Atmospheric Boundary Layer Observations of Tropical Cyclones with the Imaging Wind and Rain Airborne Profiler

IWRAP, the Imaging Wind and Rain Airborne Profiler, is the first high-resolution dual-band airborne Doppler radar designed to study the inner core of Tropical Cyclones (TCs). IWRAP is currently operated from a National Oceanic and Atmospheric Administration (NOAA) WP-3D aircraft during missions through TCs and severe ocean storms. The system is designed to provide high-resolution, dual-polarized, multi-beam C- and Ku-band reflectivity and Doppler velocity profiles of the atmospheric boundary layer within the inner core precipitation bands of TCs and to study the effects precipitation has on ocean wind scatterometry as it applies to TCs. This dual-wavelength system also provides for the use of differential attenuation techniques to derive the rainfall rate and to characterize the drop size distribution (DSD) within TCs. IWRAP implements a very unique measurement strategy; it profiles simultaneously at four separate incidence angles (approximately 30, 35, 40 and 50 degrees) while conically scanning at 60 RPM.

This instrument has already demonstrated its capability to measure the wind field and the rainfall rate over the range of winds and rain rates usually present in tropical cyclones during the CBLAST field experiments as well as the NOAA/NESDIS Ocean and Rain experiments. One of the lessons learned during these experiments was the limitation in retrieving the wind field at the lowest part of the boundary layer, since the off-nadir looking geometry makes the ocean surface return impact the precipitation measurements from which the wind field is derived. To overcome this problem, the IWRAP instrument was recently equipped with a new data acquisition system that allows us to acquire raw data and, therefore, to separate both ocean and rain contributions through spectral processing. This has enabled us to derive the wind field virtually down to the ocean surface, and for the first time creates a unique opportunity to estimate the drag coefficient in very high wind conditions. Moreover, the unique ability to estimate the rain spectrum allows us to better understand and characterize the rain processes within the inner core of TCs.

The purpose of this paper is to show how these limitations have been overcome through the experience built from the datasets acquired during the CBLAST program with the new data acquisition system. A unique set of wind field results acquired during the hurricane season 2005 will be presented, as to illustrate how this instrument has evolved during and beyond the CBLAST program, enabling us to obtain high resolution atmospheric boundary layer wind fields within the inner core of TCs.