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  1. IMPROVING THE SAFFIR-SIMPSON SCALE AND OUR RESPONSE TO HURRICANES Author: Logan Giles Published: 22nd December 22nd, 2018 Abstract: This paper analyzes why the Saffir-Simpson Scale remains outdated and dangerous, and will offer a suggestion for a new scale. Currently, the global scale on how we measure hurricanes and tropical systems continues to be the Saffir-Simpson Scale. This scale, first created in 1971 provides a means for the public to better assess the severity of these storms. The Saffir-Simpson Scale uses one factor – wind. The higher the wind, the stronger the storm, and therefore, the higher assigned number. While wind speed certainly remains important, it should not be the only factor used to determine the severity of a storm. Additional weather threats should factor into the analysis to better account for potential damage hurricanes and tropical storms bring. By including these additional threats, the public will be better informed to make decisions that affect their safety and that of their families. Link to full paper: Link to Blog On This paper:
  2. How Global Warming and Arctic Ice Melt Intensify Hurricanes - YouTube Presentation Presented By: Dr Jennifer Francis (Research Professor at Rutgers University's Institute of Marine and Coastal Sciences since 1994) Interviewed By: Greg Wilpert Broadcast Team: The Real News Network Presentation Date: 21st September, 2018 Link to YouTube presentation (13 minutes): You can click on the chart above or use this link:
  3. 2017 Atlantic Hurricane Forecasts From a High-Resolution Version of the GFDL fvGFS Model: Evaluation of Track, Intensity, and Structure Authors: Andrew T. Hazelton, Morris Bender, Matthew Morin, Lucas Harris, and Shian-Jiann Lin Published: Aug 2018 Abstract: The 2017 Atlantic hurricane season had several high-impact tropical cyclones (TCs), including multiple cases of rapid intensification (RI). A high-resolution nested version of the GFDL fvGFS model (HifvGFS) was used to conduct hindcasts of all Atlantic TCs between August 7 and October 15. HifvGFS showed promising track forecast performance, with similar error patterns and skill compared to the operational GFS and HWRF models. Some of the larger track forecast errors were associated with the erratic tracks of Jose and Lee. A case study of Maria found that although the track forecasts were generally skillful, a right-of-track bias was noted in some cases associated with initialization and prediction of ridging north of the storm. The intensity forecasts showed large improvement over the GFS and global fvGFS models, but were somewhat less skillful than HWRF. The largest negative intensity forecast errors were associated with the RI of Irma, Lee, and Maria, while the largest positive errors were found with recurving cases that were generally weakening. The structure forecasts were also compared with observations, and HifvGFS was found to generally have wind radii larger than observations. Detailed examination of the forecasts of Hurricanes Harvey and Maria showed that HifvGFS was able to predict the structural evolution leading to RI in some cases, but was not as skillful with other RI cases. One case study of Maria suggested that inclusion of ocean coupling could significantly reduce the positive bias seen during and after recurvature. Link to full paper:
  4. What Does "Invest" Mean With Tropical Weather? Prepared and Updated By: Mark Bingaman on the "Sciencing" website Last Update: January 9th, 2018 Review: This is one of a number of simple fact sheets produced by "Sciencing". This one provides a simple explanation of what "Invest" meaning when used in terms of tropical cyclone development. The artical covers identification, effects and invest names. there are links to other science facts sheets (some are weather related), other sites and papers. Link to Full Fact Sheet:
