John F. Gamache

Low-Wavenumber Structure and Evolution of the Hurricane Inner Core Observed by Airborne Dual-Doppler Radar

Abstract The asymmetric dynamics of the hurricane inner-core region is examined through a novel analysis of high temporal resolution, three-dimensional wind fields derived from airborne dual-Doppler radar. Seven consecutive composites of Hurricane Olivia’s (1994) wind field with 30-min time resolution depict a weakening storm undergoing substantial structural changes. The symmetric and asymmetric mechanisms involved in this transformation are considered separately. To zeroth order the weakening of the primary circulation is consistent with the axisymmetric vortex spindown theory of Eliassen and Lystad for a neutrally stratified atmosphere. Vertical shear, however, increased dramatically during the observation period, leading to a strong projection of the convection onto an azimuthal wavenumber 1 pattern oriented along the maximum vertical shear vector. Recent theoretical ideas elucidating the dynamics of vortices in vertical shear are used to help explain this asymmetry. The role of asymmetric vorticity d...

Eastern Pacific Hurricanes Jimena of 1991 and Olivia of 1994: The Effect of Vertical Shear on Structure and Intensity

Abstract Shear is a key inhibitor of tropical cyclone intensification. Although its signature is readily recognized in satellite imagery and theoretical or modeling studies provide some insight, detailed observations have been limited. Airborne radar and in situ observations in Hurricanes Jimena of 1991 and Olivia of 1994 are a step toward better understanding. Each storm was observed on two consecutive days. Initially, both had small eyes, 16–18-km radius, and maximum winds of ∼57 m s−1 over sea surface temperatures (SST) >28°C in easterly environmental shear. Jimena maintained constant intensity or weakened gradually for 2 days in 13–20 m s−1 easterly shear. Olivia intensified in 8 m s−1 shear on the first day. Overnight, the shear diminished to reverse and became westerly. On the second day, Olivia weakened as the shear increased to >15 m s−1 from the west, the storm moved over cooler SST, and became surrounded by dryer air. As convection weakened and the outer rainbands ceased to be effective barriers...

The Intensity Forecasting Experiment: A NOAA Multiyear Field Program for Improving Tropical Cyclone Intensity Forecasts

Abstract In 2005, NOAAs 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 NOAAs 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 d...

Comparison of Three Airborne Doppler Sampling Techniques with Airborne In Situ Wind Observations in Hurricane Gustav (1990)

Abstract Three different airborne Doppler radar sampling strategies were tested in Hurricane Gustav (1990) on 29 August 1990. The two new strategies were the fore-aft scanning technique (FAST) and airborne dual-platform Doppler sampling. FAST employs radar mans in cones pointing alternately fore and aft of the vertical plane that is perpendicular to the flight track. The airborne dual-platform sampling uses two Doppler radars, each aboard a separate aircraft. The Doppler radars scan strictly in the vertical plant normal to the flight track. The aircraft fly simultaneously along different, preferably perpendicular, tracks. The third strategy tested in Hurricane Gustav was single-platform sampling, which uses one Doppler radar on one aircraft that flies two consecutive, usually orthogonal, flight tracks. The antenna scans in the plane normal to the flight track. The third technique had been used previously in hurricanes and other disturbed weather. The rms differences between the aircraft in situ winds and ...