Ocean Initialization System for Coupled Hurricane-Ocean Models and its Transition to HWRF Isaac Ginis and Richard Yablonsky University of Rhode Island 61st Interdepartmental Hurricane Conference 6 March, 2007
Why Proper Ocean Initialization in a Coupled Hurricane-Ocean Model is Needed for Accurate Hurricane Forecasting? • Wind-induced mixing of the upper ocean generates the cold wake by entraining cooler water from below • The entrainment rate is greatly dependent on the depth of the upper ocean warm layer.
Ocean Model Initialization Challenge Depth of 26oC isotherm on September 15, 2005 (courtesy of Michelle Mainelli, TPC) Accurate representation of mesoscale oceanic features is needed, such as the penetration of the Loop Current into the Gulf of Mexico and the shedding of Loop Current eddies.
Ocean Initialization Methods that can be Utilized in a Hurricane-Ocean Coupled Model • A continuously run ocean model with assimilation of in situ and/or remotely-sensed observationsthat is capable of predicting the position of the LC and LCEs. • A feature-based modeling procedure that is capable to adjust the background LC position and structure and directly insert LCEs with real-time in situ and/or remotely-sensed observations.
Feature-Based Initialization Procedure • This procedure is based on a feature modeling approach that allows a realistic simulation of the cross-frontal temperature, salinity and velocity of oceanic fronts and eddies. • We developed an innovative method of cross-frontal “sharpening” of the background temperature and salinity fields, which is guided by observed cross-sections obtained in specialized field experiments.
Identifying LC and LCEs in the Gulf of Mexico from Sea Surface Height • During the winter months, the LC and LCE positions are often identifiable from the SST field, however during the hurricane season the SST is nearly homogeneous in the Gulf. • The LC and LCEs can be identified by the difference in the sea surface height. The daily sea-surface height anomaly (SHA) is produced at Stennis Space Center in Mississippi by blending multiple ground tracks from satellites. • The altimetric SHA is used daily by the Tropical Prediction Center to calculate the depth of the 26°C isotherm and integrate it into the Statistical Hurricane Intensity Prediction Scheme (SHIPS)
Identifying LC and LCEs in the Gulf of Mexico from Sea Surface Height Daily NCEP SST Analysis SST GDEM September climatology Depth of 26C isotherm derived at TPC SHA from satellite altimetry September 15, 2005
AXBT Measurements In advance of hurricane Rita, on 15 September 2005, HRD scientists deployed AXBTs AXBTs provide accurate measurements of the ocean temperature to a depth of ~300-400 m. Temperature profiles for this study were provided by E. Uhlhorn at HRD.
Results of Feature-Based Model Initialization on Sep. 15, 2005 Model TPC Analysis Depth of 26oC isotherm
Evaluation of the Feature-based Initialization Scheme and Comparison with RSMAS HYCOM Prediction System • Evaluation is done by comparing the model and observed temperature profiles at each AXBT location. • RSMAS HYCOM dataset from 15 September 2005 was downloaded from the HYCOM consortium data server website. RSMAS HYCOM assimilates SST and satellite altimetry.
75-m Temp. (SSH/SST-assimilated) with AXBT locations 1 2 3 7 6 5 4
75-m Temp. (SSH/SST-assimilated) with AXBT locations 8 9 10 13 12 11
75-m Temp. (SSH/SST-assimilated) with AXBT locations 14 15 16 18 17
Ocean Heat Content Feature-based initialization OHC provides a quantitative measure of the heat energy available to an approaching tropical cyclone RSMAS HYCOM OHC is the integrated heat content from the ocean’s surface to 26oC
Evaluation of OHC at AXBT Locations TCHP rmse: Feature-based: 9.5 kJ cm-2 RSMAS HYCOM: 26.0 kJ cm-2
Operational implementation Feature-based initialization of the LC and LCEs was implemented operationally in the GFDL coupled model in 2006.
Examples of LC and LCE parameters input for the ocean model initialization TABLE 1. LC parameters. TABLE 2. LCE parameters.
Summary • Using a feature-based modeling approach that assimilates satellite-derived SSH and SST, a new ocean initialization has been developed and implemented operationally for the GFDL coupled model in 2006. • Feature-based initialization provides more accurate estimates of OHC than a continuously run three-dimensional ocean model (i.e. RSMAS HYCOM) • This initialization technique is being transitioned to the HWRF system for the 2007 hurricane season.