  5. Revisiting the 26.5°C Sea Surface Temperature Threshold for Tropical Cyclone Development Authors: Ron McTaggart-Cowan, Emily L. Davies, Jonathan G. Fairman Jr., Thomas J. Galarneau Jr. and David M. Schultz Published: 4th November, 2014 Abstract: A high sea surface temperature is generally accepted to be one of the necessary ingredients for tropical cyclone development, indicative of the potential for surface heat and moisture fluxes capable of fueling a self-sustaining circulation. Although the minimum 26.5°C threshold for tropical cyclogenesis has become a mainstay in research and education, the fact that a nonnegligible fraction of storm formation events (about 5%) occur over cooler waters casts some doubt on the robustness of this estimate. Tropical cyclogenesis over subthreshold sea surface temperatures is associated with low tropopause heights, indicative of the presence of a cold trough aloft. To focus on this type of development environment, the applicability of the 26.5°C threshold is investigated for tropical transitions from baroclinic precursor disturbances in all basins between 1989 and 2013. Although the threshold performs well in the majority of cases without appreciable environmental baroclinicity, the potential for development is underestimated by up to 27% for systems undergoing tropical transition. An alternative criterion of a maximum 22.5°C difference between the tropopause-level and 850-hPa equivalent potential temperatures (defined as the coupling index) is proposed for this class of development. When combined with the standard 26.5°C sea surface temperature threshold for precursor-free environments, error rates are reduced to 3%–6% for all development types. The addition of this physically relevant representation of the deep-tropospheric state to the ingredients-based conceptual model for tropical cyclogenesis improves the representation of the important tropical transition-based subset of development events. Link to full paper: Link to supplement:
  6. Are Multiple Tropical Cyclone Events Similar among Basins? Authors: Benjamin A. Schenkel Published: 22nd May, 2017 Abstract: The present study intercompares multiple tropical cyclone event (MTCE) characteristics among each global tropical cyclone (TC) basin using best-track data. Specific focus is placed on examining the number of MTCEs and TCs during MTCEs, the zonal distance between TCs during MTCEs, and the spatiotemporal separation between genesis events during MTCEs. The results suggest that the ratio of MTCEs relative to single TCs is substantially higher in the eastern North Pacific (ENP), western North Pacific (WNP), and south Indian Ocean (SI) basins compared to the North Atlantic (NA) and South Pacific (SP). The prolific nature of ENP, WNP, and SI MTCE activity results in approximately half of TCs occurring during MTCEs. During new TC genesis, the majority of preexisting TCs are generally located westward at a consistent zonal distance from new TC genesis for MTCEs within each basin with median values between −1620 and −1961 km. TC-induced Rossby wave dispersion may set this zonal length scale as implied by its moderate-to-strong correlations (R = 0.38–0.85; p < 0.05) with the shallow-water zonal wavelength of TC-induced stationary Rossby waves. A substantial majority of TC genesis events occur progressively eastward during ENP, WNP, and SP MTCEs, whereas NA and SI MTCEs exhibit no such tendency. Last, the temporal separation between the genesis of preexisting and new TCs is generally similar among basins with median values between 3 and 4 days. Together, these results are indicative of unusual similarity in MTCE characteristics among basins despite differences in environmental and TC characteristics in each basin. Link to full paper:
  7. Kelvin Waves and Tropical Cyclogenesis: A Global Survey Authors: Carl J. Schreck III Published: 15th June, 2015 Abstract: Convectively coupled atmospheric Kelvin waves are among the most prominent sources of synoptic-scale rainfall variability in the tropics, but large uncertainties surround their role in tropical cyclogenesis. This study identifies the modulation of tropical cyclones relative to the passage of a Kelvin wave’s peak rainfall (i.e., its crest) in each basin. Tropical cyclogenesis is generally inhibited for 3 days before the crest and enhanced for 3 days afterward. Composites of storms forming in the most favorable lags illustrate the dynamical impacts of the waves. In most basins, the tropical cyclone actually forms during the convectively suppressed phase of the wave. The 850-hPa equatorial westerly anomalies provide the cyclonic vorticity for the nascent storm, and 200-hPa easterly anomalies enhance the outflow. The wind anomalies persist at both levels longer than the Kelvin wave’s period and are often related to the Madden–Julian oscillation (MJO). The onset of these wind anomalies occurs with the Kelvin wave passage, while the MJO apparently establishes their duration. Many of the composites also show evidence of an easterly wave from which the tropical cyclone develops. The composite easterly wave amplifies or even initiates within the Kelvin wave crest. These results show the importance of Kelvin waves interacting with the MJO and easterly waves during tropical cyclogenesis. Given that Kelvin waves often circumnavigate the globe, these results show promise for long-range forecasting of tropical cyclogenesis in all basins. Link to full paper:
