Tropical cyclone formation has been referred to as one of the great remaining mysteries in atmospheric dynamics (Emanuel 2005). It is a particularly difficult observational problem because it tends to occur in remote oceanic regions; most tropical waves never form TCs; and the gestation process can require many days, yet ultimately occur within only a few hours. Two recent field campaigns, T-PARC/TCS08 (Elsberry and Harr, 2008) and PREDICT, sought to examine developing and non-developing tropical disturbances in the context of new concepts that link the convective-scale and mesoscale dynamics (Dunkerton et al. 2009).
T-PARC (2008) focused on the Western North Pacific while PREDICT (2010) focused on the Atlantic basin. Typhoon Sinlaku became the most intensively observed tropical cyclone in the Pacific, with 22 research missions, 6 by the NRL P-3 with the ELDORA radar, spanning 12 days as it evolved from a fledgling tropical storm through typhoon intensity to extratropical transition. PREDICT was the first project that was able to sample tropical waves for several days and watch their structure organize or deteriorate, documenting the mesoscale circulation and thermodynamic environment. Several journal articles have already appeared from both T-PARC and PREDICT concerning tropical cyclone formation. The key findings about TC formation are the importance of humidification and thermodynamic stabilization of the inner cores (i.e. within about 200 km) of recirculation regions (pouches).
Logistical achievements in these projects were significant. T-PARC coordinated aircraft from multiple agencies using multiple bases of operation from a remote operations center. Operations literally were planned simultaneously on three continents through real-time web-based participatory video conferencing that had not been used for a field project on such a scale before. PREDICT was able to successfully vector the NSF/NCAR GV aircraft around developing tropical convective systems without the need for companion radar information from another aircraft by using advanced satellite derived products from the Naval Research Laboratory and the University of Wisconsin. The implication from operations in these projects is that TC investigations can effectively take place anywhere in the world.
While the prediction of TC track remains a challenge, recent attention has focused on improving and understanding intensity change within a system. RAINEX, T-PARC and ITOP (D'Asaro, 2011) were all related to various aspects of the intensification process. RAINEX obtained the highest-resolution observations to date of the formation of secondary eyewalls and of the detailed convective structure of rainbands. T-PARC observed the surprising reintensification of typhoon Sinlaku in strong vertical wind shear. Unprecedented observations of the upper ocean, especially during ITOP, will provide a reference point for future modeling studies of the upper ocean response to hurricanes, and documentation of the processes involved. Of note in ITOP was the widespread use of dropsondes and AXBTs to provide a continuous profile of the atmosphere and upper ocean.
The importance of special field observations for improving tropical cyclone prediction is being evaluated currently. Recent results by Aberson (2011) suggest a surprising positive influence of locally enhanced observations at remote locations. The analysis by Wu et al. (2012) pointed to improved track results of Sinlaku derived from an improvement in the vortex structure itself. Advanced data assimilation of field data in tropical cyclones is being used to quantify effective observation strategies for a variety of scales related to tropical cyclone prediction.
Numerical results foreshadow future analysis of field observations of TCs in a global context. Many questions remain about the role of tropical cyclones in the general circulation of the atmosphere. Even more questions persist about the effects of climate change on TCs? What observations can address these questions? Detailed observations of stratosphere-troposphere exchange above tropical cyclones are now possible, especially with the combined assets of aircraft like the GV and the new generation of NASA unmanned aircraft (e.g. Global Hawk). Recent theories (Emanuel and Rotunno 2012) link the details of tropical cyclone outflow to hurricane intensity. But the details of this outflow, especially its thermodynamics, are not typically well observed, yet they could be with enhanced airborne resources. The outflow and the mixing in the upper ocean are arguably the most important ways that TCs modify the earth system on large spatial and long time scales. These areas are particularly ripe for new investigation or detailed analysis of recently collected data.