Derek Ortt and Shuyi S. Chen University of Miami Rosentiel School of Marine and Atmospheric Science (RSMAS) (305) 361-4071 dortt@rsmas.miami.edu, schen@rsmas.miami.edu

Dry Air Intrusions and Related Tropical Cyclone Intensity Changes

For years, it has been assumed that tropical cyclones (TCs) interacting with dry air environments are more likely to weaken than tropical cyclones in moist environments. However, there has been little quantification regarding what constitutes a dry air environment or how much a tropical cyclone can be expected to weaken once it encounters a dry environment. To quantify this, total integrated precipitable water from the satellite SSMI data has been analyzed, along with data from the GPS dropsondes from the NOAA G-IV aircraft. The SSMI data has a spatial resolution of ¼ degree. The dropsondes collect data every ½ second. The sample set of storms contains selected Atlantic Ocean tropical cyclones since 1987. Storms analyzed from 1987 to 1997 were analyzed solely with SSMI data, while since 1998, the dropsonde data has also been available for selected TCs. The sample set of storms in which dropsondes are available is small compared to the overall set. The purpose of using dropsondes to supplement the SSMI data is that the dropsondes provide a vertical profile of the entire troposphere at a selected location, indicating at which layer any possible dry air may be located and how deep the dry layer is. Furthermore, the dropsondes have been analyzed based upon which storm relative quadrant they were deployed in. The overall sample set was then further stratified into tropical storms and hurricanes. Preliminary results indicate that in order for dry air to weaken the TCs, the air needs to be located within 150km of the center. This can happen either due to a dry air intrusion, which is typical of hurricanes (e.g. Georges 1998 and Humberto 1995), or moving into an area of very dry air, more typical of tropical storms (e.g. Fran 1990, Dolly 2002). The tropical storms that encounter the dry air often dissipate, while the hurricanes that have been analyzed at the present time experience significant 24-hour pressure rises, indicative of significant weakening. This analysis will be continued and quantitative descriptions for the effects of dry air intrusion on TC intensity change will be provided.