  8. Understanding African easterly waves: a moist singular vector approach Authors: Rosalind J. Cornforth and Brian J. Hoskins Published: 11th September, 2009 Abstract: Moist singular vectors (MSV) have been applied successfully to predicting mid‐latitude storms growing in association with latent heat of condensation. Tropical cyclone sensitivity has also been assessed. Extending this approach to more general tropical weather systems here, MSVs are evaluated for understanding and predicting African easterly waves, given the importance of moist processes in their development. First results, without initial moisture perturbations, suggest MSVs may be used advantageously. Perturbations bear similar structural and energy profiles to previous idealised non‐linear studies and observations. Strong sensitivities prevail in the metrics and trajectories chosen, and benefits of initial moisture perturbations should be appraised. Copyright © 2009 Royal Meteorological Society Link to full paper:
  9. Examining Tropical Cyclone–Kelvin Wave Interactions Using Adjoint Diagnostics Authors: Carolyn A. Reynolds, James D. Doyle, and Xiaodong Hong Published: 16th August, 2016 Abstract: The initial-state sensitivity and interactions between a tropical cyclone and atmospheric equatorial Kelvin waves associated with the Madden–Julian oscillation (MJO) during the DYNAMO field campaign are explored using adjoint-based tools from the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS). The development of Tropical Cyclone 5 (TC05) coincided with the passage of an equatorial Kelvin wave (KW) and westerly wind burst associated with an MJO that developed in the Indian Ocean in late November 2011. COAMPS 18-h adjoint sensitivities of low-level kinetic energy to changes in initial state winds, temperature, and water vapor are analyzed for both TC05 and the KW to document when the evolution of each system is sensitive to the other. Time series of sensitivity patterns confirm that TC05 and the KW low-level westerlies are sensitive to each other when the KW is to the southwest and south of TC05. While TC05 is not sensitive to the KW after this, the KW low-level westerlies remain sensitive to TC05 until it enters the far eastern Indian Ocean. Vertical profiles of both TC05 and KW sensitivity indicate lower-tropospheric maxima in temperature, wind, and moisture, with KW sensitivity typically 20% smaller than TC05 sensitivity. The magnitude of the sensitivity for both systems is greatest just prior to, and during, their closest proximity. A case study examination reveals that adjoint-based optimal perturbations grow and expand quickly through a dynamic response to decreased static stability. The evolution of moist-only and dry-only initial perturbations illustrates that the moist component is primarily responsible for the initial rapid growth, but that subsequent growth rates are similar. Link to full paper:
  10. Intraseasonal Tropical Cyclogenesis Prediction in a Global Coupled Model System Authors: Xianan Jiang, Baoqiang Xiang, Ming Zhao, Tim Li, Shian-Jiann Lin, Zhuo Wang and Jan-Huey Chen Published: 10th April, 2018 (published online; 10th July, 2018) Abstract: Motivated by increasing demand in the community for intraseasonal predictions of weather extremes, predictive skill of tropical cyclogenesis is investigated in this study based on a global coupled model system. Limited intraseasonal cyclogenesis prediction skill with a high false alarm rate is found when averaged over about 600 tropical cyclones (TCs) over global oceans from 2003 to 2013, particularly over the North Atlantic (NA). Relatively skillful genesis predictions with more than 1-week lead time are only evident for about 10% of the total TCs. Further analyses suggest that TCs with relatively higher genesis skill are closely associated with the Madden–Julian oscillation (MJO) and tropical synoptic waves, with their geneses strongly phase-locked to the convectively active region of the MJO and low-level cyclonic vorticity associated with synoptic-scale waves. Moreover, higher cyclogenesis prediction skill is found for TCs that formed during the enhanced periods of strong MJO episodes than those during weak or suppressed MJO periods. All these results confirm the critical role of the MJO and tropical synoptic waves for intraseasonal prediction of TC activity. Tropical cyclogenesis prediction skill in this coupled model is found to be closely associated with model predictability of several large-scale dynamical and thermodynamical fields. Particularly over the NA, higher predictability of low-level relative vorticity, midlevel humidity, and vertical zonal wind shear is evident along a tropical belt from the West Africa coast to the Caribbean Sea, in accord with more predictable cyclogenesis over this region. Over the extratropical NA, large-scale variables exhibit less predictability due to influences of extratropical systems, leading to poor cyclogenesis predictive skill. Link to full paper:
