Wind Radii Research Articles

Application of Surface-Adjusted GOES Low-Level Cloud-Drift Winds in the Environment of Atlantic Tropical Cyclones. Part II: Integration into Surface Wind Analyses

The Cooperative Institute for Meteorological Satellite Studies at the University of Wisconsin—Madison recently (1997 season) began providing real-time Geostationary Operational Environmental Satellite (GOES) low-level cloud-drift winds in the vicinity of tropical cyclones on an experimental basis to the National Oceanic and Atmospheric Administration's (NOAA) Hurricane Research Division (HRD). The cloud-drift winds are derived from sequential high-resolution GOES visible channel imagery. These data were included in many of HRD's real-time tropical cyclone surface wind objective analyses, which were sent to NOAA's National Hurricane Center and the Central Pacific Hurricane Center on an experimental basis during the 1997–2001 hurricane seasons. These wind analyses were used to support the forecasters' tropical cyclone advisories and warnings. The satellite wind observations provide essential low-level coverage in the periphery of the tropical cyclone circulation where conventional in situ observations (e.g., ships, buoys, and Coastal-Marine Automated Network stations) are often widely spaced or nonexistent and reconnaissance aircraft do not normally fly. Though winds derived from microwave channels on polar orbiting satellites provide valuable surface wind data for HRD surface wind analyses, their swath coverage and orbital passes are limited spatially and temporally. GOES low-level visible (GLLV) winds offer nearly continuous spatial and temporal coverage in the western Atlantic and eastern Pacific basins. The GLLV winds were extrapolated to the surface using a planetary boundary layer model developed at HRD. These surface-adjusted satellite data were used in real-time surface wind analyses of 1998 Hurricane Georges, as well as in poststorm analyses of 1996 Hurricane Lili and 1997 Tropical Storm Claudette. The satellite observations often helped to define the spatial extent of the 17.5 m s−1 (34 kt) surface wind radii and also redefined the 25.7 m s−1 (50 kt) wind radius for one case. Examples of the impact of these data on real-time hurricane surface wind fields provided to the NHC will be discussed.

Operational Prediction of Typhoon Track at KMA using Barotropic Adaptive-grid Typhoon Simulation (BATS) Model.

Since the adoption of the barotropic adaptive-grid typhoon simulation model in the KMA numerical weather prediction system in 1997, the model has been increasingly recognized to be quite useful in forecasting tropical cyclone tracks. This paper presents the description of the model and history of the model upgrades and the corresponding performance in track predictions of tropical cyclones. There have been two major model upgrades. The first upgrade is on the modification of the steering current. The model is run for 12 hours to catch the initial tendency of the vortex motion in the model and evaluate its departure from the observed one. The vector difference is added to the original steering current and then the model is rerun using the modified steering current. The improvement is mainly for the short-term forecast. Secondly, we let the circular bogus area vary with respect to the 30 kt wind radius (referred to as the storm size) reported from the RSMC Tokyo. For tropical cyclones whose size is larger than 440 km we let the circular bogus area of 1.2 times the 30 kt wind radius from the RSMC tropical cyclone bulletin. For storms smaller than 440 km, we use a constant bogus area of 600 km radius. This provides us a great improvement over the incumbent operational version of the model. The improvement ranges from at least 10% and up to 15% during the entire forecast period of 60 hours.

Numerical modelling and computer visualization of the storm surge in and around the Croatan-Albemarle-Pamlico estuary system produced by hurricane Emily of August 1993

Hurricane Emily unleashed its fury on the Outer Banks of North Carolina on 31 August 1993. Storm surge was a major cause of damage along the Outer Banks. The highest flood water (11-11.5ft) occurred in the Buxton area near Cape Hatteras, North Carolina. It was reported that this flood water was from storm surges along the sound side of the barrier islands. An experimental forecast was conducted for this event in real time using Croatan-Albemarle-Pamlico estuary systems (CAPES) storm surge prediction model developed at North Carolina State University (NCSU). It uses as input parameters the projected hurricane track, minimum center pressure, maximum sustained wind speed and radius of maximum wind speed provided by the National Hurricane Center (NHC). The forcing of the model also includes fresh water input from sound system rivers, and of coastal waters intruding into the sound via Ocracoke, Hatteras and Oregon inlets. The predicted maximum surge along the sound side of the Outer Banks was within 85-90% of the post-storm highwater-mark survey data provided by the U.S. Geological Survey (USGS). Albeit, an after the fact simulation using the post-storm analysis of the track of Emily provided by the NHC, the maximum storm surge along the sound side of the Outer Bancks predicted by the model was within 95-98% of the maximum highwater mark data. The location of the predicted maximum surge for both pre and first model runs was near Cape Hatteras, which agreed well with USGS's survey data. We conclude that the CAPES storm surge model is capable of providing accurate storm surge forecasts in and around the CAPES, but such forecasts are sensitive to not only the observed storm size and intensity but in particular, the projected storm track.

Synchrotron radiation from the winds of O supergiants - TB = 10 to the 7.6th K at 60 stellar radii

Results are presented on VLBI measurements of the nonthermal radio components around two O supergiant stars: Cyg OB2 No. 9 and HD 167971. The measurements were used to characterize the brightness temperature of the emission and to measure the size of compact 5-10 mJy components in these stars, reported by Bieging et al. (1989). The sizes found for the 5-10 mJy components are consistent with the free-free wind radii, indicating that the compact companions are not the sources of nonthermal radiation. Results suggest that there is a small fractional population (10 to the -4th to 10 to the -7th) of ultrarelativistic electrons (Teff of about 10 to the 11th K) coexisting with the stellar wind, which emit optically thin synchrotron radiation. This is in agreement with the synchrotron model of White (1985). 21 refs.

Typhoon Structure as Revealed by Aircraft Reconnaissance. Part II: Structural Variability

Abstract This is the second of two papers on the structure of northwest Pacific tropical cyclones as revealed by U.S. Air Force aircraft reconnaissance. This paper describes the varying structure of the tropical cyclone's outer-radius wind profile in relation to its inner-core intensity or minimum sea level pressure (MSLP) and eye-size characteristics. We explore this inner- to outer-radius structural relationship and its variability for the full range of cyclone central pressures, outer-core (1°–2.5° radius) wind strengths, and for radial extent of 15 m s−1 (30 kt) and 25 m s−1 (50 kt) surface winds. Results show that outer-radius wind strength and inner-core intensity can vary greatly and that there is only a weak relationship between these parameters. However, if information is available on whether an eye-wall cloud exists and what the size of the eye is, then a significant reduction in the wide variance between MSLP and outer wind radius is observed.

The Hα profile in Zeta Puppis

The Hα profile in the spectrum of Zeta Puppis is shown in Figure 1 (Conti and Frost 1977). It is of the P Cygni type, with a violet displaced absorption component that extends to about 300 km s−1 from line center. An unusual feature is the broad wing emission which can be seen out to 1500 km s−1 on both sides of the line. Of the roughly 2.5 Å equivalent width of Hα, 1.8 Å is due to velocity displacements |δv| > 300 km s−1. The presence of wing emission shortward of the violet absorption edge has been noted before in a Wolf-Rayet star, where it is attributed to non-coherent electron scattering (Castor, Smith and Van Blerkom 1970). In the present case, an optical depth in electrons of nearly 1.5 is required to produce the extensive wings by scattering a normal P Cygni profile. This exceeds by a factor of three the electron scattering optical thickness through the entire envelope, assuming published values of the mass loss rate, terminal velocity of the wind and photospheric radius. Thus an alternative explanation is required.