The Impact of Superimposed Synoptic to Meso-Gamma Scale Motions on Extreme Snowfall over Western Maryland and Northeastern West Virginia during the 2003 Presidents' Day Winter Storm

orographic precipitation enhancement;unbalanced flow;multi-scale;jet dynamics;heavy precipitation mechanisms;snowstorm

Kiefer, Michael Thomas ; Gary Lackmann, Committee Member,Michael Kaplan, Committee Member,Yuh-Lang Lin, Committee Chair,Kiefer, Michael Thomas ; Gary Lackmann ; Committee Member ; Michael Kaplan ; Committee Member ; Yuh-Lang Lin ; Committee Chair

University:North Carolina State University

关键词: orographic precipitation enhancement;    unbalanced flow;    multi-scale;    jet dynamics;    heavy precipitation mechanisms;    snowstorm;   

Others  :  https://repository.lib.ncsu.edu/bitstream/handle/1840.16/305/etd.pdf?sequence=1&isAllowed=y

美国|英语

来源: null

【 摘 要 】

During the second Presidents' Day winter storm of 15-18 February 2003, snowfall totals exceeding 100 cm were reported across a relatively small region of western Maryland and northeastern West Virginia (hereafter the region of interest). This numerical modeling study considers the role of two juxtaposed low-level jet/front systems in influencing the development and/or modification of meso-alpha to meso-gamma scale circulations producing these extreme snowfall totals. The goal in each chapter is to link each mechanism to the interaction of the aforementioned low-level jet/front systems, the maritime and continental, in order to synthesize the rather complicated conceptual model presented, and possibly in the future enable operational forecasters to better assess the likelihood of fine-scale extreme snowfall. All numerical simulations are performed with the Non-hydrostatic Mesoscale Atmospheric Simulation System (NHMASS) model.The first chapter considers the development of two low-level jet/front systems. The continental (southwesterly) jet/front system is shown to result from a combination of diabatic and adiabatic processes, including (1) formation of a polar stream lee cyclone through adiabatic compression and surface sensible heating and the resulting horizontal circulation, and (2) secondary circulations formed within an unbalanced subtropical jet (STJ) exit region due to generation of mid-level mass perturbations forced by latent heating and adiabatic compression. The maritime (easterly) low-level jet/front system was found to develop as a result of a strong southward directed pressure gradient force between a strong anticyclone over southern Quebec and a deepening coastal trough (with convection and low-level latent heating-induced pressure falls) accelerating parcels exiting in the polar jet (PJ) right entrance region. The unbalanced (with respect to geostrophic and gradient wind balance) STJ exit region was shown to impact not only the formation of the continental low-level jet, but also the meso-α scale lift through generation of a region of strong upper-level divergence.The second chapter built on the first chapter by considering the impact of the twolow- level jet/front systems becoming superimposed over the mid-Atlantic U.S. The low- level jet structure was shown to be conducive to maintenance of apparent inertia-gravity wave activity generated within the unbalanced STJ exit region, with the wave activity acting to prolong lift over the region of interest. Two other mechanisms, frontal lifting and frontogenesis, produced narrow bands of strong lift over the region, through a process wherein confluent deformation produced locally steeper slopes of the continental front with the continental jet forced up these steep frontal inclines. Secondary circulations generated as a result of intense bands of diabatic frontogenesis reinforced the primary frontogenetical circulation produced through confluent deformation.The third chapter evaluated the finest scale circulations, each tied to the interactions of the continental and maritime low-level jets with the complex terrain within the region of interest. Pure upslope flow was considered and generally discarded as a contributing mechanism in this case due to the shallow layer of upslope and large vertical separation between upslope-induced ascent and deep lift forced by the larger-scale mechanisms. The primary mechanism proposed, termed the multi-ridge mountain wave mechanism, occurs wherein air parcels within the maritime low-level jet are directed normal to a series of 100-300 m deep terrain ridges and forced to pass through multiple hydrostatic mountain-wave positive phases, condensing out additional liquid water with the passing of each ridge in the already saturated lower-troposphere present in the region. The cloud condensate is then advected downstream by the continental low-level jet. A second mechanism, surface convergence banding in the lee of the terrain, was proposed but is considered to be of secondary importance.