  11. Impacts of Convectively Coupled Kelvin Waves on Environmental Conditions for Atlantic Tropical Cyclogenesis Authors: Michael J. Ventrice, Christopher D. Thorncroft and Carl J. Schreck III Published: 2nd February, 2012 Abstract: High-amplitude convectively coupled equatorial atmospheric Kelvin waves (CCKWs) are explored over the tropical Atlantic during the boreal summer (1989–2009). Focus is given to the atmospheric environmental conditions that are important for tropical cyclogenesis. CCKWs are characterized by deep westerly vertical wind shear to the east of its convectively active phase and easterly vertical wind shear to the west of it. This dynamical signature increases vertical wind shear over the western tropical Atlantic ahead of the convectively active phase, and reduces vertical wind shear after its passage. The opposite is true over the eastern tropical Atlantic where the climatological vertical wind shear is easterly. Positive total column water vapor (TCWV) anomalies progress eastward with the convectively active phase of the CCKW, whereas negative TCWV anomalies progress eastward with the convectively suppressed phase. During the passage of the convectively active phase of the CCKW, a zonally oriented strip of low-level cyclonic relative vorticity is generated over the tropical Atlantic. Two days later, this strip becomes more wavelike and moves back toward the west. This signature resembles a train of westward-moving easterly waves and suggests CCKWs may influence such events. Strong CCKWs over the tropical Atlantic tend to occur during the decay of the active convection associated with the Madden–Julian oscillation over the Pacific. This relationship could be used to provide better long-range forecasts of tropical convective patterns and Atlantic tropical cyclogenesis. Link to full paper:
  12. On the Influence of Convectively Coupled Kelvin Waves on African Easterly waves - Presentation A presentation at the AMS 32nd Conference on “Hurricanes and Tropical Meteorology” held at the Condado Plaza Hilton in Puerto Rico between 17th and 22nd April, 2016. Presenters: Chris D. Thorncroft  Presentation Date: 18th April, 2016 Presentation Summary: While Convectively Coupled Kelvin Waves (CCKWs) are generally weaker in Boreal Summer than in Boreal Spring in the tropical West African region, previous reseach has shown that they can have a significant impact on African Easterly Waves (AEWs) in the West African and tropical Atlantic regions. This talk will highlight the significance of CCKWs in determining variability in AEW behaviour including how they impact: (i) Initiation of AEWs, (ii) Convection within existing AEWs and (iii) Development of favorable AEW structures for tropical cyclogenesis in the tropical Atlantic. Reanalysis and satellite datasets will be combined to shed light on these interactions from both a climatological and a case-study perspective. A major conclusion from this work is the strong recognition that forecasters in the region should be closely monitoring the propagation of CCKWs into the region and that medium-range weather prediction efforts in the tropics should be paying close attention to the fidelity of models to represent CCKWs. Link to conference video presentation (16 minutes):;uniqueid=Paper293941&amp;entry_password=390692 Link to full conference agenda:
