Air-Sea Interactions in Tropical Cyclones Workshop




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A.2 Surface Wave Modeling at NCEP (Hendrik Tolman, NCEP)


This presentation consisted of four parts. First, a general review of common wind-wave modeling practices was given. Second, a brief overview of WAVEWATCH III was given that covered its history, and its main features (numerics, physics, island blocking). Third, operational models at NCEP were discussed where special attention was focused on hurricane wave models, and on the quality of the hindcasts and forecasts. Finally, future plans were discussed that focused on hurricane wave prediction, e.g., the development of a multi-scale wave model, new approaches to nonlinearities in the wave model, and physics of wind-wave interaction at high wind speeds.


Since 1994, NCEP has been running a third-generation ocean wave model. The decision was made to

build a new model termed WAVEWATCH based on the WAM, but with significant differences.

WAVEWATCH became the operational ocean wave model in 2000. WAVEWATCH is initialized by

global model winds and has versions customized for the western North Atlantic and the eastern North

Pacific Ocean. All of the models use 24 directions and 25 frequencies for wave forecasts. Some aspects

of the models must be parameterized due to the lack of both computing power and physics limitations.

For example, capillary waves are critically important, but are not included in the dynamic model and

must be parameterized.


Hourly wind fields are essential to forecast the wave field development, especially for rapidly moving, intense, small-scale hurricanes. Hourly wind field analyses were implemented in 2002 for the Atlantic hurricane season. Such hourly analyses are necessary to ensure continuity of the wave model and for swell forecasts. The ocean and atmosphere are coupled through various fluxes that are also a function of surface waves. Some examples of the coupling are:


1. Momentum is transferred from the atmosphere to the ocean by surface stress and subsequent wave actions.


2. The wave breaking and spray influence the fluxes of mass and heat.


3. Wave breaking also is a large source of turbulent energy in the upper ocean and the momentum in the waves is released during the breaking action.


One of the scientific issues that needs serious attention is that the momentum transfer and drag coefficients included in the models are extrapolated from moderate wind conditions. Information is not available at the low and high ends of the wind velocities. The errors introduced by this extrapolation have a first-order impact on wave growth rates.


A.3 Operational Ocean Modeling at NCEP (Carlos Lozano, NCEP)

High-resolution ocean forecast systems for nowcast and short term (5 days) forecast in the Atlantic and the Pacific Ocean basin will form the backbone for the regional ocean model components of the coupled hurricane models. The ocean forecast system for the Atlantic Ocean basin (25oS-76oN) is being prepared for daily oceanic operations with resolution of ~5 to 7 km along the path of most hurricanes. Hindcast ocean simulations (uncoupled) during Isabel and Frances hurricanes illustrate the advantages of a high-resolution ocean model to capture sea surface temperature cooling due to turbulent mixing and near-inertial pumping on the thermocline. Initial ocean conditions will be provided by operational nowcasts that will include the assimilation of SST (remotely sensed and in situ), sea surface height anomalies from altimeters (Jason, GFO, etc), and in situ temperature and salinity observations (CTD, AXBT, buoys and drifters). There is a clear requirement for comprehensive ocean and atmosphere observations in a storm-coordinate system to evaluate coupled hurricane models, and for the development of efficient deployment strategies that provided these gridded observations.

A.4 Data Assimilation Efforts at NCEP (John Derber, NCEP)


The basic objective of the NWP data assimilation is to combine all relevant information from any source to produce an estimate of the most likely state of the atmosphere at the beginning of the forecast cycle. Generically, the “cost” or “fit” function optimizes the fit of the background field with the observations and other constraints. In some data-sparse areas of the world, the background field is as good as the observations.


1. The first step is to convert the analysis (background field) to observation-like information and compare that information to observations. Forward models to make the conversions can be such things as a simple interpolation scheme, a complex radiative transfer function, or a precipitation algorithm.


2. The weights given to the various terms in the equations are related to the estimates of the error covariances.


3. The final terms are lumped under “other constraints”, for example, to force the moisture values to be non-negative, and keep a balance between the mass and momentum in mid-latitudes. Significant differences exist in the data assimilation issues for large-scale processes in the tropics and for hurricanes. For the most part, the issues are mesoscale or smaller in nature. Clearly, data assimilation for hurricanes is much more difficult than for the larger-scale tropical circulations.


