Browsing by Author "Anantha Aiyyer, Committee Member"

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Analysis and prediction of Atlantic tropical cyclone activity
(2008-11-09) Keith, Elinor Whitney; Lian Xie, Committee Chair; Anantha Aiyyer, Committee Member; Montserrat Fuentes, Committee Member
This work begins with the development of a statistical prediction model of numbers of tropical storms, hurricanes and major hurricanes per year in several regions of the Atlantic: the entire Atlantic, the Caribbean Sea and the Gulf of Mexico, as well as landfalling storms along the US Gulf of Mexico, Southeast and Northeast coasts. The methodology involves of cross-correlating variables against Empirical Orthogonal Functions (EOFs) of the Hurricane Track Density Function (HTDF) to select predictors. The model performs well in the basin-wide predictions over the entire Atlantic and Caribbean, with the predictions showing an improvement over climatology and random chance at a 95% confidence level. Over the Gulf of Mexico, only named storms showed that level of predictability. Predicting landfalls proves more difficult, and only the prediction of named storms along the US Southeastern and Gulf Coasts shows an improvement over random chance at the 95% confidence level. Tropical cyclone activity along the U.S. Northeastern Coast is found to be unpredictable in this model; with the rarity of events, the model is unstable. In order to provide some physical basis for many of the connections found statistically, the second section is a case study of the 2004-07 Atlantic hurricane seasons. It is found that 2005 had the most favorable SST and vertical wind shear conditions over the main development region. 2004 and 2006 had intermediate levels of SST and wind shear and, outside of the month of August, similar levels of activity. Activity in 2007 was generally suppressed: although more tropical storms formed than in 2006, they were very short-lived. On average, tropical storms in 2007 survived less than 2.5 days. The strength of the subtropical anticyclone is a very important factor: in 2005, a weak subtropical high allowed for unusually high SST in the main development region, while in 2007 a strong subtropical high over the east Atlantic cooled SST and increased vertical wind shear. The strength of the subtropical cyclone may be related to the heat release of the African monsoon. This finding also emphasizes the importance of factors relating to the strength of the subtropical high pressure in hurricane prediction.
Analysis and Prediction of West African Moist Events During the Boreal Spring of 2009.
(2010-08-16) Mera, Roberto Javier; Fredrick Semazzi, Committee Chair; Lian Xie, Committee Member; Anantha Aiyyer, Committee Member; Arlene Laing, Committee Member
The Application and Evaluation of the Global Weather Research and Forecasting Model
(2009-07-09) Hemperly, Joshua John; Yang Zhang, Committee Co-Chair; Anantha Aiyyer, Committee Member; Nicholas Meskhidze, Committee Co-Chair
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
Evaluation of TRMM Satellite Precipitation Retrievals and Satellite-Observed Characteristics of Marine Shallow Clouds
(2007-11-15) Miller, Matthew Allen; Sandra Yuter, Committee Chair; Anantha Aiyyer, Committee Member; Sankar Arumugam, 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.
Momentum Transport in Mesoscale Convective Systems
(2009-08-27) Mahoney, Kelly Marie; Anantha Aiyyer, Committee Member; Sandra Yuter, Committee Member; Gary Lackmann, Committee Chair; Matthew D. Parker, Committee Member
The transport of horizontal momentum by vertical motions within a mesoscale convective system (MCS) affects storm dynamics, sensible weather, and the connection between the system and its surrounding environment. Earlier works have examined this process for a number of purposes, but understanding of its significance to both MCS motion and the generation of convectively-driven surface winds remains incomplete. This study describes the convective momentum transport (CMT) process both qualitatively and quantitatively; this is pursued through the analysis of quasi-idealized numerical simulations. Momentum budgets illustrate that the motion of a numerically-simulated MCS is significantly impacted by CMT within the MCS. Vertical advection of the perturbation wind is found to contribute largely to the momentum field at the leading edge of the cold pool, which is the region in which the resulting accelerated winds drive system motion. Results also show that the pressure gradient acceleration and, to a lesser degree the vertical advection of the background environmental wind, contribute to the acceleration of rear-to-front-directed momentum in the middle- to rearward portions of the storm, thereby generating and reinforcing transport of the perturbation flow into the cold pool and accelerating the MCS. The second part of this dissertation uses a series of experimental simulations to examine the sensitivity of CMT, MCS motion, and surface wind speed generation to environmental humidity and microphysical processes. Results reveal modest changes in MCS motion, but marked differences in the generation of convectively-driven surface winds. Drier air at mid-levels increases descent within the trailing stratiform region and enhances CMT; this