Convective Meteorology (Mesoscale Dynamics)

Improving High-Resolution Tropical Cyclone Prediction using a Cycled, GSI-based Hybrid Ensemble-Variational Data Assimilation System for HWRF with Vortex Scale Observations

Xu Lu
OU School of Meteorology

03 April 2015, 2:00 PM

National Weather Center, Room 5600
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK

Tropical cyclones (TCs) are among the most costly forms of natural disaster. While steady progress has been made in improving TC track forecasts, difficulties still remain. This can be attributed to deficiencies in models, deficiencies in current operational data assimilation systems and lack of effective utilization of observations.

A hybrid EnKF–variational data assimilation system was recently developed for HWRF based on the operational GSI. As an initial effort, the system was first tested with a single, fixed domain. Experiments have been conducted with cases during 2012-2013 hurricane seasons with the assimilation of airborne radar observations with a detailed study of hurricane Sandy (2012). The hybrid system was able to correct both the wind and mass fields in a dynamically and thermodynamically coherent fashion. Hybrid using self-consistent HWRF EnKF ensemble improved the track, minimum sea level pressure and Maximum wind forecasts relative to GSI-3DVar. The hybrid ingesting GFS ensemble improved upon the GSI 3DVar, but was still inferior to the hybrid ingesting HWRF’s own EnKF ensemble. The new hybrid system provided the largest positive impact of the airborne radar data.

Encouraged by the promising results, the HWRF hybrid system was further developed to include storm following, continuous cycling capability using a new moving strategy, dual resolution data assimilation capability, and vortex relocation capability. This system facilitates to address the following questions: a) what is the best DA configuration for high resolution TC data assimilation; b) what is the best way to utilize the observations especially vortex scale observation in hurricane DA. Experiments were conducted with four hurricane cases ( Arthur 2014, Edouard 2014, Sandy 2012 and Isaac 2012) and tail Doppler radar observations. It was found that a) the newly extended system improved in intensity forecast compared to the operational HWRF forecast; b) forecast was further improved by adding conventional prepbufr data in addition to the Tail Doppler Radar data; c) vortex relocation further improved the forecast especially when the vortex locations in the first guess ensemble was largely deviated from the observed location. These findings will be discussed in this seminar.

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Convective Meteorology (Mesoscale Dynamics) Seminar Series website