Air-Sea Interactions in Tropical Cyclones Workshop




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Table of Contents

Introduction\: 1

Workshop Report and Overview 4

Executive Summary 4

Break-out Group I Key Recommendations on Data Needs for the WRF 6

.0 Welcome, Introduction and Purpose of the Workshop (Stephen Lord/Naomi Surgi, NWS Environmental Modeling Center) 10

Introduction\: 11

.0 Summary of San Diego Workshop (Russell Elsberry, Naval Postgraduate School) 11

.0 Summary of Presentations 12

A. Forecaster Assessment\: Present and future forecast challenges 12

A.1 Operational Hurricane Forecasting at the NWS (Richard Pasch, TPC/NHC) 12

B.2 Surface Waves\: The WAVEWATCH model (Henrik Tolman, NCEP) 12

B.3 Data Assimilation ( John Derber, NCEP) 13

A. Forecaster Assessment\: Present and future forecast challenges 16

Appendix A\: Agenda for Hurricane WRF Workshop 29 – 30 May 2002 22

A. Forecaster assessment\: Present and future forecast challenges 22

A. Forecaster assessment\: Present and future forecast challenges 22

Appendix B\: Working Groups 25

Appendix C\: Workshop Organizers 26

Appendix D\: Full Report of Working Group I – Data needs for the Hurricane WRF 27

Appendix D\: Full Report of Working Group I – Data needs for the Hurricane WRF 27

Appendix E\: Full Report of Working Group II – Modeling physical processes for a high resolution coupled air/sea/land WRF hurricane prediction system 29

Appendix E\: Full Report of Working Group II – Modeling physical processes for a high resolution coupled air/sea/land WRF hurricane prediction system 30



Air-Sea Interactions in Tropical Cyclones Workshop

Camp Springs, DC 24-25 May 2005


Workshop Report and Overview


Executive Summary

The Weather and Research forecast system (WRF) is currently under development by the U.S. research and operational modeling communities and will replace current mesoscale modeling capabilities for various mesoscale The Weather Research and Forecast (WRF) system is currently under development by the U.S. weather research and operational modeling communities. The WRF will replace current mesoscale modeling capabilities for various mesoscale forecast applications at operational Numerical Weather Prediction centers including NCEP and AFWA beginning in 2004. The WRF for hurricane forecasts (HWRF) will replace the Geophysical Fluid Dynamics Laboratory hurricane prediction system at NWS/NCEP and will be coupled to the Hybrid Community Ocean Model (HYCOM) in 2007. This coupled system will serve as the nation’s next-generation operational hurricane prediction system, as well as serve as the primary research hurricane model.

Although significant progress has been made over the past several decades in advancing our Nations hurricane track forecasting capability, many forecast challenges remain that need to be addressed by the next generation Although significant progress has been made over the past several decades in advancing our nation’s hurricane track forecasting capability, scientific and forecast challenges remain that need to be addressed by the next-generation coupled ocean-wave-atmosphere hurricane prediction system, which includes understanding the role of the upper ocean on hurricane intensity through the air-sea interface and the atmospheric boundary layer. Given a spectrum of differing track scenarios such as erratically moving storms, storms that accelerate, and storms that stall, any improvements to the hurricane intensity forecast must not degrade track forecasting. In the case of a tropical cyclone interacting with the upper ocean, any subsequent intensity change is sensitive to the track forecasts. Notwithstanding, when the forecast track is fairly certain within 36 hours of landfall, understanding the ocean’s role on the intensity change through air-sea interactions becomes of paramount importance as deep ribbons of high oceanic heat content water surround the US coastline. By providing better initial conditions ocean conditions, and improving air-sea parameterization schemes in the coupled models, we may expect improved forecast of the tropical cyclone surface wind field, the ensuing storm surges and the inland flooding, which accounts for a majority of the Nation’s hurricane-related fatalities.

To meet the above forecast challenges significant advances must concurrently occur in advanced observations, data assimilation techniques and model development for both the hurricane environment and the hurricane core To meet these forecast challenges, significant advances must concurrently occur in observations, data assimilation techniques; and model development for both the hurricane environment and the hurricane core to properly simulate the complex interactions between the physical and dynamical processes on different scales of motion that determine the hurricane motion, and to forecast intensity changes over the open and coastal ocean during hurricane landfall. The HWRF will be a high-resolution, coupled air/sea/land hurricane prediction model with advanced physics. Other planned advancements in the HWRF system include a local advanced atmospheric data assimilation capability to address the next generation initialization of the hurricane-core circulation. It is envisioned a similar process must occur for oceanic data assimilation on the basin scale, such as from the ongoing Global Ocean Data Assimilation Experiments (GODAE).