Some of the data assimilation challenges for the tropics and hurricanes include:


1. Balance equations: In the tropics (and for mesoscale in general), balance is dominated by moist processes and is much more complex than for the larger scales. Failure to properly treat the balance issues will result in a rapid loss of useful information at the beginning of the forecast. The increase in non-linearity due to moist processes make the tropical/hurricane problem more difficult to solve.


2. Analysis variables: To accurately analyze variables in the tropics such as cloud liquid water and cloud ice, a balance has to be achieved and all the fields involved need to be initialized, which also means the surface and ocean fields must be correctly specified. The ability to achieve a realistic balance is not as straightforward as for the larger scales.


3. Background error covariance: For the tropics, it is essential to have circulation-dependent error covariances, but they are difficult to determine. For example, the structure of the background error covariances for cloud and surface fields are almost certainly to be dependent on small-scale dynamics that are not well known. Further, it is critical to include in the background error covariances the relationships between the variables (e.g., water vapor and clouds).


Significant progress has been made over the past 18 months in the development of the operational WRF 3DVAR data assimilation system. . The research state-of-the-art version of the WRF 3DVAR is scheduled for 2006. The advanced four-dimensional data assimilation (A4DDA) scheme is likely to be implemented by 2010. The coupled ocean model data assimilation will focus on:


1. Upper ocean and mixed layer as being of primary importance,


2. Skin temperature, which is a primary measurement from satellites,


3. Bulk water temperatures obtained from ship observations. The satellite retrievals are calibrated to the bulk temperature, and


4. Profiles of the thermal (and salinity) structure and mixed layer depth which are provided by floats and expendable conductivity temperature and depth probes.


In summary, the improved specification of the background error covariance has top priority and not all of the observations are useful. Significant progress over the last few years has been made in how to assimilate data from a wide range of sources. However, recent observations are still not being used to the maximum extent possible, and some new observations have not yet been incorporated into the data assimilation system. The task of achieving an effective data assimilation scheme for a new observational data set may be on the order of 1-2 years from the time the data are reliably available. Data assimilation systems can and should be as transportable to different platforms as are the models.


A.5 Coupled Modeling at URI/NCEP (Isaac Ginis, URI)


In 2001 the GFDL/URI coupled hurricane-model model was implemented at NCEP/EMC for operational forecasting in the Atlantic basin. Since then joint research efforts of the URI, GFDL, and NCEP scientists have been focusing on further improvements of the GFDL/URI model. In this presentation, the latest research and development efforts are highlighted.


1. New ocean model initialization method:


A new ocean data assimilation and initialization package has been developed to improve simulations of the Loop Current (LC) in the GFDL/URI operational coupled hurricane prediction system. This procedure is based on feature modeling and involves cross-frontal “sharpening” of background temperature and salinity fields according to data obtained in specialized field experiments. It allows specifying the position of the LC in the Gulf of Mexico using available observations. The new initialization procedure will be tested during the 2005 hurricane season in the Atlantic basin. It is planned for operational implementation in 2006.


2. Improving air-sea momentum flux parameterization:


In the GFDL hurricane model, the air-sea momentum flux is parameterized with a constant non-dimensional surface roughness (or Charnock coefficient, where is the roughness length, is the friction velocity and g is the gravitational acceleration) and the stability correction based on the Monin-Obukhov similarity theory, regardless of wind speeds or sea states. This parameterization assumes an increase in Cd with wind speed. However, a number of studies have suggested that the value of the Charnock coefficient depends on the sea state represented by the wave age. Lively debate is ongoing in the research community over this relationship. The major reason leading to the discrepancies among different studies is the paucity of in situ observations, especially in high wind speeds and young seas.


We investigated the Charnock coefficient under hurricane conditions using a coupled wind-wave (CWW) model that includes the spectral peak in the surface wave directional frequency from WAVEWATCH III and a parameterized high frequency part of the spectrum using a recently developed model. The wave spectrum was then introduced to the wave boundary layer model to estimate the Charnock coefficient at different wave evolution stages. We found that the drag coefficient levels off at very high wind speeds, which is consistent with recent field observations. The most important finding of this study is that the relationship between the Charnock coefficient and the input wave age (wave age determined by the peak frequency of wind energy input) is not unique, but strongly depends on wind speed. The regression lines between the input wave age and the Charnock coefficient show a negative slope at low wind speeds but a positive slope at high wind speeds. This behavior of the Charnock coefficient in high winds provides a plausible explanation why the drag coefficient under tropical cyclones, where seas tend to be extremely young, may be significantly reduced in high wind speeds.


