Vertical wind shear is computed between the 28 possible pairs of the first 8 mandatory radiosonde levels. A hail severity index is defined. Statistically significant correlations between strong shear and hail-free days are found in 11 shear layers. Strong shears are also found to occur more frequently with low and high severity indices than with intermediate severity indices. [...]

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Determining the factors affecting drizzle formation in marine boundary layer (MBL) clouds remains a challenge for both observation and modeling communities. To investigate the roles of vertical wind shear and buoyancy (static instability) in drizzle formation, ground-based observations from the Atmospheric Radiation Measurement Program at the Azores are analyzed for two types of conditions. The type I clouds should last for at least 5h and more than 90% time must be nondrizzling and then followed by at least 2h of drizzling periods, while the type II clouds are characterized by mesoscale convection cellular structures with drizzle occur every 2 to 4h. By analyzing the boundary layer wind profiles (direction and speed), it was found that either directional or speed shear is required to promote drizzle production in the type I clouds. Observations and a recent model study both suggest that vertical wind shear helps the production of turbulent kinetic energy (TKE), stimulates turbulence within cloud layer, and enhances drizzle formation near the cloud top. The type II clouds do not require strong wind shear to produce drizzle. The small values of lower tropospheric stability (LTS) and negative Richardson number (R-i) in the type II cases suggest that boundary layer instability plays an important role in TKE production and cloud-drizzle processes. By analyzing the relationships between LTS and wind shear for all cases and all time periods, a stronger connection was found between LTS and wind directional shear than that between LTS and wind speed shear.

This dissertation reports on studies on the role of the mean state in modulating climate variability at interannual and decadal time scales. In the atmosphere, the nonlinear superposition of mean flow and anomalous flow has important implications for many phenomena associated with variables that are nonlinear by definition, such as the vertical wind shear and surface wind speed. In the first part of this dissertation, the influence of mean flow and anomalous flow on vertical wind shear variability is studied in observations and numerical model simulations. At interannual timescales, the ENSO-shear relationship is compared between observations and numerical model simulations. It is shown that there is strong influence of mean flow on the ENSO-shear relationship. For same anomalous flow, different mean flows could give rise to a different ENSO-shear relationship. The nonlinear superposition of mean flow and anomalous flow also helps explains the dipole mode of tropical Atlantic vertical wind shear variability seen in observations and models, which implies opposite variation of vertical wind shear over the two sides of the tropical Atlantic. This has important implications for predicting phenomena such as Atlantic hurricanes, whose variations are modulated by vertical shear variability. The dissertation also addresses the role of the mean surface wind in decadal variability and predictability, as manifested through the Wind-Evaporation-SST (WES) feedback. The nonlinear superposition of anomalous surface wind on the mean trade wind can give rise to a positive WES feedback, which can amplify tropical climate variability. To study this feedback, we carried out ensembles of decadal climate predictions using the CAM3 atmospheric model coupled to a slab ocean model (CAM3- SOM) with prescribed ocean transport and simple extrapolative prescriptions of future external forcings. Mechansitic sensitivity runs using the CAM3-SOM were also carried out, where the WES feedback was switched off by prescribing climatological surface wind. Results suggest that switching off the WES feedback enhances the prediction skill over some regions, especially over the eastern tropical Pacific, by increasing the signal- to-noise ratio. To address the issue of cold bias noted in the decadal prediction experiments, we carried out additional sensitivity experiments where we used an adaptive formulation for the prescribed oceanic heat transport (Q-flux) in the slab ocean. The results from these experiments demonstrate that the mean oceanic heat transport plays a crucial role in influencing decadal predictability, by helping improve predictions of the trend component of decadal variations.

Indiana University-Purdue University Indianapolis (IUPUI)