chapter 7: convective initiation squall line development in illinois a visible satellite image loop...
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Chapter 7: convective initiationsquall line development in Illinois
a visible satellite image loop of CI in the eastern US
35°N
103°W
Fig. 7.2
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Fig. 7.10
35°N
103°W
97.5
°W
92
°W
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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.
average rainfall frequency (June-August 1996-2006) source: http://locust.mmm.ucar.edu/episodes/Hovmoller
time UTC
David Ahijevych
35°N
40°N
JJA108.8°W
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annual cycle of lower-tropospheric stability & BL moisture across N America at 35°N
Fig. 7. 1: numbers refer to months (1 … 12)
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7.1 CI requisitesunderstanding destabilization: lapse rate tendency
equation
First law of thermodynamics
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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. 7.7: term IV: effect of stretching
Fig. 7.8: term V: effect of latent heat release.
Fig. 7.5: term II: effect of vertical lapse rate advection plotted on a skew T.
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benign severe
convective inhibition
LFC
equilibriumlevel
no convection
LNB
LCL
parceldzBCAPE
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sensitivity of CAPE / CIN to choice of “parcel”surface-based CAPE / CIN mixed-layer CAPE / CIN
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how to derive the MU CAPE
(most unstable CAPE)
WLR: wet-bulb lapse rate
deep convectionsource layer
shaded area: MU
CAPE
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7.4 elevated convection
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destabilization without lapse rate changes: the effect of LL moisture & heating, and the lifting of a potentially-unstable
layer
note that LL moistening & warming not only reduce CIN, but also increase CAPE
three ways to remove CIN:
LL convergence, CBL deepening adding water vapor to the CBL CBL heating (sfc sensible heat flux)
Fig. 7. 9
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potential instability, layer lifting, and convective initiation
potential instability:
0dz
d wor0
dz
d e
Lifting a potentiallyunstable layer yields CAPE
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d
dz< 0
e*
Conditional instability:
Typical wet-season tropical sounding
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7.2 Mesoscale circulations and boundaries affecting CI
Atkins et al. 1995
Fig. 7.11: Sea breeze, HCR’s, and convective initiation (CI)
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)
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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)
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another example of BL variability due to mesoscale circulations and boundaries
dry
line
gravity wave ridges
old outflow boundary
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predicting CI from a sounding
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.
Tw
real parcel?
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CI failure
Misocyclones(Marquis et al 2007)
Fig. 7.15: CI failure. The Forth Worth sounding suggest no CIN, plenty of CAPE. CI did occur further north.
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destruction of embryonic convection by shearw
ind p
rofile
win
d p
rofile
win
d p
rofile
win
d p
rofile
tick marks every 2 km on x axis every 1 km on z axis
Fig. 7.20 and 21
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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.