27th Conference abstracts


Combining airborne wave observations with WAVEWATCH III model output



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Combining airborne wave observations with WAVEWATCH III model output

Edward J. Walsh, NASA/GSFC, Wallops Island, VA; and C. W. Wright, I. Ginis, Y. Fan, and H. L. Tolman


Since 1998 the NASA Scanning Radar Altimeter (SRA) has been providing targeted observations of the directional wave spectrum in the vicinity of hurricanes from a NOAA WP-3D hurricane research aircraft. Because the directional wave spectrum is the air-sea interface, knowledge of it is important in studying the coupled marine boundary layers. One of the most important days of the CBLAST experiment was 1 September 2004 when Hurricane Frances passed over an array of in-situ sensors air-deployed in its path. Because of the long transit from Barbados, the NOAA aircraft was only able to make three eye penetrations and data from important areas near the eyewall were lost on the two whose flight lines extended into the right side of the track of Frances because of rain attenuation at the 3 km height. By combining the model output of WAVEWATCH III with the SRA observations from Frances and Hurricane Ivan on 14 September 2004, when six eye penetrations were made in that storm of similar size, strength, and forward speed, a much better picture of the wave field variation in the vicinity of Frances is determined. Comparisons such as these are important in establishing the accuracy of WAVEWATCH III in the coupled GFDL hurricane-wave-ocean model that will be transitioned to the Hurricane WRF model.

Surface wave processes in high winds and hurricanes

W. Kendall Melville, SIO/Univ. of California, La Jolla, CA; and J. M. Kleiss and L. Romero


During the 2002-2004 Atlantic Hurricane seasons we participated in the ONR-sponsored Coupled Boundary-layer Air-Sea Transfer (CBLAST) experiment flying video imaging equipment on the NOAA P3 aircraft to study wave breaking and sea-surface foam in hurricanes. In February 2004 we undertook the Gulf of Tehuantepec Experiment (GOTEX) off the Pacific coast of Mexico, flying the NASA airborne terrain mapper (ATM) for surface wave measurements and imaging equipment to study the coupled development of the atmospheric boundary layer and the surface waves in high winds. The two studies are complementary and together were designed to better understand the role of surface wave breaking in high winds and hurricanes. The role of wave breaking in the momentum and energy fluxes between the atmosphere and the ocean may prove significant in the dynamics and thermodynamics of air-sea interaction in high winds. While surface wave processes in hurricanes are complicated by the effects of turning winds, the “Tehuano” winds in the Gulf of Tehuantepec are characterized by the fact that they blow consistently offshore for several days on an almost weekly cycle during winter, and may reach hurricane force at times. In this talk we present a summary of the measurements of the evolution of the surface wave fields in GOTEX, especially the incidence of breaking, the occurrence of extreme or “rogue” waves, and the implications for surface wave processes and their modeling in hurricanes.

A model of the effect of breaking waves on the air-sea momentum flux

Tobias Kukulka, University of Rhode Island, Narragansett, RI; and T. Hara and S. E. Belcher


