27th Conference abstracts
Numerical experiments on the predictability of tropical-cyclone intensification
Download 100.04 Kb.
|
Numerical experiments on the predictability of tropical-cyclone intensificationSang Van Nguyen, Department of Physics, University of Munich, Munich, Germany; and R. K. Smith Numerical model experiments are carried out to investigate the predictability of vortex growth in the prototype problem for tropical-cyclone intensification, which considers the evolution of a prescribed, initially cloud-free, axisymmetric vortex on an f-plane in an environment at rest. The calculations are carried out in a large square domain with impervious boundaries with the vortex axis initially at the centre. They are performed on a square horizontal grid. The model used is the Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model (MM5). As in many previous calculations, the vortex evolution begins with a gestation period during which the vortex slowly decays due to surface friction, but moistens due to evaporation from the underlying sea surface. Subsequently, moist convection begins near the radius of maximum tangential wind speed and there ensues a period during which the vortex rapidly intensifies. At the end of this period, which is typically 48 h in our calculations, the vortex attains a quasi-steady state in which the vortex exhibits many realistic features of a mature tropical cyclone, with spiral bands of convection surrounding an approximately symmetric eyewall and a central convection-free eye. Our interest here is focused on the structure of the asymmetries and their evolution. During the gestation period the flow remains close to axisymmetric with a weak azimuthal wavenumber-4 asymmetry that necessarily arises from the representation of a circular flow on a square grid. For a relatively coarse horizontal resolution of 15 km, saturation occurs first with a wavenumber-4 pattern, but other wavenumbers quickly emerge, first wavenumber-2 and then other wavenumbers including wavenumber-1. As the mature stage is approached, the flow consolidates into a monopole vortex once again, much as in numerical experiments using the shallow-water approximation described by Guinn and Schubert. As the horizontal resolution is increased while keeping the horizontal diffusivity as low as possible, the initial pattern of convection has increasing azimuthal wavenumber: for example with a 5 km grid, the pattern has wavenumber-12, but again other wavenumbers rapidly emerge. We have found that the asymmetries that develop are highly sensitive to the surface moisture distribution. If a random moisture perturbation is added in the boundary layer at the initial time with a magnitude that is below the accuracy with which moisture can be measured, the pattern of evolution of the flow asymmetries is dramatically changed. We conclude that the flow is not deterministic and only those features that survive in an ensemble average of many realizations can be regarded as robust features. There are clear implications for the possibility of deterministic forecasts of the mesoscale structure of hurricanes, which may have a large impact on the intensity and on rapid intensity changes. Rapid Intensity Change in Hurricane Lili (2002) Mélicie Desflots, Univ. of Miami/RSMAS, Miami, FL Hurricane Lili (2002) developed in the western Caribbean Sea and intensified rapidly from a Category 2 to a Category 4 hurricane within 18 hr in the Gulf of Mexico and rapidly weakened to a Category 1 storm before making landfall at the Gulf coast. Lili intensified in the wake of Hurricane Isidore (2002) when the SST was relatively cooler than its “normal” conditions. The internal dynamics, i.e., the eye and eyewall structure may have played an important role in the rapid intensification in Lili. It was observed that Lili had a relatively small eye when it reached the maximum intensity. Most major hurricanes intensify as the eye and eyewall contract. However, some maintain a relatively larger eye than others. The question is what determines how far an eye can contract and how rapidly a storm can intensify? In the case of Hurricane Lili, the vertical wind shear asscociated with a through of the Texas/Lousiane coast prevented further intensification of the storm and played an important part in the rapid weakening of the storm. In this study, we use a high-resolution model to investigate some aspects of the physical processes responsible for the rapid contraction/intensification of Hurricane Lili (2002) and the following weakening. Based on the current analysis and previous studies from Schubert and Hack (1982), we hypothesize that the conversion from heating to warming at the edge of the eye-eyewall region, is particularly efficient. The model simulation shows that during the intensification stage the warming in the eye expands vertically from the lower levels (about 3km height) to the upper levels (about 15 km height) and reaches 3°C throughout this entire column in 18 hours. As showed by Shapiro and Willoughby 1982, this warming will cause the isobaric surface to fall rapidly (slowly) inside (outside) the