3. Improving air-sea heat and humidity flux parameterization:


Heat and humidity flux parameterizations are a crucial factor in the hurricane-ocean coupling. In high wind conditions, heat and humidity exchange coefficients ch and ce can be directly related to the roughness lengths of temperature and water vapor (ZT and Zq). We have tested various parameterizations ZT and Zq in the GFDL hurricane model and found that for simulations of very intense hurricanes with maximum wind speeds exceeding 50 m s-1, large values of CH are necessary, with ch/cd >1. For example, testing the parameterization of ZT and Zq used in the GFS global model for Isabel (2003) indicates that the storm fails to intensify beyond 50 m s-1, while the observed maximum winds reached about 70 m s-1. It is possible that sea spray, which is neglected in these experiments, may provide an additional heat and moisture source.


4. Development of a coupled hurricane-wave-ocean model:


We have coupled the GFDL hurricane model, the Princeton Ocean Model (POM), and the WAVEWATCH III model with the URI wind-wave boundary layer model. The highest resolution of the model in the movable inner-most mesh is 1/12 degree. We have developed a movable nested grid configuration of the wave model. The inner mesh of higher resolution follows the storm center, as it is done in the GFDL hurricane model. It is a necessary step to reduce significant computational requirements of the WW3 model. Although the testing and evaluation of the new coupled system has just begun, the first numerical experiments show encouraging results.


B. Researcher Overviews:

B.1 Upper Ocean Observations (L. K. (Nick) Shay, UM)


Over the past two decades, it has been fairly well documented that ocean current and the shear field play an important role in cooling and deepening of the oceanic mixed layer (negative feedback regimes). By contrast where the ocean mixed layer (and depth of the 26oC isotherm) is deeper, the ocean current shears may not be large enough to significantly cool the upper ocean. Such positive (or less negative) feedback regimes, which provide more of a sustained heat flux to the atmosphere during hurricane passage, are usually associated with deep warm fronts and eddies that are often characterized as deep ribbons of high oceanic heat content water. That is, advection of thermal gradients by the strong currents may counterbalance upwelling and mixing effects and provide more heat to the atmosphere.


1. Thin Mixed Layers : (Negative Feedback):


In negative feedback regimes, wind-driven vertical current shear induces mixing of the oceanic mixed layer and top of the thermocline (i.e., entrainment heat flux). Strong shear events lower the Richardson number to below criticality and thereby cause the ocean mixed layer to deepen and cool as cooler water from the thermocline is mixed with the warmer ocean mixed layer water. In this case, the sea-surface temperature represents a proxy for the ocean mixed layer temperature even using crude bulk ocean mixed layer models. Thus, the available ocean heat content, which is defined as the amount of heat from the surface to the depth of the 26oC isotherm, decreases and then negatively impacts storm intensity.


Despite the importance on the physics of the air-sea interactions (available heat and moisture), only seven aircraft-based experiments have specifically focused on measuring current and current shear for this important aspect of the tropical cyclone-ocean interaction problem. Airborne eXpendable Current Profilers were first launched in 1984 and 1985 as part of an oil company-consortium initiative Ocean Response To A Hurricane Program (ORTAH) in Norbert (eastern Pacific Ocean), and Josephine and Gloria (western Atlantic Ocean). Since that experimental effort, AXCPs have only been deployed in Hurricanes Gilbert (1988), Isidore and Lili (2002). As described below, profiling floats with electromagnetic current and shear measuring capability were recently deployed in Frances, and measured strong near-inertial currents and vertical shears over several days in the cold wake region.


2. Deep Warm Mixed Layers: Positive Feedback

Coupled ocean-atmosphere measurements were acquired during an NSF/NOAA-sponsored Hurricane Air-Sea Interaction Experiment. Dual-aircraft experiments mapped 3-dimensional fields using expendable profilers (AXCPs, AXCTDs, AXBTs, GPS) deployed from NOAA research aircraft as Isidore and Lili moved into the Gulf of Mexico in September and October 2002. As the storms encountered the Loop Current, Isidore intensified to a category 3 and Lili rapidly intensified to a category 4 storm. Even at these levels of intensity, the upper ocean response was minimal SST decreases of less than 1oC and ocean heat content (OHC) losses less than 10 KJ cm-2. That is, advection of thermal gradients may have counterbalanced shear-induced mixing processes associated with forced near-inertial motions. In this positive (less negative) feedback regime, advection of the thermal structure by the strong currents has time scales of less than a day with large geostrophically balanced currents transporting high OHC water (150 KJ cm-2) from the Caribbean Sea into the Gulf of Mexico to form the Loop Current core. As Lili moved northwest of the Loop Current, the upper ocean cooled by more than 2oC with a net OHC loss of about 30 KJ cm-2 due primarily to shear-induced mixing across the base of a thin ocean mixed layer in the Gulf Common water. The storm subsequently weakened prior to landfall to a category 1 storm due in part to the entrainment of drier air as well as interacting with an ocean previously cooled by earlier tropical storms Hanna and Isidore.


