Browsing by Author "Dr. Fredrick Semazzi, Committee Chair"

Filter results by typing the first few letters
Now showing 1 - 3 of 3
  • No Thumbnail Available
    Isentropic Descent beneath the Saharan Air Layer and its Impact on Tropical Cyclogensis
    (2009-11-24) Diaz, Michael; Dr. Lian Xie, Committee Member; Dr. Anantha Aiyyer, Committee Member; Dr. Fredrick Semazzi, Committee Chair
    We investigate the driving mechanism behind strong climatological isentropic descent in the eastern Atlantic and how it affects tropical cyclogenesis from African Easterly Waves (AEW). Our results suggest that this isentropic descent is forced by the warm thermal structure associated with the Saharan Air Layer (SAL) combined with northerly flow on the eastern flank of the Azores high. Since this northerly flow travels from the drier middle troposphere at higher latitudes to the lower troposphere at lower latitudes, it provides a nearly continuous source of dry air off the West African coast. Thus, AEWs traveling south of the SAL often ingest dry air from the middle latitudes into their circulation. Being dry, this air mass may suppress the moist convection required for tropical cyclogenesis. Although this process is intimately linked with the SAL, the air mass involved is distinctly different; it originates from the middle latitudes and travels beneath the SAL. In contrast, previous research emphasizes the negatives impact of the SAL itself on tropical cyclogenesis and concentrates primarily on how strong vertical wind shear, dry mid-level air, and high static stability suppress tropical cyclone convection. In this study, we use the Global Forecast System (GFS) analyses from 2000 to 2008 to perform a back trajectory analysis of air within 191 AEW cases to determine dominant air mass source regions. We ﬠnd that AEWs contain a large fraction of low level air mass which has undergone isentropic descent along the African coast. Our results suggest that AEWs containing larger amounts of this air mass tend to have weaker convection and a lower probability of tropical cyclogenesis. We then investigate the role of sea surface temperature along the northwest African coast north of where AEWs track in moistening the dry air from isentropic descent and thus counteracting its inhibiting impact on convection and tropical cyclogenesis. Based on a series of numerical modeling case studies, we ﬠnd that warming (cooling) SST north of 15◦ N along the African coast increases (decreases) the probability that an AEW will become a tropical cyclone.
  • No Thumbnail Available
    Modeling the Variability of the Climate System over Lake Victoria Basin
    (2005-10-18) Anyah, Richard Ochieng; Dr. Fredrick Semazzi, Committee Chair
    The physical mechanisms associated with the diurnal to inter-annual variability of Lake Victoria Basin and regional climate are examined based on a three-tier modeling approach. First, diagnosis of eastern and the Horn of Africa climate variability, in which the Lake basin climate is intimately embedded, is performed based on two GCM ensemble simulations. The goal was to identify some of the systematic errors inherent in the GCMs before downscaling their output using regional climate model. The second part evaluates the downscaling ability of RegCM3 over eastern Africa based on multi-year ensemble simulations of the short rains season. Finally, the physical mechanisms associated with Lake Victoria Basin climate variability are investigated using a fully coupled RegCM3-3D lake modeling system. Overall, the RegCM3 simulated monthly and seasonal rainfall climatology during the short rains season are consistent with the observed in both the simulations forced by GCM output and NCEP reanalysis over specific homogeneous climate sub-regions. However, over central Kenya highlands the model simulates drier than normal conditions throughout the season. The simulated latitude-time (north-south) rainfall evolution over East Africa is quite consistent with the observed during the entire season. This is also in tandem with the expected ITCZ-driven southward migration of regions of rainfall maxima as the season progresses. A particularly distinct feature is the persistent wet conditions simulated over the equatorial belt, between 1oS and 2oN, throughout the season. The wet conditions are apparently associated with local convection induced by topography and Lake Victoria, since there is substantial reduction in the simulated rainfall amount in the 'no-lake' simulations. In the fully coupled RegCM3-POM simulations, it is evident that topography along the eastern border of Lake Victoria, large-scale moisture transported via the prevailing easterly trades and changes in the physical characteristics of the Lake are among the principal mechanisms associated with Lake Basin climate variability. In particular, one of the most unique evidence from our simulations is that changes in large-scale moisture transported via prevailing trades through the eastern boundary of our model domain results in significant enhancement/suppression of simulated rainfall amount over Lake Victoria Basin throughout the short rains season.
  • No Thumbnail Available
    A Regional Climate Modeling Study Over West Africa During the 2001-2006 Atlantic Hurricane Seasons
    (2008-04-02) Korte, Kurt Davidson; Dr. Fredrick Semazzi, Committee Chair; Dr. Anantha Aiyyer, Committee Member; Dr. Lian Xie, Committee Member
    The 2001-2006 base period is examined using the ICTP REGional Climate Model system version 3 (RegCM3)to investigate African easterly waves (AEWs) during the Atlantic tropical cyclone season (June-November)for the years of 2001-2006. Filtered 700mb meridional wind and relative vorticity are used to investigate AEW activity during the research period. A sea surface temperature (SST) sensitivity study was conducted to determine the impacts and effects of SSTs on rainfall, circulation and AEW strength, frequency and development. The regional climate model used in this study (RegCM3) does a good job depicting the propagation speed, frequency and development of AEWs over Western Africa and the eastern Atlantic. The AEWs in RegCM3 are stronger than those found in the NNRP1 reanalysis data and the GFS reanalysis data. It is concluded this is the result of two primary factors. First, the higher model resolution in addition to spotty and at times unreliable surface observations would result in reduced precision and accuracy of the reanalysis data in this region, resulting in weaker AEW disturbances. Secondly, the RegCM3 overpredicts precipitation over the research domain and it is concluded that the dynamic processes within the model that result from this are acting to produce stronger AEWs. The exact dynamical mechanism for this is unclear but we suspect it is in response to the baroclinic changes that occur within the area close to the Inter-tropical convergence zone (ITCZ) and the African Easterly Jet (AEJ) which results in stronger AEWs. A sensitivity study was also conducted to determine the impacts of warm sea surface temperatures (SSTs) in the eastern Atlantic portion of the domain during the historically active 2005 Atlantic hurricane season. To study this, a SST climatology model run was completed using averaged 2001-2006 weekly SSTs. All other variables and parameters remained the same, including using the 2005 NNRP1 reanalysis for boundary forcing. The results from this investigation show the dramatic impact of warm SSTs in the tropical eastern Atlantic on both AEW activity over Western Africa and the eastern tropical Atlantic. We also note a change in the location of the ITCZ through the early part of the study (June through August) in response to a strong SST dipole between the warm (2C+) pool off the Senegal and Mauritania coastlines and the cool tongue extending into the Gulf of Guinea. The location of the ITCZ trended back to normal during the second half of the research period (September through November) as the SST dipole over the eastern Atlantic and Gulf of Guinea weakened due to climatologically average SSTs returning to the Gulf of Guinea. Our results also show a change in the location and strength of the AEJ. When the model was run with observed 2005 SSTs, we note a more northerly and weaker AEJ in comparison to the climatology run. It has been found in previous studies that a more northerly and weaker AEJ corresponds with stronger AEWs and more precipitation over the Sahel region (Grist⁄Nicholson⁄Barcilon). Because of these findings we conclude that the AEJ and ITCZ respond to the strength and location of sea surface temperature anomalies and this manifests itself in determining the strength and frequency of AEWs.