Environmental Influences on Hurricane Intensification

Merrill, Ronald T.

doi:10.1175/1520-0469(1988)045<1678:eiohi>2.0.co;2

Cited by 199 publications

(155 citation statements)

References 66 publications

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“…Indeed, the primary determinant of intensity change appears to be the relationship of the storm to the upper-tropospheric flow features, particularly those that constrain storm outflow and enhance vertical wind shear, in agreement with many previous studies (e.g., Rodgers and Gentry 1983;Holland and Merrill 1984;Merrill 1988;Molinari and Vollaro 1989;Rodgers et al 1991;Hanley et al 2001;Kimball and Evans 2002;Rappin et al 2010). FIG.…”

Section: A Composite View Of the Sal And Hurricanessupporting

confidence: 88%

“…They suggested that the SAL negatively impacts tropical cyclones in the following ways: 1) the enhanced low-level temperature inversion, maintained by radiative warming of dust, suppresses deep convective development; 2) vertical wind shear caused by an increase in the lowlevel easterlies associated with the AEJ inhibits tropical cyclone intensification, based upon studies that have shown that shear tends to weaken storms (Gray 1968;Merrill 1988;DeMaria andKaplan 1994, 1999;Frank and Ritchie 2001;Rogers et al 2003;Braun and Wu 2007); and 3) intrusions of dry SAL air into tropical cyclones foster enhanced cold downdrafts (Emanuel 1989;Powell 1990) and lower the convective available potential energy within tropical cyclones. While it was not Dunion and Velden's intention to imply that the SAL's impacts were always negative or were the dominant factor affecting hurricane activity (J.…”

Section: Introductionmentioning

confidence: 99%

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Reevaluating the Role of the Saharan Air Layer in Atlantic Tropical Cyclogenesis and Evolution

Braun

2010

Monthly Weather Review

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The existence of the Saharan air layer (SAL), a layer of warm, dry, dusty air frequently present over the tropical Atlantic Ocean, has long been appreciated. The nature of its impacts on hurricanes remains unclear, with some researchers arguing that the SAL amplifies hurricane development and with others arguing that it inhibits it. The potential negative impacts of the SAL include 1) vertical wind shear associated with the African easterly jet; 2) warm air aloft, which increases thermodynamic stability at the base of the SAL; and 3) dry air, which produces cold downdrafts. Multiple NASA satellite datasets and NCEP global analyses are used to characterize the SAL's properties and evolution in relation to tropical cyclones and to evaluate these potential negative influences. The SAL is shown to occur in a large-scale environment that is already characteristically dry as a result of large-scale subsidence. Strong surface heating and deep dry convective mixing enhance the dryness at low levels (primarily below ;700 hPa), but moisten the air at midlevels. Therefore, mid-to-upper-level dryness is not generally a defining characteristic of the SAL, but is instead often a signature of subsidence. The results further show that storms generally form on the southern side of the jet, where the background cyclonic vorticity is high. Based upon its depiction in NCEP Global Forecast System meteorological analyses, the jet often helps to form the northern side of the storms and is present to equal extents for both strengthening and weakening storms, suggesting that jet-induced vertical wind shear may not be a frequent negative influence. Warm SAL air is confined to regions north of the jet and generally does not impact the tropical cyclone precipitation south of the jet.Composite analyses of the early stages of tropical cyclones occurring in association with the SAL support the inferences from the individual cases noted above. Furthermore, separate composites for strongly strengthening and for weakening storms show few substantial differences in the SAL characteristics between these two groups, suggesting that the SAL is not a determinant of whether a storm will intensify or weaken in the days after formation. Key differences between these cases are found mainly at upper levels where the flow over strengthening storms allows for an expansive outflow and produces little vertical shear, while for weakening storms, the shear is stronger and the outflow is significantly constrained.

