Air-Sea Interactions in Tropical Cyclones Workshop

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3. Sampling Approach

In support of these recommendations, and the recent successes of the ONR CBLAST and the NSF/NOAA program, the minimum sampling approach must include pre-storm, during and post-storm measurements to understand the impacts of the ocean forcing on the atmosphere, and the oceanic response to the tropical cyclone forcing. Specifically, the required types of observations and platforms to address this problem are:

  1. Pre-storm survey (oceanic temperature (T), velocity (u,v) including shear, salinity (S), pressure (p)) using a combination of Lagrangian floats and drifters, and aircraft expendables. These ocean measurements should be at least through the main thermocline (upper 250 m or deeper in warm ocean features) and preferably to as deep as 1000 m to resolve the background flows through the dynamic height field. Boundary layer structure must also be observed with GPS sondes and remote sensors such as the Stepped Frequency Microwave Radiometer (SFMR);

  1. Storm survey (T, u, v, S, p) from floats and drifters, and expendables profiling through at least 250 m; wave measurements from floats and aircraft remote sensors; boundary layer structure from GPS sondes; surface winds from Lagrangian drifters and aircraft remote sensing; sea-spray down to 50 m; and turbulence flux profiles from aircraft-based (or UAV) measurements; and,

  1. Post-storm (T,u,v, S, p) from floats and drifters, and expendables over the domain of the storm survey; surface wave and wind measurements from floats and remote sensors; and GPS sondes for the atmospheric boundary layer structure.

This approach emphasizes observations over both space and time using floats, drifters, and expendables that are capable of measuring current, temperature, and salinity. What is lost in time from the spatial snapshots from the aircraft is gained by high time resolution in floats and drifters. Thus, the measurement approach emphasizes flexibility to optimize the data acquisition efforts since no single approach will resolve the coupling issues.

Upper-ocean measurements of T, u, v, and S should have a vertical resolution of less than 4-m to accurately estimate current shears and Richardson numbers and the floats should measure these profiles at least at hourly intervals. In terms of horizontal resolution during the storm, the oceanic and atmospheric profiles should be spaced no more than 0.5 radii of maximum winds (Rmax). Remote sensing and flux measurements must be as rapidly as possible.

For non-events and more routine measurements for model evaluation and validation of the basic state of the ocean, both groups recommended enhancements to the NDBC buoy program to measure temperature and current profile time series, surface wind stress and directional surface waves; continued enhancements of the ARGO/Electro-Magnetic Autonomous Profiling EXplorer (APEX) floats (including mixed layer floats) and drifters as part of Integrated Ocean Observing System (IOOS); utilization of the growing network of High Frequency (HF) Coastal Radars deployed as part of the Coastal Ocean Observing System (COOS) to measure currents, waves and winds; and NWS WSR-88D radar networks.

4. Ocean Model Initialization and Mixing Parameterizations:

Based on recent findings, particularly in the Loop Current and warm core ring complex in the Gulf of Mexico, initializing ocean models with the correct background states represents a challenge for the modeling community. To improve the understanding of the role of the upper ocean on tropical cyclone intensity, the background state must be specified with the correct thermal and density structure that will then give rise to ocean features where energetic currents occur along frontal boundaries. Such features include the Loop Current, Florida Current and Gulf Stream which lead to the transport warm, high oceanic heat content from the tropics to the mid-latitudes as part of the climate cycle. One method is adjusting the model is through data assimilation of the sea-surface height from satellite radar altimetry and sea-surface temperature fields and projecting the surface height field vertically as is currently done in the HYCOM model. In many instances, this approach has worked reasonably well. However, assimilating T/S profiles from float data (or other routine measurements) is an opportunity that must be fully explored to get the correct basic state in terms of the thermal, haline, density and velocity structures. Time-dependent behavior of these warmer oceanic features (significantly reduced negative feedback to the hurricane) must also be evaluated with these data sets to ensure that the initial model fields are correct prior to the passage of a tropical cyclone.

