Drylines and ConvectionAndy Rye and Jeff DudaIntroduction– Characteristics– Formation– Movement– Role in initiating convection• Large-scale along-dryline variability• Small-scale along-dryline variabilityCharacteristics• Dewpoint difference– Change of 3°C at rate of .5°C per hour up to 18°C• Wind shift– Sharp wind shift on west with weak winds to the east• The zone between two air masses or boundary layers– Depth of boundary layers oscillateCharacteristics• Doppler velocity used to study– Low level convergence and upper level divergence– Hot air from west overflowed to east– Secondary circulation with descent to the east• Driven by vertical potential temperature gradient– Westward tilt with height of vertical vorticityFormation• Negatively tilted longwave trough at 500 mb over western U.S.• Weak jet at 500 mb across southern CO• 850 mb southerly winds over Texas from Gulf• Appearance of strong inversion at 1200Z • Terrain slope, heat fluxes, and soil moisture affect formation and movementMovement• Warm dry air on west side overruns and forms cap• Dry air creates higher PBL• Moves by mixing and lowering dewpoint• Can retrograde at nightMovement• Move smoothly in morning and jump in afternoon• Bulges can form• Evidence of other boundaries forming near dryline with similar characteristics– Double dryline possibleConvective Initiation• Large-scale along-dryline variability– Soil Moisture– Pressure Gradient Force– Cloud Streets• Small-scale along-dryline variability– Horizontal Convective Rolls (HCRs)– Misocyclones• Air ParcelsHorizontal Convective Rolls(HCRs)• Tubes of horizontal vorticity• Generated by convective instability and wind shear• Aligned with boundary layer shear vector• Cloud streets, reflectivity fine lines• Near-Surface Moisture Convergence (modeled)• Enhanced surface convergence → enhanced upward motion!HCR Conceptual ModelFrom Xue and Martin (2006)HCRs on RadarHCRs on Radar (cont.)HCRs on Satellite (Cloud Streets)A Closer LookNear-Surface Moisture ConvergenceMisocyclones• Vertical vorticity tubes < 4km in diameter• Relationship near HCRs along boundary– Aid in bending boundary to wavelike shape• Control where updrafts can exist due to downward-directed pressure gradient at core• Can spawn non-supercell tornadoes when established updraft core/storm collocates with misocycloneMisocyclonesFrom Murphey et al. (2006)Air ParcelsThe “So what?” of it all• Even in the presence of enhanced convergence and vertical motion, convection can still be rejected– Parcels need to be forced to their LFC– hlcland hlfc– Large-scale subsidence at a ridge– Capping– CIN, and too much of itReferencesCrawford, T. M. and H. B. Bluestein, 1996: Characteristics of Dryline Passage during COPS-91. Mon. Wea. Rev., 125, 463-477.Murphey, H. V., R. M. Wakimoto, C. Flamant, and D. E. Kingsmill, 2006: Dryline on 19 June 2002 during IHOP, Part I: Airborne Doppler and LEANDRE II Analyses of the Thin Line Structure and Convection Initiation. Mon. Wea. Rev., 134, 406-430.Wakimoto, R. M., H. V. Murphey, E. V. Browell, and S. Ismail, 2006: The “Triple Point” on 24 May 2002 during IHOP. Part I: Airborne Doppler and LASE Analyses of the Frontal Boundaries and Convection Initiation. Mon. Wea. Rev., 134, 231-249. Weiss, C. C., H. B. Bluestein, and A. L. Pazmany, 2006: Finescale Radar Observations of the 22 May 2002 Dryline during the International H20 Project (IHOP). Mon. Wea. Rev., 134, 273-293.Xue, M., and W. J. Martin, 2006a: A High-Resolution Modeling Study of the 24 May 2002 Dryline Case during IHOP, Part I: Numerical Simulation and General Evolution of the Dryline and Convection. Mon. Wea. Rev., 134, 149-171.____, and ____, 2006b: A High-Resolution Modeling Study of the 24 May 2002 Dryline Case during IHOP, Part II: Horizontal Convective Rolls and Convective Initiation. Mon. Wea. Rev., 134, 172-191.Ziegler, C. L., and E. N. Rasmussen, 1998: The Initiation of Moist Convection at the Dryline: Forecasting Issues from a Case Study Perspective. Wea. Forecasting, 13,