Examination of environmental characteristics associated with secondary eyewall formation and their implications on intensity forecasts

The formation of a second eyewall around a preexisting smaller eyewall is generally the precursor to an eyewall replacement cycle. These cycles often cause dramatic and rapid changes in intensity and are very important to recognize in a forecasting setting, particularly when a hurricane is approaching land. Presently, there is no comprehensive forecast algorithm for secondary eyewall formation (SEF).

An SEF climatology was developed by analyzing over 4,500 SSMI, SSMIS, TRMM, and AQUA microwave images in combination with aircraft, radar, and additional satellite data (when available). Using these data, nearly 150 SEF cases were identified for 176 major Northern Hemisphere tropical cyclones (TCs) from 1997-2006. Over half of all major TCs (V ≥ 100 kt) exhibited at least one case of SEF.

In an effort to add skill to the climatology, environmental parameters and satellite imagery were examined for the 12 hours leading up to the formation of a concentric eyewall structure. When compared to the non-SEF periods for major TCs the differences between many parameters were found to be significant. These environmental features were applied to Bayesian methodology to produce a probability of SEF for each TC. Preliminary results indicate that the Bayesian method provides significant skill in forecasting the likelihood of SEF.