Second MSC/COMET Winter Weather Course, Boulder, Slantwise Convection: An Operational Approach The Release of Symmetric Instability

Second MSC/COMET Winter Weather Course, Boulder, 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, 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, The atmosphere can be inertially and gravitationally stable but be symmetrically unstable

Second MSC/COMET Winter Weather Course, Boulder, 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, 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 M g = v + fx

Second MSC/COMET Winter Weather Course, Boulder, CSI Theory (cont.) y-component of the eqn. of motion:  M is conserved following a parcel. x- and z-components of eqn. of motion

Second MSC/COMET Winter Weather Course, Boulder, CSI Criteria Slope of M g 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, lifted parcel lower temp than surroundings - sinks - gravitationally stable lifted parcel along M surface higher temp than surroundings - rises - symmetrically unstable

Second MSC/COMET Winter Weather Course, Boulder, 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, Alternative Diagnosis or 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 (Martin, Locatelli, Hobbs, 1992)

Second MSC/COMET Winter Weather Course, Boulder, substitute for the geostrophic absolute vorticity assume f j small compared to vertical wind shear and

Second MSC/COMET Winter Weather Course, Boulder, 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

Second MSC/COMET Winter Weather Course, Boulder, 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

Second MSC/COMET Winter Weather Course, Boulder, 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, 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, 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, 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, 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, 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, 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”