Elevated Thunderstorm Identification

My goal is to:

  • - identify ETS from observations at many Australian sites (soundings which aren’t outflow/rain contaminated)

  • - apply identification methods to Era Int to see how many ETS Era Int identifies

  • - Use Era Int to answer the following questions:

    1. Where and when (monthly, diurnally, seasonally) do ES occur in Australia?

    2. What are the typical synoptic situations associated with Australian ES (monthly, diurnally, seasonally)?

    3. (Time permitting) Which types of instabilities (moist gravitational, moist symmetric, inertial instabilities) are associated with Australian ES?

Colman 1990 - climatology EL TS identified by: Surface reports of thunderstorms, and 00Z and 12Z sfc, 850hPa, 700 hPa, 50hPa pressure and θe, and the following criteria: 1. Report lies on the cold side of an analysed front, 2. station temp, pressure, dew point consistent with surroundings (reports in mountainous terrain are excluded by this criteria). 3. surface air in warm sector must have higher θe than air on the surface cold side. 4. surface air lifted psuedoadiabatically to 50hPa must have temperatures LESS than 850hPa lifted air temperatures lifted to 50hPa (eliminate poorly analysed surface fronts). Associated with each event were observations of 850 and 50 hPa horizontal wind shear, confluence or diffluence. Reports=1093 Events=497 events (2 rep/event).


Thunderstorm Sounding Thermodynamic Indicies Creation

At Australian Capital City sites, all soundings between 2008-2015 were extracted from the BoM. Thunderstorm soundings were identified using GPATS lightning strike data. A sounding was flagged as a thunderstorm sounding if, (a) 1 or more strikes occurred within a 0.5 degree lat/lon box centered about the sounding site, and (b) within +-1hour from the sounding time. A total of 316 thunderstorm soundings were identified using this method.

For each thunderstorm sounding the following was calculated:

  • Theta Surface (K)

  • Theta Max upper (K)

  • Lev Theta Max upper (hPa)

  • Theta E Surface (K)

  • Theta E Max upper (K)

  • Lev Theta E Max upper (hPa)

  • CAPE Surface (J)

  • CIN Surface (J)

  • Lev Surface (hPa)

  • T Surface (C)

  • TD Surface (C)

  • MUCAPE (J)

  • MUCIN (J)

  • Lev MUCAPE (hPa)

  • T MUCAPE (C)


Upper Thermodynamic Index Calculations

Typical MUCAPE calculations use the parcel which has the highest equivalent potential temperature value in the lowest 300hPa of the sounding, i.e. from 1000-700hPa (REF). However some Elevated Thunderstorm case study soundings (REF) have identified the most unstable parcel occuring above 700hPa.

Using thetae through large depth of atmosphere can be problematic. Thetae asymptotes to theta when the atmophere is very dry. Since theta increases with height, if a dry layer exists the maximum thetae value level within the upper dry layer. Using such a dry parcel would underestimate MUCAPE. Theta and Thtae calculations are deliberately restricted to the lowest 300hPa, to ensure no dry layers are encountered.

MUCAPE therefore was calculated by lifting parcels at every sounding level between 1000-500hPa.

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These discriminators include the Most Unstable Convective Available Potential Energy and the equivalent potential temperature (hereafter refereed to as MUCAPE and θe respectively). These quantities are simple to calculate from both Bureau of Meteorology soundings and EraInt soundings, both of which are readily available. Discriminators based on these quantities will be used to separate surface-based conditionally unstable environments from conditionally-unstable environments which occur only for parcels that originate above the surface. To test EraInt’s ability to capture these indices, a comparison to observed sounding indices at all Australian Capital City airports from 2008-2014 has been created. Figure 1 shows various indices calculated from EraInt soundings, compared to the same indices derived from observed soundings with matching EraInt vertical resolution. With good agreement between EraInt and observed θe values, the criterion surface θe being less than Upper θe could be a robust EraInt discriminator between surface and elevated storms. Whilst there is poor agreement between values of observed and EraInt CAPE/MUCAPE, CIN/MUCIN, and the pressure level where the maximum upper θe values occur, the sign of the difference between surface and upper indices could be a promising discriminator.