In 2005, NOAA's Hurricane Research Division (HRD), part of the Atlantic Oceanographic and Meteorological Laboratory, began a multiyear experiment called the Intensity Forecasting Experiment (IFEX). By emphasizing a partnership among NOAA's HRD, Environmental Modeling Center (EMC), National Hurricane Center (NHC), Aircraft Operations Center (AOC), and National Environmental Satellite Data Information Service (NESDIS), IFEX represents a new approach for conducting hurricane field program operations. IFEX is intended to improve the prediction of tropical cyclone (TC) intensity change by 1) collecting observations that span the TC life cycle in a variety of environments; 2) developing and refining measurement technologies that provide improved real-time monitoring of TC intensity, structure, and environment; and 3) improving the understanding of the physical processes important in intensity change for a TC at all stages of its life cycle. This paper presents a summary of the accomplishments of IFEX during the 2005 hurricane season. New and refined technologies for measuring such fields as surface and three-dimensional wind fields, and the use of unmanned aerial vehicles, were achieved in a variety of field experiments that spanned the life cycle of several tropical cyclones, from formation and early organization to peak intensity and subsequent landfall or extratropical transition. Partnerships with other experiments during 2005 also expanded the spatial and temporal coverage of the data collected in 2005. A brief discussion of the plans for IFEX in 2006 is also provided.

This study is an observational analysis of the inner-core structure, sea surface temperature, outflow layer, and atmospheric boundary layer of an intense tropical cyclone whose intensity and structure is consistent with recent numerical and theoretical predictions of superintense storms. The findings suggest new scientific challenges for the current understanding of hurricanes. Unprecedented observations of the category-5 Hurricane Isabel (2003) were collected during 12–14 September. This two-part article reports novel dynamic and thermodynamic aspects of the inner-core structure of Isabel on 13 September that were made possible by analysis of these data. Here, a composite of the axisymmetric structure of the inner core and environment of Isabel is estimated using global positioning system dropwindsondes and in situ aircraft data. In Part II, an extreme wind speed observation on the same day is discussed in the context of this work. The axisymmetric data composite suggests a reservoir of high-entropy air inside the low-level eye and significant penetration of inflowing near-surface air from outside. The analysis suggests that the low-level air penetrating the eye is enhanced thermodynamically by acquiring additional entropy through interaction with the ocean and replaces air mixed out of the eye. The results support the hypothesis that this high-entropy eye air “turboboosts” the hurricane engine upon its injection into the eyewall clouds. Recent estimates of the ratio of sea-to-air enthalpy and momentum exchange at high wind speeds are used to suggest that Isabel utilized this extra power to exceed the previously assumed intensity upper bound by 10–35 m s−1 for the given environmental conditions. Additional study with other datasets is encouraged to further test the superintensity hypothesis.

An update of the progress achieved as part of the NOAA Intensity Forecasting Experiment (IFEX) is provided. Included is a brief summary of the noteworthy aircraft missions flown in the years since 2005, the first year IFEX flights occurred, as well as a description of the research and development activities that directly address the three primary IFEX goals: 1) collect observations that span the tropical cyclone (TC) life cycle in a variety of environments for model initialization and evaluation; 2) develop and refine measurement strategies and technologies that provide improved real-time monitoring of TC intensity, structure, and environment; and 3) improve the understanding of physical processes important in intensity change for a TC at all stages of its life cycle. Such activities include the real-time analysis and transmission of Doppler radar measurements; numerical model and data assimilation advancements; characterization of tropical cyclone composite structure across multiple scales, from vortex scale to turbulence scale; improvements in statistical prediction of rapid intensification; and studies specifically targeting tropical cyclogenesis, extratropical transition, and the impact of environmental humidity on TC structure and evolution. While progress in TC intensity forecasting remains challenging, the activities described here provide some hope for improvement.

