LASE Measurements of Water Vapor, Aerosol, and Cloud Distributions in Saharan Air Layers and Tropical Disturbances

Ismail, Syed; Ferrare, R. A.; Browell, Edward V.; Chen, Gao; Anderson, B. E.; Kooi, S. A.; Notari, A.; Butler, C. F.; Burton, S. P.; Marta, Fenn; Dunion, Jason; Heymsfield, G.; Krishnamurti, T. N.; Biswas, Mrinal K.

doi:10.1175/2009jas3136.1

Cited by 36 publications

(45 citation statements)

References 45 publications

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“…AOT values for the 30 vertical profiles over the MDR ranged from 0.1 to 0.7, with a median of 0.4. These estimates are highly consistent with DC-8-based LASE observations (at ambient RH) reported by Ismail et al (2010) which provided observations with a much more extensive geographical coverage. The in-situ based AOT estimates also fall within the range of monthly averaged AERONET observations on the Sal Island for the period of 1994-1999(Tanré et al, 2003 and agree well with the average AOT values of 0.3-0.4 derived from MODIS Terra and Aqua data for the NAMMA time period and study area (i.e., 15-30 • N and 15-30 • W), encompassing the NAMMA sampling region.…”

Section: Assessment Of Saharan Dust Vertical Distributionssupporting

confidence: 88%

Observations of Saharan dust microphysical and optical properties from the Eastern Atlantic during NAMMA airborne field campaign

et al. 2011

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Abstract. As part of the international project entitled "African Monsoon Multidisciplinary Analysis (AMMA)", NAMMA (NASA AMMA) aimed to gain a better understanding of the relationship between the African Easterly Waves (AEWs), the Sahara Air Layer (SAL), and tropical cyclogenesis. The NAMMA airborne field campaign was based out of the Cape Verde Islands during the peak of the hurricane season, i.e., August and September 2006. Multiple Sahara dust layers were sampled during 62 encounters in the eastern portion of the hurricane main development region, covering both the eastern North Atlantic Ocean and the western Saharan desert (i.e., 5-22 • N and 10-35 • W). The centers of these layers were located at altitudes between 1.5 and 3.3 km and the layer thickness ranged from 0.5 to 3 km. Detailed dust microphysical and optical properties were characterized using a suite of in-situ instruments aboard the NASA DC-8 that included a particle counter, an Ultra-High Sensitivity Aerosol Spectrometer, an Aerodynamic Particle Sizer, a nephelometer, and a Particle Soot Absorption Photometer. The NAAMA sampling inlet has a size cut (i.e., 50% transmission efficiency size) of approximately 4 µm in diameter for dust particles, which limits the representativeness of the NAMMA observational findings. The NAMMA dust observations showed relatively low particle number densities, Correspondence to: G. Chen (gao.chen@nasa.gov) ranging from 268 to 461 cm −3 , but highly elevated volume density with an average at 45 µm 3 cm −3 . NAMMA dust particle size distributions can be well represented by tri-modal lognormal regressions. The estimated volume median diameter (VMD) is averaged at 2.1 µm with a small range of variation regardless of the vertical and geographical sampling locations. TheÅngström Exponent assessments exhibited strong wavelength dependence for absorption but a weak one for scattering. The single scattering albedo was estimated at 0.97 ± 0.02. The imaginary part of the refractive index for Sahara dust was estimated at 0.0022, with a range from 0.0015 to 0.0044. Closure analysis showed that observed scattering coefficients are highly correlated with those calculated from spherical Mie-Theory and observed dust particle size distributions. These values are generally consistent with literature values reported from studies with similar particle sampling size range.

