Applications of Satellite-Derived Ocean Measurements to Tropical Cyclone Intensity Forecasting

Abstract : Sudden tropical cyclone (TC) intensification has been linked with high values of upper ocean heat content contained in mesoscale features, particularly warm ocean eddies, provided that atmospheric conditions are also favorable. Although understanding of air-sea interaction for TCs is evolving, this manuscript summarizes some of the current work being carried out to investigate the role that the upper ocean plays in TC intensification and the use of ocean parameters in forecasting TC intensity.


Applications of Satellite-Derived Ocean Measurements to tropical cyclone intensity Forecasting B y G u S tAV O G O N i , M A r k D E M A r i A , J O h N k N A F F, c h A r l E S S A M p S O N , i S A A c G i N i S , F r A N c i S B r i N G A S ,
A l B E r t O M AV u M E , c h r i S l A u E r , i .-i .l i N , M .M .A l i , pA u l S A N D E r y, S i lVA N A r A M O S -B u A r q u E ,  (DeMaria et al., 2005).Leipper and Volgenau (1972) first recognized the importance of ocean thermal structure in TC intensification.Although sea surface temperature (SST) plays a role in TC genesis, the ocean heat content contained between the sea surface and the depth of the 26°C isotherm (D26), also referred to as tropical cyclone heat potential (TCHP), has been shown to play a more important role in TC intensity changes (Shay et al., 2000).TCHP shows high spatial

NOrth AtlANtic OcEAN
An operational satellite-altimetry-based TCHP analysis was implemented at the National Oceanic and Atmospheric Administration (NOAA) National Hurricane Center (NHC) in 2004 (Mainelli et al., 2008).This approach uses sea surface height anomaly fields derived from altimetry and historical hydrographic observations in a statistical regression analysis to determine the depth of the main thermocline, usually the 20°C isotherm in tropical regions (Goni et al., 1996) (Mainelli et al., 2008).A validation performed on 685 Atlantic SHIPS forecasts from [2004][2005][2006][2007] shows that the average improve-   (Powell et al., 1998) to resolve the inner-core structure of the storm.Investigation of global ocean variability and, in particular, of sea height and SST has become increasingly important.Regional variability of these parameters indicates, for example, that TCHP time series in the Gulf of Mexico exhibit an increase of 0.20 ± 0.05 kJ cm -2 per year since 1993 (Goni, 2008).This increase in TCHP values (Figure 4) could be related to a more pronounced intrusion of the LC into the Gulf of Mexico and to the generation of a larger number of rings.It is known that the warm rings in the Gulf contribute to TC intensification; hence, further investigation is required to determine whether this trend also contributes to additional intensification occurrences.Prediction System (TCLAPS) forecasting model (Davidson and Weber, 2000), the Ocean-Atmosphere-Sea-Ice-Soil (OASIS) coupler (Valcke et al., 2003), and a regional version of the BLUElink>  SST fields to 419 km using SHA fields (Ali et al., 2007).

FuturE wOrk
The current open ocean observing system was mainly designed for climate and not for TC intensification studies.
Although there are efforts underway to rEFErENcES