Since PDT-5 report (Marks and Shay, 1998), only two focused oceanic and atmospheric experiments in hurricanes have measured current and shear along with temperature and salinity. While there have been fortuitous encounters where tropical cyclones have passed over mooring deployed in support of other experiments such as Frederic (1979), Allen (1983), Gloria (1985), Georges (1999) and Ivan (2004). Developing, evaluating, validating, and implementing accurate ocean/coupled models will require a more systematic measurement approach to accurately represent the response to the atmospheric forcing and understand the levels of negative (and positive) feedback to the atmosphere over the life cycles of several storms. Measurements of T,S and velocity (u,v) acquired in grids are needed for the models to adequately represent parameter space, assess the ocean mixing schemes, and evaluate performance. Without the current and shear measurements, models will not necessarily improve (i.e., thermal structure is not enough).


B.2 Atmospheric Boundary Layer Observations (Gary Barnes, UH)


Observations collected over the last decade with the NOAA-AOC aircraft (directed by members of the NOAA/AOML/ Hurricane Research Division and university investigators) and recent specific experiments such as CBLAST, supported by NOAA, NSF, and ONR are revealing fresh details of the hurricane boundary layer. The observations are the result of new aircraft deployment strategies and the interpretation of new instrumentation that includes the SFMR, Doppler, fast response wind, temperature, and humidity sensors, and especially the Global Positioning System (GPS) sondes. At this early stage, the GPS sondes have been exploited more than the other sensors. Over 300 hundred vertical profiles of wind speed in numerous hurricanes reveal that roughness length (and therefore the drag coefficient) does not continue to increase with increasing wind speed as previously believed. Above 35 m s-1 the drag coefficient remains constant. The profiles also identify that 10-m winds are typically 0.8 -0.9 of the wind maximum that is typically located 500 to 700 m altitude.


GPS sondes deployed in Hurricane Bonnie (1998) provided the first views of vortex-scale horizontal maps of temperature, specific humidity, equivalent potential energy, and radial and tangential wind components from 10 m to 2 km altitude. These maps have a variety of structures that include non-isothermal inflow to the eyewall, rapid moistening of offshore flow, and the depth and energy content of the inflow layer to the eyewall. An energy budget of the inflow reveals that the increase of energy occurs within 50 km of the radius of maximum winds, demonstrating the importance of understanding the air-sea fluxes in the inner core of the hurricane.


Profiles of potential and equivalent potential temperature have structures in the inner core of the hurricane that depart from the typical undisturbed tropical conditions. Positive lapse rates of equivalent potential temperature well below the mid-tropospheric minimum, moist absolutely unstable layers, very shallow mixed layers, and nearly saturated and super-adiabatic surface layers have been observed. The GPS sonde results have the potential to serve as a test field for the GFDL and HWRF hurricane models used for operational and research tasks. Comparisons between the model fields and the observations may ultimately lead to improvements in key parameterizations, and thus result in improved intensity forecasts.


B.3 Ocean Modeling (S. Daniel Jacob, UMBC)


Modeling and evaluation of the ocean response to tropical cyclones are crucial to coupled hurricane intensity prediction. Since the ocean component in the prediction system provides the lower boundary conditions that affect the fluxes for rapid intensification or weakening. This overview focused on uncertainties in the state-of-the-art ocean response models from a physical and numerical perspective and their applications to coupled prediction system. Due to the availability of observations prior, during, and after the passage of the storms, simulations were conducted for Gilbert (1988), Isidore and Lili (2002) using the Hybrid Coordinate Ocean Model (HYCOM) to quantify the range of uncertainties relevant to coupled modeling.