Above the air-sea interface, within the constant stress layer, the momentum flux partitions into turbulent and wave-induced components. Non-breaking surface waves induce a wave-like pressure perturbation in the air, while ahead of a breaking crest the airflow separates, causing a pressure drop on the leeside of the wave. Both pressure perturbations lead to energy and momentum fluxes from wind to waves. By conserving airside momentum and energy and also imposing the wave action balance, we derive coupled equations governing the turbulent stress, wind speed, wave spectrum, and breaking wave distribution (length distribution of breaking crests per unit surface area as a function of wave number). Furthermore, we assume that smaller scale breaking waves are sheltered from wind forcing if they are in airflow separation regions of longer breaking waves (spatial sheltering effect). The saturation spectrum and the breaking wave distribution approach constant values for large wave numbers. Our model also yields the normalized roughness length (Charnock coefficient), which is roughly consistent with earlier observations. Model results suggest that breaking waves can support a significant fraction of the momentum flux, especially for younger seas.
In situ measurements of 3D turbulence in Hurricanes Frances and Ivan using a pressure-sphere anemometer
Richard M. Eckman, NOAA/ARL, Idaho Falls, ID; and R. J. Dobosy, T. W. Strong, and P. G. Hall
As part of a joint effort between NOAA and the CBLAST Hurricane program, the NOAA Air Resources Laboratory has developed an Extreme Turbulence (ET) probe for measuring 3D turbulence and fluxes in hurricanes. This probe is a surface-based system that uses pressure-sphere anemometry, which is commonly employed for aircraft gust probes. It uses a 43 cm fiberglass sphere with 30 pressure ports distributed over its surface. Special modifications were developed to prevent water and spray from fouling the ports during a hurricane deployment. Turbulence data from the system are archived at 50 Hz.
In September 2004, a set of ET probes was successfully deployed into Hurricanes Frances and Ivan as they made landfall along the U.S. Coast. Three probes were deployed along the Atlantic Coast of Florida for Hurricane Frances. They were located to the north of the eye and collected data over approximately a 30 hour period during the landfall. Two probes were deployed near the coast at the Florida-Alabama border for Ivan. These probes operated for over 50 hours and passed near Ivan's eyewall.
Analysis of the data from the deployments indicates that the data are of high quality. Only rarely did the wind data show any evidence of the pressure ports being fouled by water. Velocity spectra computed from the ET measurements show the expected -5/3 inertial subrange, which is further evidence that the water fouling was minimal. In Ivan, the probes measured average pressures as low as 950 hPa and wind gusts up to about 50 m s-1. A continuous 30 hour record of the turbulent kinetic energy (TKE) was generated from the Ivan data, showing that the TKE increased by a factor of ten as the storm came ashore. At its peak, the TKE was about 30 m2 s-2.
http://www.noaa.inel.gov/capabilities/etprobe/etprobe.htm
Tower and Doppler Radar Observations from the Boundary Layer of Hurricanes Isabel (2003) and Frances (2004)
Sylvie Lorsolo, Texas Tech Univ., Lubbock, TX; and J. L. Schroeder
Research on sub-kilometer coherent features found in the boundary layer of hurricanes at landfall has been problematic, as it involves structures for which the data collection has proven to be quite challenging. For a long time, technical limitations prevented the identification of such features. Recently, with the improvement of remote sensing instruments such as mobile Doppler radars, small-scale linear features of the HBL have been identified and documented, revealing the prevalence of such features in the HBL. Although some physical characteristics of these linear features are now better understood, their impact on the surface windfield is still a topic that raises numerous questions. To be able to fully understand the relationship between the HBL small-scale features and underlying surface windfield, it is essential to acquire near surface wind data, along with data from the general HBL. Instrumented towers have been valuable instruments to collect surface wind data, but their limited vertical extent does not allow any access to the general HBL. On the other hand, Doppler radars have been very efficient in documenting HBLs, however, technical limitations such as ground clutter has made data acquisition in the lowest part of the HBL very challenging. In an attempt to address those issues, the Texas Tech University Hurricane Intercept Team (TTUHIT) has combined the two types of instruments to document the influence of the HBL small-scale features with the surface windfield. During the landfalls of Hurricanes Isabel (2003) and Frances (2004), the TTUHIT deployed SMART radars and instrumented towers close to each other to collect a coupled dataset. The processing of the Doppler velocity data revealed very fine coherent features in the HBL. The goal of the study is to be able to correlate these features' kinematic signature to the surface windfield recorded by the instrumented towers.
Preliminary Comparison of DOW and In Situ Wind Measurements in Hurricane Rita
Joshua Wurman, Center for Severe Weather Research, Boulder, CO; and C. Alexander, P. Robinson, and F. Masters
A Center for Severe Weather Research (CSWR) Doppler On Wheels (DOW) mobile radar was deployed to Port Arthur near the point of landfall of Hurricane Rita (2005). High temporal and spatial resolution data were collected for several hours as the eyewall and eye came ashore. The DOW was deployed 8 km downstream of two Florida Coastal Meteorology Program (FCMP) instrumented towers, also in Port Arthur, which collected wind data at 10 m agl at better than 1 Hz time resolution. The DOW conducted scans over the towers at 1.2 deg elevation (~150 m agl) every 12 sec resulting in an over 2400 point time series of Doppler winds which could be compared to tower data.
Comparisons of the DOW and tower data will be presented. A couple preliminary comparison plots are shown below, illustrating the long term correspondence of winds and the correspondence of short period gusts.
Figures can be found at: http://www.cswr.org/rita/rita-preliminary-plots-2005-1013.pdf

