Browsing by Author "Fredrick Semazzi, Committee Member"

Now showing 1 - 7 of 7

Climate and Tropical Cyclones.
(2010-08-20) Hill, Kevin; Gary Lackmann, Committee Chair; Lian Xie, Committee Member; Fredrick Semazzi, Committee Member; Anantha Aiyyer, Committee Member
Inundation Mapping Employing One Dimensional Hydraulic Modeling and Geographic Information System: Study Cases on Neuse River and Tar River.
(2010-10-28) Cepero-Perez, Keren; Jing-pu Liu, Committee Chair; Fredrick Semazzi, Committee Member; Stacy Arnold Nelson, Committee Member; Seann Reed, Committee Member
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.
Mesoscale Disturbances and Orographic Precipitation Distribution: Three Special Case Scenarios
(2007-04-05) Reeves, Heather Dawn; Yuh-Lang Lin, Committee Chair; Richard Rotunno, Committee Member; Gerald Janowitz, Committee Member; Fredrick Semazzi, Committee Member
Flow patterns and precipitation distribution in the vicinity of mountains are often attributed to whether the flow is in a blocked or unblocked regime, which, in turn, is dictated by the wind speed and static stability far upstream of the mountain and the mountain height. Herein, the notion that mesoscale disturbances immediately upstream of mountains and/or terrain inhomogeneities could alter the flow patterns significantly, resulting in a precipitation distribution that does not fit typical patterns for blocked vs. unblocked flows, was studied via consideration of specific case studies and idealized numerical experiments. The first scenario considered that of an orographically trapped stable layer upstream of a mountain. A series of quasi-idealized simulations suggests that flow patterns and precipitation distribution in the vicinity of a mountain are affected not only by the presence of such a layer, but by the strength of the capping inversion of this layer. Part two of this research investigated the effects of a pre-existing convective system upstream of a mountain for conditionally unstable flow, where a pre-existing convective system is defined as one that is triggered by some mechanism other than lifting by the mountain in question. Idealized numerical experiments with differing upstream wind speeds reveal that if the flow was in a blocked regime, the introduction of a pre-existing precipitation system had little effect on the precipitation distribution and flow patterns in the vicinity of the mountain. If the flow was in an unblocked regime, the introduction of a pre-existing precipitation system was associated with flow reversal upstream of the mountain and an upstream shift of the locus of maximum precipitation. The last part of the thesis considered the effects of terrain inhomogeneities on precipitation distribution via full-physics simulations of two heavy precipitation events along the western face of the Sierra Nevada mountains. Of the terrain features that were tested, the greatest sensitivity in the precipitation distribution was to directional changes in slope orientation along the upstream face of the range. In sensitivity tests where these undulations were removed, isolated maxima along the upslope that were present in the corresponding reference simulations were either reduced or non-existant. The overriding conclusion from this thesis is that although the wind speed and static stability far upstream of a mountain are important, mesoscale flow phenomena immediately upstream of a mountain can significantly change the flow and precipitation patterns.
Modeling Tropical Cyclone Induced Inland Flooding at Tar Pamlico River Basin of North Carolina.
(2010-07-27) Tang, Qianhong; Lian Xie, Committee Chair; Fredrick Semazzi, Committee Member; Sethu Raman, Committee Member; Gary Lackmann, Committee Member
Simulation of Ocean Circulation around the Galápagos Archipelago Using a Hybrid Coordinate Ocean Model (HYCOM).
(2010-05-17) Liu, Yanyun; John Morrison, Committee Chair; Lian Xie, Committee Chair; Fredrick Semazzi, Committee Member; Daniel Kamykowski, Committee Member
Synoptic and Mesoscale Environments for Orographic Rainfall Associated with MAP IOP-8
(2003-07-18) Chen, Shu-Yun; Yuh-Lang Lin, Committee Chair; Fredrick Semazzi, Committee Member; Larry D. Carey, Committee Member
In this study, we have adopted Penn State/NCAR Mesoscale Model version 5 (MM5) to simulate the synoptic and mesoscale environments conducive to orographic rainfall associated with Mesoscale Alpine Programme (MAP) Intensive Observation Period 8 (IOP-8). The model sensitivity tests on cumulus parameterization schemes, microphysical parameterization schemes, and terrain resolution were also included in this study. A deep trough system associated with low-level jet approached the Lago Maggiore target area at 0000UTC 20 October 1999. During the same time period, a high pressure system was located to the east of the trough system at the same time. The high-low pressure system then remained quasi-stationary through 1200 UTC 20 October and 1200UTC 21 October. The southerly flow advected conditionally unstable air, i.e. high [subscript e], up to the Po Valley and the southern Alpine slopes. The sounding upstream of the Ligurian Apennines appears to contain high convective available potential energy (CAPE). Meanwhile, an easterly flow penetrated the Po Valley along the foothill of the southern Alps. The easterly flow met the southerly flow near the northern coast of the Adriatic Sea and Ligurian Sea to help enhance the orographically induced low-level convergence. As a result, the low-level convergence near the Ligurian Apennines was stronger. The easterly flow was confined in the Po Valley between Alps and Apennines and kept moving toward the west. Eventually, it flowed out through the gap between Maritime Alps and Ligurian Apennines and formed a mesoscale vortex with the southerly flow around the western Po Valley. The relative cold and stable easterly flow then piled beneath to provide a stable environment. It was proved that the cold air serves as a cold dome to make the southerly flow easily ride on it. Therefore, the upward motion near the southern Alpine slopes was very weak and not able to produce convective rainfall. Only shallow clouds developed and stratiform precipitation was shown from both model results and observations. On the other hand, the southerly flow produced heavy orographic rainfall over the Ligurian Apennines. Along with the low-level convergence, which was enhanced by the confluence of easterly and southerly flow near the Ligurian Sea and Apennines, the upper-level divergence also played an important role in triggering and maintaining the convective systems near this region. The right entrance of jet streak was co-located with the Ligurian Apennines surrounding area through the model integration. As seen in the model simulation, the coupling of upper-level and lower-level forcing was essential for producing rainfall over Ligurian Apennines and Ligurian Sea during IOP-8. Based on model sensitivity tests on microphysical parameterization schemes, we found that the Reisner scheme tended to underpredict the precipitation over the southern Alpine slopes. The Goddard LFO scheme produced a reasonable amount of snow particles but overpredicted the amount of graupel, which may help explain the overprediction of rainfall over the southern Alpine slopes. As shown by the cumulus parameterization schemes sensitivity tests, the Grell scheme was not active enough to produce enough rainfall. Most of the rainfall was produced via microphysical parameterization scheme. In comparison, the Kain-Fritsch scheme did produce a fair amount of precipitation near the southern Alps and the ocean. However, it appears the Grell scheme is more suitable for the stable environment near the Lago Maggiore target area because it produced a reasonable amount of rainfall. For the sensitivity tests on Ligurian Apennines, we found that it has a significant impact on the rainfall distribution associated with MAP IOP-8. Without the Ligurian Apennines, the cold air was spread toward the ocean and the heavy rainfall was shifted toward the southern Alpine slopes.