Influence of Environmental Water Vapor on Tropical Cyclogenesis

Improving the prediction of tropical cyclogenesis remains a goal of the tropical meteorology community. The processes that lead to genesis are yet to be entirely understood. Prediction of cyclogenesis is very important for business as many businesses, such as deep water drilling locations, require lead times of up to 7 days to make storm preparations. Storms that form near locations are of great concern as the full preparation lead time is not provided. Thus, it is essential to warn these customers prior to genesis of a potential storm.

There are many factors that affect tropical cyclogenesis. This study will focus on the role of environmental water vapor (WV). Previous studies (e.g. Ortt 2007) have established a relationship between WV and TC intensity and intensity change. In particular, intensifying TCs are on average located in environments with more environmental water vapor than are steady or weakening TCs. In addition, category 3-5 hurricanes are on average located within more moist environments than weaker TCs. The study also showed that the greatest moisture differences were located within the middle levels of the troposphere. However, the study only addressed TCs of tropical storm intensity or greater. No analysis was conducted of weaker TCs or disturbances. This study will extend the work from Ortt (2007) to evaluate whether the previously established relationship extends to the cyclogenesis phase. In particular, this study will evaluate the differences in environmental water vapor between systems that develop within 48 hours (developing) and those that do not (non-developing) tropical disturbances in the Atlantic basin from the 2010 and 2011 hurricane seasons. The working hypothesis from monitoring total precipitable water (TPW) images in real time during the seasons is that developing systems are located within more moist environments than non-developing disturbances. This study seeks to test this hypothesis and quantify any differences. This would allow for improved genesis forecasts, providing additional lead time to clients.

To perform the study, SSM/I TPW data from Remote Sensing Systems with a horizontal resolution of .25 degree will be used. The method of analysis will be similar to that used in Ortt (2007) with two differences. The first is that the study will calculate the mean TPW within 500 km of the center, instead of a 200-600km annulus. This is because disturbances are often disorganized, and the deep convection may not be located over the center. Therefore, there is not likely to be as much of a false signal due to convection as there is for developed TCs. The second is that the best track database cannot be used to determine the positions, since non-developing disturbances are not included. Instead, the center position will be from ImpactWeather disturbance statements and daily briefings. SSM/I TPW passes will be included only there is 65 percent or greater data coverage. The study will evaluate the differences between the developing and non developing disturbances by calculating the differences in the mean TPW, as well as differences in the respective TPW distributions.