Bulletin of the American Physical Society
75th Annual Meeting of the Southeastern Section of APS
Volume 53, Number 13
Thursday–Saturday, October 30–November 1 2008; Raleigh, North Carolina
Session PC: Geophysics and Atmospheric Physics |
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Chair: Yuh-Lang Lin, North Carolina A&T University Room: Holiday Inn Brownstone Lincoln |
Saturday, November 1, 2008 10:30AM - 10:42AM |
PC.00001: Using Seismic Refraction Method to Image the Subsurface Structure of the Eastern Bushveld Complex in South Africa Abebaw Belay, Andrew Nyblade, Aubreya Adams, Mulugeta Dugda Few undergraduate students had the opportunity to go to the field and image the subsurface structure of the eastern Bushveld Complex in South Africa. We applied the seismic refraction method which is a method based on a seismic wave refracted by 90 degrees (critical angle) and will generate a head wave that will be picked up by geophones as detectors. Sledgehammer was used to generate seismic waves (artificial pulses). Software called SeisModule Controller was used to record and process the seismic data. The idea is to model the subsurface structure and the modeling gave us the depth and velocity of different layers. We also utilized aeromagnetic and borehole information to make comparison with the results obtained. Each student in the field had a chance to utilize four different geophysical methods (electrical resistivity, magnetic, gravity and seismic refraction) but focused more on one of those methods in this geologically remarkable place that is rich in minerals. [Preview Abstract] |
Saturday, November 1, 2008 10:42AM - 10:54AM |
PC.00002: New Crustal Thickness for Djibouti, Afar, Using Seismic Techniques Mulugeta Dugda, Solomon Bililign Crustal thickness and Poisson's ratio for the seismic station ATD in Djibouti, Afar, has been investigated using two seismic techniques (H-$\kappa $ stacking of receiver functions and a joint inversion of receiver functions and surface wave group velocities). Both techniques give consistent results of crustal thickness 23$\pm $1.5 km and Poisson's ratio 0.31$\pm $0.02. We also determined a mean P-wave velocity (Vp) of $\sim $6.2 km/s but $\sim $6.9-7.0 km/s below a 2 - 5 km thick low velocity layer at the surface. Previous studies of crustal structure for Djibouti reported that the crust is 6 to 11 km thick while our study shows that the crust beneath Djibouti is between 20 and 25 km. This study argues that the crustal thickness values reported for Djibouti for the last 3 decades were not consistent with the reports for the other neighboring region in central and eastern Afar. Our results for ATD in Djibouti, however, are consistent with the reports of crustal thickness in many other parts of central and eastern Afar. We attribute this difference to how the Moho (the crust-mantle discontinuity) is defined (an increase of Vp to 7.4 km/s in this study vs. 6.9 km/s in previous studies). [Preview Abstract] |
Saturday, November 1, 2008 10:54AM - 11:06AM |
PC.00003: Orographic Effects on the Evolution of AEW-MCSs Across Northern Africa James Spinks, Wilson Jones, Yuh-Lang Lin, Guoqing Tang The pre-development period of Tropical Storm Debby (2006) initially formed over the Ethiopian Highlands (EH) region is studied in conjunction with the African easterly wave (AEW) disturbance and the embedded mesoscale convective systems (MCS). Based on infrared satellite imagery and numerically simulated results, several convective genesis and lysis periods are identified. With the mountains in central and western North Africa replaced by flat terrains, we found that the embedded MCS is weakened and the convective cycles less apparent. Similar effects are found with the reduced moisture content. It is found that the PBL effects and moisture availability are important for maintaining AEW-MCS system as it travels to the west and supporting sufficient moisture by allowing for surface moisture fluxes to affect these systems and aid in convective development. The AEW is slightly modulated by orography downstream from the EH region. Both the EH region and the PBL effects are essential in the proper development and propagation of the AEW and its subsequent disturbances. [Preview Abstract] |
Saturday, November 1, 2008 11:06AM - 11:18AM |
