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

Fig. 7.10

35°N

103°W

97.5

°W

92

°W

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

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 requisitesunderstanding 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. 7.7: 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.

benign severe

convective inhibition

LFC

equilibriumlevel

no convection

LNB

LCL

parceldzBCAPE

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 convectionsource layer

shaded area: MU

CAPE

7.4 elevated convection

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

potential instability, layer lifting, and convective initiation

potential instability:

0dz

d wor0

dz

d e

Lifting a potentiallyunstable layer yields CAPE

d

dz< 0

e*

Conditional instability:

Typical wet-season tropical sounding

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)

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

dry

line

gravity wave ridges

old outflow boundary

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?

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.

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

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.