A second important issue deals with the oceanic mixing parameterizations in ocean models. Given the number of mixing schemes available at the present time, choosing the most appropriate scheme for an oceanic or coupled system requires careful examination of simulated fields with observed profiles to understand the sensitivity of the oceanic response to these mixing schemes. With the exception of the bulk schemes, the turbulent kinetic energy schemes depend primarily on shear at the base of the oceanic mixed layer to parameterize entrainment heat flux. This shear term has been shown to contribute between 60 to 80% to the observed oceanic mixed layer cooling in predominately negative feedback regimes (away from frontal boundaries). Vertical shear effects in the ocean are similar to those of atmospheric shear, although the shear must be calculated over much smaller vertical scales. Large values of ocean shear, will lower the Richardson numbers to below criticality, and force the upper ocean to mix and cool. Recent simulations from each of these schemes has shown that for the same initial ocean conditions and same forcing, the amount of cooling in the ocean mixed layer differs considerably in terms of magnitude and structure. The amount of ocean cooling impacts the available air-sea fluxes that provide heat and moisture for the storm. The amount of uncertainties in surface fluxes from these various mixing parameterizations is unacceptable for an ocean model or fully coupled system because they will lead to larger uncertainties in intensity and structure of a tropical cyclone. In addition, the oceanic mixed layer heat, mass, and momentum budgets are affected by advection of the gradients in frontal regimes due to the background and wind-forced currents. In these cases, the oceanic mixed layer budgets are not 1-D, rather they are 3-D. Thus, careful attention must be devoted to mixing schemes and considerably more current and shear data are required to represent parameter space for oceanic models.

Little attention has been given to the impact of the surface wave interactions with the surface current field, and the impact of the wave coupling on the oceanic mixed layer processes through breaking waves. For example, in strong oceanic current regimes, the vorticity field associated with background and wind-forced currents will impact the surface waves through wave-current interactions. This may have a positive or a negative impact on air-sea fluxes depending on whether the wave heights are increased (caustics) or decreased (shadow region). To investigate such interactions numerically and their possible impact on air-sea fluxes will require much higher resolution Large Eddy Simulation (LES) oceanic models similar to those developed in the CBLAST program.

Air-Sea Interactions in Tropical Cyclones Workshop

Camp Springs, DC 24-25 May 2005

Final Report

  1. .0 Welcome, Introduction and Purpose of the Workshop (Stephen Lord/Naomi Surgi, NWS Environmental Modeling Center)

This workshop was convened to address the near- and far-term theoretical, observing, and modeling challenges in developing the next-generation coupled ocean-hurricane prediction system to become operational at the National Weather Service/National Centers for Environmental Prediction (NWS/NCEP) during 2007. A broad cross-section of researchers, numerical modelers, operational forecasters, and managers of governmental and university research programs gathered at NCEP in May 2005 to identify the scientific challenges associated with coupled models and to discuss potential avenues for addressing those challenges.

The context for this workshop, in both the NWS and US Weather Research Program (USWRP) frameworks was provided by Steve Lord (NCEP/Environmental Modeling Center) and Naomi Surgi (NCEP/EMC). Nick Shay (Rosenstiel School of Marine and Atmospheric Science/University of Miami) provided the overall charge of the workshop, which is to assess progress in the Air-Sea Interaction Community based on field programs and modeling studies sponsored by National Science Foundation, Office of Naval Research and National Oceanic and Atmospheric Administration through the USWRP Hurricane Landfall program, and to identify pressing scientific issues related to improving the physics of the air-sea interaction problem under strong winds in a hurricane and forecast models. Central to this theme is how the forecasting community can use these data sets to improve predictions from coupled ocean-atmosphere models. A clear objective of the workshop was to open the dialogue between the forecasting and research communities, and understand each group’s needs. Breakout groups were designed to maximize discussions between forecasters and researchers in addressing cross-cutting issues.

To set the scene for the challenges that lie ahead, the workshop began with a series of overview presentations from NCEP-related activities: Operational Modeling (Surgi), Wave Modeling (Tolman), Ocean Modeling (Lozano), Data Assimilation (Derber), and Coupled Modeling (Ginis-University of Rhode Island). A recurrent theme was that any potential improvements for intensity forecasts must not degrade track forecasts. The two breakout groups in the afternoon focused on model forecasting and required observations. The second day focused on research issues and their importance for forecasting issues: Oceanic Observations (Shay-UM), Atmospheric Boundary Layer Observations (Barnes-University of Hawaii), Ocean Modeling (Jacob-University of Maryland-Baltimore County), and Sea Spray Parameterization Schemes (Fairall-Environmental Technology Laboratory). In addition, two brief talks were given by Girton (University of Washington-Applied Physics Laboratory) and Terrill (Scripps Institution of Oceanography) on profiling floats that were deployed in the ONR-Coupled Boundary Layer Air-Sea Transfer (CBLAST) program. This session emphasized the need for continued observations to improve our understanding of physical processes and model parameterizations prior to implementation in forecast models. The afternoon breakout sessions focused on setting priorities and refining focused recommendations discussed in Plenary on the first day.

