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Response of MCSs and low-frequency gravity waves to vertical wind shear and nocturnal thermodynamic environments




Groff, Faith, author
Schumacher, Russ S., advisor
Adams-Selin, Rebecca D., advisor
Rasmussen, Kristen L., committee member
Nelson, Peter A., committee member

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Low-frequency gravity waves have been found to both increase and decrease environmental favorability ahead of mesoscale convective systems (MCSs) based on their associated vertical motions. The strength and timing of these waves is determined by the internal dynamics of the MCS. This study investigates the sensitivities of MCSs to changes in the vertical wind and thermodynamic profiles through idealized cloud model simulations, highlighting how internal MCS processes impact low-frequency gravity wave generation, propagation and environmental influence. A common feature among all of the simulations is that fluctuations within the internal latent heating profile, the generation mechanism behind n = 1 (N1) waves, display concurrent cellularity with the MCS updrafts. Spectral analysis is performed on the rates of latent heat release, updraft velocity, and deep-tropospheric descent ahead of the convection as a signal for N1 wave passage. Results strongly suggest that perturbations in mid-level descent up to 100 km ahead of the MCS occur at the same frequency as N1 gravity wave generation due to fluctuations of latent heat release caused by the cellular variations of MCS updrafts. The introduction of deep vertical wind shear does not change this connection nor impact the lifecycle of daytime MCS updrafts and associated N1 wave generation, however within a nocturnal environment, the frequency of the cellularity of the updrafts increases, subsequently increasing the frequency of N1 wave generation. In response to surges of latent cooling within the lower half of the troposphere, n = 2 (N2) low-frequency gravity waves are generated, however this only occurs with cooling contributions from both evaporation and melting of hydrometeors. Results indicate that in environments with minimal upper-level wind shear atop more pronounced shear below, the N2 wave generation mechanisms and environmental influence behave similarly among daytime and nocturnal MCSs. Within environments that incorporate deep vertical wind shear, many of the N2 waves are strong enough to support cloud development ahead of the MCS as well as sustain and support convection within the domain.


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gravity waves
convection initiation
mesoscale convective systems


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