Thermodynamic precursors to rapidly intensifying Atlantic Basin hurricanes

A relationship between unusually high equivalent potential temperature (theta e) measurements collected by reconnaissance aircraft in the core of tropical cyclones, and subsequent episodes of rapid intensification (R.I.), was proposed by Dunnavan (1981) and Sikora (1976) years ago using data gathered in a large sample of Western Pacific typhoons. Since 1987, all such operational reconnaissance missions have been flown in the either the Atlantic Basin or the Central and Eastern Pacific (with only a limited number of flights off the western Mexican coast or in what have been remarkably quiet waters near Hawaii (where there has been no reconnaissance data gathered within an intense hurricane since 1994)).

Dunnavan used 700 mb theta e values measured by WestPac reconnaissance aircraft. In the Atlantic Basin, there is also some data gathered at 850 mb prior to episodes of R.I. in addition to the 700 mb level. This study will attempt to determine whether such a relationship exists in the Atlantic data, and perhaps more importantly, whether a prognostic tool can be developed to help develop some skill in predicting R.I., one of the most difficult challenges in the field of TC intensity forecasting.

A brief examination will propose processes that might be involved in the gathering of thermodynamic potential that manifests itself in a fashion that can be readily detected during routine reconnaissance. It is clear that many instances of R.I. are characterized by gradually increasing flight level temperature and dew point in the eye accompanied by increasing organization of the surrounding eyewall (often facilitated by decreasing vertical shear) as the MSLP gradually drops. At some point in this process, there is an explosion of subsidence that often quickly clears cloudiness out of the eye of all but a low stratocumulus deck as the inversion is driven down close to the ocean surface and the flight level dew point depression above the inversion reaches values of 20C or more. This extreme subsidence event is accompanied by MSLP falls of 30 millibars or more within very short time intervals.

If a hurricane is still well below its Maximum Potential Intensity (MPI), is showing advanced inner core organization with high values of theta e, but has yet to undergo this pronounced 700 mb drying event, it can be thought of as a “loaded gun” with the thermodynamic potential in place for one or two millibars of pressure fall possible for every degree of drying that has yet to occur.

Rita (2005) is perhaps the most extreme case of this phenomenon on record in the Atlantic Basin with a 700 mb dew point depression of as much as 34 degrees accompanying its R.I. down to 895 mb MSLP (a temperature of 31C with a corresponding dew point of -3C). Another interesting aspect that will be examined is the increase in near-surface theta e values in the boundary layer accompanying these events. Dropsondes released prior to and during the R.I. of Rita into the eye show boundary layer dew points as high as 29C. Accumulation of extremely warm, moist air below the inversion in the eye has been proposed as a means to fuel adjacent convective towers along the inner edge of the eyewall.