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Joshua B. Wadler, Joseph J. Cione, Jun A. Zhang, Evan A. Kalina, and John Kaplan

Abstract

The relationship between deep-layer environmental wind shear direction and tropical cyclone (TC) boundary layer thermodynamic structures is explored in multiple independent databases. Analyses derived from the tropical cyclone buoy database (TCBD) show that when TCs experience northerly component shear, the 10-m equivalent potential temperature θe tends to be more symmetric than when shear has a southerly component. The primary asymmetry in θe in TCs experiencing southerly component shear is radially outward from 2 times the radius of maximum wind speed, with the left-of-shear quadrants having lower θe by 4–6 K than the right-of-shear quadrants. As with the TCBD, an asymmetric distribution of 10-m θe for TCs experiencing southerly component shear and a symmetric distribution of 10-m θe for TCs experiencing northerly component shear was found using composite observations from dropsondes. These analyses show that differences in the degree of symmetry near the sea surface extend through the depth of the boundary layer. Additionally, mean dropsonde profiles illustrate that TCs experiencing northerly component shear are more potentially unstable between 500- and 1000-m altitude, signaling a more favorable environment for the development of surface-based convection in rainband regions. Analyses from the Statistical Hurricane Intensity Prediction Scheme (SHIPS) database show that subsequent strengthening for TCs in the Atlantic Ocean basin preferentially occurs in northerly component deep-layer environmental wind shear environments whereas subsequent weakening preferentially occurs in southerly component wind shear environments, which further illustrates that the asymmetric distribution of boundary layer thermodynamics is unfavorable for TC intensification. These differences emphasize the impact of deep-layer wind shear direction on TC intensity changes that likely result from the superposition of large-scale advection with the shear-relative asymmetries in TC structure.

Significance Statement

This research investigates how the direction of the winds surrounding the storm impacts the strength of a tropical cyclone. Analyses from this study illustrate that when the winds come from the south the atmospheric boundary layer has a cool and dry side along with a warm and moist side. When the large-scale winds come from the north, temperature and moisture conditions are more uniform throughout the boundary layer. Consequently, results from tropical cyclone climatology show that winds observed to come from the north favor subsequent intensification. These relationships illustrate that tropical cyclone structure and intensity are directly influenced by their surrounding environments and that knowledge of the wind environment could help to improve future forecasts of tropical cyclone intensity change.

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Tyler K. West and W. James Steenburgh

Abstract

The Sea of Japan (SOJ) coast and adjoining orography of central Honshu, Japan, receive substantial snowfall each winter. A frequent contributor during cold-air outbreaks (CAOs) is the Japan Sea polar airmass convergence zone (JPCZ), which forms downstream of the highland areas of the Korean Peninsula (i.e., the Korean Highlands), extends southeastward to Honshu, and generates a mesoscale band of precipitation. Mesoscale polar vortices (MPVs) ranging in horizontal scale from tens (i.e., meso-β-scale cyclones) to several hundreds of kilometers (i.e., “polar lows”) are also common during CAOs and often interact with the JPCZ. Here we use satellite imagery and Weather Research and Forecasting Model simulations to examine the formation, thermodynamic structure, and airflow of a JPCZ that formed in the wake of an MPV during a CAO from 2 to 7 February 2018. The MPV and its associated warm seclusion and bent-back front developed in a locally warm, convergent, and convective environment over the SOJ near the base of the Korean Peninsula. The nascent JPCZ was structurally continuous with the bent-back front and lengthened as the MPV migrated southeastward. Trajectories illustrate how air–sea interactions and flow splitting around the Korean Highlands and channeling through low passes and valleys along the Asian coast affect the formation and thermodynamic structure of the JPCZ. Contrasts in airmass origin and thermodynamic modification over the SOJ affect the cross-JPCZ temperature gradient, which reverses in sign along the JPCZ from the Asian coast to Honshu. These results provide new insights into the thermodynamic structure of the JPCZ, which is an important contributor to hazardous weather over Japan.

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George R. Alvey III, Michael Fischer, Paul Reasor, Jonathan Zawislak, and Robert Rogers

Abstract

Dorian’s evolution from a weak, disorganized tropical storm to a rapidly intensifying hurricane is documented through a unique multiplatform synthesis of NOAA’s P-3 tail-Doppler radar, airborne in situ data, and Météo-France’s Martinique and Guadeloupe ground radar network. Dorian initially struggled to intensify with a misaligned vortex in moderate midtropospheric vertical wind shear that also allowed detrimental impacts from dry air near the inner core. Despite vertical wind shear eventually decreasing to less than 5 m s−1 and an increasingly symmetric distribution of stratiform precipitation, the vortex maintained its misalignment with asymmetric convection for 12 h. Then, as the low-level circulation (LLC) approached St. Lucia, deep convection near the LLC center dissipated, the LLC broadened, and precipitation expanded radially outward from the center temporally coinciding with the diurnal cycle. Convection then developed farther downtilt within a more favorable, humid environment and deepened appreciably at least partially due to interaction with Martinique. A distinct repositioning of the LLC toward Martinique was induced by a spinup of a mesovortex into a small, compact LLC. It is hypothesized that this somewhat atypical reformation event and the repositioning of the vortex into a more favorable environment, farther from detrimental dry midtropospheric air, increased its favorability for the rapid intensification that subsequently ensued. Although the repositioning resulted in tilt reducing to less than the scale of the vortex itself, the preexisting broad mid- to upper-level cyclonic envelope remained intact with continued misalignment observed between the midlevel center and repositioned LLC even during the early stages of rapid intensification.

