Use of Doppler Radar Data for Tropical Storm Initialization

Jin-Luen Lee

NOAA Forecast Systems Laboratory

A technique is presented for initialization of a tropical storm vortex using horizontal wind velocities through a deep layer of the atmosphere obtained from doppler radar. The technique, which is a balance initialization approach, uses two new innovations. The first is the use of the mesoscale vorticity equation to diagnose the vertical velocity and divergent wind. The divergent wind inferred dynamically and the rotational wind derived from observations constitute total wind fields, including the vertical velocity. The second is the use of the Bounded Derivative Initialization to obtain two dynamic constraints, one each for gravity and sound waves. With the fast waves controlled, a nonhydrostatic model can be initialized to allow a smooth and balanced start. A four-dimensional data assimilation system based on Newtonian relaxation is used to generate the dynamically consistent heating and clouds. The results are tested using the 1.5 km resolution MM5 model with radar data from Hurricane Danny as it approached the Gulf Coast in 1997. Numerical experiments show positive radar data impacts on track and intensity forecasts, in particular, substantial improvements on wind fields, can be achieved with the initialization scheme. Future work to improve the initialization and extend it to larger scales using satellite data will be discussed.