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Thomas J. Galarneau Jr.

Abstract

Analysis of a predecessor rain event (PRE) over the Straits of Florida ahead of Tropical Cyclone (TC) Isaac on 25 August 2012 is presented. This PRE is unique compared to previously documented PREs in midlatitudes because it occurred over the oceanic subtropics and impacted the track of an approaching TC. The Isaac PRE developed in conjunction with a tropical moisture plume with precipitable water values over 60 mm that intersected a region of mid- and upper-level frontogenesis and warm air advection on the southeast flank of an upper-level trough. The PRE occurred in an environment with more abundant tropical moisture and weaker synoptic-scale forcing for ascent compared to the environments in which midlatitude PREs developed.

The Isaac PRE contributed to the fracture of the upper-level trough through negative potential vorticity advection by convectively driven divergent outflow. Fracture of the upper-level trough and midlevel cyclonic vorticity amplification associated with the PRE acted to steer Isaac south of Florida into the Gulf of Mexico. Forecasts from the National Centers for Environmental Prediction–Global Forecast System (NCEP–GFS) initialized at 0000 UTC 21–24 August 2012 failed to predict the PRE and as a result recurved TC Isaac over Florida and the eastern Gulf of Mexico rather than continued Isaac on a northwest course toward southeast Louisiana. Vorticity inversion and detailed diagnosis of the NCEP–GFS TC track forecast initialized at 0000 UTC 22 August is also presented to assess the relationship between the PRE and TC Isaac’s track.

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Thomas J. Galarneau Jr. and Thomas M. Hamill

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Analysis and diagnosis of the track forecasts for Tropical Cyclone (TC) Rita (2005) from the Global Ensemble Forecast System (GEFS) reforecast dataset is presented. The operational numerical weather prediction guidance and GEFS reforecasts initialized at 0000 UTC 20–22 September 2005, 2–4 days prior to landfall, were all characterized by a persistent left-of-track error. The numerical guidance indicated a significant threat of landfall for the Houston, Texas, region on 24 September. The largest mass evacuation in U.S. history was ordered, with the evacuation resulting in more fatalities than TC Rita itself. TC Rita made landfall along the Texas–Louisiana coastal zone on 24 September. This study utilizes forecasts from the GEFS reforecast and a high-resolution regional reforecast. The regional reforecast was generated using the Advanced Hurricane Weather Research and Forecasting Model (AHW) with the GEFS reforecasts providing the initial and boundary conditions. The results show that TC Rita’s track was sensitive to errors in both the synoptic-scale flow and TC intensity. Within the GEFS reforecast ensemble, the nonrecurving members were characterized by a midlevel subtropical anticyclone that extended too far south and west over the southern United States, and an upper-level cutoff low west and anticyclone east of TC Rita that were too weak. The AHW regional reforecast ensemble further highlighted the role of intensity and steering-layer depth in TC Rita’s track. While the AHW forecast was initialized with a TC that was too weak, the ensemble members that were able to intensify TC Rita more rapidly produced a better track forecast because the TCs followed a deeper steering-layer flow.

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Christopher A. Davis and Thomas J. Galarneau Jr.
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Thomas J. Galarneau Jr. and Xubin Zeng

Abstract

A synoptic analysis and soil moisture (SM) sensitivity experiment is conducted on the record-breaking rainstorm in Texas associated with Hurricane Harvey on 26–30 August 2017. The rainstorm occurred as the moist tropical air mass associated with Harvey was lifted along a frontogenetical near-surface coastal baroclinic zone beneath the equatorward entrance region of an upper-level jet streak. The weak steering winds in Harvey’s environment, allowing Harvey to remain nearly stationary, were associated with a deformation steering flow pattern characterized by a trough to the north and flanking ridges to the west and east. This pattern has occurred with other notable tropical cyclone rainstorms along the Gulf Coast, except in Harvey’s case it contributed to the collocation of deep tropical moisture and a persistent midlatitude lifting mechanism. Motivated by marked increases in SM during the rainstorm, a suite of six numerical simulations is used to test the sensitivity of the Harvey rainstorm (track, intensity, and rainfall) to varying SM. These simulations include dry, realistic, and wet SM conditions and an additional three runs with the initial SM held constant throughout the simulations. The results showed that track and prelandfall intensity were most sensitive to SM. Decreased SM resulted in the 1) development of an anticyclone in the southern plains that steered Harvey farther southwest in Texas, and 2) interruption in the intensification of Harvey in the Gulf of Mexico as dry air in the Yucatan Peninsula was entrained into Harvey’s circulation, contributing to a weaker system at landfall. Implications of these findings on the evolution of tropical systems are also discussed.

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F. Martin Ralph and Thomas J. Galarneau Jr.

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Between North America’s Sierra Madre and Rocky Mountains exists a little-recognized terrain “gap.” This study defines the gap, introduces the term “Chiricahua Gap,” and documents the role of easterly transport of water vapor through the gap in modulating summer monsoon precipitation in southeastern Arizona. The gap is near the Arizona–New Mexico border north of Mexico and is approximately 250 km wide by 1 km deep. It is the lowest section along a 3000-km length of the Continental Divide from 16° to 45°N and represents 80% of the total cross-sectional area below 2.5 km MSL open to horizontal water vapor transport in that region. This study uses reanalyses and unique upper-air observations in a case study and a 15-yr climatology to show that 72% (76%) of the top-quartile (decile) monsoon precipitation days in southeast Arizona during 2002–16 occurred in conditions with easterly water vapor transport through the Chiricahua Gap on the previous day.

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James P. Fowler and Thomas J. Galarneau Jr.

