scholarly journals Model Sensitivity Study of the Direct Radiative Impact of Saharan Dust on the Early Stage of Hurricane Earl

Atmosphere ◽  
2021 ◽  
Vol 12 (9) ◽  
pp. 1181
Author(s):  
Jianyu Liang ◽  
Yongsheng Chen ◽  
Avelino F. Arellano ◽  
Abdulla Al Mamun
Keyword(s):  
Early Stage ◽  
Sensitivity Study ◽  
Development Stage ◽  
Saharan Dust ◽  
African Easterly Waves ◽  
Pressure System ◽  
Easterly Waves ◽  
Aerosol Distribution ◽  
Low Pressure System ◽  
Model Treatment

Current studies report inconsistent results about the impacts of Saharan dust on the development of African Easterly Waves (AEWs), the African Easterly Jet (AEJ), and tropical cyclones (TCs). We present a modeling case study to further elucidate the direct radiative impacts of dust on the early development stage of a TC. We conducted experiments using the Weather Research and Forecasting model coupled with chemistry (WRF-Chem-V3.9.1) to simulate Hurricane Earl (2010) which was influenced by the dusty Saharan Air Layer (SAL). We used the aerosol product from ECMWF MACC-II as the initial and boundary conditions to represent aerosol distribution, along with typical model treatment of its radiative and microphysical effects in WRF. Our simulations at 36-km resolution show that, within the first 36 h, the presence of dust weakens the low-pressure system over North Africa by less than 1 hPa and reduces its mean temperature by 0.03 K. Dust weakens and intensifies the AEJ at its core and periphery, respectively, with magnitudes less than 0.2 m/s. Dust slightly shifts the position of 600 hPa AEW to the south and reduces its intensity prior to impacting the TC. Finally, TC with dust remains weaker.

scholarly journals A Characterization of African Easterly Waves on 2.5–6-Day and 6–9-Day Time Scales

2013 ◽  
Vol 26 (18) ◽  
pp. 6750-6774 ◽  
Author(s):  
Man-Li C. Wu ◽  
Oreste Reale ◽  
Siegfried D. Schubert
Keyword(s):  
Time Scales ◽  
Ensemble Empirical Mode Decomposition ◽  
African Easterly Waves ◽  
Pressure System ◽  
Easterly Waves ◽  
African Easterly Jet ◽  
The North ◽  
Easterly Wave ◽  
Mode Decomposition ◽  
The Tropics

Abstract This study shows that the African easterly wave (AEW) activity over the African monsoon region and the northern tropical Atlantic can be divided in two distinct temporal bands with time scales of 2.5–6 and 6–9 days. The results are based on a two-dimensional ensemble empirical mode decomposition (2D-EEMD) of the Modern-Era Retrospective Analysis for Research and Applications (MERRA). The novel result of this investigation is that the 6–9-day waves appear to be located predominantly to the north of the African easterly jet (AEJ), originate at the jet level, and are different in scale and structure from the well-known low-level 2.5–6-day waves that develop baroclinically on the poleward flank of the AEJ. Moreover, they appear to interact with midlatitude eastward-propagating disturbances, with the strongest interaction taking place at the latitudes where the core of the Atlantic high pressure system is located. Composite analyses applied to the mode decomposition indicate that the interaction of the 6–9-day waves with midlatitude systems is characterized by enhanced southerly (northerly) flow from (toward) the tropics. This finding agrees with independent studies focused on European floods, which have noted enhanced moist transport from the ITCZ toward the Mediterranean region on time scales of about a week as important precursors of extreme precipitation.

scholarly journals Saharan Dust Transport during the Incipient Growth Phase of African Easterly Waves

Geosciences ◽  
2019 ◽  
Vol 9 (9) ◽  
pp. 388 ◽  
Author(s):  
Terrence R. Nathan ◽  
Dustin F. P. Grogan ◽  
Shu-Hua Chen
Keyword(s):  
Wave Field ◽  
Growth Phase ◽  
Wrf Model ◽  
Reanalysis Data ◽  
Saharan Dust ◽  
Critical Surface ◽  
African Easterly Waves ◽  
Analytical Analysis ◽  
Dust Transport ◽  
Easterly Waves

