Second MSC/COMET Winter Weather Course, Boulder, 2002-2-26 Slantwise Convection: An Operational Approach The Release of Symmetric Instability
Jan 01, 2016
Second MSC/COMET Winter Weather Course, Boulder, 2002-2-26
Slantwise Convection: An Operational Approach
The Release of Symmetric Instability
Second MSC/COMET Winter Weather Course, Boulder, 2002-2-26
Overview
• Atmospheric Instability, CSI and slantwise convection
• Theory and conceptualization
• Precipitation in complex terrain
• Operational approach and challenges
• Operational application lab
Second MSC/COMET Winter Weather Course, Boulder, 2002-2-26
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
Second MSC/COMET Winter Weather Course, Boulder, 2002-2-26
The atmosphere can be inertially and gravitationally stable but be
symmetrically unstable
Second MSC/COMET Winter Weather Course, Boulder, 2002-2-26
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)
Second MSC/COMET Winter Weather Course, Boulder, 2002-2-26
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
Second MSC/COMET Winter Weather Course, Boulder, 2002-2-26
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
( )
Second MSC/COMET Winter Weather Course, Boulder, 2002-2-26
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
Second MSC/COMET Winter Weather Course, Boulder, 2002-2-26
lifted parcel lower temp than surroundings - sinks - gravitationally stable
lifted parcel along M surface highertemp than surroundings - rises - symmetrically unstable
Second MSC/COMET Winter Weather Course, Boulder, 2002-2-26
Observations
• Layer of instability often not sufficiently thick to produce liquid precipitation
• Responsible for substantial portion of snowfall in typical subsidence regions
Second MSC/COMET Winter Weather Course, Boulder, 2002-2-26
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)
Second MSC/COMET Winter Weather Course, Boulder, 2002-2-26
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
Second MSC/COMET Winter Weather Course, Boulder, 2002-2-26
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
Second MSC/COMET Winter Weather Course, Boulder, 2002-2-26
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
Second MSC/COMET Winter Weather Course, Boulder, 2002-2-26
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
Second MSC/COMET Winter Weather Course, Boulder, 2002-2-26
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
Second MSC/COMET Winter Weather Course, Boulder, 2002-2-26
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
Second MSC/COMET Winter Weather Course, Boulder, 2002-2-26
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.
Second MSC/COMET Winter Weather Course, Boulder, 2002-2-26
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)
Second MSC/COMET Winter Weather Course, Boulder, 2002-2-26
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
Second MSC/COMET Winter Weather Course, Boulder, 2002-2-26
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”