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Zhuo Wang
,
Michael T. Montgomery
, and
Cody Fritz

Abstract

In support of the National Science Foundation Pre-Depression Investigation of Cloud-systems in the tropics (NSF PREDICT) and National Aeronautics and Space Administration Genesis and Rapid Intensification Processes (NASA GRIP) dry run exercises and National Oceanic and Atmospheric Administration Hurricane Intensity Forecast Experiment (NOAA IFEX) during the 2009 hurricane season, a real-time wave-tracking algorithm and corresponding diagnostic analyses based on a recently proposed tropical cyclogenesis model were applied to tropical easterly waves over the Atlantic. The model emphasizes the importance of a Lagrangian recirculation region within a tropical-wave critical layer (the so-called pouch), where persistent deep convection and vorticity aggregation as well as column moistening are favored for tropical cyclogenesis. Distinct scenarios of hybrid wave–vortex evolution are highlighted. It was found that easterly waves without a pouch or with a shallow pouch did not develop. Although not all waves with a deep pouch developed into a tropical storm, a deep wave pouch had formed prior to genesis for all 16 named storms originating from monochromatic easterly waves during the 2008 and 2009 seasons. On the other hand, the diagnosis of two nondeveloping waves with a deep pouch suggests that strong vertical shear or dry air intrusion at the middle–upper levels (where a wave pouch was absent) can disrupt deep convection and suppress storm development.

To sum up, this study suggests that a deep wave pouch extending from the midtroposphere (~600–700 hPa) down to the boundary layer is a necessary condition for tropical cyclone formation within an easterly wave. It is hypothesized also that a deep wave pouch together with other large-scale favorable conditions provides a sufficient condition for sustained convection and tropical cyclone formation. This hypothesized sufficient condition requires further testing and will be pursued in future work.

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Weiwei Li
,
Zhuo Wang
, and
Melinda S. Peng

Abstract

Tropical cyclone (TC) forecasts from the NCEP Global Ensemble Forecasting System (GEFS) Reforecast version 2 (1985–2012) were evaluated from the climate perspective, with a focus on tropical cyclogenesis. Although the GEFS captures the climatological seasonality of tropical cyclogenesis over different ocean basins reasonably well, large errors exist on the regional scale. As different genesis pathways are dominant over different ocean basins, genesis biases are related to biases in different aspects of the large-scale or synoptic-scale circulations over different basins. The negative genesis biases over the western North Pacific are associated with a weaker-than-observed monsoon trough in the GEFS, the erroneous genesis pattern over the eastern North Pacific is related to a southward displacement of the ITCZ, and the positive genesis biases near the Cape Verde islands and negative biases farther downstream over the Atlantic can be attributed to the hyperactive Africa easterly waves in the GEFS. The interannual and subseasonal variability of TC activity in the reforecasts was also examined to evaluate the potential skill of the GEFS in providing subseasonal and seasonal predictions. The GEFS skillfully captures the interannual variability of TC activity over the North Pacific and the North Atlantic, which can be attributed to the modulation of TCs by the El Niño–Southern Oscillation (ENSO) and the Atlantic meridional mode (AMM). The GEFS shows promising skill in predicting the active and inactive periods of TC activity over the Atlantic. The skill, however, has large fluctuations from year to year. The analysis presented herein suggests possible impacts of ENSO, the Madden–Julian oscillation (MJO), and the AMM on the TC subseasonal predictability.

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Zhuo Wang
,
John Walsh
,
Sarah Szymborski
, and
Melinda Peng