An analytical analysis is combined with numerical modeling simulations in order to expose the physical and dynamical processes that control the zonal-mean transport of Saharan mineral dust aerosols during the incipient growth phase of African easterly waves. The analytical analysis provides the theoretical basis for understanding and predicting how the waves and background flow combine to affect the zonal-mean eddy transports of dust. The analytically derived transport equations―which are valid for any wave field, irrespective of its spatial or temporal scale―predict that the eddy transports of dust are largest where the maximum in the background dust gradients coincide with a critical surface, i.e., where the Doppler-shifted frequency of the wave field vanishes. Linear simulations of the eddy dust transports are conducted using a mechanistic version of the Weather Research and Forecasting (WRF) model coupled to an interactive dust model. The simulations show that the eddy dust transports are directed down the background dust gradients and that the meridional transports of dust dominate over the vertical transports. The numerical simulations confirm the theoretical predictions. The predictions are used to explain recent statistical analyses of reanalysis data for dust-coupled African easterly waves.

scholarly journals Effects of Saharan Dust on African Easterly Waves: The Impact of Aerosol-Affected Satellite Radiances on Data Assimilation

2021 ◽  
Author(s):  
Dustin Francis Phillip Grogan ◽  
Cheng-Hsuan Lu ◽  
Shih-Wei Wei ◽  
Sheng-Po Chen
Keyword(s):  
Data Assimilation ◽  
Large Scale ◽  
West African Monsoon ◽  
Forecast Model ◽  
Saharan Dust ◽  
African Easterly Waves ◽  
Radiative Effects ◽  
Easterly Waves ◽  
Brightness Temperatures ◽  
The Impact

Abstract. This study incorporates time-varying aerosols into satellite radiance calculations within the Global Data Assimilation System (GDAS) to investigate its impact on African easterly waves (AEWs) and their environment. Comparison of analysis fields from the aerosol-aware experiment and an aerosol-blind control during August 2017 showed that the aerosol-affected radiances accelerated the African easterly jet and West African monsoon flow; warmed the Saharan boundary layer; and modified the AEW vorticity structure, with increases in the northern circulation and decreases in the southern circulation. Analysis fields from each experiment were used in the Global Forecast System (GFS) to examine differences in forecasting two AEW cases that developed hurricanes over the Atlantic, but were structurally different over North Africa. The aerosol-aware experiment reduced errors in forecasting the AEW case whose northern circulation interacted with a large-scale Saharan dust plume; neutral improvement was found for the other AEW, which did not contain a northern circulation nor interacted with a dust plume. The changes to the analysis fields by the aerosol-aware assimilation are reminiscent of dust radiative effects that operate on AEWs and their environment. That is, the aerosol-affected radiances produce corrections to the brightness temperatures that modify the analysis fields like dust aerosols that are radiatively coupled to the atmospheric variables in the forecast model. We show qualitatively that dust radiative effects are captured by the aerosol-affected radiances for the AEW case that interacted with a dust plume, which served to improve forecasts of the wave downstream.

scholarly journals Passive versus Active Transport of Saharan Dust Aerosols by African Easterly Waves

Atmosphere ◽  
2021 ◽  
Vol 12 (11) ◽  
pp. 1509
Author(s):  
Dustin F. P. Grogan ◽  
Terrence R. Nathan
Keyword(s):  
Active Transport ◽  
Conservation Equation ◽  
Saharan Dust ◽  
Weather Research And Forecasting ◽  
Critical Surface ◽  
African Easterly Waves ◽  
Dust Transport ◽  
Easterly Waves ◽  
Active Case ◽  
Theoretical Predictions

Theory and modeling are combined to reveal the physical and dynamical processes that control Saharan dust transport by amplifying African easterly waves (AEWs). Two cases are examined: active transport, in which the dust is radiatively coupled to the circulation; passive transport, in which the dust is radiatively decoupled from the circulation. The theory is built around a dust conservation equation for dust-coupled AEWs in zonal-mean African easterly jets. The theory predicts that, for both the passive and active cases, the dust transports will be largest where the zonal-mean dust gradients are maximized on an AEW critical surface. Whether the dust transports are largest for the radiatively passive or radiatively active case depends on the growth rate of the AEWs, which is modulated by the dust heating. The theoretical predictions are confirmed via experiments carried out with the Weather Research and Forecasting model, which is coupled to a dust conservation equation. The experiments show that the meridional dust transports dominate in the passive case, while the vertical dust transports dominate in the active case.