Starting from 2003, a new typhoon surveillance program, Dropwindsonde Observations for Typhoon Surveillance near the Taiwan Region (DOTSTAR), was launched. During 2004, 10 missions for eight typhoons were conducted successfully with 155 dropwindsondes deployed. In this study, the impact of these dropwindsonde data on tropical cyclone track forecasts has been evaluated with five models (four operational and one research models). All models, except the Geophysical Fluid Dynamics Laboratory (GFDL) hurricane model, show the positive impact that the dropwindsonde data have on tropical cyclone track forecasts. During the first 72 h, the mean track error reductions in the National Centers for Environmental Prediction’s (NCEP) Global Forecast System (GFS), the Navy Operational Global Atmospheric Prediction System (NOGAPS) of the Fleet Numerical Meteorology and Oceanography Center (FNMOC), and the Japanese Meteorological Agency (JMA) Global Spectral Model (GSM) are 14%, 14%, and 19%, respectively. The track error reduction in the Weather Research and Forecasting (WRF) model, in which the initial conditions are directly interpolated from the operational GFS forecast, is 16%. However, the mean track improvement in the GFDL model is a statistically insignificant 3%. The 72-h-average track error reduction from the ensemble mean of the above three global models is 22%, which is consistent with the track forecast improvement in Atlantic tropical cyclones from surveillance missions. In all, despite the fact that the impact of the dropwindsonde data is not statistically significant due to the limited number of DOTSTAR cases in 2004, the overall added value of the dropwindsonde data in improving typhoon track forecasts over the western North Pacific is encouraging. Further progress in the targeted observations of the dropwindsonde surveillances and satellite data, and in the modeling and data assimilation system, is expected to lead to even greater improvement in tropical cyclone track forecasts.

Since 1997, the Tropical Prediction Center and the Hurricane Research Division have conducted operational synoptic surveillance missions with a Gulfstream IV-SP jet aircraft to improve numerical forecast guidance. Due to limited aircraft resources, optimal observing strategies for these missions must be developed. In the current study, the most rapidly growing modes are represented by areas of large forecast spread in the NCEP bred-vector ensemble forecasting system. The sampling strategy requires sampling of the entire target region with regularly spaced dropwindsonde observations. Three dynamical models were employed in testing the targeting and sampling strategies. With the assimilation into the numerical guidance of all the observations gathered during the surveillance missions, only the 12-h Geophysical Fluid Dynamics Laboratory Hurricane Model forecast showed statistically significant improvement. Assimilation of only the subset of data from the subjectively found fully sampled target regions produced a statistically significant reduction of the track forecast errors of up to 25% within the critical first 2 days of the forecast. This is comparable with the cumulative business-as-usual improvement expected over 18 yr.

A unique dataset of targeted dropsonde observations was collected during The Observing System Research and Predictability Experiment (THORPEX) Pacific Asian Regional Campaign (T-PARC) in the autumn of 2008. The campaign was supplemented by an enhancement of the operational Dropsonde Observations for Typhoon Surveillance near the Taiwan Region (DOTSTAR) program. For the first time, up to four different aircraft were available for typhoon observations and over 1500 additional soundings were collected. This study investigates the influence of assimilating additional observations during the two major typhoon events of T-PARC on the typhoon track forecast by the global models of the European Centre for Medium-Range Weather Forecasts (ECMWF), the Japan Meteorological Agency (JMA), the National Centers for Environmental Prediction (NCEP), and the limited-area Weather Research and Forecasting (WRF) model. Additionally, the influence of T-PARC observations on ECMWF midlatitude forecasts is investigated. All models show an improving tendency of typhoon track forecasts, but the degree of improvement varied from about 20% to 40% in NCEP and WRF to a comparably low influence in ECMWF and JMA. This is likely related to lower track forecast errors without dropsondes in the latter two models, presumably caused by a more extensive use of satellite data and four-dimensional variational data assimilation (4D-Var) of ECMWF and JMA compared to three-dimensional variational data assimilation (3D-Var) of NCEP and WRF. The different behavior of the models emphasizes that the benefit gained strongly depends on the quality of the first-guess field and the assimilation system.