PC.00004: Formation of AEWs and MCSs over Eastern Africa and its Implication to Tropical Cyclogenesis over Eastern Atlantic Ocean Guoqing Tang, Yuh-Lang Lin, James Spinks, Wilson Jones The formation of African easterly waves (AEWs) and mesoscale convective systems (MCSs) in eastern North Africa and its impacts on the tropical cyclogenesis over the eastern Atlantic Ocean is studied. Based on numerical simulations using WRF model, the AEWs during the hurricane season may be generated by the orography and shear zone established by the Somali jet and anticyclonic circulation associated with the Asian monsoon to the north of the Ethiopian Highlands (EH) and Red Sea. The WRF is employed to simulate the pre-hurricane AEW-MCS system of Tropical Storm Debby (2006) near the EH. Finer-resolution numerical simulations demonstrate that the vortex generated on the lee and MCS over the mountain eventually merge and become an AEW-MCS system serving as a precursor of tropical cyclone. The larger-scale environments conducive to the formation of the AEW-MCS system are also investigated. [Preview Abstract] |
Saturday, November 1, 2008 11:18AM - 11:30AM |
PC.00005: Orographic Effects on the Evolution of African Easterly Wave-Mesoscale Convective Systems Across Africa James Spinks, Wilson Jones, Yuh-Lang Lin, Guoqing Tang Effects of northern African mountains on the evolution of the Pre-Tropical Storm Debby (2006) African easterly wave-mesoscale convective system (AEW-MCS) are studied using a mesoscale numerical model. Specifically, we test the hypothesis, as proposed by previous studies, that moisture and vertical velocity played a major role in the generation and maintenance of the MCS convective cycles. With the mountains in central and western North Africa removed or the moisture reduced, we found that the embedded MCS is weakened and the convective cycles less apparent. Effects of PBL are also found to play a major role in modulating the AEW-MCS system. Both orographic and PBL effects are essential in the proper development and propagation of the AEW and its subsequent disturbances. This work is mainly supported by UCAR SOARS Program and partially supported by a grant from the National Oceanic and Atmospheric Administration, Educational Partnership Program under the cooperative agreement NA06OAR4810187. [Preview Abstract] |
Saturday, November 1, 2008 11:30AM - 11:42AM |
PC.00006: Formation of African Easterly Waves and Mesoscale Convective Systems over Eastern Africa Guoqing Tang, Yuh-Lang Lin, James Spinks, Wilson Jones Based on simulations of Tropical Storm Debby (2006) using the Weather Research and Forecast (WRF) model, the African Easterly Waves (AEWs) during the hurricane season may be generated by the orography and shear zone established by the Asian monsoon currents. The vortices often form on the lee of major mountain ranges. The MCSs are originated from the moist convection over the major mountain ranges as triggered by diurnal sensible heating. An MCS may merge with a vortex in the shear zone and form a coupled AEW-MCS system. The larger-scale environments conducive to the formation of the AEW-MCS system are also investigated, but using a much larger numerical model domain. This work is supported by a grant from the National Oceanic and Atmospheric Administration, Educational Partnership Program under the cooperative agreement NA06OAR4810187. [Preview Abstract] |
Saturday, November 1, 2008 11:42AM - 11:54AM |
PC.00007: Effects of Southern Appalachian Mountains on the Rainfall Associated with the Passage of Hurricane Iavn (2004) Alan Covell, Yuh-Lang Lin Effects of Appalachian Mountains on Hurricane Ivan (2004) are investigated using the Weather Research and Forecast (WRF) model. A single domain with 12 km resolution is used to simulate the storm from 0000UTC 16 to 1200UTC 18 September with a time interval of 3 h. The Appalachian Mountains have a much more profound effect at 1000mb than at 300mb, as revealed in vorticity fields. The simulated fields compare well with satellite imagery, however, the simulated track is slightly west of the actual track. The simulated rainfall indicates that most of the precipitation appears to be focused over the Appalachian mountains and far western N.C., as shown in the observed data, which is also known as rain-shadow phenomenon. The storm weakens as it passes over and around the mountains. The Appalachian mountains appear to have a focusing effect on getting the precipitation concentrated around the mountains. The mountains also seem to influence the path of the storm, causing it to move farther westward as it heads north. This work is supported by a grant from the~National Oceanic and Atmospheric Administration, Educational Partnership Program under the cooperative agreement NA06OAR4810187. [Preview Abstract] |
