27th Conference abstracts
Validation of QuikSCAT wind retrievals in hurricanes
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Validation of QuikSCAT wind retrievals in hurricanesChristopher C. Hennon, Univ. of North Carolina, Asheville, NC; and D. G. Long and F. J. Wentz QuikSCAT wind speed retrievals in over a dozen tropical cyclones are validated against Stepped Frequency Microwave Radiometer (SFMR) observations and wind speed analyses (H*Wind). Four QuikSCAT retrieval products are evaluated: 25.0 km resolution Near Real Time (NRT), 12.5 km NRT, 2.5 km Ultra Hi-Resolution, and an Ultra Hi-Res product empirically adjusted for hurricane environments. Tropical cyclones (TCs) present a difficult challenge for active scatterometry due to heavy rain and wind speeds beyond the upper design limits of retrieval algorithms. Rain effects have qualitatively been well documented but little validation work has been performed due to difficulty in obtaining reliable surface truth measurements in TCs. This study exploits SFMR observations taken from National Oceanic and Atmospheric Administration (NOAA) research aircraft from within tropical cyclones. In addition, comparisons are also made to H*Wind analyses. H*Wind is a data assimiliation and analysis tool that produces a gridded (~6 km resolution) surface wind speed snapshot. Recent Atlantic Basin tropical cyclone seasons were searched for collocations of QuikSCAT passes with available SFMR data. Results will be presented that show a consistent low bias in the QuikSCAT retrievals near the tropical cyclone core. This suggests that attenuation of the signal by rain dominates over any enhanced backscatter. Retrievals outside of the tropical cyclone core are remarkably accurate. A statistical comparison of the four retrieval algorithms shows that the low resolution NRT product has approximately a 40% lower root mean square error overall. However, there are large differences in algorithm performance from case to case. A Recalculation of MPI Using Upper—Ocean Depth—Averaged temperatures: climatology and Case Studies Michael C. Watson, Florida State University, Tallahassee, FL; and R. E. Hart Tropical cyclone track forecasts have improved greatly in recent years. However, intensity forecasts still pose a large problem in tropical meteorology. Several theories have been developed over the past fifty years which attempt to arrive at an upper bound or Maximum Potential Intensity (MPI) of tropical cyclones. Emanuel (1986, 1988a,b, 1991, 1995a, 1997), in particular, uses the SST and atmospheric sounding to arrive at a maximum intensity for the hurricane using a Carnot Engine cycle approach to the energetics of the storm. While this approach has captured the upper bound of a hurricanes intensity reasonably well, the use of SST for MPI calculations may be overestimating the maximum intensity. Hurricanes draw their energy from a significant depth of the upper ocean, the average temperature of which is almost always less than the SST. Consequently, there may be more cases of tropical cyclones approaching or exceeding their MPI than an SST-based MPI would dictate alone. Presumably, a purely SST-based MPI calculation would only be valid for intense storms if the entire layer affected by hurricane mixing was isothermal. A recalculation of the MPI climatology acknowledging this oceanic variability is performed here in an attempt to better isolate those storms that have approached or exceeded a more realistic MPI. Using the NODC (Levitus) World Ocean Atlas Data (1994) oceanic temperatures over five layers to a depth of 50m have been weighted producing a climatology of mean upper ocean temperature over the globe. This new SST is then used in a new calculation of the Emanuel MPI. The weighting of oceanic layers chosen is supported by observations taken during the passage of Felix over the Bermuda testbed mooring on 15 August 1995 (Dickey, et al. 1998) and will be compared to simulations in a one-dimensional ocean model (Kantha and Clayson, 1994) driven by fluxes typical of a tropical cyclone. A revised MPI climatology for the named tropical systems in the North Atlantic basin from the years 1982-2003 is produced. Results have shown that there is a dramatic impact upon the MPI climatology when the mean upper oceanic temperature is used instead of just SST. With only the skin SST being used for MPI, the only storms to actually exceed their MPI are recurving, poleward moving systems, a consequence of the storm accelerating rapidly and moving over cooler waters while weakening more slowly than the timescale required for the storm to come into thermodynamic balance with the decreasing SST. With the new MPI calculation using a mean oceanic temperature, several storms in the Atlantic basin actually exceed their MPI over waters south of 35° N. Several examples of these will be examined (including Isabel 2003) in an attempt to determine whether the ocean, the atmospheric environment (trough interaction), the internal dynamics (eyewall contraction or vortex Rossby wave forcing) were the cause of intensification. Further, it was discovered that for strong storms of category three or larger, the most intense estimate of the MPI was not found under the storm at analysis time, but rather 10-11 days before the storm actually reached the location. This shows that strong tropical cyclones begin to affect the ambient environment well before the TC center arrives at the location, and that the thermodynamic stabilization process of the environment begins a week or more prior to a strong TC's passage. It is unclear whether this stabilization is a result of the TC outflow itself, or of the modification of the intensity of the Hadley and Walker circulations in the process of TC generation