Figure 1 .
Figure 1.Global map showing the tracks of tropical cyclones (category 1 and above) during the period 2000-2008, with green circles indicating where they formed.The background color is the satellite-derived mean sea surface temperature during the same years, for June through November in the Northern hemisphere, and November through April in the Southern hemisphere.
and temporal variability associated with oceanic mesoscale features.TC intensification has been linked with high values of TCHP contained in these mesoscale features, particularly warm ocean eddies, provided that atmospheric conditions are also favorable.Because sustained, in situ ocean observations alone cannot resolve global mesoscale features and their vertical thermal structures, different indirect approaches and techniques are used to estimate TCHP.Most of these techniques use sea surface height observations derived from satellite altimetry, a parameter that provides information on upper ocean dynamics and verticalABStr Act.Sudden tropical cyclone (TC) intensification has been linked with high values of upper ocean heat content contained in mesoscale features, particularly warm ocean eddies, provided that atmospheric conditions are also favorable.Although understanding of air-sea interaction for TCs is evolving, this manuscript summarizes some of the current work being carried out to investigate the role that the upper ocean plays in TC intensification and the use of ocean parameters in forecasting TC intensity.thermal structure.This article highlights the importance of collecting a variety of data, particularly satellite-derived observations, for tropical cyclone intensification studies.
ment of SHIPS due to the inclusion of TCHP and Geostationary Operational Environmental Satellite (GOES) SST data is as much as 3% for the 96-h forecast (Figure 2, left).Nearly all improvements at the longer forecast intervals are due to TCHP because that input is averaged along the storm track.Although not as large as the sample of just the category 5 hurricanes, this result indicates that TCHP input improved the operational SHIPS forecasts, especially at the longer forecast intervals.Altimetry observations are also used to initialize the ocean component of a coupled hurricane prediction model with fields extracted from data-assimilative ocean hindcasts generated as part of the Global Ocean Data Assimilation Experiment (GODAE).These hindcasts rely heavily on altimetry to properly locate mesoscale features, such as ocean currents and eddies.Halliwell et al. (2008) examined this initialization approach in ocean model simulations of the response to hurricane Ivan (2004) in the Northwest Caribbean and Gulf of Mexico.This simulation was driven by quasi-realistic forcing generated by blending fields extracted from the Navy Coupled Ocean/Atmospheric Mesoscale Prediction System atmospheric model with higher-resolution fields obtained from the NOAA/Atlantic Oceanographic Halliwell et al. (2008) concluded that for the ocean component of the Hurricane Weather Research and Forecast System (HWRF;Surgi et al., 2006) to correctly forecast intensity, it must correctly forecast the rate of SST cooling in the coupled forecast runs.This capability can only be realized if ocean features are correctly initialized in the ocean model.Yablonsky and Ginis (2008) created a new feature-based ocean initialization procedure to account for spatial and temporal variability of mesoscale oceanic features in the Gulf of Mexico, including the Loop Current (LC) and eddies.Using this methodology, nearreal-time maps of sea surface height and/or D26 derived from altimetry are used to adjust the position of the LC and insert these eddies into the background climatological ocean temperature field prior to the passage of a hurricane.For the 2008 Atlantic hurricane season, the full version of this procedure was implemented in the NOAA Geophysical Fluid Dynamics Laboratory (GFDL) and HWRF models, which can also assimilate real-time, in situ data such as airborne-dropped expendable bathythermograph profiles.GFDL coupled hurricane-ocean model sensitivity experiments for selected hurricanes were run with and without altimeter data assimilation to evaluate the impact of assimilating mesoscale oceanic features on both the SST cooling under the storm and the subsequent change in storm intensity.For Hurricane Katrina (2005), the presence of the LC and of a warm ring, as given by the assimilated altimeter data (Figure 3, left panel), reduced SST cooling along the hurricane track and allowed the storm to become more intense (Figure 3, right panel).This assimilation improved the actual storm's intensity forecast with respect to that obtained without assimilating the altimetry fields.

Figure 2 .Figure 3
Figure 2. (left) percent improvement of the 2004-2007 operational Statistical hurricane intensity prediction Scheme (ShipS) forecasts for the Atlantic sample of over-water cases west of 50°w due to the inclusion of input from altimetry-derived tropical cyclone heat potential (tchp) and GOES-derived sea surface temperature fields.(right) percent improvement resulting from the use of tchp information in the Statistical typhoon intensity prediction Scheme (StipS).This homogeneous comparison between StipS with tchp and StipS without tchp is based on forecasts of 63 western North pacific tropical cyclones.The number of cases used at each forecast time is given at the top of each bar.
ocean forecasting system.Preliminary results show that TC intensity is sensitive to ocean heat content, and that fluctuations in the lowest central pressure (LCP) between 10-20 hPa is related to variability in mesoscale upper ocean thermal structure and feedback into the storm via air-sea heat fluxes.Use of more accurate SSTs from the reanalysis is also important in improving TC intensity forecasting.The coupled simulation produced a less-intense and faster-moving storm than the uncoupled simulation due to feedback of cool SSTs.In the simulation, the rapid rise in LCP after 50 h occurred when the storm made landfall over Cape York Peninsula, Australia.Further work is being done with the CLAM system to couple a wave model and to improve ocean initialization and model physics at the air-sea interface and in the oceanic mixed layer.The link between TC intensification and TCHP has also been identified in the North Indian Ocean, showing that TCs intensify (dissipate) after traveling over anticyclonic (cyclonic) eddies.Results obtained for tropical cyclone 01A in the Arabian Sea in 2002 show a correlation of 0.92 between the intensity and the sea height anomaly (SHA) values under the track of the tropical cyclone.However, the correlation value is only 0.07 between