Abstract

Large sea ice loss on the synoptic time scale is examined in various subregions in the Arctic as well as at the pan-Arctic scale. It is found that the frequency of large daily sea ice loss (LDSIL) days is significantly correlated with the September sea ice extent over the Beaufort–Chukchi–Siberian Seas, the Laptev–Kara Seas, the central Arctic, and the all-Arctic regions, indicating a link between the synoptic sea ice variability and the interannual variability of the annual minimum sea ice extent. A composite analysis reveals dipoles of anomalous cyclones and anticyclones associated with LDSIL days. Different from the well-known Arctic dipole pattern, the east–west dipoles are found over the corresponding regions of LDSIL in the Arctic marginal seas and are associated with the increasing occurrence of Rossby wave breaking and atmospheric rivers. The anticyclones of the dipoles are persistent and quasi-stationary, reminiscent of blocking. The anomalous poleward flow between the cyclone and the anticyclone enhances the poleward transport of heat and water vapor in the lower troposphere. Although enhanced downward shortwave radiation, associated with reduced cloud fraction, is found in some regions, it is not collocated with the regions of LDSIL. In contrast, enhanced downward longwave radiation owing to increasing column water vapor shows good spatial correspondence with LDSIL, indicating the importance of atmospheric rivers in LDSIL events. Lead/lag composites with respect to the onset of LDSIL episodes reveal precursor wave trains spanning the midlatitudes. The wave trains have predominantly zonal energy propagation in the midlatitudes and do not show a clear link to tropical or subtropical forcing.

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Ziyu Yan
,
Zhuo Wang
,
Melinda Peng
, and
Xuyang Ge

Abstract

Polar lows (PLs) are intense mesoscale cyclones over high-latitude oceans. The PL motion and track characteristics over the North Atlantic in the cold season are examined based on ∼1700 PLs during 1979–2016. Our analysis shows that PL motion is mainly controlled by the environmental flow in the lower troposphere. In particular, the steering flow defined over 950–550 hPa within a 450-km radius best matches the PL motion. Meanwhile, 700 hPa is an optimal single level to assess the steering flow. Cluster analysis based on a linear regression mixture model is utilized to describe the PL tracks. Four distinct clusters are identified, and they are characterized by northeastward motion (NE), eastward motion (E), southward motion (S), and slow motion without a dominating direction (SM), respectively. Although PLs in the four clusters have similar lifespans, SM-type PLs have much shorter tracks than the other clusters, due to their slow translation speeds. Further analysis shows that there are no distinctive geographic differences in genesis locations for the four clusters. The track differences can be largely explained by the associated synoptic-scale environmental circulations. Additionally, the S-type PLs tend to develop in a reverse shear environment, the NE-type and E-type PLs are associated with forward shear and left shear environments, respectively, while the SM-type PLs do not show a preference in the environmental shear. The link between the PL tracks and the North Atlantic weather regimes is also investigated. The NAO+ regime is associated with the most frequent PL occurrence over the North Atlantic, while the Scandinavian blocking regime is associated with lowest PL frequency.

Significance Statement

Polar lows (PLs) are intense mesoscale cyclones and pose hazards to high-latitude coastal areas. A better understanding of polar low motion can help improve polar low forecasts and reduce the hazardous impacts of the storms. Based on a PL track dataset over the North Atlantic basin during 1979–2016, it is found that PL motion is mainly controlled by the environmental flow in the lower troposphere and that the steering flow defined over 950–550 hPa within a 450-km radius best matches the PL motion. In addition, four distinct types of tracks are identified using cluster analysis, and they are characterized by northeastward motion, eastward motion, southward motion, and slow motion without a dominating direction, respectively. The different background flows can largely explain the propagation directions of different track types. Furthermore, the links between the PL motion and the North Atlantic weather regimes are also investigated.

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Kevin Boyd
,
Zhuo Wang
, and
John E. Walsh

Abstract

Polar lows (PLs) are intense maritime mesocyclones that typically develop during marine cold-air outbreak events over the high latitudes. The impacts posed by these systems to humans and the broader environment demand a robust understanding of the environmental factors that promote PL formation and, in turn, skillful prediction of PL activity. We hypothesize that the variability of PL activity is associated with some key large-scale climate variables skewed toward “extreme” values, which can provide predictable information on PL activity beyond the synoptic time scale. A PL genesis potential index (PGI) is developed that relates the climatological spatial distribution of PL genesis frequency and key climate variables in a Poisson regression framework. The optimal set of predictors consists of a static stability parameter and an environmental baroclinicity parameter. The optimal predictor categories are shown to be robust across different reanalyses and PL track datasets. The observed spatial distribution and seasonal cycle of PL genesis frequency are represented well by the PGI, and the interannual variability of PL activity is captured skillfully. The effects of the Arctic Oscillation (AO), El Niño–Southern Oscillation (ENSO), and a few other climate modes on the interannual variability of PL activity are explored. Overall, our results suggest that the PGI may be used to inform skillful subseasonal to seasonal prediction of PL activity.

