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Slantwise Convection: An Operational Approach


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

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Title: Slantwise Convection: An Operational Approach

Slantwise Convection An Operational Approach
  • The Release of Symmetric Instability

  • Atmospheric Instability, CSI and slantwise
  • 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 lt 0
  • symmetric
  • combination of gravitational and inertial

The atmosphere can be inertially and
gravitationally stable but be symmetrically
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)

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
  • gt M is conserved following a parcel.
  • x- and z-components of eqn. of motion

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
  • 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 higher temp than
surroundings - rises - symmetrically unstable
  • Layer of instability often not sufficiently thick
    to produce liquid precipitation
  • Responsible for substantial portion of snowfall
    in typical subsidence regions

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

assume fj small compared to vertical wind shear
substitute for the geostrophic absolute vorticity
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
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
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
  • wide variation in precipitation over small

  • 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