Improving Ocean State Initialization in Coupled Tropical Cyclone Forecast Models

Coupled ocean-atmosphere models do not accurately forecast tropical cyclone (TC) intensity at present, partly due to inadequate representation of the ocean. Intensity depends in part on sea surface temperature (SST) since latent heat provided by sea surface evaporation is the primary TC energy source. Assuming constant SST, the additional power required by an intensifying storm is supplied by the increase in evaporation rate as wind speed increases. This tendency for the ocean to support intensification will be reversed if SST cools rapidly enough beneath the storm. As a result, SST along a projected TC path cannot be used alone to predict whether the ocean will promote or inhibit intensification. The thickness and temperature (heat content) of the upper-ocean warm layer along with the entrainment rate of colder water into the ocean surface mixed layer are important factors determining SST cooling rate and the net oceanic contribution to intensity change. A key factor for improving TC intensity forecasts by coupled models is accurate initialization of the state of the ocean. We explore this issue in the northwest Caribbean and southeast Gulf of Mexico for September 2002 just prior to hurricane Isidore. Initialization from climatology is inadequate because: (1) climatologies do not accurately represent the location of variable oceanographic features associated with different heat content such as the warm Loop Current; and (2) very anomalous conditions were present in the upper ocean prior to Isidore. In particular, the warm layer was much thicker than normal in the northwest Caribbean and in the Loop Current as documented by pre-storm Airborne Expendable Conductivity Temperature and Depth (AXCTD) profiles. We evaluate one Global Ocean Data Assimilation Experiment (GODAE) nowcast product for providing the initial ocean state, specifically the Atlantic Ocean nowcast produced by Planning Systems, Inc. and the Naval Research Laboratory that assimilates satellite altimetry (with vertical information projection) and SST into the HYbrid Coordinate Ocean Model (HYCOM). Comparison of nowcast temperature and salinity fields to aircraft observations and to Navy Modular Ocean Data Assimilation System (MODAS) analysis fields demonstrates that the present HYCOM-GODAE product accurately represents the location of important features such as the Loop Current, but does not reproduce the anomalous ocean state as accurately as the MODAS analysis. Salinity is inaccurately represented in both HYCOM and MODAS due to insufficient observations. Significant improvement is expected in the next generation HYCOM-GODAE product because it will use a more sophisticated assimilation procedure that also assimilates subsurface ocean observations. This study demonstrates the critical importance of sufficient observational coverage to initialize, evaluate, and improve the ocean component of TC prediction models for improving intensity forecast

US GODAE: Global Ocean Prediction with the HYbrid Coordinate Ocean Model (HYCOM)

During the past five to ten years, a broad partnership of institutions under NOPP sponsorship has collaborated in developing and demonstrating the performance and application of eddy-resolving, real-time global- and basin-scale ocean prediction systems using the HYbrid Coordinate Ocean Model (HYCOM). The partnership represents a broad spectrum of the oceanographic community, bringing together academia, federal agencies, and industry/commercial entities, and spanning modeling, data assimilation, data management and serving, observational capabilities, and application of HYCOM prediction system outputs. In addition to providing real-time, eddy-resolving global- and basin-scale ocean prediction systems for the US Navy and NOAA, this project also offered an outstanding opportunity for NOAA-Navy collaboration and cooperation, ranging from research to the operational level. This paper provides an overview of the global HYCOM ocean prediction system and highlights some of its achievements. An important outcome of this effort is the capability of the global system to provide boundary conditions to even higherresolution regional and coastal models

Estimates of surface drifter trajectories in the equatorial Atlantic: a multi-model ensemble approach

International audienceWe compared the estimates of surface drifter trajectories from 1 to 7 days in the equatorial Atlantic over an 18-month period with five eddying ocean general circulation model (OGCM) reanalyses and one observational product. The cumulative distribution of trajectory error was estimated using over 7,000 days of drifter trajectories. The observational product had smaller errors than any of the individual OGCM reanalyses. Three strategies for improving trajectory estimates using the ensemble of five operational ocean analysis and forecasting products were explored: two methods using a multi-model ensemble estimate and also spatial low-pass filtering. The results were insensitive to the method used to create the ensemble estimates, and by most measures, the results were better than the observational product. Comparison of relative skill of the various OGCM reanalyses suggested promising avenues for exploration for further improvements: forcing with higher frequency wind stress and quality control of input data. One of the lowest horizontal resolution OGCMs, with 1/4° longitude horizontal resolution, made the best trajectory estimates. The individual OGCMs were dominated by errors at spatial scales smaller than about 100 to 200 km, i.e., less than the local deformation radius. But buried in those errors were valuable signals that could be retrieved by combining all the OGCM velocity fields to produce a multi-model ensemble-based estimate. This estimate had skill down to spatial scales about 75 km. Results from this study are consistent with previous work showing that ensemble-mean forecast skill is superior to individual forecasts

Approaches to Data Assimilation within GODAE

Abstract – The main purpose of this paper is to review some key issues and challenges for ocean data assimilation in the perspective of GODAE. This includes the “high-resolution challenge ” and simultaneous trend towards the use of dynamically-consistent methods, the problem of modelling multivariate error covariances in “simple ” OI-type schemes, verification of physical consistency, internal consistency checks via the calculation of residual (analysis minus observation) statistics, further extraction of information by adaptive algorithms, effect of assimilation on unobserved variables, forecasting skill, error dynamics in “advanced” schemes, as well as considerations on the performance of the observational network. GODAE is expected to boost the inter-comparison exercises between assimilating systems, as well as to provide a forum to discuss the areas where progress is needed in the next years

Airborne Ocean Surveys of the Loop Current Complex From NOAA WP‐3D in Support of the Deepwater Horizon Oil Spill

This chapter contains sections titled:IntroductionAirborne Ocean SurveysObserved Oceanic VariabilityModels and Data AssimilationSummar

US GODAE: Global Ocean Prediction with the HYbrid Coordinate Ocean Model (HYCOM)

During the past five to ten years, a broad partnership of institutions under NOPP sponsorship has collaborated in developing and demonstrating the performance and application of eddy-resolving, real-time global- and basin-scale ocean prediction systems using the HYbrid Coordinate Ocean Model (HYCOM). The partnership represents a broad spectrum of the oceanographic community, bringing together academia, federal agencies, and industry/commercial entities, and spanning modeling, data assimilation, data management and serving, observational capabilities, and application of HYCOM prediction system outputs. In addition to providing real-time, eddy-resolving global- and basin-scale ocean prediction systems for the US Navy and NOAA, this project also offered an outstanding opportunity for NOAA-Navy collaboration and cooperation, ranging from research to the operational level. This paper provides an overview of the global HYCOM ocean prediction system and highlights some of its achievements. An important outcome of this effort is the capability of the global system to provide boundary conditions to even higher-resolution regional and coastal models