The role of negative buoyancy and urbanization in warm season precipitation processes over the US.

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Ganeshan, Manisha
Murtugudde, Raghu
This thesis investigates some important processes for better understanding and modeling warm season rainfall characteristics over the US. In the first part, the causes for commonly observed biases in the simulation of the diurnal cycle of warm season rainfall are explored. Model sensitivity analyses are carried out to identify potential deficiencies in two popular cumulus parameterization schemes, viz. Betts-Miller-Janjic (BMJ) and Kain-Fritsch (KF) schemes, considered suitable for use in mesoscale simulations. A novel approach using remote sensing observations to better understand the relevant trigger processes for convection is demonstrated. The convective trigger in both schemes is found to include weak, implicit constraints above the lifting condensation level (LCL), which may contribute to premature, light rain. In order to adjust for this behavior, a simple modification is made to the KF scheme to allow moist convection to begin only from the level of free convection (LFC). Even with the seemingly strict constraint, the scheme performs adequately in a mesoscale seasonal simulation producing an improvement in the nocturnal phase propagation of rainfall in the Central Plains region. The resolvable processes in the mesoscale model are able to overcome the negative buoyancy below the LFC, thereby reducing biases caused by sensitivity of the scheme's trigger to the grid-scale forcing at the LCL. In the future, such a modified scheme will be tested in regional and global simulations, to evaluate its robustness in varying convective regimes. In the second part of this thesis, a multi-city analysis using high-resolution surface observations over the US, investigates the impact of the Urban Heat Island (UHI) on warm season precipitation. Statistical methods are employed to study the rainfall anomalies associated with propagating and non-propagating storms. A strong variability is observed in the UHI-influence on rainfall based on geographical setting and diurnal forcing mechanisms. Coastal cities may experience a more pronounced positive rainfall anomaly during daytime due to the UHI-sea breeze (or lake breeze) interaction. Apart from the late-afternoon rainfall enhancement, a nocturnal rainfall increase occurs over and downwind of inland cities. The nocturnal urban instability, and its interaction with propagating thunderstorms, is explored in detail using model sensitivity analyses. It appears that urban areas act as "hot spots" during the nighttime favoring convergence of propagating storm cells due to the UHI and enhanced frictional drag. In the future, a better understanding of the contribution of thermal and dynamical effects is needed while planning strategies to reduce the urban land cover impact on climate. The results of this study also suggest that the varying influence of the UHI for coastal and inland cities must be further investigated for improving forecasts, urban water resources management, flood disaster planning, etc.