Analysis of the influence of Saharan air layer on tropical cyclone intensity using AIRS/Aqua data

Shu, Shoujuan; Wu, Liguang

doi:10.1029/2009gl037634

Cited by 57 publications

(65 citation statements)

References 20 publications

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“…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. Dunion and C. Velden 2009, personal communication), it appears from recent research papers (e.g., Jones et al 2007;Sun et al 2008Sun et al , 2009Reale et al 2009;Shu and Wu 2009) that that view is becoming more widespread.…”

Section: Introductionmentioning

confidence: 99%

“…A frequent assumption made in many recent SAL papers (Jones et al 2007;Dunion and Marron 2008;Reale et al 2009;Shu and Wu 2009;Sun et al 2008;Sun et al 2009) is that very dry low-to midlevel tropical air is necessarily of Saharan origin. Dunion and Marron (2008) separated soundings at four Caribbean sites into SAL and non-SAL categories, with the separation being accomplished based upon Geostationary Operational Environmental Satellite (GOES) multichannel SAL analyses (DV).…”

Section: A Dry Tropical Air Is Sal Airmentioning

confidence: 99%

“…Early studies (e.g., Karyampudi and Carlson 1988;Karyampudi and Pierce 2002) suggested a potential positive influence on the growth of easterly waves and tropical cyclones in the Atlantic. A more recent study by Dunion and Velden (2004, hereafter DV) described several potentially negative influences of the SAL, and Wu et al (2006), Jones et al (2007), and Shu and Wu (2009) assumed the general applicability of the DV results to describe SAL impacts on hurricanes. The reduced Atlantic hurricane activity of 2006 and 2007 compared to 2004 and 2005, particularly as it affected the United States, has led to speculation in some research papers (e.g., Lau and Kim 2007a,b;Sun et al 2008) that dustiness or dry air from increased SAL activity contributed significantly to the decline in hurricane activity in those two years.…”

Section: Introductionmentioning

confidence: 99%

“…The link between the SAL's properties and its supposed negative impacts (DV;Shu and Wu 2009;Sun et al 2008) has been built largely on very limited evidence and some false assumptions. Alternative causes of storm weakening or lack of intensification, such as vertical shear or weak convective activity not associated with the SAL, and hurricane-induced ocean cooling, were not examined.…”

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: Introductionmentioning

confidence: 99%

Section: A Dry Tropical Air Is Sal Airmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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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“…This air mass is considered hostile to Atlantic tropical cyclones (Dunion and Velden 2004;Evan et al 2006;Jones et al 2007;Wu 2007;Dunion and Marron 2008;Sun et al 2009;Reale et al 2009;Shu and Wu 2009), primarily due to the characteristically very dry and dusty midlevel air. Satellite-derived products designed to depict the SAL based on multispectral approaches to isolate the dust and dry air effects on radiances are available during the PREDICT period (i.e.…”

Section: −1mentioning

confidence: 99%

The Pre-Depression Investigation of Cloud-Systems in the Tropics (PREDICT) Experiment: Scientific Basis, New Analysis Tools, and Some First Results

Montgomery¹,

Davis²,

Dunkerton³

et al. 2012

Bull. Amer. Meteor. Soc.

153

132

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Mineral dust transport toward Hurricane Helene (2006)

et al. 2016

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This study investigates the transport of mineral dust from its source regions in West Africa toward the developing tropical cyclone Helene (2006) and diagnoses the resulting properties of the air influencing the tropical cyclonegenesis. The model system COSMO-ART (Consortium for Small-Scale Modelling-Aerosols and Reactive Trace gases) in which the emission and transport of mineral dust as well as the radiation feedback are taken into account, was used. The emission of mineral dust between 9 and 14 September 2006 occurred in association with the relatively strong monsoon flow and northeasterly trade winds, with gust fronts of convective systems over land, and with the Atlantic inflow. Additionally, increased surface wind speed was linked to orographical effects at the Algerian Mountains, Atlas Mountains, and the Hoggar. The dust, as part of the Saharan air layer, is transported at low levels by the monsoon flow, the Harmattan, the northeasterly trade winds, and the monsoon trough, and is transported upward in the convergence zone between Harmattan and monsoon flow, in the baroclinic zone along the West African coastline, and by convection. At around 700 hPa the dust is transported by the African easterly jet. Dry and dust-free air is found to the north-northwest of the developing tropical depression due to descent in an anticyclone. Based on the model data, it was possible to distinguish between dry (from the anticyclone), dry and dusty (from the Harmattan and northeasterly trade winds), and dusty and moist air (from the monsoon flow and in the tropical depression due to convection). middle and upper levels in the region. The dry air over the eastern and subtropical Atlantic is partly a result of large-scale descent on the eastern side of the Azores high-pressure system [Braun, 2010].center and in the vicinity of Helene originates. Additionally, the model data allows the differences between dry air, dry, and dusty air, as well as dusty and moist air to be distinguished. Hence, in this study we are going to address the following question: How is the mineral dust transported from its source regions to the developing tropical storm and what are the implications of this transport for the properties of the air influencing the tropical cyclogenesis? Model Setup and Observations Numerical ModelThe dust events between 9 and 14 September 2006 and the associated weather systems over West Africa were simulated using the Consortium for Small-Scale Modelling (COSMO) model (http://www.cosmo-model.org/) with the Aerosols and Reactive Trace gases (ART) module. COSMO is a nonhydrostatic limited area numerical model, based on the primitive equations that describe the compressible flow in a moist atmosphere [Doms and Schättler, 2002]. A generalized terrain-following height coordinate with rotated geographical coordinates and a grid stretching in the vertical are used for the coordinate system. The subgrid-scale moist convection is parametrized using the Tiedtke mass flux scheme [Tiedtke, 1989] with equilibrium closure based on mois...

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Analysis of the influence of Saharan air layer on tropical cyclone intensity using AIRS/Aqua data

Cited by 57 publications

References 20 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

The Pre-Depression Investigation of Cloud-Systems in the Tropics (PREDICT) Experiment: Scientific Basis, New Analysis Tools, and Some First Results

Mineral dust transport toward Hurricane Helene (2006)

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