Figure 4 .
Figure 4. time series showing the monthly residuals (anomalies with the seasonal cycle removed) of tropical cyclone heat potential values in the Gulf of Mexico from 1993-2008.These values exhibit an increase that may be partly related to a more western intrusion of the loop current into the Gulf of Mexico as revealed by contours of the jet of this current and associated rings obtained from altimetry observations for 1996 and 2004 (maps in the upper panels).
analyses of cyclone track data in the Mozambique Channel for 1994-2007 by author Mavume and colleagues allowed identification of 15 intense cyclones with landfall in Mozambique or Madagascar.There is no doubt that high TCHP values in the region are important.However, an assessment of these 15 TCs did not show a clear tendency for intensification over warm eddies, but there was intensification over cyclonic eddies, similar to what was found in the Northwest Pacific Ocean.It was hypothesized that improved knowledge of the vertical density profile, and not just of temperature, is necessary to further understand the ocean's role in TC intensification because of the effect of salinity on density and mixed-layer depth.The ocean's role in TC intensification can be investigated globally using high- improve this system to investigate TC genesis regions, current sustained in situ ocean observations (e.g., XBTs, Argo floats, moorings, surface drifters) do not fully support TC intensification studies.Therefore, indirect methodologies that employ satellite observations and numerical modeling are being used to monitor the upper ocean for TC intensification research.Studies performed in all ocean basins indicate an ocean role in TC intensification that still needs to be adequately investigated and quantified.Future work will include detailed analysis of other upper ocean parameters, such as heat content and mean temperature in the mixed layer to different depths or isotherms, including isotherms below 26°C.Models based on statistical methodologies show a correlation between upper ocean thermal structure and TC intensification, where mesoscale ocean features with minimum TCHP values of ~ 50 kJ cm -2 may contribute to intensification of strong storms.It is clear that improved estimates of TCHP in ocean and ocean-atmosphere coupled models are critical for improvement in TC intensity forecasting.Results from some of the current efforts presented here highlight the importance of continuous support for altimetric missions able to resolve mesoscale features.Several observational research efforts are also underway to better understand TCs' boundary layers and air-sea interaction.For example, one of the goals of the Intensity Forecast Experiment is to develop and refine technologies to improve real-time monitoring of TC intensity, structure, and environment(Rogers et al., 2006).Other observational efforts reveal the importance of innercore SST with regard to intensification(Cione and Uhlhorn, 2003).Improved numerical model and data assimilation algorithms, and understanding of the ocean's role in TC intensification will help set up the requirements for observations through the execution of an Observations System Simulation Experiment.Improved TC monitoring will also aid in storm surge prediction, whose errors decrease with correct forecasting of TC tracks and intensities.AckNOwlEDGEMENtS Some of the work of GG, MDM, JK, CS, and FB was supported by NOAA/ NESDIS through the Research to Operations Program.Part of GG's work was done during a rotational assignment at the NOAA/IOOS Program Office.Research and development of OceanMAPS and CLAM is supported by the BLUElink> project, Australian Bureau of Meteorology, CSIRO, and the Royal Australian Navy.The Indian National Centre for Ocean Information Services sponsored the project on North Indian Ocean tropical cyclone studies.Analysis carried out by PSV Jagadeesh and Sarika Jain in this project is grate- Ali is Scientist and Head, Oceanography Division, National Remote Sensing Centre, Hyderabad, India.Paul Sandery is a member of the ocean forecasting team, Center for Australian Weather and Climate Research, Melbourne, Australia.
Atmospheric Administration (NOAA) Atlantic Oceanographic and Meteorological Laboratory (AOML), Miami, FL, USA.Mark DeMaria is Chief, NOAA National Environmental Satellite, Data, and Information Service (NESDIS), Regional and Mesoscale Meteorology Branch, Fort Collins, CO, USA.John Knaff is Meteorologist, NOAA NESDIS Regional and Mesoscale Meteorology Branch, Fort Collins, CO, USA.Charles Sampson is Meteorologist, Marine Meteorology Division, Naval Research Laboratory, Monterey, CA, USA.Isaac Ginis is Professor of Oceanography, University of Rhode Island, Graduate School of Oceanography, RI, USA.Francis Bringas is Research Associate, Cooperative Institute for Marine and Atmospheric Sciences, University of Miami, Miami, FL, USA.Alberto Mavume is Chair, Marine Science and Oceanography Group, Eduardo Mondlane University, Maputo, Mozambique.Chris Lauer provides computer programming and technical support to the NOAA National Hurricane Center, Tropical Prediction Center, Miami, FL, USA.I.-I.Lin is Associate Professor, Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan.M.M. Springs, MD, USA.Eric Chassignet is Professor and Director, Center for Ocean-Atmospheric Prediction Studies, Florida State University, Tallahassee, FL, USA.George Halliwell is Research Scientist, NOAA AOML, Miami, FL, USA, and Professor, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL, USA.