Forecasting
convective outbreaks
using thermodynamic
diagrams.
Anthony R. Lupo
Atms 4310 / 7310
Lab 10
Forecasting convective outbreaks using
thermodynamic diagrams.
 *Recall early in the semester we talked
about determining the LCL, CCL, LFC,
and EL using a thermodynamic diagram.
 We also talked about calculating the LI
and the SI. These indicies based on the
principle that warmer air parcels (parcels
warmer that the environment will rise
under their own power).
Forecasting convective outbreaks using
thermodynamic diagrams.
 The warmer the air parcels are over the
environment the stronger the bouyant force.
This indicates greater atmospheric instability
and is correlated with severe weather
outbreaks.
 If parcels are cooler than the environment, the
cooler parcels fall back to their original place.
 Convective Available Potential Energy (CAPE)
Forecasting convective outbreaks using
thermodynamic diagrams.
 Convective Available Potential Energy (CAPE)
 CAPE  Positive (+ ) - then parcels warmer
than their environment. The environment gains
energy from the air parcels.

 Negative (-) parcels cooler than
their environment. Environment must do work to
lift parcels and loses energy.
Forecasting convective outbreaks using
thermodynamic diagrams.
 The bouyant force can be defined as:
Tv( z )  T v( z )
Bg
T v( z )
 where g is gravity (the restoring force)
 and Tv’ is the parcel virtual temperature.
Forecasting convective outbreaks using
thermodynamic diagrams.
 Thus it should be apparent that:
  if you are cooler than the
environmental temperature, then B is
negative
  if you are warmer than the
environmental temperature, then B is
positive.
Forecasting convective outbreaks using
thermodynamic diagrams.
 If we integrate from LFC to EL (invoke il
serpente):
Tv( z )  T v( z )
CAPE   Bdz   g
dz
T v( z )
LFC
LFC
EL
EL
 CAPE is given in units of Joules. (Or
Joules per unit mass).
Forecasting convective outbreaks using
thermodynamic diagrams.
 CAPE is rapidly becoming the index of
choice for severe weather forecasters
(Pass out Rochette et al. 2000)
 Many different empirical CAPE quantities
– such as BEST CAPE, or MU CAPE –
calculate by starting with the parcel with
the highest qe
Forecasting convective outbreaks
using thermodynamic diagrams.
 Now let’s take a look at the dynamical
effect of B and relate to vertical motion
(ignore x,y):
dw
w
dt
 Invoke “uzh”
w
z
 1 2
 z  2 w dz   Bdz
1 2
w  CAPE
2
wmax  2CAPE
Forecasting convective outbreaks
using thermodynamic diagrams.
 Typical values of CAPE that lead to severe weather. Of
course these can be different for different locales, and
different types of CAPE (what color is your CAPE?), but
in general:
 Values of CAPE:
  Moderate to strong convection occurs with CAPE
values of 1000 – 3000 J/kg
  Very strong convective outbreaks 3000 – 5000 J/ kg
  Max observed CAPEs. 5000 – 7000 J/kg
Forecasting convective outbreaks
using thermodynamic diagrams.
 For CAPE of 2500 J/kg:

wmax = square root of 2 CAPE or
approximately 70 m/s
 The computation of CAPE is sensitive to
location of LCL and LFC which means that this
is sensitive to moisture and dew points (mixing
ratios) in the lowest 500 m of the PBL.
Forecasting convective outbreaks
using thermodynamic diagrams.
 If there is a “capping inversion” or stable layer
above a moist unstable boundary layer, This is
Convective Inhibition (CIN) In this Case: B is (-).
 Then the integral from LCL to LFC is CIN, it’s
the measure of the strength of the capping
inversion. From that we can calculate the
strength of the forced vertical motion necessary
to “bust the cap”.
Forecasting convective outbreaks
using thermodynamic diagrams.
 The measurement of convective
inhibition:
Tv( z )  T v( z )
CIN   Bdz   g
dz
T v( z )
LCL
LCL
LFC
LFC
 1 2
 z  2 w dz   Bdz
1 2
w  CIN
2
wmax  2CIN
Forecasting convective outbreaks
using thermodynamic diagrams.
 Thus for a CIN of 200 J/kg a forced rise of 20
m/s is needed to “bust the cap”.
 Typical severe weather soundings (Bluestein
Vol II p 445 – 455)
 The Air Force (Col. Miller, 1940’s through
1970’s) did much severe weather research
including the compilation of many soundings
and their characteristics w/r/t severe weather
(hail, wind gust etc.)
Forecasting convective outbreaks
using thermodynamic diagrams.
 Miller Type 1  “loaded gun” sounding
  Moist well mixed (q, and RH) PBL separated
from a very dry layer above by a stable layer.
  Lapse rate is nearly dry adiabatic Above
“cap” stable layer.
  Depth of the mosit layer typically 1500 m
(150 hPa), and sfc dewpoints typically 10 C or
better near sfc and 8 C or better at top of PBL
(well-mixed!!)
Forecasting convective outbreaks
using thermodynamic diagrams.
 Sounding
Forecasting convective outbreaks
using thermodynamic diagrams.

High qe laspe rates and CAPEs high, and SI and LI
low.
  Common for plains region, potential for very
strong intense updrafts. Strong cool
downdrafts likely.
 Miller Type II
  Common in tropics and more common to
severe weather east of Mississippi river.
Forecasting convective outbreaks
using thermodynamic diagrams.
  Deep layer of moisture up to 7 km. (RH > 65%)
  Sounding either moist adiabtic or conditionally
unstable. Lower CAPEs
  No capping inversion.
 Miller Type III
  Similar to type II (no cap and moist adiabatic or
conditionally unstable)
Forecasting convective outbreaks
using thermodynamic diagrams.
 Sounding (big outbreak 2011)
Forecasting convective outbreaks
using thermodynamic diagrams.
  but temps 10 – 15 C cooler, and the moist layer is
more shallow
  Cold air sounding, more typical of cold lows near cold
core closed Ls or troughs. Thus convection is not
widespread. (LOWER CAPEs)
 Miller Type IV 

Common in high plains and desert in summer.
 Warm dry CT airmass, under cool maritime polar
airmass.
Forecasting convective outbreaks
using thermodynamic diagrams.
 Sounding
Forecasting convective outbreaks
using thermodynamic diagrams.
 Sounding type 4
Forecasting convective outbreaks
using thermodynamic diagrams.
  PBL dry adiabatic, layer above moist
adiabatic.
 Very high based convection,frequently no rain
but lightning and strong downdrafts.
 Lifted Index
 Showalter Index
 Energy Index:
Forecasting convective outbreaks
using thermodynamic diagrams.
 The End!
Forecasting convective outbreaks
using thermodynamic diagrams.
 Questions?
 Comments?
 Criticisms?
 LupoA@missouri.edu
Forecasting convective outbreaks
using thermodynamic diagrams.