Continuing Studies of Mesoscale Gravity Waves and Precipitation Bands

本文主要研究了两个重要的中尺度现象：中尺度重力波和气旋西北象限的重雪带。更好地了解这两者将有助于更好地预测各种冬季天气事件。中尺度重力波是波长在30到250公里之间的波扰动，周期在0.5到4小时之间，振幅可以超过几毫巴。虽然许多研究已经记录了中尺度重力波的发生，但关于这些波是如何形成的，存在相当大的争论。首席研究人员提出了一个在先前支持下发展起来的假设，该假设解决了这些波的起源。主要研究人员将测试和完善这一假设，并通过两种不同但互补的建模方法解决有关中尺度重力波成因的关键问题。大雪带通常发生在气旋西北象限的狭窄区域，在那里锋面结构和相关的锋面环流受到强变形流的改变。许多这样的过程发生在比操作资源所能观察到的规模更小的规模上。1997年至1998年冬季在密歇根湖附近开展的雪带动力学项目收集了前所未有的数据，以记录和了解这些大雪带的结构。研究人员将利用这些观察结果和互补的模型研究：1)研究气旋西北象限和密歇根湖西部“逆湖效应”地区强降水带的动力和热力学结构；2)确定等熵上升、与锋生有关的横向地转环流、重力波和边界层过程在迫使垂直运动形成雪带中的相对作用；3)评价对流和条件对称不稳定性在波段组织和增强中的重要性。另一个目标是确定来自湖泊的热量和水分的边界层通量如何改变湖泊附近的锋面结构、稳定性和降水手动力学。

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.