Structural Changes in the African Easterly Jet and Its Role in Mediating the Effects of Saharan Dust on the Linear Dynamics of African Easterly Waves

2019 ◽  
Vol 76 (11) ◽  
pp. 3351-3365 ◽  
Author(s):  
Dustin F. P. Grogan ◽  
Terrence R. Nathan ◽  
Shu-Hua Chen
Keyword(s):  
Growth Rates ◽  
Structural Changes ◽  
Numerical Results ◽  
Saharan Dust ◽  
Vertical Shear ◽  
African Easterly Waves ◽  
Linear Dynamics ◽  
Easterly Waves ◽  
Zonal Scale ◽  
African Easterly Jet

Abstract Analytical and numerical analyses are used to examine how structural changes to the African easterly jet (AEJ) mediate the effects of Saharan mineral dust aerosols on the linear dynamics of African easterly waves (AEWs). An analytical expression for the generation of eddy available potential energy (APE) is derived that exposes how the AEJ and dust combine to affect the energetics of the AEWs. The expression is also used to interpret the numerical results, which are obtained by radiatively coupling a simplified version of the Weather Research and Forecasting Model to a conservation equation for dust. The WRF-Dust model is used to conduct linear simulations based on five observationally consistent zonal-mean AEJs: a reference AEJ and four other AEJs that are obtained by perturbing the maximum meridional and vertical shear. For a dust distribution consistent with summertime observations over North Africa, the numerical simulations show the following: (i) Irrespective of the AEJ structure or the zonal scale of the AEWs, the dust increases the growth rates of the AEWs. (ii) The growth rates of the AEWs are optimized when the ratio of baroclinic to barotropic energy conversions is largest. (iii) When the energy conversions are sufficiently large, the zonal scale of the fastest-growing AEW shortens. The numerical results confirm the analytical analysis, which shows that the dust effects, which are modulated by the Doppler-shifted frequency, are strongest north of the AEJ axis, a region where the dust augments the preexisting meridional temperature gradient.

scholarly journals Direct Radiative Effect of Mineral Dust on the Development of African Easterly Waves in Late Summer, 2003–07

2012 ◽  
Vol 51 (12) ◽  
pp. 2090-2104 ◽  
Author(s):  
Po-Lun Ma ◽  
Kai Zhang ◽  
Jainn Jong Shi ◽  
Toshihisa Matsui ◽  
Albert Arking
Keyword(s):  
Potential Energy ◽  
Radiative Forcing ◽  
Late Summer ◽  
Saharan Dust ◽  
Primary Role ◽  
African Easterly Waves ◽  
Radiative Effect ◽  
Easterly Waves ◽  
Available Potential Energy ◽  
Episodic Events

AbstractEpisodic events of both Saharan dust outbreaks and African easterly waves (AEWs) are observed to move westward over the eastern tropical Atlantic Ocean. The relationship between the warm, dry, and dusty Saharan air layer on the nearby storms has been the subject of considerable debate. In this study, the Weather Research and Forecasting model is used to investigate the radiative effect of dust on the development of AEWs during August and September, the months of maximum tropical cyclone activity, in years 2003–07. The simulations show that dust radiative forcing enhances the convective instability of the environment. As a result, most AEWs intensify in the presence of a dust layer. The Lorenz energy cycle analysis reveals that the dust radiative forcing enhances the condensational heating, which elevates the zonal and eddy available potential energy. In turn, available potential energy is effectively converted to eddy kinetic energy, in which local convective overturning plays the primary role. The magnitude of the intensification effect depends on the initial environmental conditions, including moisture, baroclinity, and the depth of the boundary layer. The authors conclude that dust radiative forcing, albeit small, serves as a catalyst to promote local convection that facilitates AEW development.

scholarly journals Effects of Saharan Dust on the Linear Dynamics of African Easterly Waves

