27th Conference abstracts


Overview of RAINEX Modeling of 2005 Hurricanes



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Overview of RAINEX Modeling of 2005 Hurricanes

Shuyi S. Chen, Univ. of Miami/RSMAS, Miami, FL


The Hurricane Rainband and Intensity Change Experiment (RAINEX) is a coordinated observational and modeling study of hurricane intensity changes in relation to inner-core vortex dynamics, formation of secondary wind maxima in the outer rainband region, eyewall replacement cycles, and dynamic/thermodynamic feedbacks of outer rainbands. The two main objectives are: 1) to examine structures (both dynamic and thermodynamic) of hurricane outer rainband and inner core regions simultaneously, and 2) to investigate the interactions of the rainbands and primary hurricane vortex circulation and the role of these interactions in hurricane intensity changes. The RAINEX field program took place from 15 August-30 September 2005, which is during the most active hurricane seasons in the Atlantic basin on record. RAINEX employs three aircraft equipped with dual-Doppler radar and the GPS dropsondes. It involves the NRL P3 with the NCAR ELDORA dual-Doppler system plus the two dual-Doppler equipped NOAA WP-3D aircraft. It was conducted in collaboration with the partners at NOAA. RAINEX conducted multi-day and multi-aircraft missions into Hurricanes Katrina, Ophelia, and Rita with well-designed fly patterns that target important features in both inner core and outer rainband regions. The data collected in RAINEX provides a unique opportunity for evaluating and validating high-resolution hurricane prediction models that are capable of resolving the eye, eyewall, and rainbands. An ensemble of high-resolution MM5 and WRF model forecasts was conducted in real-time during RAINEX. The models (at 1.6 km grid resolution) were able to capture the rapid intensification in Katrina and Rita as well as the formation of the secondary eyewall and eyewall replacement cycle in Rita. These forecasts were extremely valuable in the RAINEX mission planning in real-time. Detailed analysis of the dynamic and thermodynamic fields in the rainband and inner regions is underway. The goal is to provide an overview of the RAINEX modeling effort aimed to understand and better predict the storm structure and intensity changes.

Effect of ocean surface waves on storm surge and coastal flooding

Huiqing Liu, North Carolina State University, Raleigh, NC; and L. Xie, L. J. Pietrafesa, and M. Peng


The effects of wave-current interaction on storm surge and coastal flooding are examined by using a three-dimensional (3-D) wave-current coupled modeling system. The 3-D storm surge and inundation modeling component of the coupled system is based on the Princeton Ocean Model (POM), whereas the wave modeling component is based on the third generation wave model, Simulating WAves Nearshore (SWAN). The 3-D wave-current coupled modeling system is applied to simulate storm surge and inundation induced by several historical hurricanes. The results indicate that it is important to introduce wave-current interaction effects into storm surge and inundation prediction modeling systems. Consideration of wave-induced wind stress, bottom shear stress, and 3-D radiation stress in storm surge and inundation modeling can lead to significantly improvement in storm surge and coastal flooding prediction.

Surface Turbulent Stress Derived from GPS Dropsondes

Mark A. Bourassa, COAPS/Florida State Univ., Tallahassee, FL


GPS Dropsonde wind profiles are used to determine the friction velocity and displacement height (vertical offset of the log-wind profile). The roughness length is also determined; however, it is a strong function of the surface boundary condition on speed. Uncertainties are estimated for each of these terms. These results are compared to friction velocities, roughness lengths and displacement heights determined from theory combined with observations of waves and high wind speeds in the North Atlantic. The friction velocities are compared for similar wind speeds in North Atlantic and hurricane conditions. The friction velocity for weak hurricane condition is much less than that for North Atlantic storms, suggesting that the weak hurricanes have a surface boundary condition on speed that is greater than expected, thereby reducing the wind shear and the stress. Similarly, the displacement height for weak hurricanes is greater than expected from the observed waves. These results indicate that sea spray, wave breaking, or some yet to be determined process acts as a stress-reducing interface between the wavecrests and the winds aloft. Such an interface has been modeled for very strong hurricanes, but was not anticipated for weak hurricanes.

Accuracy of tropical cyclone intensity forecasts in the North Pacific and Atlantic

Mark A. Boothe, Naval Postgraduate School, Monterey, CA; and T. Lambert, J. Blackerby, and R. L. Elsberry