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Section: A Composite View Of the Sal And Hurricanessupporting

confidence: 88%

Section: Introductionmentioning

confidence: 99%

Reevaluating the Role of the Saharan Air Layer in Atlantic Tropical Cyclogenesis and Evolution

Braun

2010

Monthly Weather Review

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“…3. This figure shows that for a given SST in the indicated SST range the minimum surface pressure revealed as the maximum intensity curve for western North Pacific storms is lower than the potential minimum surface pressures for North Atlantic storms analyzed by Merrill (1987Merrill ( , 1988 and DK. This is also true for the 99th intensity percentile curve except for the slightly lower minimum surface pressure in Merrill's curve in the 29C SST group.…”

Section: Data and Analysis Methodsmentioning

confidence: 74%

A Climatology of Sea Surface Temperature and the Maximum Intensity of Western North Pacific Tropical Cyclones

Baik

Paek

1998

Journal of the Meteorological Society of Japan

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Using a 31-year (1960-1990) sample of western North Pacific tropical cyclones and monthly mean sea surface temperature (SST) for each year, an empirical relationship between SST and the maximum intensity of western North Pacific storms is determined and used to calculate the relative intensity, a measure of how close a storm reaches its maximum potential intensity. The analysis method in this study follows that by DeMaria and Kaplan (1994b) and results are compared with observations over the North Atlantic and theoretical studies.Similar to previous studies, an upper bound of storm intensity for a given SST was determined. It is shown that a larger fraction of Pacific storms are observed over warm waters than Atlantic storms and the maximum potential intensity of Pacific storms tends to be stronger than that of Atlantic storms or theoretically calculated storms. The analyses of the relative intensity at the time of each storm's life-time maximum intensity indicate that the maximum intensity of Pacific storms is well below the maximum potential intensity. The average relative intensity of the total sample is 37% (47%) when the regression curve for the maximum (99th) intensity percentile is used to compute the relative intensity, implying that environmental influences appear to be more important than SST in determining the maximum intensity of Pacific storms. The average relative intensity of late-season storms tends to be, as in the Atlantic, larger than that of early-season storms, and the yearly-averaged relative intensity shows to some extent interannual variability but with little correlation either with quasi-biennial oscillation or with El Nino.

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“…For the western North Pacific, Baik and Paek (1998) advanced a proportion of 37%. This clearly signifies that factors such as a strong vertical wind shear between the lower and the upper troposphere, the intrusion of dry air or the weakness of the divergence in the upper troposphere have a considerable influence in limiting the intensification of cyclones (Merril, 1988). In the northern Indian Ocean, if one simply considers cyclones having reached a minimum of 65 knots, only 5.66% (3 of 53) approached or attained their maximum potential intensity over the last three decades.…”

Section: Intense Cyclones Considerably Influenced By Nearby Land Massesmentioning

confidence: 99%

Intense tropical cyclone activities in the northern Indian Ocean

Hoarau

Bernard

Chalonge

2011

Intl Journal of Climatology

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This research concerning the northern Indian Ocean demonstrates the variability of intense tropical cyclones (categories 3-5) both on an inter-annual and intra-seasonal scale. All the cyclones intensity have been re-analysed with the Dvorak technique using both National Oceanic and Atmospheric Administration and geostationary satellites with a 4-km infrared resolution. During the period from 1980 to 2009, 21 cyclones became intense. The decade from 1990 to 1999 was by far the most active with 11 intense cyclones while 5 intense cyclones formed in each of the other two decades. There has been no trend towards an increase in the number of categories 3-5 cyclones over the last 30 years. An early study, not based on re-analysed data, found a significant increase in the number of intense cyclones. Thirteen cyclones became intense when the Oceanic Nino Index was negative and matched with lower vertical wind shear. And, La Nina events have had a noticeable influence with eight intense cyclones. The monthly distribution is bimodal with seven and eight cyclones, respectively in the months of May and November. No intense cyclones were observed from July to September, being the peak of the monsoon season. Over these 3 months, the vertical wind shear is too strong to allow a significant intensification of storms. Despite particularly warm ocean waters, only 6% of categories 1-5 cyclones approached their maximum potential intensity. However, extreme intensities (05B at 155 knots in 1999) are comparable to other basins despite lower in terms of activity level. The proximity of land limits most cyclones from reaching a greater intensity. However, 16 of the 21 cyclones reached land with sustained winds of 100 knots and more. India and Bangladesh have been hit frequently by intense cyclones. Since 1990s, there is an increasing hit in Burma and Pakistan.

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Environmental Influences on Hurricane Intensification

Cited by 199 publications

References 66 publications

Reevaluating the Role of the Saharan Air Layer in Atlantic Tropical Cyclogenesis and Evolution

Reevaluating the Role of the Saharan Air Layer in Atlantic Tropical Cyclogenesis and Evolution

A Climatology of Sea Surface Temperature and the Maximum Intensity of Western North Pacific Tropical Cyclones

Intense tropical cyclone activities in the northern Indian Ocean

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