Slantwise Convection: An Operational Approach

Second MSC/COMET Winter Weather Course, Boulder, 2002-2-26

Slantwise Convection: An Operational Approach

The Release of Symmetric Instability


Overview

• Atmospheric Instability, CSI and slantwise convection

• Theory and conceptualization

• Precipitation in complex terrain

• Operational approach and challenges

• Operational application lab


Atmospheric Instability

• gravitational– pure, potential, conditional– vertical parcel displacement– determined by lapse rate and saturation

• inertial– horizontal parcel displacement– absolute vorticity < 0

• symmetric– combination of gravitational and inertial


The atmosphere can be inertially and gravitationally stable but be

symmetrically unstable


Slantwise Convection

• Banded clouds and precipitation

• Sometimes associated with extratropical fronts

• Single or multiple bands isolated or embedded

• Length 100 to >500 km

• Width 5 to 40 km

• Bands observed in regions where the atmosphere is gravitationally stable

• Bennetts and Hoskins (1979), Emanuel (1983)


CSI Theory

• Idealized Framework with u = 0

• Consider 2-D cross section W-E

• Saturated environment

• Unidirectional southerly geostrophic wind flow increasing with height.

• Baroclinic atmosphere (cold air to west)

• Define geostrophic momentum Mg = v + fx


CSI Theory (cont.)• y-component of the eqn. of motion:

M is conserved following a parcel.

• x- and z-components of eqn. of motion

dv

dtfu

dp

dy

dM

dt

10

du

dtfv f M Mgag ( )

dw

dt

g g

vv

vv vp

( )


CSI Criteria

• Slope of Mg surface shallower than e surface• Strong vertical wind shear and weak stability

• Near saturation

• Weakly conditionally stable

• Absolute vorticity small (weak inertial stability)

If conditions met, banded clouds oriented parallel to thermal wind as CSI released


lifted parcel lower temp than surroundings - sinks - gravitationally stable

lifted parcel along M surface highertemp than surroundings - rises - symmetrically unstable


Observations

• Layer of instability often not sufficiently thick to produce liquid precipitation

• Responsible for substantial portion of snowfall in typical subsidence regions


Alternative Diagnosisor Math with a Purpose

• Negative EPV implies presence of CSI (Moore and Lambert, 1993)

• Vector equations not easy to understand

• McCann (1995) provides manipulations to aid in comprehension

EPV gng e

(Martin, Locatelli, Hobbs, 1992)


EPV

gV f

y

v

pi

u

p

w

xf j

v

x

u

yf k

xi

yj

pk

v

p x

u

p y

v

x

u

yf

p

g e

g g g g

j

g g

ke e e

g e g e g g

ke

( )

[( ) ( ) ( ) ] ( )

( )

v

x

u

yf

g g

k g

v

p x

u

p yk

V

pg e g e g

p e ( )

EPV g kV

p pg

p e ge [ ( ) ]

substitute for the geostrophic absolute vorticity

assume fj small compared to vertical wind shear and g 0


V

p fk

g 1 is the thermal wind and, on a constant pressure

surface

(ln ) (ln )

m

de

V

p fk

g

k e

m

dp e

1

EPV g kV

p pg

p e ge [ ( ) ]

the relation between theta and theta-e on a constant pressure surface

the thermal wind equation becomes


EPV gf pk e

m

dp e g

e ( )1 2

substitute for the thermal wind into EPV equation and use a fewvector identities to yield

Although difficult to compute, this form of EPV is easy to interpret qualitatively

EPV varies with horizontal and vertical temperature gradients


Evaluating CSI from Observations

• Wind speed increases with height

• Temperature profile near neutral and near saturation for a significant layer

• Layer is well mixed (no discontinuities) due to unstable processes

• Single or multiple bands oriented parallel to thermal wind


Precipitation in Complex Terrain

• Mechanisms for precipitation– orographic uplift– warm frontal lift– ana-type cold fronts– upright convection– synoptic scale vertical motion– slantwise convection

• In mountain valleys in winter, most of these do not occur


CSI Assessment in the Mountains

• mesoscale precipitation bands

• forcing more on the synoptic scale

• Forcing often in mid-levels of atmosphere therefore less affected by terrain

• Valleys may get more snow due greater residence time of crystals in boundary layer

• NWP capable of predicting potential for slantwise convection even in the mountains


Observational Example

• Alberta study – Reuter and Akarty (MWR, Jan 95)– 40% of winter precipitation soundings were conv

stable, yet symmetrically unstable,– producing about ½ of total snowfall amounts

• In typically subsidence regions of Western NOAM, speculate that significant portion of annual snowfall produced by slantwise convection

• CSI and CI often co-exist.- CI will typically dominate.


Slantwise Convection Checklist

• S or SW flow, little directional shear, windspeed increasing with height

• weak gravitational and inertial stability• at or near saturation• Strong thermal gradient• M/theta-e or EPV from model data• take cross-section perpendicular to thermal

wind (or actual wind/height field)


Operational Pitfalls

• Slantwise convection often occurs well ahead of approaching warm fronts

• Can be coupled with ana-type cold fronts although not often in Canada

• Without directional shear, bands nearly stationary

• wide variation in precipitation over small distances


Summary

• Operational forecast capability sufficient to recognize slantwise convection potential

• Satellite imagery often of limited use

• Radar can be used for very short range forecasts – positions of bands

• Current structure of public forecasts limits ability to “tell what we know”

Documents

Slantwise Convection: An Operational Approach