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Robert L. Creasey and Russell L. Elsberry

Abstract

A method is developed to calculate the zero-wind center (ZWC) position from a sequence of Yankee High Density Sounding System (HDSS) dropwindsondes deployed during a high-altitude overpass of a tropical cyclone. The approach is similar to the Willoughby and Chelmow technique in that it utilizes the intersections of bearings normal to the wind directions across the center to locate the ZWC position. Average wind directions over 1-km layers are calculated from the accurate global positioning system (GPS) latitude–longitude positions as the HDSS sonde falls from the 60 000-ft flight level of the NASA WB-57 to the ocean surface. An iterative procedure is used to also account for the storm translation, which is necessary to put these high-frequency HDSS observations into a storm-relative coordinate system. The Tropical Cyclone Intensity (TCI-15) mission into Hurricane Joaquin on 4 October 2015 is examined here. The ZWC positions from two center overpasses indicate the vortex tilts from 1- to 10-km elevation and rotates cyclonically with height.

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Patricia M. Pauley, Robert L. Creasey, Wallace L. Clark, and Gregory D. Nastrom

Abstract

This study examines the consistency between VHF horizontal wind measurements and those interpolated from routine objective analyses. First, the agreement between the two U components and between the two V components measured on opposing beams (here referred to as the beam-to-beam intercomparison) by the Flatland 49.8-MHz wind profiler is examined to determine the beam-to-beam consistency and relative precision of this radar. This part of the study demonstrates the ability of this technique to detect system problems affecting only one radar beam and provides a benchmark for comparison with radar systems operating near the Front Range of the Rockies and for the comparison in the second part of this study. This second comparison is between the Flatland observations and the spatially smooth winds from the National Meteorological Center's (NMC) regional objective analysis for July through November 1990. The location of the Flatland profiler near Champaign-Urbana, Illinois, is free of significant orographic features, in contrast to the proximity to the Colorado Rockies of many of the radars employed in earlier studies.

The beam-to-beam intercomparison is presented in terms of the mean and standard deviation of the differences between the measurements made on opposing beams. The Flatland difference standard deviations of about 0.8 m s−1 are roughly one-third of those for radars in the lee of the Rocky Mountains, reflecting reduced vertical velocities. However, the mean differences are approximately −0.25 m s−1 for both the U and V components, consistent with the tropospheric monthly mean downward motion of 2–6 cm s−1 indicated in the Flatland vertical beam measurements since its construction, including the period of this study. In fact, when the data were stratified into periods with and without precipitation based on estimates of latent heating from the NMC data, the precipitation periods showed standard deviations of about 1.3 m s−1, with mean differences two to three times that for nonprecipitation cases. This behavior is consistent with larger downward velocities during precipitation, whether from clear-air or hydrometeor scatterers. Thus, these vertical-motion biases, which the authors believe are of atmospheric origin (whether bulk motion or reflectivity effects), must be accounted for in long-term climatological studies.

Finally, for the Flatland–NMC comparison, 4-h averages of Flatland winds were chosen to better correspond to the spatially smooth NMC winds. The correlation coefficients, larger than 0.95, indicate very good agreement, but not as good as the 0.99 found in the beam-to-beam intercomparison. The larger 2.3 m s−1 difference standard deviations are similar to those found in studies comparing profiler and rawinsonde winds near the Front Range of the Rockies, indicating the applicability of the Taylor hypothesis implicit in this comparison of the 4-h temporally averaged Flatland winds and the spatially avenged NMC-analyzed winds. The consistency between these two datasets implies that increases in accuracy of objective analyses may result more from the increased time resolution of the profiler data rather than from an inherent increase in accuracy of the observations.

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Eric A. Hendricks, Russell L. Elsberry, Christopher S. Velden, Adam C. Jorgensen, Mary S. Jordan, and Robert L. Creasey

Abstract

The objective in this study is to demonstrate how two unique datasets from the Tropical Cyclone Intensity (TCI-15) field experiment can be used to diagnose the environmental and internal factors contributing to the interruption of the rapid decay of Hurricane Joaquin (2015) and then a subsequent 30-h period of constant intensity. A special CIMSS vertical wind shear (VWS) dataset reprocessed at 15-min intervals provides a more precise documentation of the large (~15 m s−1) VWS throughout most of the rapid decay period, and then the timing of a rapid decrease in VWS to moderate (~8 m s−1) values prior to, and following, the rapid decay period. During this period, the VWS was moderate because Joaquin was between large VWSs to the north and near-zero VWSs to the south, which is considered to be a key factor in how Joaquin was able to be sustained at hurricane intensity even though it was moving poleward over colder water. A unique dataset of High Definition Sounding System (HDSS) dropwindsondes deployed from the NASA WB-57 during the TCI-15 field experiment is utilized to calculate zero-wind centers during Joaquin center overpasses that reveal for the first time the vortex tilt structure through the entire troposphere. The HDSS datasets are also utilized to calculate the inertial stability profiles and the inner-core potential temperature anomalies in the vertical. Deeper lower-tropospheric layers of near-zero vortex tilt are correlated with stronger storm intensities, and upper-tropospheric layers with large vortex tilts due to large VWSs are correlated with weaker storm intensities.

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James D. Doyle, Jonathan R. Moskaitis, Joel W. Feldmeier, Ronald J. Ferek, Mark Beaubien, Michael M. Bell, Daniel L. Cecil, Robert L. Creasey, Patrick Duran, Russell L. Elsberry, William A. Komaromi, John Molinari, David R. Ryglicki, Daniel P. Stern, Christopher S. Velden, Xuguang Wang, Todd Allen, Bradford S. Barrett, Peter G. Black, Jason P. Dunion, Kerry A. Emanuel, Patrick A. Harr, Lee Harrison, Eric A. Hendricks, Derrick Herndon, William Q. Jeffries, Sharanya J. Majumdar, James A. Moore, Zhaoxia Pu, Robert F. Rogers, Elizabeth R. Sanabia, Gregory J. Tripoli, and Da-Lin Zhang

Abstract

Tropical cyclone (TC) outflow and its relationship to TC intensity change and structure were investigated in the Office of Naval Research Tropical Cyclone Intensity (TCI) field program during 2015 using dropsondes deployed from the innovative new High-Definition Sounding System (HDSS) and remotely sensed observations from the Hurricane Imaging Radiometer (HIRAD), both on board the NASA WB-57 that flew in the lower stratosphere. Three noteworthy hurricanes were intensively observed with unprecedented horizontal resolution: Joaquin in the Atlantic and Marty and Patricia in the eastern North Pacific. Nearly 800 dropsondes were deployed from the WB-57 flight level of ∼60,000 ft (∼18 km), recording atmospheric conditions from the lower stratosphere to the surface, while HIRAD measured the surface winds in a 50-km-wide swath with a horizontal resolution of 2 km. Dropsonde transects with 4–10-km spacing through the inner cores of Hurricanes Patricia, Joaquin, and Marty depict the large horizontal and vertical gradients in winds and thermodynamic properties. An innovative technique utilizing GPS positions of the HDSS reveals the vortex tilt in detail not possible before. In four TCI flights over Joaquin, systematic measurements of a major hurricane’s outflow layer were made at high spatial resolution for the first time. Dropsondes deployed at 4-km intervals as the WB-57 flew over the center of Hurricane Patricia reveal in unprecedented detail the inner-core structure and upper-tropospheric outflow associated with this historic hurricane. Analyses and numerical modeling studies are in progress to understand and predict the complex factors that influenced Joaquin’s and Patricia’s unusual intensity changes.

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