13th Cyclone Workshop 25 October 2005 Pacific Grove, CA1 A Study of the Effect of Horizontal Contrasts in Static Stability on Frontal Behavior Mark T. Stoelinga University of Washington

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA2 Vertical cross section through an idealized lower-tropospheric cold front → consistent with many observed cold fronts, textbooks, and well-established theory (e.g., Hoskins and Bretherton 1972) Vertical cross section through an idealized lower-tropospheric forward- tilted cold front → variants described by Sienkiewicz et al. (1989); Hobbs et al. (1990,1996); Martin et al. (1990); Steenburgh and Mass (1994); Locatelli et al. (1995, 1998, 2002a, 2002b); Neiman et al. (1998); Schultz and Steenburgh (1999); Koch (2001); Stoelinga et al. (2002; 2003) SFC 700 hPa SFC 700 hPa 200 km

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA3 Methodology: 1.Using examples, establish a characteristic static stability pattern associated with forward-tilted cold fronts 2.Examine the effects of that distribution by applying the Sawyer-Eliassen equation to an idealized front with and without the characteristic static stability pattern.

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA4 Question: Can the differences in circulation shed light on the plausibility of 1.Development of a forward-tilt in the lower troposphere? 2.A stronger baroclinic zone above the surface than at the surface?

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA5 The “Perfect CFA Storm”: October 2004 Surface chart and radar summary 0600 UTC 30 Oct 2004 NOAA Surface Analysis and Radar Composite

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA6 The “Perfect CFA Storm”: October 2004

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA7 The “Perfect CFA Storm”: October 2004 Surface chart and radar summary 0600 UTC 30 Oct 2004 NOAA Surface Analysis and Radar Composite

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA8 The “Perfect CFA Storm”: October km WRF model forecast valid 06 UTC 30 Oct 2004 Colors: θe; contours: θ; vectors: horizontal winds Surface Front Squall Line

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA9 The “Perfect CFA Storm”: October 2004 Surface chart and radar summary 1200 UTC 29 Oct 2004 NOAA Surface Analysis and Radar Composite

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA10 The “Perfect CFA Storm”: October km WRF model forecast valid 18 UTC 29 Oct 2004 Colors: θe; contours: θ; vectors: horizontal winds

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA11 The “Perfect CFA Storm”: October km WRF model forecast valid 21 UTC 29 Oct 2004 Colors: θe; contours: θ; vectors: horizontal winds

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA12 The “Perfect CFA Storm”: October km WRF model forecast valid 00 UTC 30 Oct 2004 Colors: θe; contours: θ; vectors: horizontal winds

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA13 Other Examples… Benjamin and Carlson (1986) Low Stability High Stability

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA14 Other Examples… Martin et al. (1990) High Stability Low Stability

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA15 Other Examples… High Stability Low Stability

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA16 Other Examples… High StabilityLow Stability Schultz and Steenburgh (1990)

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA17 Idealized Front y z

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA18 Idealized Front Cross-front distance (km) Height (km) Eliassen’s (1962) front

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA19 Sawyer-Eliassen Circulation Static Stability Shear/ Baroclinity Absolute Vorticity Shear Forcing Deformation Forcing y z

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA20 Modified Front  Reduces thermal gradient  Reduces forcing at low levels - PV+ PV  PV altered  Could alter reponse

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA21 Modified Front y z

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA22 Control Front y z

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA23 Modified minus Control Front y z Circulation due only to altered forcing, without altered response coefficients.

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA24 Frontogenesis in Control Front

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA25 Frontogenesis in Modified Front

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA Maximum total frontogenesis at each height level Frontogenesis (K/(100 km)/hour) Height (km) ControlFull perturbation Half perturbation

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA27 Conclusions 1.Forward-tilted cold fronts are characterized by a thermal gradient which is both farther ahead of, and stronger than, that at the surface. 2.They also appear to exhibit a characteristic lower-tropospheric potential temperature perturbation, with statically more stable air in the warm air and less stable air in the cold air. 3.The Sawyer-Eliassen circulation implied by this perturbation (with constant geost. deformation) inhibits frontal motion at the surface relative to that aloft. This is due mainly to the change in forcing (Q), rather than the change in response modulation (PV). 4.The total frontogenesis (geost. + ageost.) resulting from this perturbation moves the maximum frontogenesis away from the surface, to a height near the top of the perturbation. 5.Frontogenesis is maximum above the surface as long as the potential temperature gradient at the surface is no more than half that aloft.

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA28 Ongoing Work 1.Examine effects of static stability contrasts in time-dependent frontogenesis, using 2-D frontal models (e.g. Keyser and Pecnick 1988) and 3-D baroclinic wave simulations. 2.Examine boundary-layer and precipitation processes that lead to, or feed back into, static stability contrasts.

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA29

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA30 Why are fronts supposed to be tilted over the cold air and strongest at the surface? Deformation Frontogenesis (QG) y z

13th Cyclone Workshop 25 October 2005 Pacific Grove, CA31 Deformation Frontogenesis (SG) Why are fronts supposed to be tilted over the cold air and strongest at the surface? y z