream.
George’s K Index (J.J. George) “Weather Forecasting for Aeronautics’ (1960).
Severe Weather Part II: The Synoptic Scale Sets the Table
The K-Index appears in the NCEP composite moisture chart (Upper Left-hand corner under the lifted Index). They were impressed enough with the index’s performance to include it.
K = (T850 – T500) + Td850 – (T700 – Td700)
K = (T850 – T500) + Td850 – DD700
Severe Weather Part II: The Synoptic Scale Sets the Table
Takes into account stability, moisture content of lower troposphere. The DD700 tries to account for “deep layer” moisture assuming that a deeper moisture layer means more fuel for the fire.
The K-Index turns out to be a better indicator of airmass type-thunderstorms as opposed to baroclinic type severe weather.
Severe Weather Part II: The Synoptic Scale Sets the Table
The “K” index also forecasts thunderstorm probability
Advantages to all these indicies:
1) Quick, one only need look at two maps and have a pen and paper handy, (or calculator)
2. Rooted in thermodynamic principles
Severe Weather Part II: The Synoptic Scale Sets the Table
Disadvantages:
1) can differ in differing regions, seasons, and situations.
2) Not “coverall” indicies
3) We look at lower troposphere only.
4) excludes upper air dynamics
Severe Weather Part II: The Synoptic Scale Sets the Table The SweaT (Severe Weather Threat ) Index
SW = 12 * D + 20 (TT – 49) + 2f8 + f5 + 125(S + 0.2)
This index tries to take into account everything we learn in “cookbook” approaches.
Plenty of Lowlevel moisture:
D = 850 hPa dewpoint (Use 0 if Td < = 0 C)
Severe Weather Part II: The Synoptic Scale Sets the Table
Stability and bouyancy:
TT – 49 : Total Totals index – 49.
The higher, the better!
TT – 49 = 0 if TT < = 49, there are negative numbers used here.
Severe Weather Part II: The Synoptic Scale Sets the Table
Strong moist, low level winds:
2*f8 = twice the 850 hPa wind speed.
Strong vertical speed shear (strong jet stream):
F5 = wind speed at 500 hPa
Severe Weather Part II: The Synoptic Scale Sets the Table
Strong baroclinicity or Directional shear
S = sine of the (500 hPa – 850 hPa) wind direction.
Set S equal to zero if the following are not met:
a) 850 wind direction 130 – 250 b) 500 wind direction 210 – 310 c) 500dir – 850dir < = 0
Severe Weather Part II: The Synoptic Scale Sets the Table
Advantanges:
1) includes upper air dynamics as well as thermodynamics.
2) empirical number
Severe Weather Part II: The Synoptic Scale Sets the Table
Disadvantages:
1) Not a coverall index.
2) more complex, cannot calculate relatively quickly.
Severe Weather Part II: The Synoptic Scale Sets the Table
Interpretation
Studied 189 tornado cases
Studied 402 severe thunderstorm cases
Summary: SW > 400 necessary for severe weather, Severe
weather not a threat untill SW > 500, tonadoes not a threat until SW > 600.
Severe Weather Part II: The Synoptic Scale Sets the Table
The end!
Severe Weather Part II: The Synoptic Scale Sets the Table
Questions?
Comments?
Criticisms?