radius of maximum winds (RMW). The largest gradient of isobaric height fall will be located inside of the RMW and supported a peak of the tangential wind tendency. The maximum of tangential wind propagates inward in response to warming in the eye. During Lili's weakening phase the vertical wind shear is shown to create asymmetries in the storm and to dissipate the warm temperature anomaly in the eye. Budget analyses of key dynamic and thermodynamic variables are currently underway to understand the sequence of events involved in the rapid intensification and weakening of Hurricane Lili (2002). Interaction of hurricane and cloud scales: contribution to hurricane intensityL. Stefanova, Florida State Univ., Tallahassee, FL; and T. Krishnamurti and S. Pattnaik The overwhelming amount of kinetic energy of tropical cyclones is contained within the azimuthal wave number zero, representing the symmetric flow, and azimuthal wave numbers one and two, representing the principal asymmetries. The spatial scale of the symmetric and asymmetric components of a hurricane's circulation is on the order of several hundreds of kilometers, while the individual cloud scales are on the order of a few kilometers. The clouds however are organized on the scale of the hurricane, i.e. at azimuthal wave numbers zero, one and two. The kinetic energy budget of hurricanes' wave numbers one and two is calculated using scale interaction approach. MM5 simulations of hurricanes Charley of 2004 and Katrina of 2005 are used to diagnose the kinetic energy interactions involving azimuthal wave numbers one and two, which represent the dominant asymmetries. We find that the wave-mean kinetic energy interactions and the potential to kinetic energy conversions are the dominant processes affecting the kinetic energy at wave numbers one and two. In an area-averaged sense the wave-mean interactions prevail. The wave-wave interactions are found to be large only in localized regions but their area-averaged contribution is found to be small. There appears to be a relationship between the kinetic energy tendency of wave numbers one and two and the hurricanes' intensification, particularly in the case of Katrina, which was a significantly stronger and larger storm with a much better defined asymmetric component. Robust and interpretable statistical models for predicting the intensification of tropical cyclones Kyriakos C. Chatzidimitriou, Colorado State Univ., Fort Collins, CO; and C. W. Anderson and M. DeMaria Hurricane intensity prediction has proven to be a challenging task for the tropical cyclone forecasting community. One way this problem has been approached is through the use of statistical models. In this paper, three different aspects of intensity forecasting through statistical inference will be explored. To date, the prediction of intensity change for up to five days has been sufficiently addressed using multiple linear regression (MLR) in practice, while efficient non-linear regression methods, like neural networks (NNs), can be found in the literature. On the other hand, the procedures for reporting the prediction performance of the models and the feature selection techniques have not been investigated to a great extent, in the sense that (1) they widely vary and (2) the derived models have an inherent bias that prohibits good generalization behavior. Machine learning theory and a wide range of experiments in both artificial and real life domains have provided us with a large repository of algorithms and heuristics that help improve upon the methodologies used so far, while simultaneously building more robust models. Based on that, a large range of prediction performance and feature selection methods are investigated and the results are presented against the reference model, the Statistical Hurricane Intensity Prediction Scheme (SHIPS). As a final contribution, recently developed rule based regression techniques are applied to the dataset in order to identify more elaborate structure behind the intensity predictions. MLR and NNs fail to provide the human expert with interpretable results regarding possible multiple interdependencies of the inputs and the output. In contrast, rule based methods are not only competitive with respect to prediction performance, denoting that the rules are of good quality, but also support the capability of identifying multiple correlations in the dataset in an easy to read and validate manner. Directory: pub pub -> Project Scoring Template Pre-Construction Phase New Core/Branch Campus Projects pub -> Acm word Template for sig site pub -> The german unification, 1815-1870 pub -> Baltic Olympiads in Informatics: Challenges for Training Together pub -> Preparation of Papers for ieee transactions on medical imaging pub -> Forthcoming meetings and conferences pub -> Asilomar Conference on Circuits, Systems & Computers, Pacific Grove, ca, November 1978, pp. 55 58 pub -> Forthcoming meetings and conferences pub -> Harmonised compatibility and sharing conditions for video pmse in the 7 9 ghz frequency band, taking into account radar use Download 100.04 Kb. Share with your friends:
|