1. Model and Simulations:


Two configurations of HYCOM were used to simulate the upper ocean response to hurricanes Gilbert, Isidore and Lili for different vertical resolution. Since Isidore and Lili occurred closely spaced in time, these two were combined into a 20-day simulation in contrast to a 6-day simulation in the Gilbert case. Momentum forcing in this study was derived by combining environmental winds from an atmospheric general circulation model with aircraft-reduced and buoy-observed winds using the Hurricane Research Division wind analysis program. While realistic initial conditions for the Gilbert case were derived from in situ data, background fields from a data assimilative basin-scale HYCOM run provide the conditions in the Isidore and Lili cases. Numerical simulations were conducted to quantify uncertainties for realistic and quiescent initial conditions and differing entrainment mixing schemes that parameterize sub-grid scale processes. Initial conditions were evaluated with data acquired one day prior to the storm passage.


2. Results:


Realistic initial conditions for the three cases considered here included the deep warm layers of the western Caribbean Sea, Loop Current, and the Warm Core Rings that separate from it in the Gulf of Mexico. Evaluation of initial conditions in the Gilbert case indicates that they are reproduced accurately for ocean response modeling. While the location of oceanic features are reproduced by the assimilative basin-scale model and the vertical thermal structure is comparable to Levitus and GDEM climatologies, pre-Isidore expendable probe data indicate a much warmer upper layer in the ocean. Consequently, simulated cooling is more than the observed cooling by about 0.5° C. This result highlights the need for routine pre-storm observations for evaluation of initial conditions used in the ocean component of the coupled model.


While the magnitude of upper-ocean cooling simulated for quiescent initial conditions compares reasonably well with observations, the pattern and extent of simulated cooling are modulated by pre-storm mesoscale variability. While attempts were made in the current ocean component of the operational coupled model to prescribe a condition that resolves the Gulf Stream system, the Loop Current eddies are not initialized in the present system. Results from Gilbert simulations suggest eddies are a necessity for more accurate prediction of the upper-ocean heat content evolution in the Gulf of Mexico. An additional effect of the pre-storm velocity structure is to reduce the frequency of the near-inertial internal waves generated by the storm and therefore the phasing of strong shears contributing to significant mixing will be delayed and make larger more fluxes available to the atmosphere.


One of the significant effects on the upper-ocean heat budget and the fluxes to the atmosphere is the choice of entrainment mixing parameterization. For quiescent initial conditions, the range of fluxes in the directly forced region of the storm exceeded 500 Wm-2 for different schemes. Comparative statistics suggest that the three higher-order mixing schemes considered will lead to a more accurate ocean response simulation. These comparisons are limited by data availability, and therefore routine measurements are necessary to evaluate the ocean component of the coupled system. Similar to the post-season track and intensity verification analysis, more ocean observations must be acquired to evaluate the different schemes on a post-season basis to build a statistical base of comparisons. Given the large range in the simulated surface fluxes for different schemes, this is a crucial step toward reducing this uncertainty. The approach of stand-alone ocean simulations using derived realistic atmospheric forcing used here allowed us to focus on and evaluate the ocean model and associated parameterizations. Since the boundary layer structure forcing from the atmospheric component of the coupled model is subject to additional uncertainties, this approach based on observations will lead to reduction in uncertainties of the ocean component in the coupled system.


B.4 Sea Spray Parameterization Schemes (Chris Fairall, NOAA ETL)


For the last decade, the NOAA Environmental Technology Laboratory (ETL) has been developing a hierarchy of models of the production of sea spray at high winds and the subsequent thermodynamic effects of the evaporation of spray on hurricane boundary layers. The three steps in this process are: 1) characterization of the size spectrum of droplets produced by the ocean as a function of the forcing (wind speed, stress, wave breaking, etc); 2) computation of the exchanges of heat and moisture between the droplets and an unperturbed near-surface layer structure; and, 3) accounting for the ‘subgrid-scale’ distortion of the standard surface layer T/RH structure by the droplets (a process referred to as ‘feedback’). Our present sea spray source strength parameterization is derived from the Fairall-Banner physical sea spray model (which predicts the size spectrum of sea spray produced by the ocean in terms of wind speed, surface stress, and wave properties). The Fairall-Banner spectrum has been parameterized into a simple mass flux representation in terms of friction velocity. The unperturbed thermodynamic effects are based on integrals of the ratios of thermodynamic and suspension time constants following Andreas. Finally, the diagnostic feedback parameterization has been developed to characterize the way evaporating droplets of various sizes modify the stratification of the air near the surface, which in turn reduces further droplet evaporation but enhances sensible heat flux carried by the droplets. The present form of the parameterization has two tuning coefficients: one that scales the magnitude of the source strength and the other that affects the partitioning of enthalpy flux between sensible and latent heat.


