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 < 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 >500 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: => M is conserved following a parcel. • x- and z-components of eqn. of motion

8. 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

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 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

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”