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The aim of this study is to examine the development of four tropical cyclones (TCs) in the North Atlantic basin in late August and early September 2010. This period is of interest because four consecutive easterly waves emerged from West Africa and resulted in a multiple TC event (MTCE) over the North Atlantic. The first two TCs—Danielle and Earl—quickly developed into TCs east of 40°W and eventually intensified into major hurricanes. Conversely, the last two TCs—Fiona and Gaston—developed more slowly reaching only weak tropical storm intensity at their peak. The close proximity and differing evolution of these four TCs provides a unique opportunity to examine how these TCs interacted with each other and their surrounding environment, which influenced their development as they moved westward across the North Atlantic. The results showed that concurrent extratropical cyclogenesis events over the western and eastern North Atlantic and the recurvature of TC Danielle produced increased meridional flow over the midlatitude North Atlantic. This increased meridional flow resulted in subsynoptic-scale regions of increased vertical wind shear in the subtropics, which delayed Earl’s development and led to Fiona’s demise. Additionally, increased meridional flow in midlatitudes contributed to anomalous drying of the subtropics. This dry air was entrained into Gaston’s circulation leading to reduced convection and weakening. These TC–TC and TC–environment interactions highlight the difficult challenge of forecasting TC genesis and position posed by MTCEs in a rapidly evolving synoptic-scale flow.

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Thomas J. Galarneau Jr. and Christopher A. Davis

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This paper reports on the development of a diagnostic approach that can be used to examine the sources of numerical model forecast error that contribute to degraded tropical cyclone (TC) motion forecasts. Tropical cyclone motion forecasts depend upon skillful prediction of the environment wind field, and by extension, the synoptic-scale weather systems nearby the TC. While previous research suggests that the deep-layer mean (DLM) steering flow typically approximates the actual TC motion, it is shown that the motion of even mature TCs can depart from the DLM steering flow. An optimal environmental steering flow is defined, which varies the vertical extent of the steering layer and the radius over which TC vorticity and divergence are removed.

Errors in predicted TC motion are quantified using a diagnostic equation that accounts for not only differences in the synoptic-scale flow, but also differences in the depth and radius used to define the steering flow. Differences in the latter two parameters are interpreted in terms of errors in predicted TC structure or errors in proximate mesoscale flow features. Results from an analysis of 24-h forecasts from the Advanced Hurricane Weather Research and Forecasting Model during the 2008–10 North Atlantic TC seasons show that forecast motion errors are dominated by errors in the environment wind field. Contributions from other terms are occasionally large and are interpreted from a vorticity perspective. The utility of this new diagnostic equation is that it can be used to assess TC motion forecasts from any numerical modeling system.

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Thomas J. Galarneau Jr. and Morris L. Weisman

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Convection-allowing simulations of two warm seclusion cyclones are used to elucidate the vorticity dynamics that contribute to intensification of these systems. The rapidly intensifying oceanic “bomb” cyclone on 4–5 January 1989 and the super derecho on 8 May 2009 are the subject of this study. While these systems occupy different spatial scales, they both acquire characteristics of a warm seclusion cyclone. The aim of this study is to compare the basic structure and determine the dynamics driving increases in system-scale vertical vorticity during the intensification of these systems. Results from a vorticity budget show that system-scale stretching and the lateral transport of vertical vorticity to the cyclone center contribute to increases of system-scale low-level vertical vorticity during the intensification of the oceanic cyclone. The intercomparison of the oceanic cyclone and the super derecho shows that the relative contributions to increases in system-scale vertical vorticity by stretching and tilting as a function of height differ among the two cases. However, the lateral transport of vertical vorticity to the cyclone center is a key contributor to increases in low-level system-scale vertical vorticity for both cases. We hypothesize that this process may be common among a wide array of intense cyclonic systems across scales ranging from warm seclusion extratropical cyclones to some mesoscale convective systems.

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Christopher A. Davis and Thomas J. Galarneau Jr.

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Simulations of two cases of developing mesoscale convective vortices (MCVs) are examined to determine the dynamics governing the origin and vertical structure of these features. Although one case evolves in strong vertical wind shear and the other evolves in modest shear, the evolutions are remarkably similar. In addition to the well-known mesoscale convergence that spins up vorticity in the midtroposphere, the generation of vorticity in the lower troposphere occurs along the convergent outflow boundary of the parent mesoscale convective system (MCS). Lateral transport of this vorticity from the convective region back beneath the midtropospheric vorticity center is important for obtaining a deep column of cyclonic vorticity. However, this behavior would be only transient without a secondary phase of vorticity growth in the lower troposphere that results from a radical change in the divergence profile favoring lower-tropospheric convergence. Following the decay of the nocturnal MCS, subsequent convection occurs in a condition of greater relative humidity through the lower troposphere and small conditional instability. Vorticity and potential vorticity are efficiently produced near the top of the boundary layer and a cyclonic circulation appears at the surface.

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Russ S. Schumacher and Thomas J. Galarneau Jr.

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Global ensemble forecasts from The Observing System Research and Predictability Experiment (THORPEX) Interactive Grand Global Ensemble (TIGGE) are used to quantify the magnitude of moisture transport into North America ahead of recurving tropical cyclones (TCs). Two cases in which a predecessor rain event (PRE) occurred ahead of the recurving TC—Erin (2007) and Ike (2008)—are analyzed, with ensemble members correctly predicting TC recurvature contrasted from those predicting the TC to weaken or turn southward. This analysis demonstrates that TC-related moisture transport can increase the total water vapor in the atmosphere over North America by 20 mm or more, and that the moisture transport takes place both in the boundary layer and aloft. The increased moisture does not always correspond to increased rainfall in the ensemble forecasts, however, as the location and strength of baroclinic zones and their attendant secondary circulations that can lift this moist air are also crucial to the development of heavy rains.

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