P1.26 Role of Warm Ocean Features on Intensity Change: Hurricane Opal (original) (raw)
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Sea Surface Temperature Variability in Hurricanes: Implications with Respect to Intensity Change
Monthly Weather Review, 2003
Scientists at NOAA's Hurricane Research Division recently analyzed the inner-core upper-ocean environment for 23 Atlantic, Gulf of Mexico, and Caribbean hurricanes between 1975 and 2002. The interstorm variability of sea surface temperature (SST) change between the hurricane inner-core environment and the ambient ocean environment ahead of the storm is documented using airborne expendable bathythermograph (AXBT) observations and buoy-derived archived SST data. The authors demonstrate that differences between inner-core and ambient SST are much less than poststorm, ''cold wake'' SST reductions typically observed (i.e., ϳ0Њ-2ЊC versus 4Њ-5ЊC). These findings help define a realistic parameter space for storm-induced SST change within the important high-wind inner-core hurricane environment. Results from a recent observational study yielded estimates of upper-ocean heat content, upper-ocean energy extracted by the storm, and upper-ocean energy utilization for a wide range of tropical systems. Results from this analysis show that, under most circumstances, the energy available to the tropical cyclone is at least an order of magnitude greater than the energy extracted by the storm. This study also highlights the significant impact that changes in inner-core SST have on the magnitude of airsea fluxes under high-wind conditions. Results from this study illustrate that relatively modest changes in innercore SST (order 1ЊC) can effectively alter maximum total enthalpy (sensible plus latent heat) flux by 40% or more.
Observations of Air–Sea Interaction and Intensity Change in Hurricanes
Monthly Weather Review, 2013
Recent enhancements to the tropical cyclone-buoy database (TCBD) have incorporated data from the Extended Best Track (EBT) and the Statistical Hurricane Intensity Prediction Scheme (SHIPS) archive for tropical cyclones between 1975 and 2007. This information is used to analyze the relationships between large-scale atmospheric parameters, radial and shear-relative air-sea structure, and intensity change in strengthening and weakening hurricanes. Observations from this research illustrate that the direction of the large-scale vertical wind shear at mid-to low levels can impact atmospheric moisture conditions found near the surface. Drier low-level environments were associated with northerly shear conditions. In a separate analysis comparing strengthening and weakening hurricanes, drier surface conditions were also found for the intensifying sample. Since SST conditions were similar for both groups of storms, it is likely that the atmosphere was primarily responsible for modifying the near-surface thermodynamic environment (and ultimately surface moisture flux conditions) for this particular analysis.
Journal of Atmospheric and Oceanic Technology, 2015
The Princeton Ocean Model for Tropical Cyclones (POM-TC), a version of the three-dimensional primitive equation numerical ocean model known as the Princeton Ocean Model, was the ocean component of NOAA’s operational Hurricane Weather Research and Forecast Model (HWRF) from 2007 to 2013. The coupled HWRF–POM-TC system facilitates accurate tropical cyclone intensity forecasts through proper simulation of the evolving SST field under simulated tropical cyclones. In this study, the 2013 operational version of HWRF is used to analyze the POM-TC ocean temperature response in retrospective HWRF–POM-TC forecasts of Atlantic Hurricanes Earl (2010), Igor (2010), Irene (2011), Isaac (2012), and Leslie (2012) against remotely sensed and in situ SST and subsurface ocean temperature observations. The model generally underestimates the hurricane-induced upper-ocean cooling, particularly far from the storm track, as well as the upwelling and downwelling oscillation in the cold wake, compared with o...
Large-Scale Characteristics of Rapidly Intensifying Tropical Cyclones in the North Atlantic Basin
Weather and Forecasting, 2003
The National Hurricane Center (NHC) and Statistical Hurricane Intensity Prediction Scheme (SHIPS) databases are employed to examine the large-scale characteristics of rapidly intensifying Atlantic basin tropical cyclones. In this study, rapid intensification (RI) is defined as approximately the 95th percentile of over-water 24-h intensity changes of Atlantic basin tropical cyclones that developed from 1989 to 2000. This equates to a maximum sustained surface wind speed increase of 15.4 m s Ϫ1 (30 kt) over a 24-h period. It is shown that 31% of all tropical cyclones, 60% of all hurricanes, 83% of all major hurricanes, and all category 4 and 5 hurricanes underwent RI at least once during their lifetimes. The mean initial (t ϭ 0 h) conditions of cases that undergo RI are compared to those of the non-RI cases. These comparisons show that the RI cases form farther south and west and have a more westward component of motion than the non-RI cases. In addition, the RI cases are typically intensifying at a faster rate during the previous 12 h than the non-RI cases. The statistical analysis also shows that the RI cases are further from their maximum potential intensity and form in regions with warmer SSTs and higher lower-tropospheric relative humidity than the non-RI cases. The RI cases are also embedded in regions where the upper-level flow is more easterly and the vertical shear and upper-level forcing from troughs or cold lows is weaker than is observed for the non-RI cases. Finally, the RI cases tend to move with the flow within a higher layer of the atmosphere than the non-RI cases. A simple technique for estimating the probability of RI is described. Estimates of the probability of RI are determined using the predictors for which statistically significant differences are found between the RI and non-RI cases. Estimates of the probability of RI are also determined by combining the five predictors that had the highest individual probabilities of RI. The probability of RI increases from 1% to 41% when the total number of thresholds satisfied increases from zero to five. This simple technique was used in real time for the first time during the 2001 Atlantic hurricane season as part of the Joint Hurricane Testbed (JHT).