2016 ◽  
Vol 73 (2) ◽  
pp. 891-911 ◽  
Author(s):  
Dustin F. P. Grogan ◽  
Terrence R. Nathan ◽  
Shu-Hua Chen
Keyword(s):  
Wrf Model ◽  
Saharan Dust ◽  
Weather Research And Forecasting ◽  
Critical Surface ◽  
African Easterly Waves ◽  
Linear Dynamics ◽  
Analytical Analysis ◽  
Easterly Waves ◽  
Spatial Gradients ◽  
Dust Distribution

Abstract The direct radiative effects of Saharan mineral dust aerosols on the linear dynamics of African easterly waves (AEWs) are examined analytically and numerically. The analytical analysis combines the thermodynamic equation with a dust continuity equation to form an expression for the dust-modified generation of eddy available potential energy . The dust-modified is a function of the transmissivity and spatial gradients of the dust, which are modulated by the Doppler-shifted frequency. The expression for predicts that for a fixed dust distribution, the wave response will be largest in regions where the dust gradients are maximized and the Doppler-shifted frequency vanishes. The numerical analysis uses the Weather Research and Forecasting (WRF) Model coupled to an online dust model to calculate the linear dynamics of AEWs. Zonally averaged basic states for wind, temperature, and dust are chosen consistent with summertime conditions over North Africa. For the fastest-growing AEW, the dust increases the growth rate from ~15% to 90% for aerosol optical depths ranging from τ = 1.0 to τ = 2.5. A local energetics analysis shows that for τ = 1.0, the dust increases the maximum barotropic and baroclinic energy conversions by ~50% and ~100%, respectively. The maxima in the generation and conversions of energy are collocated and occur where the meridional dust gradient is maximized near the critical surface—that is, where the Doppler-shifted frequency is small, in agreement with the prediction from the analytical analysis.

scholarly journals Early Detection of Encroaching Woody Juniperus virginiana and Its Classification in Multi-Species Forest Using UAS Imagery and Semantic Segmentation Algorithms

2021 ◽  
Vol 13 (10) ◽  
pp. 1975
Author(s):  
Lin Wang ◽  
Yuzhen Zhou ◽  
Qiao Hu ◽  
Zhenghong Tang ◽  
Yufeng Ge ◽  
...  
Keyword(s):  
Early Detection ◽  
Early Development ◽  
Spatial Resolution ◽  
Early Stage ◽  
Semantic Segmentation ◽  
Development Stage ◽  
Unmanned Aircraft ◽  
Economic Losses ◽  
Juniperus Virginiana ◽  
Segmentation Algorithms

Woody plant encroachment into grasslands ecosystems causes significantly ecological destruction and economic losses. Effective and efficient management largely benefits from accurate and timely detection of encroaching species at an early development stage. Recent advances in unmanned aircraft systems (UAS) enabled easier access to ultra-high spatial resolution images at a centimeter level, together with the latest machine learning based image segmentation algorithms, making it possible to detect small-sized individuals of target species at early development stage and identify them when mixed with other species. However, few studies have investigated the optimal practical spatial resolution of early encroaching species detection. Hence, we investigated the performance of four popular semantic segmentation algorithms (decision tree, DT; random forest, RF; AlexNet; and ResNet) on a multi-species forest classification case with UAS-collected RGB images in original and down-sampled coarser spatial resolutions. The objective of this study was to explore the optimal segmentation algorithm and spatial resolution for eastern redcedar (Juniperus virginiana, ERC) early detection and its classification within a multi-species forest context. To be specific, firstly, we implemented and compared the performance of the four semantic segmentation algorithms with images in the original spatial resolution (0.694 cm). The highest overall accuracy was 0.918 achieved by ResNet with a mean interaction over union at 85.0%. Secondly, we evaluated the performance of ResNet algorithm with images in down-sampled spatial resolutions (1 cm to 5 cm with 0.5 cm interval). When applied on the down-sampled images, ERC segmentation performance decreased with decreasing spatial resolution, especially for those images coarser than 3 cm spatial resolution. The UAS together with the state-of-the-art semantic segmentation algorithms provides a promising tool for early-stage detection and localization of ERC and the development of effective management strategies for mixed-species forest management.

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