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Section: Assessment Of Saharan Dust Vertical Distributionssupporting

confidence: 88%

Observations of Saharan dust microphysical and optical properties from the Eastern Atlantic during NAMMA airborne field campaign

et al. 2011

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“…This zone of intermediate humidity within the SAL can be partly explained by westward transport of relatively moist midlevel SAL air. In addition, as suggested by Ismail et al (2010) (see also Fig. 8c), southerly flow just above the AEJ and in association with AEWs may transport moister air from convective regions to the south.…”

Section: B Dry Air Over the Atlantic Ocean And Within The Salmentioning

confidence: 61%

“…These results have led to assumptions in several studies (Jones et al 2007;Reale et al 2009;Sun et al 2008Sun et al , 2009) that the SAL is very dry throughout its depth. As was shown in sections 3 and 4, the SAL in the eastern Atlantic is often characterized by a midlevel layer of more elevated humidity (Carlson and Prospero 1972;Ismail et al 2010;Messager et al 2009). Furthermore, gradual net cooling of the SAL as it moves westward should lead to a rise in relative humidity at low levels.…”

Section: B the Sal Is Dry Throughout Its Depthmentioning

confidence: 71%

“…The dry region off of the northwest African coast also shows signs of warming in a region of large-scale subsidence. Relatively high humidity is found extending northward from the ITCZ over western Africa, a possible indication of the moistening near the top of the SAL (Carlson and Prospero 1972) and perhaps horizontal transport from the ITCZ (Ismail et al 2010).…”

Section: B Dry Air Over the Atlantic Ocean And Within The Salmentioning

confidence: 96%

“…The transition from dry to relatively moist conditions occurs on average in the 700-600-hPa layer, although considerable variability likely exists in individual SAL events. Although not described in this context, Lidar Atmospheric Sensing Experiment (LASE), rawinsondes, and dropsonde measurements obtained during the NASA African Monsoon Multidisciplinary Activities experiment in 2006 (Zipser et al 2009;Ismail et al 2010) suggest that this vertical structure of humidity is often found at the west coast and downstream of Africa over the eastern Atlantic as well.…”

Section: Datamentioning

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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Saharan dust, convective lofting, aerosol enhancement zones, and potential impacts on ice nucleation in the tropical upper troposphere

et al. 2017

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Dry aerosol size distributions and scattering coefficients were measured on 10 flights in 32 clear‐air regions adjacent to tropical storm anvils over the eastern Atlantic Ocean. Aerosol properties in these regions were compared with those from background air in the upper troposphere at least 40 km from clouds. Median values for aerosol scattering coefficient and particle number concentration >0.3 μm diameter were higher at the anvil edges than in background air, showing that convective clouds loft particles from the lower troposphere to the upper troposphere. These differences are statistically significant. The aerosol enhancement zones extended ~10–15 km horizontally and ~0.25 km vertically below anvil cloud edges but were not due to hygroscopic growth since particles were measured under dry conditions. Number concentrations of particles >0.3 μm diameter were enhanced more for the cases where Saharan dust layers were identified below the clouds with airborne lidar. Median number concentrations in this size range increased from ~100 l−1 in background air to ~400 l−1 adjacent to cloud edges with dust below, with larger enhancements for stronger storm systems. Integration with satellite cloud frequency data indicates that this transfer of large particles from low to high altitudes by convection has little impact on dust concentrations within the Saharan Air Layer itself. However, it can lead to substantial enhancement in large dust particles and, therefore, heterogeneous ice nuclei in the upper troposphere over the Atlantic. This may induce a cloud/aerosol feedback effect that could impact cloud properties in the region and downwind.

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LASE Measurements of Water Vapor, Aerosol, and Cloud Distributions in Saharan Air Layers and Tropical Disturbances

Cited by 36 publications

References 45 publications

Observations of Saharan dust microphysical and optical properties from the Eastern Atlantic during NAMMA airborne field campaign

Observations of Saharan dust microphysical and optical properties from the Eastern Atlantic during NAMMA airborne field campaign

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

Saharan dust, convective lofting, aerosol enhancement zones, and potential impacts on ice nucleation in the tropical upper troposphere

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