Significance Statement

Polar lows are intense mesocyclones over high-latitude oceans, and they have destructive impacts on coastal and island communities, and maritime and air operations. However, skillful prediction of polar lows on the subseasonal and longer time scales remains challenging. This study links polar low activity to large-scale environmental conditions in the Arctic through a statistical modeling approach. This work is based on the hypothesis that a shared statistical relationship exists between the large-scale climate variables and polar low activity across the Arctic, which enables a geographical unification of the controlling factors on polar low activity. Our results reveal two dominant factors, one related to the lower-tropospheric stratification and the other to the hydrodynamic instability of the lower-tropospheric flow. This statistical framework has potential applications to climate prediction and projection of polar low activity.

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Zhuo Wang
,
C-P. Chang
,
Bin Wang
, and
Fei-Fei Jin

Abstract

Rossby wave propagation theory predicts that Rossby waves in a tropical easterly flow cannot escape from the Tropics to the extratropics. Here the authors show that a southerly flow component in the basic state (a southerly conveyor) may transfer a Rossby wave source northward; thus, a forcing embedded in the deep tropical easterlies may excite a Rossby wave response in the extratropical westerlies. It is shown that the southerly conveyor determines the location of the effective Rossby wave source and that the extratropical response is relatively insensitive to the location of the tropical forcing, provided that the tropical response can reach the southerly conveyor. A stronger southerly flow favors a stronger extratropical response, and the spatial structure of the extratropical response is determined by the extratropical westerly basic flows.

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Zhuo Wang
,
Timothy J. Dunkerton
, and
Michael T. Montgomery

Abstract

A wave-tracking algorithm is developed for northwestward-propagating waves that, on occasion, play a role in tropical cyclogenesis over the western oceans. To obtain the Lagrangian flow structure, the frame of reference is translated obliquely at the same propagation speed with the precursor disturbance. Trajectory analysis suggests that streamlines in the obliquely translated frame of reference can be used to approximate flow trajectories. The algorithm was applied to Super Typhoon Nakri (2008), Tropical Cyclone Erika (2009), and a few other examples. Diagnoses of meteorological analyses and satellite-derived moisture and precipitation fields show that the marsupial framework for tropical cyclogenesis in tropical easterly waves is relevant also for northwestward-propagating disturbances as are commonly observed in the tropical western Atlantic, the Gulf of Mexico, and the western North Pacific. Finally, it is suggested that analysis of the global model data and satellite observations in the marsupial framework can provide useful guidance on early tropical cyclone advisories.

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Ziyu Yan
,
Xuyang Ge
,
Zhuo Wang
,
Chun-Chieh Wu
, and
Melinda Peng

Abstract

Typhoon Jongdari (2018) had an unusual looping path before making landfall in Japan, which posed a forecasting challenge for operational numerical models. The impacts of an upper-tropospheric cold low (UTCL) on the track and intensity of Jongdari are investigated using numerical simulations. The storm track and intensity are well simulated in the control experiment using the GFS analysis as the initial and boundary conditions. In the sensitivity experiment (RCL), the UTCL is removed from the initial-condition fields using the piecewise potential vorticity inversion (PPVI), and both the track and intensity of Jongdari change substantially. The diagnosis of potential vorticity tendency suggests that horizontal advection is the primary contributor for storm motion. Flow decomposition using the PPVI further demonstrates that the steering flow is strongly affected by the UTCL, and the looping path of Jongdari results from the Fujiwhara interaction between the typhoon and UTCL. Jongdari first intensifies and then weakens in the control experiment, consistent with the observation. In contrast, it undergoes a gradual intensification in the RCL experiment. The UTCL contributes to the intensification of Jongdari at the early stage by enhancing the eddy flux convergence of angular momentum and reducing inertial stability, and it contributes to the storm weakening via enhanced vertical wind shear at the later stage when moving closer to Jongdari. Different sea surface temperatures and other environmental conditions along the different storm tracks also contribute to the intensity differences between the control and the RCL experiments, indicating the indirect impacts of the UTCL on the typhoon intensity.