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Thomas M. Hamill, Jeffrey S. Whitaker, Anna Shlyaeva, Gary Bates, Sherrie Fredrick, Philip Pegion, Eric Sinsky, Yuejian Zhu, Vijay Tallapragada, Hong Guan, Xiaqiong Zhou, and Jack Woollen

Abstract

NOAA has created a global reanalysis dataset, intended primarily for initialization of reforecasts for its Global Ensemble Forecast System, version 12 (GEFSv12), which provides ensemble forecasts out to +35-days lead time. The reanalysis covers the period 2000–19. It assimilates most of the observations that were assimilated into the operational data assimilation system used for initializing global predictions. These include a variety of conventional data, infrared and microwave radiances, global positioning system radio occultations, and more. The reanalysis quality is generally superior to that from NOAA’s previous-generation Climate Forecast System Reanalysis (CFSR), demonstrated in the fit of short-term forecasts to the observations and in the skill of 5-day deterministic forecasts initialized from CFSR versus GEFSv12. Skills of reforecasts initialized from the new reanalyses are similar but slightly lower than skills initialized from a preoperational version of the real-time data assimilation system conducted at the higher, operational resolution. Control member reanalysis data on vertical pressure levels are made publicly available.

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Rachel C. North, Marion P. Mittermaier, and Sean F. Milton

Abstract

Monitoring precipitation forecast skill in global numerical weather prediction (NWP) models is an important yet challenging task. Rain gauges are inhomogeneously distributed, providing no information over large swathes of land and the oceans. Satellite-based products, on the other hand, provide near-global coverage at a resolution of ∼10–25 km, but limitations on data quality (e.g., biases) must be accommodated. In this paper the stable equitable error in probability space (SEEPS) is computed using a precipitation climatology derived from the Tropical Rainfall Measuring Mission (TRMM) TMPA 3B42 V7 product and a gauge-based climatology and then applied to two global configurations of the Met Office Unified Model (UM). The representativeness and resolution effects on an aggregated SEEPS are explored by comparing the gauge scores, based on extracting the nearest model grid point, with those computed by upscaling the model values to the TRMM grid and extracting the TRMM grid point nearest the gauge location. The sampling effect is explored by comparing the aggregate SEEPS for this subset of ∼6000 locations (dictated by the number of gauges available globally) with all land points within the TRMM region of 50°N and 50°S. The forecast performance over the oceanic areas is compared with performance over land. While the SEEPS computed using the two different climatologies should never be expected to be identical, using the TRMM climatology provides a means of evaluating near-global precipitation using an internally consistent dataset in a climatologically consistent way.

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Alexander J. DesRosiers, Michael M. Bell, and Ting-Yu Cha

Abstract

The landfall of Hurricane Michael (2018) at category-5 intensity occurred after rapid intensification (RI) spanning much of the storm’s lifetime. Four Hurricane Hunter aircraft missions observed the RI period with tail Doppler radar (TDR). Data from each of the 14 aircraft passes through the storm were quality controlled via a combination of interactive and machine-learning techniques. TDR data from each pass were synthesized using the Spline Analysis at Mesoscale Utilizing Radar and Aircraft Instrumentation (SAMURAI) variational wind retrieval technique to yield three-dimensional kinematic fields of the storm to examine inner-core processes during RI. Vorticity and angular momentum increased and concentrated in the eyewall region. A vorticity budget analysis indicates that the tendencies became more axisymmetric over time. In this study, we focus in particular on how the eyewall vorticity tower builds vertically into the upper levels. Horizontal vorticity associated with the vertical gradient of tangential wind was tilted into the vertical by the eyewall updraft to yield a positive vertical vorticity tendency inward atop the existing vorticity tower, which is further developed locally upward and outward along the sloped eyewall through advection and stretching. Observed maintenance of thermal wind balance from a thermodynamic retrieval shows evidence of a strengthening warm core, which aided in lowering surface pressure and further contributed to the efficient intensification in the latter stages of this RI event.