Saturday, November 1, 2008 11:54AM - 12:06PM |
PC.00008: Comparison of Water Vapor Measurements from Ground-based and Space-based GPS Atmospheric Remote Sensing Techniques Ian Colon-Pagan, Ying-Hwa Kuo In this study, we compare precipitable water vapor (PWV) values from ground-based GPS water vapor sensing and COSMIC radio occultation (RO) measurements over the Caribbean Sea, Gulf of Mexico, and United States regions as well as global analyses from NCEP and ECMWF models. The results show good overall agreement; however, the PWV values estimated by ground-based GPS receivers tend to have a slight dry bias for low PWV values and a slight wet bias for higher PWV values, when compared with GPS RO measurements and global analyses. An application of a student T-test indicates that there is a significant difference between both ground- and space-based GPS measured datasets. The dry bias associated with space-based GPS is attributed to the missing low altitude data, where the concentration of water vapor is large. The close agreements between space-based and global analyses are due to the fact that these global analyses assimilate space-based GPS RO data from COSMIC, and the retrieval of water vapor profiles from space-based technique requires the use of global analyses as the first guess. This work is supported by UCAR SOARS and a grant from the~National Oceanic and Atmospheric Administration, Educational Partnership Program under the cooperative agreement NA06OAR4810187. [Preview Abstract] |
Saturday, November 1, 2008 12:06PM - 12:18PM |
PC.00009: A study of ocean spray lubrication effect on tropical cyclone intensity Yevgenii Rastigejev, Yuh-Lang Lin It has been shown recently (\textit{Barenblatt, et al., 2005}) that the presence of water droplets in the vortex of tropical cyclone (TC) leads to a significant reduction in turbulent intensity and consequently to a sharp flow acceleration. The developed theory has been extended by considering different mechanisms of ocean spray production, positive feedback of wind acceleration, different turbulence closure models and some other contributing factors. The sensitivity of ocean spray lubrication effect to the theoretical model has been investigated. A series of numerical experiments with an ideal hurricane model have been performed. The simulations have been run with and without spray for different theoretical models. An effort to develop a proper spray parameterization based on the theoretical consideration and the results of numerical experiments is undertaken. Consequently we will incorporate the spray parameterization in the realistic Weather Research and Forecast (WRF) numerical model in order to improve the accuracy of TC intensity prediction. This work is supported by a grant from the~ National Oceanic and Atmospheric Administration, Educational Partnership Program under the cooperative agreement NA06OAR4810187. [Preview Abstract] |
Saturday, November 1, 2008 12:18PM - 12:30PM |
PC.00010: Effects of Orography on the Genesis of Hurricane Javier (2004) in the Eastern Pacific Ocean Van Nguyen, Yuh-Lang Lin Observational evidence shows that the Eastern Pacific Ocean is the most active region of tropical cyclone genesis in the world. In this study, we will perform numerical experiments using the Weather Forecast and Research (WRF) model to investigate the orographic effects on the genesis of Hurricane Javier (2004). In particular, we test the hypothesis that the formation of Hurricane Javier is due to the merging of the orographically modified African easterly waves by Central American mountains, mesoscale convective systems (MCSs) embedded within the AEWs, and the MCSs induced by diurnal heating over the mountains. Effects of orography and moisture are studied by performing sensitivity experiments using WRF with the mountains removed and moisture reduced, respectively. Fundamental understanding of the tropical cyclogenesis over the eastern Pacific is obtained by comparing the results from the control experiment, sensitivity experiemtns, and available data. This work is supported by a grant from the~ National Oceanic and Atmospheric Administration, Educational Partnership Program under the cooperative agreement NA06OAR4810187. [Preview Abstract] |
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