Case Study in Conditional Symmetric Instability METR 4433

Case Study in Conditional Symmetric Instability METR 4433: Mesoscale Meteorology Spring 2008 Semester Dr. Kelvin Droegemeier School of Meteorology University of Oklahoma

Definition and Attributes of CSI n n The combined effects of gravitational and inertial forces leading to instability of a parcel displaced in a slantwise direction Conditional refers to an atmosphere near saturation The slantwise displacement resulting from the instability restores the atmosphere to neutrality Banded precipitation often results from CSI owing to the fact that the longitudinal instability occurs (i. e. , rolls aligned with the shear or thermal wind) – Bands typically range from 50 -100 km wide and from 75 -400 km long (slopes of 1: 100) – Typically last 1 -3 hours and remain stationary – Vertical velocities can be 10 m/s

Characteristics of CSI n n Occurs in regions where vertical wind profile shows increasing speed and veering with height (10 -20 m/s over 1 -2 km) Occurs in the presence of low static stability in the middle troposphere and a stable air mass in the boundary-layer (north of a surface frontal boundary) Occurs near a warm front and ahead of an upper-level trough Occurs where the slope of the isentropes (theta-e or s) are steeper (more vertical) than the isopleths of absolute momentum (M)

n Note weak shortwave approaching area

n Shortwave reflected at sfc as inverted trough

n Atmosphere nearly saturated (good for CSI) with strong low-level inversion

n NEXRAD VAD from Sterling, VA – note wind veering with height and increasing in speed

n CSI often occurs in regions of frontogenesis and FG reinforces the slantwise circulation and helps take it to smaller scales

900 -400 mb thickness

n CSI occurred between D-C, where strong stability is present – and thus convection would not occur!