The U.S. Weather Research Program [USWRP] was designed to bring together researchers and the operational community to meet such challenges as these. It is under the auspices of USWRP that this workshop was The the The U.S. Weather Research Program (USWRP) was designed to bring together researchers and the operational community to meet such challenges as these. It is under the auspices of the USWRP that this workshop was convened to begin discussing: (1) the state-of-the-art in tropical cyclone models and observations; (2) maximizing the usefulness of various data sets that have been acquired by various groups in hurricane environments over the past five years (ONR CBLAST in multiple hurricanes; NSF/NOAA Experiments in Isidore and Lili; NRL in Ivan; MMS in Georges, etc); and (3) upper-ocean observational strategies to acquire more complete data sets that must now include ocean current and salinity profiles as well as thermal profiles to evaluate the oceanic and coupled forecast models.


One of the key impediments that must be addressed by the community is to provide a comprehensive data archive to support the operational implementation of the HWRF in the 2007 time horizon. Specific attention must be directed toward improving air-sea parameterizations (such as drag and enthalpy coefficients), and to assess the relative importance of sea spray under fetch-limited wave fields. The development and testing of air-sea parameterizations at high wind speeds are required for the tropical cyclone forecast problem for both the large-scale environment and on the tropical cyclone scale. These were the overarching objectives presented to the broad cross-section of researchers, modelers, operational forecasters, and physicists, and managers of governmental and university research programs who gathered at the NCEP in May 2005.


Break-out groups purposely included both forecasters and researchers to deal with cross-cutting ocean-atmosphere issues. Each group focused on the same sets of questions with the intent to recommend a set of priorities to deal with the coupled ocean-wave-atmosphere part of the forecast models. Within the broad framework of ocean-atmosphere coupling, the central questions posed to each breakout group were:


  1. What is the current state-of-the-art in observations and models for the tropical cyclone problem, and how can this community maximize recently acquired data sets?

  2. What relevant time-space scales need to be resolved to improve intensity predictions in the models and what observations are needed to evaluate them with regard to vertical mixing, wave coupling, and air-sea parameterizations?

  3. What metrics need to be in place for assessment of model-generated fields, and how do we implement them in near-real time for forecasting needs?

  4. What is the design of an optimal field program for the evaluation of these models fields for in situ and satellite-based support? How can the community maximize the Global Ocean Observing System data to steadily improve ocean models?



A benefit of this approach was a similar set of recommendations were determined from both break-out groups. That is, a consensus was reached from the more than 30 participants. In addition, the informal exchanges between forecasters and researchers will inevitably foster collaborations. Such collaborative ties between academia, government, and private sectors are central to advancing our understanding of the tropical cyclone intensity forecast problem and achieving the NCEP goal of having a fully coupled HWRF/HYCOM model for operations.


Breakout Group Recommendations:


Both working groups agreed that infrastructure and resources need to be developed over the next three years. Specific recommendations are:


  1. Given the range of uncertainty in the surface drag (e.g., wave effects), heat fluxes (e.g., sea spray), and initial conditions (e.g., wind field) beyond 30 m s-1 using CBLAST data, assess how these uncertainties propagate through the coupled GFDL/HWRF model;


2. Develop an archive of data sets and model outputs and make these archives publicly available for research and operational purposes. Investigate the potential use of these data sets in assimilation, evaluation, and verification of models and parameterization schemes (e.g., HYCOM);


3. Create an in-situ tropical cyclone ocean-atmosphere observing program for pre-storm, storm, and post-storm environments. Develop optimal observing strategies and observational mix for spatial evolution of upper ocean, interface, and atmospheric fields (including secondary circulations such as roll vortices in the hurricane boundary layer); and;


4. Develop improved ocean model initialization schemes through data assimilation of satellite and in situ measurements, and test mixing parameterizations for a spectrum of ocean, wave and atmospheric conditions including the impact of waves on the surface heat, moisture and momentum fluxes and thus on the evolving ocean mixed layer.


A recurrent theme in all of the discussions from both breakout groups was that this program needs to build on recent field programs such as ONR-CBLAST experiments in multiple years and NSF/NOAA in Isidore and Lili. In this broader context the background and justification for each of these four recommendations is provided in the following four subsections.