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Weiwei Li
,
Zhuo Wang
,
Melinda S. Peng
, and
James A. Ridout

Abstract

Navy Operational Global Atmospheric Prediction System (NOGAPS) analysis and operational forecasts are evaluated against the Interim ECMWF Re-Analysis (ERA-Interim; ERAI) and satellite data, and compared with the Global Forecast System (GFS) analysis and forecasts, using both performance- and physics-based metrics. The NOGAPS analysis captures realistic Madden–Julian oscillation (MJO) signals in the dynamic fields and the low-level premoistening leading to active convection, but the MJO signals in the relative humidity (RH) and diabatic heating rate (Q1) fields are weaker than those in the ERAI or the GFS analysis. The NOGAPS forecasts, similar to the GFS forecasts, have relatively low prediction skill for the MJO when the MJO initiates over the Indian Ocean and when active convection is over the Maritime Continent. The NOGAPS short-term precipitation forecasts are broadly consistent with the Climate Prediction Center (CPC) morphing technique (CMORPH) precipitation results with regionally quantitative differences. Further evaluation of the precipitation and column water vapor (CWV) indicates that heavy precipitation develops too early in the NOGAPS forecasts in terms of the CWV, and the NOGAPS forecasts show a dry bias in the CWV increasing with forecast lead time. The NOGAPS underpredicts light and moderate-to-heavy precipitation but overpredicts extremely heavy rainfall. The vertical profiles of RH and Q1 reveal a dry bias within the marine boundary layer and a moist bias above. The shallow heating mode is found to be missing for CWV < 50 mm in the NOGAPS forecasts. The diabatic heating biases are associated with weaker trade winds, weaker Hadley and Walker circulations over the Pacific, and weaker cross-equatorial flow over the Indian Ocean in the NOGAPS forecasts.

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Zhuo Wang
,
M. T. Montgomery
, and
T. J. Dunkerton

Abstract

The formation of pre–Hurricane Felix (2007) in a tropical easterly wave is examined in a two-part study using the Weather Research and Forecasting (WRF) model with a high-resolution nested grid configuration that permits the representation of cloud system processes. The simulation commences during the wave stage of the precursor African easterly-wave disturbance. Here the simulated and observed developments are compared, while in of the study various large-scale analyses, physical parameterizations, and initialization times are explored to document model sensitivities.

In this first part the authors focus on the wave/vortex morphology, its interaction with the adjacent intertropical convergence zone complex, and the vorticity balance in the neighborhood of the developing storm. Analysis of the model simulation points to a bottom-up development process within the wave critical layer and supports the three new hypotheses of tropical cyclone formation proposed recently by Dunkerton, Montgomery, and Wang. It is shown also that low-level convergence associated with the ITCZ helps to enhance the wave signal and extend the “wave pouch” from the jet level to the top of the atmospheric boundary layer. The region of a quasi-closed Lagrangian circulation within the wave pouch provides a focal point for diabatic merger of convective vortices and their vortical remnants. The wave pouch serves also to protect the moist air inside from dry air intrusion, providing a favorable environment for sustained deep convection. Consistent with the authors’ earlier findings, the tropical storm forms near the center of the wave pouch via system-scale convergence in the lower troposphere and vorticity aggregation. Components of the vorticity balance are shown to be scale dependent, with the immediate effects of cloud processes confined more closely to the storm center than the overturning Eliassen circulation induced by diabatic heating, the influence of which extends to larger radii.

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