Chapter 7: convective initiation
squall line development in Illinois
a visible satellite image loop of CI in the eastern US
35°N
103°W
Fig. 7.2
92°W
35°N
Fig. 7.10
diurnal cycle of convective precip
•
•
Afternoon convection in the Rockies and Southeast
Nocturnal convection is prevalent over the Gulf & Gulf Stream, and in a broad
swath of the Great Plains.
time UTC
108.8°W
JJA
40°N
35°N
David Ahijevych
average rainfall frequency (June-August 1996-2006)
source: http://locust.mmm.ucar.edu/episodes/Hovmoller
annual cycle of lower-tropospheric stability & BL moisture
across N America at 35°N
Fig. 7. 1: numbers refer to months (1 … 12)
7.1 CI requisites
understanding destabilization: lapse rate tendency equation
First law of thermodynamics
Fig. 7.4: term I: shown is the 700-500 mb T
difference. Larger differences are advected
from the NW into Texas.
Fig. 7.6: term I + III: effect of
differential horizontal temp.
advection
Fig. 7.5: term II: effect of vertical lapse
rate advection plotted on a skew T.
Fig. 7.7: term IV: effect of
stretching
Fig. 7.8: term V: effect of latent
heat release.
equilibrium
level
benign
severe
LFC
no convection
convective inhibition
LNB
CAPE 
B
LCL
parcel
dz
sensitivity of CAPE / CIN to choice of “parcel”
surface-based CAPE / CIN
mixed-layer CAPE / CIN
how to derive the
MU CAPE
(most unstable CAPE)
WLR: wet-bulb lapse rate
deep convection
source layer
7.4 elevated convection
destabilization without lapse rate changes: the effect of LL
moisture & heating, and the lifting of a potentially-unstable layer
three ways to remove CIN:
LL convergence, CBL deepening
adding water vapor to the CBL
note that LL moistening &
warming not only reduce CIN, but
also increase CAPE
Fig. 7. 9
CBL heating (sfc sensible heat flux)
potential instability, layer lifting, and convective initiation
Lifting a potentially
unstable layer yields CAPE
potential instability:
d e
dz
0
or
d w
dz
0
Typical wet-season tropical sounding
*

d
e <0
Conditional instability:
dz
7.2 Mesoscale circulations and boundaries affecting CI
Fig. 7.11: Sea breeze, HCR’s,
and convective initiation (CI)
Atkins et al. 1995
CI may occur along single boundaries, or at
intersections between boundaries, or
between boundaries and HCRs
Fig. 7.16: Horizontal convective rolls
& CI (Weckwerth et al 1996)
3D structure of boundaries: core/gap, cleft & lobe,
misocyclones, and CI
Fig. 7.12 and 13 (based on the paper by Marquis,
Richardson, Markowski 2007)
another example of BL variability due to mesoscale
circulations and boundaries
gravity wave ridges
predicting CI from a sounding
real parcel?
Tw
The key reason why the parcel may follow the dashed black curve is entrainment, mainly as soon
as a shallow Cu cloud forms. Note the very dry air above the BL. The shallow Cu will be diluted
by the dry air, and the Cu temperature will cool towards the wet-bulb T (Tw) of the mixed air.
CI failure
Fig. 7.15: CI failure. The Forth Worth sounding suggest
no CIN, plenty of CAPE. CI did occur further north.
Misocyclones
(Marquis et
al 2007)
wind profile
wind profile
wind profile
wind profile
destruction of embryonic convection by shear
tick marks every 2 km on x axis every 1 km on z axis
Fig. 7.20 and 21
7.3 Moisture convergence & CI
•
changes in mixing ratio by moisture convergence in flux form:
•
Most model Cu parameterization schemes use resolved moisture
convergence & stability changes as arguments. They may not
capture the fine-scale structure of mesoscale boundaries.