Recently the parameterization was coded in F90 and implemented in the GFDL hurricane model and a version of Weather Research Forecast (WRF) model that runs at ETL. Preliminary tests on hurricanes Ivan and Isabel showed sensitivity to sea spray, but there are interdependencies with the non-droplet (direct) transfer specifications in the models. More testing is needed to understand these interdependencies (see: ftp://ftp.etl.noaa.gov/user/cfairall/onr_droplet/parameterization/ )


B.5 EM-APEX Floats (James Girton, UW/APL)


A collaborative, ONR SBIR effort between the UW/APL and Webb Research Corporation (WRC) has developed an autonomous ocean profiling float that provides exceptional vertical and temporal resolution of velocity, temperature and salinity to depths of 2000 m for deployments of many years. Electrodes were added to the exterior of standard WRC APEX floats, and electronics were added inside. The electrode voltages result from the motion of seawater and the instrument through the Earth's magnetic field. Other systems included magnetic compass, tilt, CTD, GPS, and Iridium (that allow for sampling/mission changes).


Three EM-APEX (Electromagnetic Autonomous Profiling Explorer) floats were deployed from a C-130 aircraft ahead of Hurricane Frances as part of the ONR-sponsored CBLAST experiment. The floats profiled for 10 h from the surface to 200 m, then continued profiling and then between 35 m and 200 m at hourly intervals with excursions to 500 m every half inertial period (16 hr). The velocity computations were performed onboard and saved for later transmission. After five days, the floats surfaced and then transmitted the accumulated processed observations, then the floats profiled to 500 m every half inertial period until recovery early in October that was facilitated by GPS and Iridium positioning The resulting view of the evolution of upper-ocean momentum, shear, and stratification provides an important set of constraints for testing parameterizations of wind stress and ocean mixing in coupled ocean-atmosphere hurricane models. In addition, information on the direction and amplitude of the dominant surface waves can be extracted from high-frequency velocity measurements in the upper part of the profile.


B.6 ARGO Profiling Floats (Eric Terrill, SIO)


An autonomous profiling float now exists for observations of the upper ocean and air-sea interface during hurricanes. This observational tool was developed, tested, and, deployed as part of the ONR CBLAST experiments. The air-deployable profiler measures surface waves, wave breaking, wind-speed, and rainfall (via acoustic ambient noise inversions), Lagrangian currents, and the temperature and salinity structure of the upper ocean through rapid profiling of the upper 200 m of the ocean. The platform which hosts this unique set of underwater sensors is based upon a heavily modified SOLO float, which is similar to those now deployed in large numbers for the ARGO global climate monitoring system. The air deployment package is certified for usage from WC-130 aircraft.


During the 2004 season, we deployed nine units in the path of Hurricane Frances in collaboration with the AFRC 53rd WRS using NHC model track guidance for the airdrop locations and tasking from NHC. All nine units operated reliably through the course of the storm, with some units performing beyond expectation in their ability to transmit data during the winds exceeding 50 m s-1 using the ORBCOMM telemetry system. In addition to providing reliable data telemetry, the bidirectional communication system allows commands to be sent to the profiler to alter its mission after deployment. Unique to the platform is the development of a ‘hover’ mode that keeps the instrument at a nominal 30-50 m depth so that the air-sea interface can be probed with compact sonar for the direct measurement of surface waves.


Additional sensors onboard the instrument package include an acoustic system for processing ambient noise spectra in real time, a CTD package, a sonar altimeter for computing wave spectra, and a three- axis accelerometer. Two floats were equipped with a Aanderaa Optode for measuring dissolved oxygen. All sampling, power and communication with peripherals are done using a microcontroller that is independent of the vehicle control and telemetry system. The hurricane float missions included:


  • Profile temperature and salinity to 200 m, which we anticipate is below the mixed layer

  • Rise to a neutrally buoyant depth of O(30-50)m and enter a ‘hovering’ routine. While at this depth, the acoustic ambient noise field and surface wave field are sampled using the sonar altimeter, pressure sensor, and accelerometer.

  • Profile to surface to obtain GPS position and transmit data using the ORBCOMM telemetry system.

  • Repeat cycle every 4 hours for 180 dives.


For more information see http:// www.sdcoos.ucsd.edu/hurricanefloats.


Appendix A: Agenda for Air-Sea Interactions in Tropical Cyclones Workshop


Tuesday, May 24th


08:00 – 08:10 Stephen Lord and Naomi Surgi: Welcome, introduction and purpose of workshop

08:10 – 08:20 Nick Shay Motivation/Writing Charges
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