Warm Season Convective Storm Structures over the Northeastern U.S. and their Interaction with the Marine Environment

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Lombardo, Kelly
The Graduate School, Stony Brook University: Stony Brook, NY.
The evolution of organized convective structures over the northeastern U.S. from initiation to decay is influenced by a variety of geographical features (elevated terrain, coastal boundary). Warm season convection over this region has been relatively unexplored, especially the interaction between quasi-linear convective structures (QLCSs) and the marine environment. This thesis is the first study to systematically explore the evolving convective structures over the Northeast, with particular emphasis on the coastal region from New Jersey northeastward to Rhode Island, through observational analysis as well as high-resolution simulations of 2 representative case studies. Organized convective structures over the Northeast during the warm season (May-August) were identified and classified into 3 main groups, including cells, quasi-linear convection, and nonlinear structures. Across the Northeast, the occurrence of all convection decreases from the western Appalachian slopes eastward to the Atlantic coast. Composite analysis highlights the importance of terrain during the initiation of cellular convection, with a majority developing in orographically-favored upslope areas. Linear and nonlinear convection are dynamically supported with a weaker terrain influence. Composite analyses reveal that QLCSs that decay upon encountering the Atlantic coastline organize along a surface pressure trough, collocated with a region of low-level frontogenesis. Those that maintain their intensity organize downstream of a surface trough within low-level warm air advection with higher saturation in the lowest 100 hPa compared to decaying events. Sensitivity studies of a representative decaying linear event illustrate that evaporative cooling causes the development of a strong a cold pool that overwhelms the weak ambient vertical wind shear downwind of the system causing the system to decay. During this event, the role of the marine layer appears secondary. For the maintaining event, the marine layer allows the magnitude of the surface winds to be larger compared to the land due to decreasing surface friction, which increases the vertical wind shear and helps to maintain the convection. Sensitivity tests show that by increasing the roughness length over the ocean to an equivalent land value, thus decreasing vertical wind shear, the maintaining event decays closer to the coastline compared to the control run.