Browsing by Author "Matthew Parker, Committee Member"

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    The Interaction of Moisture Fluxes and Orographic Precipitation over Northern California Associated with a Landfalling Atmospheric River.
    (2007-12-21) Smith, Barrett Lee II; Matthew Parker, Committee Member; Gary Lackmann, Committee Member; Sandra E. Yuter, Committee Chair
    Atmospheric rivers emanating from the tropics are responsible for the majority of the meridional transport of water vapor in the Northern Hemisphere, and have also been linked to episodes of heavy orographic precipitation along the mountains of the U.S West Coast. As moist air flow impinges on mountain ranges, orographic lifting converts water vapor to precipitation and can greatly reduce the moisture content of the airmass. The nearly along-coast parallel orientation of the Coastal and Sierra Nevada Ranges in Northern California, and the proximity of the Petaluma Gap to the south along the coast yield a geography, where moisture may enter the Sacramento Valley from multiple locations, complicating the quantification of airmass transformation over the region. Limitations of surface and satellite observing networks further complicate these calculations. In this study, the Weather Research and Forecasting Model (WRF) Version 2.2 is used to investigate the moisture flux and three-dimensional airmass transformation over northern California associated with the 29-31 December 2005 atmospheric river. Moisture flux analysis of the storm reveals that moisture enters the Sacramento Valley by both flowing over and around the Coastal Range. A large portion of the flow-over moisture is converted to precipitation along the windward slopes. Flow-around moisture enters through the Petaluma Gap, and then a significant portion is deflected northward by the strong barrier jet associated with the Sierra Nevada range. Moisture convergence and orographic lifting enhance precipitation along the slopes of the Sierra Nevada and Siskiyou Ranges. A drying ratio, or moisture reduction, of nearly 55% is found for the entire mountain complex, with 30% and 25% for the Coastal and Sierra Nevada Ranges, respectively. In a model sensitivity test where the Coastal Range is removed, the amount of moisture reaching the Sierras is only slightly increased compared to when the Coastal Range is present. When all terrain is removed, there is little reduction of moisture flux by the ocean/coast boundary, and the atmospheric river is able to penetrate deep into the western U.S.
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    The Mesoscale Characteristics of Tropical Oceanic Precipitation during Kelvin and Mixed Rossby-gravity Wave Events
    (2007-11-01) Holder, Christopher Thomas; Anantha Aiyyer, Committee Member; Fredrick Semazzi, Committee Member; Matthew Parker, Committee Member; Sandra Yuter, Committee Chair
    We analyze the mesoscale precipitation structures during Kelvin and mixed Rossby-gravity (MRG) wave troughs near Kwajalein Atoll (8.7 °N 167.7 °E) during the 1999-2003 rainy seasons using three-dimensional radar data (radius=157 km) and upper-air sounding data. The large region of anomalously cold cloudiness in the outgoing longwave radiation fields filtered in the wavenumber-frequency domain are suggestive of the presence of the wave trough. Mesoscale convective systems (MCSs) occur more frequently within Kelvin and MRG wave troughs compared to a multiyear rainy season climatology, but MCS activity widely varies from one trough event to another. Radar volumes during troughs contain only small, isolated rain areas at least half the time, similar to typical Kwajalein conditions and overwhelming many ensemble organizational statistics such as the size, shape, orientation, and reflectivity characteristics of individual contiguous rain areas. This suggests wave trough forcing is variable. Many MCSs contain scattered convective cores and areas of weak reflectivities embedded within the stratiform region, suggestive of perturbations in the MCS air and moisture flow field which may be homogenized away in MCS many schematics and have significant physical implications. There is an observed limit to convective precipitation area that the atmosphere near Kwajalein can support. This limit is observed in two different datasets near Kwajalein and in the west Pacific warm pool, but the physical reasons for this limit are unclear. Stratiform area fractions vary widely for small total rain areas, and as total precipitation area increases the stratiform area fraction tends to increase and is less variable. This reflects that small total rain areas contain small rain blobs which often have smaller stratiform proportions than larger blobs. Kelvin trough mesoscale precipitation structures tend to be slightly more organized than MRG. Total, convective, and stratiform rain areas and MCS rain areas are often somewhat larger during Kelvin troughs, and convective lines occur three to four times more often than during MRG troughs. Enhanced organization of mesoscale precipitation structures during Kelvin events may be linked to stronger, deeper, and more sustained convective updraft regions than MRG troughs, and to a potentially more favorable environment for convective initiation due to enhanced wave dynamics in the convective initiation region than with MRG waves.