Tellus A, 2014
This study investigates the sensitivity of tropical cyclone (TC) motion and intensity to ocean surface fluxes that, in turn, are directly related to sea surface temperatures (SSTs). The Advanced Research version of the Weather Research and Forecast (WRF-ARW) model is used with an improved parameterisation of surface latent heat flux account for ocean salinity. The WRF-ARW simulations compare satisfactorily with the NCEP/NCAR reanalysis for atmospheric fields and remotely sensed precipitation fields, with the model providing details lacking in coarse resolution observations. Among four TCs investigated, except the one redeveloped from a previous TC remnant, the stretching term dominates the relative vorticity generation, and a bottom-up propagation mechanism holds for the three TCs. For the Tropical Rainfall Measuring Mission (TRMM) precipitation, the spatial ranges are accurate but actual rainfall rates are significantly larger than those remotely sensed. This indicates the value of numerical simulation in quantitative rainfall precipitation estimation (QPE) for TCs. Sensitivity experiments are performed with altered SSTs and changes in tracks and intensity are examined. A TC-dependent threshold wind speed is introduced in defining total kinetic energy, a measure of TC intensity, so arbitrariness in domain setting is avoided and inter-basin comparisons are possible. The four TCs selected from different global basin show that the intensity increases with increasing SST. Within a domain, a powerÁlaw relationship applies. More important, warmer SST indicates a more rapid intensification, quicker formation and reduced warning issuance time for emergency services. The influence of SSTs on TC track is more complex and lacks a generic relationship. For the South Pacific basin, higher SSTs favour a more northerly track. These TCs occasionally cross continental Australia and redevelop in the southern Indian Ocean basin, affecting the resource-rich onshore and offshore industrial developments in northwest Western Australia. In the Atlantic basin (e.g. Katrina 2005), when SSTs increase, the TC tracks tend to curve over warm pools but generally have a shorter ocean-residence time. When the synthesised SST fields are raised 28C above Katrina (i.e. !328C), the possibility exists of generating two TCs in close proximity. That lack of unanimity of the impacts on TC tracks, in response to synthesised SSTs, partly arises from the complicated response of subtropical highs, which may be disintegrated into several pieces and dispersed with relatively lower pressure regions, which may become the shortcuts when a TC traces the periphery of the subtropical high.
Air-Sea Interactions in Tropical Cyclones
Global Perspectives on Tropical Cyclones: From …, 2010
Significant progress has been made in the area of upper ocean responses and air-sea interactions during the passage of tropical cyclones (TC) since the first edition published in 1992. In terms of the upper ocean impacts on intensity, considerable attention has focused on the three-dimensional cold wake structure and the negative feedback due to ocean mixed layer cooling by shear-induced vertical mixing in quiescent oceans. By contrast, in the western parts of the oceanic basins where the ocean state is not at rest, strong currents (e.g., Kuroshio, Gulf Stream) transport warm water poleward as part of the gyre circulation. In these regimes, the upper ocean current shears do not necessarily develop as in regions with shallow ocean mixed layers where significant sea surface cooling often occurs. Transports by these energetic western boundary currents tend to be resistive to shear-induced ocean mixing events since the mixed layer is already deep and the thermal response (or sea surface temperature cooling) tends to be minimized. The implication is since the oceanic mixed layers do not significantly cool during TC passage, there is a more sustained heat flux to the atmospheric boundary layer, thus representing an important mechanism for observed deepening of recently observed severe TCs. In terms of the air-sea interactions, surface waves and sea spray impact the surface drag coefficient. Recent studies have shown the surface drag coefficient to level off between 28 to 33 m s −1 at values from 2.5 to 3.5 × 10 −3. While the measurement uncertainties increase for higher wind speeds, it is clear that the surface drag cannot continue to increase with wind speeds. Moreover, for intense TCs, the ratio of the enthalpy and surface drag coefficient exceeds unity (typically 1.2 to 1.5). When this ratio is less than unity, theoretical studies suggest that TCs cannot reach their maximum potential intensity. I continue to be grateful to the NOAA Aircraft Operation Center (Dr. Jim McFadden) who make it possible to acquire high quality August 24, 2010 11:3 B-936 b936-ch03 Air-Sea Interactions in Tropical Cyclones 125