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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.
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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
CSI Criteria • Slope of Mgsurface 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
lifted parcel lower temp than surroundings - sinks - gravitationally stable lifted parcel along M surface higher temp 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 (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
assume fj small compared to vertical wind shear and 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 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”