scholarly journals Potential Impacts of the Saharan Air Layer on Numerical Model Forecasts of North Atlantic Tropical Cyclogenesis

2009 ◽  
Vol 24 (2) ◽  
pp. 420-435 ◽  
Author(s):  
Aaron S. Pratt ◽  
Jenni L. Evans
Keyword(s):  
Tropical Cyclones ◽  
North Atlantic ◽  
Forecast Accuracy ◽  
Tropical Cyclogenesis ◽  
Atlantic Basin ◽  
Easterly Waves ◽  
Saharan Air Layer ◽  
The North ◽  
Gfs Model ◽  
Environmental Prediction

Abstract Tropical cyclones have devastating impacts on countries across large parts of the globe, including the Atlantic basin. Thus, forecasting of the genesis of Atlantic tropical cyclones is important, but this problem remains a challenge for researchers and forecasters due to the variety of weather systems that can lead to tropical cyclogenesis (e.g., stalled frontal boundaries, African easterly waves, and extratropical cyclones), as well as the role of the surrounding environment in promoting or inhibiting the development into a tropical depression and beyond. In the North Atlantic, the effects of the Saharan air layer (SAL), a hot, dry dusty layer that moves into the eastern Atlantic basin, must be taken into account when forecasting whether genesis will occur. There are several characteristics of SAL that impact tropical cyclones (decreased midtropospheric moisture, increased midlevel shear, and enhanced stability). The purpose of this study is to examine the forecasting skill of the National Centers for Environmental Prediction (NCEP) Global Forecasting System (GFS) model for the 2002 and 2003 Atlantic hurricane seasons, with particular regard paid to possible SAL effects on model genesis forecast accuracy. Cyclone phase space analyses of GFS 6-hourly forecasts were divided into three possible outcomes: S (successful forecasts that verified in cyclogenesis), F1 (cyclogenesis events that were not forecast to occur), and F2 (forecasted cyclogenesis that did not occur). The spatial variabilities of these outcomes for the early, middle, and late season were analyzed for both years, as well as the background environmental conditions. The large number of F2 forecasts that were seen in both years can be partly explained by the GFS model not capturing the detrimental effects of the SAL on cyclogenesis.

scholarly journals Tropical Cyclones in the North Atlantic Basin and Yucatan Peninsula, Mexico: Identification of Extreme Events

2021 ◽  
Vol 16 (2) ◽  
pp. 145-160
Author(s):  
Gabriel Sánchez-Rivera ◽  
Oscar Frausto-Martínez ◽  
Leticia Gómez-Mendoza ◽  
Ángel Refugio Terán-Cuevas ◽  
Julio Cesar Morales Hernández
Keyword(s):  
Tropical Cyclones ◽  
North Atlantic ◽  
Extreme Events ◽  
Yucatan Peninsula ◽  
Atlantic Basin ◽  
North Atlantic Basin ◽  
The North ◽  
Yucatán Peninsula ◽  
The North Atlantic

Assessing the Influence of Upper-Tropospheric Troughs on Tropical Cyclone Intensification Rates after Genesis

2017 ◽  
Vol 145 (4) ◽  
pp. 1295-1313 ◽  
Author(s):  
Michael S. Fischer ◽  
Brian H. Tang ◽  
Kristen L. Corbosiero
Keyword(s):  
Tropical Cyclones ◽  
North Atlantic ◽  
Temporal Evolution ◽  
Tropical Cyclogenesis ◽  
Brightness Temperatures ◽  
The North ◽  
The North Atlantic ◽  
Upper Level ◽  
Infrared Brightness

Abstract The role of upper-tropospheric troughs on the intensification rate of newly formed tropical cyclones (TCs) is analyzed. This study focuses on TCs forming in the presence of upper-tropospheric troughs in the North Atlantic basin between 1980 and 2014. TCs were binned into three groups based upon the 24-h intensification rate starting at the time of genesis: rapid TC genesis (RTCG), slow TC genesis (STCG), and neutral TC genesis (NTCG). Composite analysis shows RTCG events are characterized by amplified upper-tropospheric flow with the largest upshear displacement between the TC and trough of the three groups. RTCG events are associated with greater quasigeostrophic (QG) ascent in upshear quadrants of the TC, forced by differential vorticity advection by the thermal wind, especially around the time of genesis. This pattern of QG ascent closely matches the RTCG composite of infrared brightness temperatures. Conversely, NTCG events are associated with an upper-tropospheric trough that is closest to the TC center. The distribution of QG ascent in NTCG events becomes increasingly asymmetric around the time of genesis, with a maximum that shifts downshear of the TC center, consistent with infrared brightness temperatures. It is hypothesized that the TC intensification rate after tropical cyclogenesis, in environments of upper-tropospheric troughs, is closely linked to the structure and temporal evolution of the upper-level trough. The TC–trough configurations that provide greater QG ascent to the left of, and upshear of, the TC center feature more symmetric convection and faster TC intensification rates.

Are Tropical Cyclones Less Effectively Formed by Easterly Waves in the Western North Pacific than in the North Atlantic?

