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Slantwise Convection: An Operational Approach

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Slantwise Convection: An Operational Approach The Release of Symmetric Instability – PowerPoint PPT presentation

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Title: Slantwise Convection: An Operational Approach


1
Slantwise Convection An Operational Approach
  • The Release of Symmetric Instability

2
Overview
  • Atmospheric Instability, CSI and slantwise
    convection
  • Theory and conceptualization
  • Precipitation in complex terrain
  • Operational approach and challenges
  • Operational application lab

3
Atmospheric Instability
  • gravitational
  • pure, potential, conditional
  • vertical parcel displacement
  • determined by lapse rate and saturation
  • inertial
  • horizontal parcel displacement
  • absolute vorticity lt 0
  • symmetric
  • combination of gravitational and inertial

4
The atmosphere can be inertially and
gravitationally stable but be symmetrically
unstable
5
Slantwise Convection
  • Banded clouds and precipitation
  • Sometimes associated with extratropical fronts
  • Single or multiple bands isolated or embedded
  • Length 100 to gt500 km
  • Width 5 to 40 km
  • Bands observed in regions where the atmosphere is
    gravitationally stable
  • Bennetts and Hoskins (1979), Emanuel (1983)

6
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

7
CSI Theory (cont.)
  • y-component of the eqn. of motion
  • gt M is conserved following a parcel.
  • x- and z-components of eqn. of motion

8
CSI Criteria
  • Slope of Mg surface shallower than qe 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

9
lifted parcel lower temp than surroundings -
sinks - gravitationally stable
lifted parcel along M surface higher temp than
surroundings - rises - symmetrically unstable
10
Observations
  • Layer of instability often not sufficiently thick
    to produce liquid precipitation
  • Responsible for substantial portion of snowfall
    in typical subsidence regions

11
Alternative Diagnosis or Math with a Purpose
(Martin, Locatelli, Hobbs, 1992)
  • Negative EPV implies presence of CSI (Moore and
    Lambert, 1993)
  • Vector equations not easy to understand
  • McCann (1995) provides manipulations to aid in
    comprehension

12
assume fj small compared to vertical wind shear
and
substitute for the geostrophic absolute vorticity
13
is the thermal wind and, on a constant
pressure surface
the relation between theta and theta-e on a
constant pressure surface
the thermal wind equation becomes
14
substitute for the thermal wind into EPV equation
and use a few vector identities to yield
Although difficult to compute, this form of EPV
is easy to interpret qualitatively EPV varies
with horizontal and vertical temperature gradients
15
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

16
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

17
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

18
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.

19
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)

20
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

21
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
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