Measuring environmental favorability for tropical cyclogenesis: Genesis parameters and point-downscaling

Predicting future variability of hurricane activity in a changing climate remains a difficult endeavor. The pioneering work of Gray showing the importance of tropical cyclogenesis parameters such as high sea surface temperature, low mean wind shear, and positive relative vorticity have been used in several empirically-derived genesis formulae with limited success. A separate approach involving “cyclone counting” in long-term simulations using global climate models (GCMs) are also important, however the limitations of coarse resolution and cumulus parameterizations result in significantly skewed GCM results. An alternative to these approaches merges the advantage of the genesis index approach, namely the inclusion of important cyclogenesis parameters, with the accuracy of high-resolution computer models. For each of the important indices known to be important for cyclogenesis, a ‘threshold' is found that is known to prevent an incipient vortex from reaching tropical storm strength, regardless of the favorability of the remaining indices. To find each threshold value, each index is simulated with the Weather, Research and Forecast (WRF) model by initializing the background environment using a “point-downscaling” technique. This technique uses a single sounding of temperature, wind speed, wind direction and humidity that represents the entire domain, with the exception of the weak incipient vortex. Although important details of the environment are removed such as horizontal wind shear and baroclinicity, preliminary results show this simplified approach improves on the genesis index technique and provides consistent results not seen by the more detailed ‘cyclone counting' technique.