2008 ◽  
Vol 136 (11) ◽  
pp. 4527-4540 ◽  
Author(s):  
Tsing-Chang Chen ◽  
Shih-Yu Wang ◽  
Ming-Cheng Yen ◽  
Adam J. Clark
Keyword(s):  
Tropical Cyclones ◽  
North Atlantic ◽  
North Pacific ◽  
Western North Pacific ◽  
Monsoon Trough ◽  
Easterly Waves ◽  
The North ◽  
Easterly Wave ◽  
Monsoon Gyre ◽  
The North Atlantic

Abstract It has been observed that the percentage of tropical cyclones originating from easterly waves is much higher in the North Atlantic (∼60%) than in the western North Pacific (10%–20%). This disparity between the two ocean basins exists because the majority (71%) of tropical cyclogeneses in the western North Pacific occur in the favorable synoptic environments evolved from monsoon gyres. Because the North Atlantic does not have a monsoon trough similar to the western North Pacific that stimulates monsoon gyre formation, a much larger portion of tropical cyclogeneses than in the western North Pacific are caused directly by easterly waves. This study also analyzed the percentage of easterly waves that form tropical cyclones in the western North Pacific. By carefully separating easterly waves from the lower-tropospheric disturbances generated by upper-level vortices that originate from the tropical upper-tropospheric trough (TUTT), it is observed that 25% of easterly waves form tropical cyclones in this region. Because TUTT-induced lower-tropospheric disturbances often become embedded in the trade easterlies and resemble easterly waves, they have likely been mistakenly identified as easterly waves. Inclusion of these “false” easterly waves in the “true” easterly wave population would result in an underestimation of the percentage of easterly waves that form tropical cyclones, because the TUTT-induced disturbances rarely stimulate tropical cyclogenesis. However, an analysis of monsoon gyre formation mechanisms over the western North Pacific reveals that 82% of monsoon gyres develop through a monsoon trough–easterly wave interaction. Thus, it can be inferred that 58% (i.e., 82% × 71%) of tropical cyclones in this region are an indirect result of easterly waves. Including the percentage of tropical cyclones that form directly from easterly waves (∼25%), it is found that tropical cyclones formed directly and indirectly from easterly waves account for over 80% of tropical cyclogeneses in the western North Pacific. This is more than the percentage that has been documented by previous studies in the North Atlantic.

scholarly journals Convectively Coupled Kelvin Waves and Tropical Cyclogenesis in a Semi-Lagrangian Framework

2016 ◽  
Vol 144 (11) ◽  
pp. 4131-4139 ◽  
Author(s):  
Carl J. Schreck
Keyword(s):  
North Atlantic ◽  
Phase Speed ◽  
Kelvin Waves ◽  
Tropical Cyclogenesis ◽  
Kelvin Wave ◽  
Easterly Waves ◽  
The North ◽  
Lagrangian Framework ◽  
Easterly Wave ◽  
Vertically Aligned

Abstract This study examines how convectively coupled Kelvin waves interact with the semi-Lagrangian circulation of easterly waves to modulate tropical cyclogenesis. Recent studies have shown that fewer tropical cyclones form in the three days before passage of the Kelvin wave’s peak convection and more develop in the three days thereafter. Separately, other studies have identified the recirculation of moisture and vorticity within easterly waves using a semi-Lagrangian frame of reference. That framework is achieved by subtracting the easterly wave phase speed from the earth-relative winds. This study combines these recent findings by testing whether the equatorial westerlies from Kelvin waves can help close the semi-Lagrangian circulation. Past studies have shown that Kelvin waves tilt westward with height in the troposphere such that equatorial westerlies build upward from the surface in the days following the convective peak. This study shows that the easterly wave’s semi-Lagrangian closed circulation grows upward as it intersects the Kelvin wave’s westward tilt. The Kelvin wave’s westerly anomalies reach 500 hPa about three days after the convection has passed, which establishes the deep, vertically aligned easterly wave vortex necessary for tropical cyclogenesis. This study focuses on the eastern Pacific, but similar results are found for the North Atlantic. In other basins, the Kelvin wave accentuates the westerlies from the Madden–Julian oscillation and/or the monsoon trough. Given that Kelvin waves often last weeks and circumnavigate the globe, these results may advance long-range tropical cyclogenesis forecasting.

scholarly journals Impacts of Tropical Cyclones on Longleaf Pine Ecosystems of Florida: Tropical Cyclogenesis, Landfall Frequencies, and Climate Change

2021 ◽  
Vol 9 ◽  
Author(s):  
Frank S. Gilliam
Keyword(s):  
Climate Change ◽  
Tropical Cyclones ◽  
North Atlantic ◽  
North Atlantic Ocean ◽  
Longleaf Pine ◽  
Terrestrial Ecosystem ◽  
Tropical Cyclogenesis ◽  
Human Populations ◽  
The North ◽  
The North Atlantic