slightly increases average MCS speed by ~1 ms-1, but produces a much larger number of severe surface winds. CMT is also shown to be a contributing factor to the occurrence of severe surface winds produced via the favorable superpositioning of a descending rear inflow jet and the low-level circulation associated with gust front mesovortices. The potential for a descending rear inflow jet to cause strong surface winds at locations away from the leading edge of the gust front is discussed as well. While such surface wind patterns may occur in a variety of storm environments, it is shown that the additional downward motion imparted by decreasing the relative humidity of the mid-levels leads to additional acceleration through CMT and contributes to an increase in occurrence of strong to severe surface winds. Reducing evaporation yields the most marked decrease in both MCS motion and strength of surface wind speeds, followed by the removal of melting and sublimation, respectively. The challenge of completely isolating the contribution of Ã¢â‚¬Å“cold pool dynamicsÃ¢â‚¬Â (i.e., density current propagation) from Ã¢â‚¬Å“CMT-forcedÃ¢â‚¬Â MCS motion is also discussed. Avenues for future work are outlined, with a focus on adding or improving the representation of CMT in existing cumulus parameterization schemes, incorporating CMT into conceptual models of both MCS motion and severe surface wind generation, and further exploring of the sensitivity of CMT to a greater variety of storm environments and kinematic profiles.
Multi-scale climate change modeling study over the Greater Horn of Africa
(2008-12-12) Bowden, Jared Heath; Lian Xie, Committee Member; Sankarasubramanian Arumugam, Committee Member; Fredrick Semazzi, Committee Chair; Anantha Aiyyer, Committee Member
There has been limited regional climate modeling (RCM) studies of climate change over the Greater Horn of Africa because of challenges of modeling tropical precipitation with a limited observational rainfall network. This study customized a RCM model with particular interest in precipitation process using several precipitation data sets for validation. Various convective schemes and micro-physics sensitivities were performed. It was found that the convective scheme of MIT-Emanuel in conjunction with reducing the relative humidity threshold for cloud formation provided the most realistic simulation in terms of spatial distribution, convective partition, rainfall totals and temperature bias when compared with observations. The above RCM customization was run for approximately 40 years to determine the models ability to capture inter-annual variability and the possible climate change fingerprint over the region. The RCM is able to capture the inter-annual variability for all places and seasons for temperature. However, the positive precipitation bias limits the models ability to capture inter-annual variability of precipitation. Despite, the low inter-annual precipitation correlation, the RCM is able to simulate large scale changes in the rainfall pattern associated with the possible climate change fingerprint and the annual precipitation cycle associated with the monsoon. Since the model was able to capture possible changes associated with climate change, the model was downscaled for climate change simulations. The Finite Volume GCM (FVGCM) is used as the lateral boundary forcing for A2 scenario RCM climate change simulations. The FVGCM was compared with the other IPCC models and found to perform within the range during the contemporary climate for circulation, precipitation and temperature. Our analysis concluded that the FVGCM has a cool and wet bias compared to the other GCMs. The RCM future climate simulations, using an A2 emission scenario, show that average temperature patterns in arid to semi-arid regions are likely to have the largest temperature increases when coupled to increased drying. Coastal locations are likely to experience the smallest temperature increase in response to increased likelihood of enhanced precipitation east of the Great Lakes and the response to the oceanâ€™s thermal inertia. Daily temperature mean increase of 2.5C is found with a shift toward more extreme heat waves. There is also a clear shift for more intense precipitation for the eastern GHA with some localized regions having a shift in the rainfall frequency. We caution the interpreation of the dynamical downscale results because we have only downscaled one GCM and biases in the GCM lateral boundary forcing and internal errors in the RCM itself. The approach was limited to one GCM because of the computational expense of dynamically downscaling and locating 6 hourly ICBC for both the contemporary and future climate. We test a â€œclimatologicalâ€ ICBC approach for climate change simulations to limit the cost factor of the simulations. We find the approach is likely to be of benefit in simulating the spatial distributions for the GHA region when the boundary is far removed from region of interest. This method may be applied to other regions within the tropics and likely useful dynamical downscale physics ensembles and mutliple GCMs. Based on the application of the dynamical downscale results, we have provided a framework to help provide a clear approach for future dynamical downscale climate change simulations.