Consensus methods require that the techniques have no bias and have skill. The accuracy of six statistical and dynamical model tropical cyclone intensity guidance techniques was examined for western North Pacific, eastern North Pacific, and North Atlantic tropical cyclones during the 2003-2004 seasons using the climatology and persistence techniques called ST5D or SHF5 as measures of skill. A framework of three phases: (i) initial intensification, (ii) maximum intensity with possible decay/reintensification cycles; and (iii) decay was used to examine the skill.
From an initial study for the 2003-2004 western North Pacific seasons, only about 60% of the 24-36 h forecasts during both the formation and intensification stages were within +/- 10 kt, and the predominant tendency was to under-forecast the intensity. None of the guidance techniques predicted rapid intensification well. All of the techniques tended to under-forecast maximum intensity and miss decay/reintensification cycles. Whereas about 60-70% of the 12-h to 72-h forecasts by the various techniques during the decay phase were within +/- 10 kt, the strong bias was to not decay the cyclone rapidly enough. In general the techniques predict too narrow of a range of intensity changes for both intensification and decay.
From an initial study for the 2003-2004 eastern North Pacific and Atlantic seasons, the Decay Statistical Hurricane Intensity Prediction (DSHIPS) technique was the best technique in both basins during the formation phase. When the forecast errors during formation exceed +/- 10 kt, the statistical techniques tend to over-forecast and the dynamical models tend to under-forecast. Whereas DSHIPS was also the best technique in the Atlantic during the early intensification stage, the Geophysical Fluid Dynamics Laboratory model was the best in the eastern North Pacific. All techniques under-forecast periods of rapid intensification and the peak intensity, and have an overall poor performance during decay/reintensification cycles in both basins. Whereas the DSHIPS was the best technique in the Atlantic during decay, none of the techniques excelled during the decay phase in the eastern North Pacific. All techniques tend to decay the tropical cyclones in both basins too slowly, except that the DSHIPS performed well (13 of 15) during rapid decay events in the Atlantic.
STEADY-STATE HURRICANE INTENSITY IN THE WRF MODEL: COMPARISON TO MPI THEORY AND SENSITIVITY TO PBL AND SURFACE FLUX PARAMETERIZATION
Kevin A. Hill, North Carolina State Univ., Raleigh, NC; and G. M. Lackmann
Despite significant improvements in numerical prediction models and computing technology, numerical model predictions of hurricane intensity have improved much more slowly than track predictions. The Weather Research and Forecast (WRF) modeling system is currently under development by the U.S. weather research and operational modeling communities. In addition, a high resolution, coupled air/sea/land hurricane version of WRF (HWRF) will replace the GFDL hurricane model at the NHC in the near future. Before this transition can occur, it is important to understand limitations with the current model physics in WRF that will need to be addressed. This research is designed to (i) test the ability of different Planetary Boundary Layer schemes in WRF to intensify a tropical system in an idealized testing environment to its theoretical maximum potential intensity (MPI), and (ii) analyze the individual PBL scheme formulations in order to gain an understanding of how the different parameterizations influence the model results under the extreme conditions accompanying tropical cyclones.
Emanuel's MPI theory provides a quantitative estimate of the maximum intensity that a tropical cyclone could reach if certain atmospheric/oceanic conditions were satisfied. These conditions typically are not found in the natural environment, but can be satisfied in an idealized model environment. In order to more effectively judge the accuracy of each PBL scheme, an environment was created that was consistent with the assumptions underlying Emanuel's MPI theory. An initial vortex obtained from gridded analyses prior to the formation of hurricane Ivan (2004) was inserted within an observationally-based, idealized tropical testing environment, and WRF simulations were run for 20 days until a quasi-steady maximum intensity was attained. The sensitivity of the simulations to PBL scheme choice was examined. Preliminary results indicate that both PBL scheme choices produce simulations where the vortex exceeds the theoretical MPI, with the Mellor-Yamada-Janjic (MYJ) PBL scheme generally producing a system with a lower central pressure for simulations featuring 20 km grid spacing and using the Kain-Fritsch cumulus parameterization scheme.
The simulated storm's strength and structure showed marked sensitivity to the PBL scheme choice. In order to understand these differences, values of fluxes and other variables used in surface layer parameterizations are examined. Additional experiments document sensitivity of these results to horizontal and vertical resolution, as well as the inclusion of sea-spray effects.
Three-dimensional hurricane structure change prior to landfall as revealed by automated airborne Doppler analyses
John F. Gamache, NOAA/AOML/HRD, Miami, FL; and P. G. Black and F. D. Marks Jr.

NOAA/AOML/Hurricane Research Division


Wind fields from automatically quality-controlled and analyzed airborne Doppler-radar observations were produced in "real time" (aboard the NOAA WP-3D aircraft during the flight) for Hurricanes Katrina, Rita, Ophelia, and Wilma during the 2005 Hurricane Season. Several steps are involved in the quality control and analysis:
1. Remove observations with high spectral width in the velocity observations. The spectral-width value used this season was 6.25 m/s.

2. Remove the reflection of the main and side lobes by the ocean surface.

3. Remove "speckles" of data from the observations.

4. De-alias the observations using the Bargen-Brown method, and an HRD-developed two-dimensional sweep dealiasing method.

5. Produce a wavenumber-0 and 1 analysis from these quality-controlled observations.

6. Use the low-wavenumber analysis to assist the Bargen-Brown and two-dimensional sweep dealias processes, and then produce a fully three-dimensional Doppler wind analysis

7. Use a specialized interpolation method to produce higher-resolution (1.5 km radial resolution; 150 m vertical resolution) radial-vertical cross sections of wind speed, tangential wind, radial wind and vertical wind along the flight tracks.
In this presentation we describe comparisons of automatic airborne Doppler analyses with other observations, including flight-level observations, stepped frequency microwave radiometer (SFMR) data, and GPS dropwindsonde data. These comparisons show the value of the airborne Doppler analyses in providing a context for the other observations. In particular, near landfall in Hurricanes Katrina and Rita the Doppler analyses showed a jet near the 700 mb level where the reconnaissance aircraft were flying, and a vertical motion field that suggested a relatively stratified hurricane rather than a convectively active one. Such stratification would suggest a stronger shear in the planetary boundary layer, and possibly weaker surface winds than suggested by the flight-level data. Several Doppler analyses during the season suggested that the low-level Doppler wind maximum observed during a radial flight leg was displaced to the left or right of the flight track, and thus was not detected by flight level or SFMR surface measurements. This suggests a method for quantifying the level of uncertainty in the maximum surface wind speed determined from the flight-level, sonde, and SFMR data.
The automatic process appeared to be fairly robust in its first full year of real-time testing, indicating promise as a future operational tool. It also showed its use as a quick-look, higher-resolution tool in diagnosing hurricane structure and intensity in the days following hurricane landfall. The automatic method can also be applied successfully to the large archive of airborne Doppler data, opening up a new data source for hurricane researchers.




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