Steve Lord and Naomi Surgi welcomed the participants scientific meeting on the HWRF modeling system. The overall WRF program is to provide the community with a full-range of Numerical Weather Prediction (NWP) model capabilities that include such things as model software architecture and physics, data assimilation, input/output applications interfaces, testing and verification, documentation, standards, as well as a framework within which to develop and test new theories, models, and capabilities. The WRF is a community effort that provides a modeling infrastructure to run on a number of platforms such as the IBM at the National Center for Atmospheric Research (NCAR), Alpha-Linux system at the Forecast Systems Laboratory (FSL) , and Linux PC systems. The WRF-community based models will be available to both university and governmental organizations.

Within the overall WRF activities, the hurricane modeling system is a particular subset with unique characteristics and issues. The HWRF will be a coupled land/sea/atmosphere model. The HWRF will have a nested wave model coupled to the ocean model, and the land surface model will be coupled to hydrology to address the inland flooding problem. Development of this advanced hurricane modeling system will provide a unique opportunity for collaboration between the research and operational communities to take advantage of a variety of expertise that is necessary for the successful modeling system development.

Lord and Surgi challenged the workshop participants to consider the data needs for such a hurricane prediction system – including assimilation issues, as well as considering the configuration of the optimal coupled land/sea/atmosphere model for the hurricane forecast problem, and any roadblocks to achieving either the data or modeling goals. They charged the workshop participants with beginning to design, through working group sessions, a pathway to implementation of the next-generation hurricane forecast model.

  1. .0 Summary of ONR CBLAST April 2005 Workshop (P. Black)

The ONR CBLAST workshop was held 4-6 April 2005 in Miami and brought together scientists involved in the CBLAST field and modeling studies under the sponsorship of ONR and NOAA. The goal was to discuss the progress from the field campaigns and ongoing model studies largely focused on the Isabel (2003) and Frances (2004) cases. Presentations and summaries from the breakout groups can be found at: Two important findings from that workshop are:

1. Upper ocean floats such as ARGO-Solo and the electromagnetic APEX floats developed by SIO and WEBB and UW/APL, respectively, worked well in the hurricane environment by providing measurements of T, S, u, v (current) profiles, waves and acoustical properties for the first time; and

  1. Model studies are suggesting a new wind-wave coupling parameterization in which the behavior of the drag coefficient levels off between 28 to 32 m s-1 with a range of values ranging from 2.8 to 3.5 x 10-3 at 40 m s-1.

Direct measurements of the air-sea fluxes during the CBLAST storms were valid for winds to 30 m s-1 measured in the left-front quadrant of a tropical cyclone. It is encouraging that several studies are showing this leveling off, and not a continued increase of the drag coefficient with surface winds as suggested by earlier drag coefficient parameterizations. In a broader context, the ONR-CBLAST data in 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 ocean 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 focused field experiments.

3.0 Summary of Presentations

A. Forecaster Overviews:

A.1 Operational Modeling at NCEP (Naomi Surgi, EMC/NCEP)

Naomi Surgi gave an overview of the progress on the Weather and Research Forecast system for hurricane prediction, e.g., the HWRF scheduled for operational implementation at NCEP in 2007 when it will replace the current GFDL model. This advanced hurricane prediction system is being developed at the NWS/NCEP’s EMC to address the nation’s next-generation hurricane forecast problems. The HWRF will have the capability to fully address the intensity, structure, and rainfall forecast problem in addition to advancing wave and storm surge forecasts. As continued advancements in track prediction will remain an important focus of this prediction system, any new enhancements for physical processes to improve the intensity and structure forecasts cannot be implemented if they degrade track forecasting.

The HWRF will be a high resolution coupled air-sea-land prediction model with a movable nested grid and advanced physics. To address the totality of the hurricane forecast problems noted above, the HWRF will also include coupling to an ocean surface wave model that will eventually be coupled to a dynamic storm surge model. To facilitate these requirements, the HWRF will incorporate advanced air-sea physics of the atmospheric and oceanic boundary layers. Additionally, the land-surface component will also serve as input to hydrology and inundation models to address to hurricane-related inland flooding problem. For initialization of the hurricane core circulation, an advanced data assimilation technique is under development at EMC that will make use of real-time airborne Doppler radar data from NOAA’s high altitude G-IV jet to initialize the 3-D storm-scale structure.

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