Progress in the study of coherent structures in the hurricane boundary layer

Ralph C. Foster, APL, Univeristy of Washington, Seattle, WA


Recently published observational and theoretical studies have shown that roll vortices are a common and persistent feature of hurricane and tropical cyclone boundary layer mean flow. The roll circulation is dominated by alternating bands of higher- and lower-speed near-surface flow roughly parallel to the mean azimuthal winds above the PBL. These wind speed variations are on the order of +/- 5 to 10 m/s over lateral (roughly radial) distances of 300 to 2000 m. Associated with this azimuthal flow modulation is a somewhat weaker overturning circulation in which the enhanced (reduced) azimuthal flow is correlated with the downdraft (updraft) branches of the overturning flow. Thus, the roll circulation directly enhances the transfer of momentum across the boundary layer. Since all conventional turbulence parameterizations used in numerical model PBL parameterizations assume homogeneous turbulence, it is crucial to improve our understanding of these coherent structures and their effects on the fluxes of momentum, heat and water vapor across the hurricane PBL. For example, the observations and theoretical studies estimate that the inhomogeneous roll circulation enhances the averaged surface stress and the momentum flux in the mid-PBL by no less than a factor of two over conventional estimates. We have previously shown that, despite the potentially enormous surface heat fluxes in tropical cyclones, tropical cyclone PBL rolls are the result of a primarily shear-driven instability of the mean flow that quickly establishes a new equilibrium composed of a modified mean flow with an embedded three-dimensional roll circulation. We present recent advances in our development of a nonlinear hurricane boundary layer theory that captures both the nonlinear mean flow dynamics and the nonlinear effects of the mean flow perturbations, such as rolls. If time permits, we will also present preliminary investigations into the characteristics of smaller-scale, transient linear features that also form in such highly sheared boundary layer flows.
Atmospheric Boundary Layer Observations of Tropical Cyclones with the Imaging Wind and Rain Airborne Profiler
Daniel Esteban Fernandez, NOAA, Camp Springs, MD; and Z. Jelenak, P. S. Chang, R. F. Contreras, T. Chu, P. Asuzu, and J. Carswell
IWRAP, the Imaging Wind and Rain Airborne Profiler, is the first high-resolution dual-band airborne Doppler radar designed to study the inner core of Tropical Cyclones (TCs). IWRAP is currently operated from a National Oceanic and Atmospheric Administration (NOAA) WP-3D aircraft during missions through TCs and severe ocean storms. The system is designed to provide high-resolution, dual-polarized, multi-beam C- and Ku-band reflectivity and Doppler velocity profiles of the atmospheric boundary layer within the inner core precipitation bands of TCs and to study the effects precipitation has on ocean wind scatterometry as it applies to TCs. This dual-wavelength system also provides for the use of differential attenuation techniques to derive the rainfall rate and to characterize the drop size distribution (DSD) within TCs. IWRAP implements a very unique measurement strategy; it profiles simultaneously at four separate incidence angles (approximately 30, 35, 40 and 50 degrees) while conically scanning at 60 RPM.
This instrument has already demonstrated its capability to measure the wind field and the rainfall rate over the range of winds and rain rates usually present in tropical cyclones during the CBLAST field experiments as well as the NOAA/NESDIS Ocean and Rain experiments. One of the lessons learned during these experiments was the limitation in retrieving the wind field at the lowest part of the boundary layer, since the off-nadir looking geometry makes the ocean surface return impact the precipitation measurements from which the wind field is derived. To overcome this problem, the IWRAP instrument was recently equipped with a new data acquisition system that allows us to acquire raw data and, therefore, to separate both ocean and rain contributions through spectral processing. This has enabled us to derive the wind field virtually down to the ocean surface, and for the first time creates a unique opportunity to estimate the drag coefficient in very high wind conditions. Moreover, the unique ability to estimate the rain spectrum allows us to better understand and characterize the rain processes within the inner core of TCs.
The purpose of this paper is to show how these limitations have been overcome through the experience built from the datasets acquired during the CBLAST program with the new data acquisition system. A unique set of wind field results acquired during the hurricane season 2005 will be presented, as to illustrate how this instrument has evolved during and beyond the CBLAST program, enabling us to obtain high resolution atmospheric boundary layer wind fields within the inner core of TCs.

High Resolution Airborne Radar Measurements of Hurricane Isabel

Robert F. Contreras, University of Massachusetts, Amherst, MA; and D. Esteban Fernandez, P. S. Chang, and P. G. Black


The Imaging Wind and Rain Airborne Profiler (IWRAP) is a dual-frequency, conically-scanning Doppler radar that measures high resolution profiles of rain's effective reflectivity Ze and Doppler velocity, as well as surface wind vectors via scatterometry. IWRAP was flown aboard a NOAA WP-3D aircraft during the 2003, 2004, and 2005 hurricane seasons as part of the ONR's Coupled Boundary Layers Air-Sea Transfer (CBLAST) experiment, NASA's Ocean Vector Winds research, and the NOAA/NESDIS Ocean Winds and Rain experiments. We will start with a description of IWRAP and its capabilities. Following this we will introduce a new dataset available to the CBLAST community. We will finish with high resolution radar observations of Hurricane Isabel with an emphasis on the 3-D structure of the storm, especially in the atmospheric boundary layer (ABL). In particular, IWRAP will be shown to be effective at resolving linear boundary layer structures.





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