and intensification. NCEP's Two-way-Interactive-Moving-Nest NMM-WRF modeling system for Hurricane Forecasting S.G. Gopalakrishnan, NOAA/NWS/NCEP, Camp Springs, MD; and N. Surgi, R. Tuleya, and Z. Janjic At NCEP, a preliminary version of the moving, two way interactive nested grid NMM-WRF modeling system is now being evaluated and tested for the numerical hurricane predictions. Based on horizontal mesh refinement technique, this nesting capability commonly referred to as “telescopic mesh” currently supports one and two way interactions between a lower-resolution domain and one or more higher-resolution nests and automatic grid motion of the higher-resolution nest. The dynamical system of equations and the numerical techniques are described in Janjic et al, 2001 for a uniform domain and is now extended onto the telescopic, nested domains. All parent-to-nested domain interpolations are done on a rotated lat-lon, E-grid with the reference lat-lon located at the center of the parent domain. Consequently the nested domain can be freely moved anywhere within the grid points of the parent domain, yet the nested domain lat-lon lines will coincide with the lat-lon lines of the parent domain at integral parent-to-nest ratio. This coincidence of grid points between the parent and nested domain eliminates the need for more complex, generalized remapping calculations in the WRF Advanced Software Framework (Michalakes, 2005) and is expected to aid better distributed memory performance, and portability of the modeling system. High-resolution topography and land-sea mask are redefined over the nested domain using the wrfsi dataset. To be consistent with the NMM model numerics, quasi-hydrostatic mass balancing is carried out after introducing the high resolution topography. Cubic spline technique is used to interpolate data back and forth from standard pressure surfaces on to the hybrid surfaces. The grid motion algorithm is based on the variations in dynamic pressure. The so called “stagnation point” was chosen to be the center of the storm (Gopalakrishnan et al 2002). For the two-way interactive technique, grid volume averaged mass, momentum and scalar fields from the high resolution nest is weighed and fed-back into the parent domain.Numerical evaluation of the nest motion technique yields satisfactory performance. Testing of idealized and real cases of Hurricanes of the past seasons will be presented. References: (1) S. G. Gopalakrishnan, David P. Bacon, Nash'at N. Ahmad, Zafer Boybeyi, Thomas J. Dunn, Mary S. Hall, Yi Jin, Pius C. S. Lee, Douglas E. Mays, Rangarao V. Madala, Ananthakrishna Sarma, Mark D. Turner and Timothy R. Wait. 2002: An Operational Multiscale Hurricane Forecasting System. Monthly Weather Review: Vol. 130, No. 7, pp. 1830–1847. (2) Z. I. Janjic, J. P. Gerrity Jr. and S. Nickovic. 2001: An Alternative Approach to Nonhydrostatic Modeling. Monthly Weather Review: Vol. 129, No. 5, pp. 1164–1178. (3) John Michalakes WRF SOFTWARE. 6th WRF / 15th MM5 Users' Workshop, National Center for Atmospheric Research June 27-30, 2005. Hurricane WRF model transition to operations at NCEP/EMC: Sensitivity of results to surface fluxes and convection Robert Tuleya, EMC, Norfolk, VA; and N. Surgi, S. Gopalkrishnan, and D. Johnson It has long been recognized that hurricane models are sensitive to surface energy fluxes, momentum drag and both resolvable and parameterized convective schemes. Recent generation research models such as MM5 and WRF (Weather Research and Forecasting Model) have physical schemes more advanced than the present operational GFDL hurricane model. Despite this fact it hasn't been shown that these new generation models lead to improved forecasts of track and intensity on an operational basis. In transitioning to NCEP's next generational Hurricane WRF model, the benchmark physics will be the physics package presently used in the GFDL model. This physics package includes the Simplified Arakawa convective scheme and a Monin-Obukov surface scheme. These schemes will be compared to the present Global Forecasts System (GFS) parameterizations as well as with some other parameterizations deemed appropriate for meso-scale forecasting. Emphasis will be placed on the surface package presently used in the GFDL model and it's comparison with schemes that have separate surface roughness estimates for heat and momentum. This is especially important since intensity is known to be quite sensitive to these parameterizations and that hurricane maintenance can only be sustained through surface energy fluxes, especially that of moisture. On the other hand, surface friction has a retarding effect on hurricanes. The surface exchange processes are still poorly understood and still under investigation. Recently, wave models and observations appear to indicate that the long used parameterization that increases drag with wind speed may not apply under hurricane conditions. On the other hand, surface evaporation is complicated due to the effect of spray and the chaotic nature of the ocean interface under hurricane conditions. HWRF Offline and model code comparisons indicate that surface evaporation in the GFDL model increases monotonically with wind speed while the GFS physics package increases evaporation at a lesser rate. Furthermore, the GFDL surface drag appears to be more dissipative even with a reduced coefficient in the Charnock's formulation over water. Comparisons will be shown in real data cases of HWRF for the 2005 Atlantic season. The effect of surface parameterization will also be shown on storm track. The effect of the subsurface land parameterization in HWRF will also be discussed. 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:
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