1. Air-Sea Parameterizations

A central issue in the air-sea interaction parameterizations is the appropriate value for the surface drag coefficients during high winds. It is clear from the recent experimental data that the drag coefficient does not continue to increase ad-infinitum with winds. Several investigations have shown that the coefficient seems to have a maximum between 28 to 32 m s-1 with values of 2.5 to 3.5 x 10-3. These investigations range from using Global Positioning Sondes (GPS) sonde wind profiles extrapolated to the surface, wind-wave tank results, turbulent flux measurements, numerical modeling results with coupled wave-atmospheric models and using forced upper ocean currents as a tracer of momentum flux. While the air-sea community agrees on the leveling off of surface drag values, what remains unclear is whether the drag decreases with higher winds or remains relatively constant. Measurements must be acquired on the right side of the storm where the wind and waves are presumably interacting with the ocean current field to impact the momentum flux and the surface drag, which then feeds back to hurricane intensity.


Another critical issue is the role of sea spray in high winds, and especially thermodynamic effects of the evaporation of spray in the tropical cyclone boundary layers. Three important parts in understanding the impact of sea spray are: 1) size spectrum characterization as a function of forcing (stress, wave breaking, etc); 2) exchanges of heat and moisture with an initial state at rest; and 3) sub-grid scale distortion of the surface layer temperature and moisture structure by droplets. Of particular interest is the feedback mechanism that characterizes the manner in which the various sizes of evaporating droplets modify the stratification near the oceanic surface layer, which reduces further droplet evaporation, but enhances the sensible heat fluxes. This approach introduces tuning coefficients based on the ratio of the enthalpy flux and surface drag coefficient that are a function of tropical cyclone intensity. For a high-droplet source function, this ratio decreases in the current version of the model. To get the impact of sea spray correct, measurements in the surface layer (~50 m) are required that include turbulent fluxes, mean profile structures, wave spectra, wave breaking and accurate estimates of rain rates. Measurements at 50-m levels may not be attainable from the NOAA WP-3D aircraft, but perhaps a sensor package could be potentially developed for a Unmanned Aerial Vehicle (UAV) to address this scientific question (see ftp://ftp.etl.noaa.gov/user/cfairall/onr_droplet/parameterization/).


In a broader context, the ONR-CBLAST data must be used to examine the sensitivity of the model simulations to these parameterizations. This modeling-based approach would provide insights into the parameter space of these important air-sea processes where data in the atmospheric and oceanic boundary layers can be used to constrain model solutions. This would lead to an understanding of uncertainties in observations and provide a motivation for the next series of field experiments discussed below.


2. Data Archive

The breakout working groups agreed that coupled ocean-atmosphere data sets must be archived and maintained for the community for use in model evaluation studies (the evaluation step is considered to come before the validation step). Such data sets must include:


  1. Josephine (1984) and Gloria (1985) current and temperature profiles from the Ocean Response to a Hurricane (ORTAH ) project;

  2. Gilbert (1988) current and temperature profiles prior, during, and subsequent to the storm from the ONR/NOAA project;

  3. Dennis (1999) ocean mixed layer float data acquired prior, during, and subsequent to the storm for an NSF project;

  4. Georges (1999) current and temperature profile data acquired prior, during, and subsequent to the storm for an Minerals Management Service (MMS) project;

  5. Isidore and Lili (2002) current, temperature, and salinity profiles acquired prior, during, and subsequent to the storm for an NSF/NOAA project;

  6. Frances (2004) current, temperature and salinity profiles acquired prior, during and subsequent to the storm for the ONR CBLAST project; and

  7. Ivan (2004) current, temperature, and salinity data acquired prior, during and subsequent to the storm for an NRL project in the northern Gulf of Mexico.


In summary, only in a few storms have ocean current and shear structures been observed over the past 21 years from aircraft campaigns. Moored measurements during Georges and Ivan were fortuitous encounters, as opposed to focused aircraft-based experiments in CBLAST and NSF/NOAA programs. Given its importance for shear-induced mixing processes (Richardson number instabilities), this relatively poor data base needs to be improved to advance coupled modeling during hurricane conditions.


These data sets, along with surface forcing from remote sensors (both satellite- and aircraft-based), GPS sondes and available wave data should have a permanent archive site where the community can access these valuable data sets. The NDBC surface buoy data and thermal profiles from Airborne eXpendable Bathythermographs (AXBT) from decades of measurements must be included in such an archive. The archive should also include model-generated fields from GFDL that could be used by graduate students for their thesis/dissertation research. As noted in both breakout groups, this will require resources to establish the archive, locate the data and establish quality controls, and maintain it perhaps on a password basis for investigators working with NCEP. This effort will require a full-time position for a couple of years to establish it.

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