Tropical storms and hurricanes (collectively hereafter, tropical cyclones) are among the most destructive forces in nature. These threats are of particular concern to human populations and ecosystems of coastal areas of the southeastern United States, most especially in the State of Florida. This review begins with an overview of the effects of tropical cyclones on Florida’s most conspicuous terrestrial ecosystem—longleaf pine. Environmental factors leading to tropical cyclogenesis will also be reviewed, with a specific focus on (1) landfall history in Florida, and (2) the potential relationship between climate change and the frequency/intensity of tropical cyclones in the North Atlantic Ocean. Given its geographical distribution, it is not surprising that longleaf pine has long been impacted by tropical cyclones of the North Atlantic. Tropical cyclones are formed from a complex combination of meteorological conditions, driven initially by the release of excess heat from the surface waters of the ocean, along with an unstable atmosphere comprising air temperatures decreasing and wind speeds increasing with altitude. Among the coastal counties from Texas to Maine, those of Florida have experienced by far the highest frequency of tropical cyclones, especially the southern tip of peninsular Florida, with its most populous county (Miami-Dade) receiving 25 hits from 1900 to 2010, second only to Monroe County (32 hits) during that period. Frequencies of all categories of cyclones have increased significantly from 1850 to the present. Cyclone frequencies were significantly correlated with increases in air and ocean temperatures, both of which have increased over the past, suggesting a causal relationship with anthropogenic climate change. Of future concern is how increases in frequencies and intensities of tropical cyclones will negatively affect the structure and function of these ecologically and economically important longleaf pine ecosystems.

scholarly journals Electrically Active Convection in Tropical Easterly Waves and Implications for Tropical Cyclogenesis in the Atlantic and East Pacific

2013 ◽  
Vol 141 (2) ◽  
pp. 542-556 ◽  
Author(s):  
Kenneth D. Leppert ◽  
Walter A. Petersen ◽  
Daniel J. Cecil
Keyword(s):  
Brightness Temperature ◽  
Tropical Cyclones ◽  
Tropical Rainfall Measuring Mission ◽  
Meridional Wind ◽  
Tropical Cyclogenesis ◽  
Lightning Flash ◽  
Reanalysis Dataset ◽  
Easterly Waves ◽  
East Pacific ◽  
Environmental Prediction

Abstract In this study, the authors investigated the characteristics of tropical easterly wave convection and the possible implications of convective structure on tropical cyclogenesis and intensification over the Atlantic Ocean and the east Pacific Ocean. Easterly waves were partitioned into northerly, southerly, trough, and ridge phases based on the 700-hPa meridional wind from the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis dataset. Waves were subsequently divided according to whether they did or did not develop tropical cyclones (i.e., developing and nondeveloping, respectively), and developing waves were further subdivided according to development location. Finally, composites as a function of wave phase and category were created using data from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager, Precipitation Radar (PR), and Lightning Imaging Sensor as well as infrared (IR) brightness temperature data from the NASA global-merged IR brightness temperature dataset. Results suggest that the convective characteristics that best distinguish developing from nondeveloping waves vary according to where developing waves spawn tropical cyclones. For waves that develop a cyclone in the Atlantic basin, coverage by IR brightness temperatures ≤240 and ≤210 K provide the best distinction between developing and nondeveloping waves. In contrast, several variables provide a significant distinction between nondeveloping waves and waves that develop cyclones over the east Pacific as these waves near their genesis location including IR threshold coverage, lightning flash rates, and low-level (<4.5 km) PR reflectivity. Results of this study may be used to help develop thresholds to better distinguish developing from nondeveloping waves and serve as another aid for tropical cyclogenesis forecasting.

scholarly journals North Atlantic Hurricanes Contributed by African Easterly Waves North and South of the African Easterly Jet

2008 ◽  
Vol 21 (24) ◽  
pp. 6767-6776 ◽  
Author(s):  
Tsing-Chang Chen ◽  
Shih-Yu Wang ◽  
Adam J. Clark
Keyword(s):  
Tropical Cyclones ◽  
North Atlantic ◽  
Monsoon Trough ◽  
African Easterly Waves ◽  
Easterly Waves ◽  
African Easterly Jet ◽  
The North ◽  
West African Coast ◽  
Atlantic Hurricanes ◽  
Population Ratio

Abstract A majority of tropical cyclones in the North Atlantic develop from African easterly waves (AEWs), which originate along both the southern and northern flanks of the midtropospheric African easterly jet (AEWS and AEWn, respectively). The purpose of this note is to identify the contribution of AEWSs and AEWns to North Atlantic tropical cyclones that develop from AEWs. Applying a manual backtracking approach to identify the genesis locations of AEWS, it was found that the population ratio of tropical cyclones formed from AEWSs to those formed from AEWns is 1:1.2. Because the population ratio of AEWSs to AEWns is 1:2.5, the conversion rate of the former AEWS to tropical cyclones is twice as effective as the latter waves. In addition, it was found that AEWns travel farther and take longer to transform into tropical cyclones than AEWSs, which is likely because the AEWns are drier and shallower than AEWSs. An analysis of various terms in the moisture and vorticity budgets reveals that the monsoon trough over West Africa provides moisture and enhances low-level vorticity for both AEWns and AEWSs as they move off the West African coast. The monsoon trough appears to be of particular importance in supplying AEWns with enough moisture so that they have similar properties to AEWSs after they have traveled a considerable westward distance across the tropical Atlantic.

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