Continuing Studies of Mesoscale Gravity Waves and Precipitation Bands

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This research focuses on two important mesoscale phenomena: mesoscale gravity waves and heavy snowbands in the northwest quadrant of cyclones. Better understanding of both of these will allow better forecasts of various wintertime weather events.Mesoscale gravity waves are wave disturbances with wavelengths that range from about 30 to 250 km, periods between 0.5 and 4 hours, and amplitudes that can exceed several millibars. While a number of studies have documented the occurrence of mesoscale gravity waves, there is considerable debate on how these waves form. The Principal Investigators present a hypothesis developed under prior support that addresses the genesis of these waves. The Principal Investigators will test and refine this hypothesis and address key questions concerning mesoscale gravity wave genesis via two distinct but complementary modeling methodologies.Heavy snowbands typically occur in narrow regions in the northwest quadrant in cyclones where frontal structures and associated frontal circulations are modified by strong deformation flow. Many of these processes occur on scales smaller than can be observed with operational resources. The Snowband Dynamics Project, carried out in the winter of 1997-98 near Lake Michigan, collected unprecedented data to document and understand the structure of these heavy snow bands. The researchers will use these observations and complementary modeling studies to: 1) examine the dynamic and thermodynamic structure of heavy precipitation bands in the northwest quadrant of cyclones and in the "reverse lake-effect" regions west of Lake Michigan, 2) determine the relative roles of isentropic ascent, transverse ageostrophic circulations associated with frontogenesis, gravity waves and boundary layer processes in forcing vertical motion to create the snowbands, and 3) evaluate the importance of convective and conditional symmetric instability in band organization and intensification. An additional goal is to determine how boundary layer fluxes of heat and moisture from the lake modify frontal structure, stability and precipitation hand dynamics in the vicinity of the lake.