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Nicholas H. Grosfeld, Shayne McGregor, and Andréa S. Taschetto

Abstract

Cutoff low pressure systems have been found to be the synoptic system responsible for the majority of rainfall in southeastern Australia during the cool season (April–October inclusive). Meanwhile, rainfall in southeastern Australia at the seasonal and interannual scale is known to be related to remote climate drivers, such as El Niño–Southern Oscillation, the Indian Ocean dipole, and the southern annular mode. In this study, a new automated tracking scheme to identify synoptic scale cutoff lows is developed, and then applied to 500-hPa geopotential height data from the NCEP1 and ERA-Interim reanalyses, to create two databases of cool-season cutoff lows for southeastern Australia for the years 1979–2018 inclusive. Climatological characteristics of cutoff lows identified in both reanalyses are presented and compared, highlighting differences between the NCEP1 and ERA-Interim reanalyses over the Australian region. Finally, cool-season and monthly characteristics of cutoff low frequency, duration, and location are plotted against cool-season and monthly values of climate driver indices (oceanic Niño, dipole mode, and Antarctic Oscillation indices), to identify any evidence of linear correlation. Correlations between these aspects of cutoff low occurrence and the remote drivers were found to be statistically significant at the 95% level for only a single isolated month at a time, in contrast to results predicted by previous works. It is concluded that future studies of cutoff low variability over SEA should employ identification criteria that capture systems of only upper-level origin, and differentiate between cold-cored and cold-trough systems.

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John Uehling, Vasubandhu Misra, Amit Bhardwaj, and Nirupam Karmakar

Abstract

In this study, we introduce a localized definition of the onset and retreat of the northern Australian rainy season that is solely based on gridded rainfall analysis. Our analysis shows that the local onset/retreat of the rainy season has considerable spatial heterogeneity. Onset is earlier and the length of the rainy season is longer to the west of the Gulf of Carpentaria than to its east. Furthermore, we also find the local onset/retreat is influenced by the wet and dry spells of the 30–60-day intraseasonal oscillation. Much of the retreat of the rainy season occurs in the dry phases of the intraseasonal oscillation. However, intriguingly, a majority of the local onset of the rainy season occurs during dry phases of the intraseasonal oscillation. The ENSO teleconnection with the variable-length northern Australian rainy season also exhibits spatial heterogeneity and significant differences from rainfall anomalies using the fixed-length boreal winter season. The onset, the retreat, the length, and the seasonal rainfall anomalies of the rainy season display a stronger correlation with the ENSO SST anomalies for the region east of 140°E relative to its west. The strong covariability of the local onset date with the corresponding seasonal length and seasonal rainfall anomalies over northern Australia offers the advantage of monitoring the onset of the northern Australian rainy season to provide an outlook for the forthcoming season. The proposed local definition of onset/retreat of the northern Australian rainy season is simple, objective, and unambiguous and is ideally suited for real-time monitoring of the evolution of the season.

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Daniel P. Stern, George H. Bryan, Chia-Ying Lee, and James D. Doyle

Abstract

Recent studies have shown that extreme wind gusts are ubiquitous within the eyewall of intense tropical cyclones (TCs). These gusts pose a substantial hazard to human life and property, but both the short-term (i.e., during the passage of a single TC) and long-term (over many years) risk of encountering such a gust at a given location is poorly understood. Here, simulated tower data from large-eddy simulations of idealized TCs in a quiescent (i.e., no mean flow or vertical wind shear) environment are used to estimate these risks for the offshore region of the United States. For both a category 5 TC and a category 3 TC, there is a radial region where nearly all simulated towers experience near-surface (the lowest 200 m) 3-s gusts exceeding 70 m s−1 within a 10-min period; on average, these towers respectively sample peak 3-s gusts of 110 and 80 m s−1. Analysis of an observational dropsonde database supports the idealized simulations, and indicates that offshore structures (such as wind turbines) in the eyewall of a major hurricane are likely to encounter damaging wind speeds. This result is then incorporated into an estimate of the long-term risk, using analyses of the return period for major hurricanes from both a best-track database and a statistical–dynamical model forced by reanalysis. For much of the nearshore region of the Gulf of Mexico and southeastern U.S. coasts, this analysis yields an estimate of a 30%–60% probability of any given point experiencing at least one 70 m s−1 gust within a 30-yr period.

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Buo-Fu Chen, Christopher A. Davis, and Ying-Hwa Kuo

Abstract

Idealized numerical studies have suggested that in addition to vertical wind shear (VWS) magnitude, the VWS profile also affects tropical cyclone (TC) development. A way to further understand the VWS profile’s effect is to examine the interaction between a TC and various shear-relative low-level mean flow (LMF) orientations. This study mainly uses the ERA5 reanalysis to verify that, consistent with idealized simulations, boundary layer processes associated with different shear-relative LMF orientations affect real-world TC’s intensity and size. Based on analyses of 720 TCs from multiple basins during 2004–16, a TC affected by an LMF directed toward downshear-left in the Northern Hemisphere favors intensification, whereas an LMF directed toward upshear-right is favorable for expansion. Furthermore, physical processes associated with shear-relative LMF orientation may also partly explain the relationship between the VWS direction and TC development, as there is a correlation between the two variables. The analysis of reanalysis data provides other new insights. The relationship between shear-relative LMF and intensification is not significantly modified by other factors [inner-core sea surface temperature (SST), VWS magnitude, and relative humidity (RH)]. However, the relationship regarding expansion is partly attributed to environmental SST and RH variations for various LMF orientations. Moreover, SST is critical to the basin-dependent variability of the relationship between the shear-relative LMF and intensification. For Atlantic TCs, the relationship between LMF orientation and intensification is inconsistent with all-basin statistics unless the analysis is restricted to a representative subset of samples associated with generally favorable conditions.

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