A Numerical Investigation of Supercells in Landfalling Tropical Cyclones.
(2011-01-14) Morin, Matthew; Matthew Parker, Committee Chair; Gary Lackmann, Committee Member; Anantha Aiyyer, Committee Member
Satellite Observations of Low Marine Clouds.
(2010-07-13) Miller, Matthew Allen; Sandra Yuter, Committee Chair; Scott Braun, Committee Member; Anantha Aiyyer, Committee Member; Sankarasubramanian Arumugam, Committee Member
Sensitivity of WRF Simulations of Hurricane Ivan to Horizontal Resolution
(2007-08-01) Gentry, Megan Suzanne; Gary M. Lackmann, Committee Chair; Fred Semazzi, Committee Member; Anantha Aiyyer, Committee Member
As finer resolutions become possible in numerical modeling, it has become increasingly common to turn off the cumulus parameterization scheme in favor of explicit simulation of convection. To the author's knowledge, the grid spacing at which it is appropriate to do so in a tropical cyclone (TC) case has not been systematically investigated. Therefore, this study examines the sensitivity of explicit model simulations of Hurricane Ivan (2004) to changes in horizontal grid spacing, when grid spacing between 12 and 2 km is used. As grid spacing decreases, the minimum central pressure of Ivan deepens, dropping by approximately 20 hPa as grid spacing decreases from 4 to 2 km. However, the 8-, 6-, and 4-km simulations have intensity differences of only around 10 hPa between them. The structure shown by model-simulated radar, as well as model-simulated satellite infra-red (IR) temperatures, shows that the eyewall of the coarser resolution simulations (12- to 6-km) is highly asymmetrical and elliptically-shaped, with two large maxima (minima) in reflectivity (cloud top temperature) rotating about the TC center. The 4- and 2-km runs have more circular eyewalls, with more numerous and larger maxima (minima) in reflectivity (cloud top temperature) embedded within the eyewall, as well as better developed spiral bands. Temporal and spatial averaging, done at a given radius over azimuth, show the system-averaged quanitites in cross-section and reveal differences in the structure of the TC core and eyewall. The finer resolution simulations have larger updrafts and more subsidence within the eye. However, the warming of the eye, relative to the other runs, is confined to the upper levels of the troposphere. The eyewall of the TC in the finer resolution runs slopes radially outward less with height, as the horizontal temperature gradient changes little with height, compared with the coarser simulations. This lack of warming in the lower- and mid-levels of the TC eye indicates a ventillation mechanism at work in the finer resolution runs, acting to mix high potential temperature (θe) air from the eye into the eyewall. Such air could act as a fuel source for buoyant convection within the eyewall (Persing and Montgomery 2003; Eastin et al. 2005b; Yang et al. 2007). Fine-scale eyewall and eye features are examined at high temporal resolution in order to further analyze changes in the TC structure as grid resolution increases. Wind, θe, and potential vorticity (PV) anomalies in the finer resolution simulations tend to be smaller in size and larger in magnitude, especially in the 2-km simulation. The PV field in the 2-km simulation appears to have several wave-like features moving throughout the eyewall, suggesting that smaller-scale processes, such as vortex Rossby waves (VRWs) and buoyant convection, areat least partially resolved at this grid spacing. VRWs, waves that propagate along a PV gradient, are further explored as a possible ventillation mechanism acting in the lower TC eye. The presence of VRWs is tested by visual analysis, as well as by a subjective estimate of the motion of PV features and a PV budget. Both of these analyses show the properties of these PV features to be consistent with the theoretical and observed properties of VRWs. A spectral decomposition of kinetic energy shows that the higher resolution simulations distribute energy to specific wavenumbers where organized wave motions are simulated. However, the coarser runs distribute lower amounts of power over more wavenumbers, some of which are not even fully-resolved at that grid spacing. There is some convergence in the model solution for the basic TC structure and intensity at grid spacings between 8- and 4-km, suggesting that these grid spacings might be appropriate for an operational NWP environment. For research purposes, where the time needed for numerical integration is less constrained, 4-km is the largest grid spacing that could be considered appropriate to partially resolve physical process within the eyewall. However, as the minimum central pressure of the 2-km simulation is significantly deeper than all other simulations, small-scale physical processes important to the intensification.