  13. Effect of remote sea surface temperature change on tropical cyclone potential intensity Authors: Gabriel A. Vecchi and Brian J. Soden (Nature volume 450, pages 1066–1070) Published: 13th December, 2007 Abstract: The response of tropical cyclone activity to global warming is widely debated. It is often assumed that warmer sea surface temperatures provide a more favourable environment for the development and intensification of tropical cyclones, but cyclone genesis and intensity are also affected by the vertical thermodynamic properties of the atmosphere. Here we use climate models and observational reconstructions to explore the relationship between changes in sea surface temperature and tropical cyclone ‘potential intensity’—a measure that provides an upper bound on cyclone intensity and can also reflect the likelihood of cyclone development. We find that changes in local sea surface temperature are inadequate for characterizing even the sign of changes in potential intensity, but that long-term changes in potential intensity are closely related to the regional structure of warming; regions that warm more than the tropical average are characterized by increased potential intensity, and vice versa. We use this relationship to reconstruct changes in potential intensity over the twentieth century from observational reconstructions of sea surface temperature. We find that, even though tropical Atlantic sea surface temperatures are currently at a historical high, Atlantic potential intensity probably peaked in the 1930s and 1950s, and recent values are near the historical average. Our results indicate that—per unit local sea surface temperature change—the response of tropical cyclone activity to natural climate variations, which tend to involve localized changes in sea surface temperature, may be larger than the response to the more uniform patterns of greenhouse-gas-induced warming. Link to full paper:;rep=rep1&amp;type=pdf
  14. Projected changes in African easterly wave intensity and track in response to greenhouse forcing Authors: Christopher Bryan Skinnera and Noah S. Diffenbaugha Published: 28th April, 2014 Abstract: Synoptic-scale African easterly waves (AEWs) impact weather throughout the greater Atlantic basin. Over the African continent, AEWs are instrumental in initiating and organizing precipitation in the drought-vulnerable Sahel region. AEWs also serve as the precursors to the most intense Atlantic hurricanes, and contribute to the global transport of Saharan dust. Given the relevance of AEWs for the climate of the greater Atlantic basin, we investigate the response of AEWs to increasing greenhouse gas concentrations. Using an ensemble of general circulation models, we find a robust increase in the strength of the winds associated with AEWs along the Intertropical Front in West Africa by the late 21st century of the representative concentration pathway 8.5. AEW energy increases directly due to an increase in baroclinicity associated with an enhanced meridional temperature gradient between the Sahara and Guinea Coast. Further, the pattern of low-level warming supports AEW development by enhancing monsoon flow, resulting in greater convergence and uplift along the Intertropical Front. These changes in energetics result in robust increases in the occurrence of conditions that currently produce AEWs. Given relationships observed in the current climate, such changes in the location of AEW tracks and the magnitude of AEW winds carry implications for the relationship between AEWs and precipitation in the Sahel, the mobilization of Saharan dust, and the likelihood of cyclogenesis in the Atlantic. Our results therefore suggest that changes in AEW characteristics could play a critical role in shaping the response of Atlantic basin climate to future increases in greenhouse gas concentrations. Significance: African easterly waves (AEWs) are ubiquitous weather disturbances over the tropical Atlantic basin during the summer season. However, despite the critical influence of AEWs on Sahel rainfall, transport of Saharan dust, and initiation of Atlantic hurricanes, little is known about how global warming influences AEW activity. We use an ensemble of general circulation models to investigate the response of AEWs to elevated greenhouse gas concentrations. We find that increases in regional temperature gradients and the strength of convergence and uplift along the Intertropical Front of Africa lead to increases in the strength of AEWs. Given the relationships that are observed in the current climate, changes in AEW strength could influence climate over both West Africa and the larger Atlantic basin. Link to full paper: Link to supplement with supporting information:
  15. Subcritical Destabilization of African Easterly Waves by Saharan Mineral Dust Authors: Terrence R. Nathan Published: 23rd December, 2016 Abstract: A theoretical framework is presented that exposes the radiative–dynamical relationships that govern the subcritical destabilization of African easterly waves (AEWs) by Saharan mineral dust (SMD) aerosols. The framework is built on coupled equations for quasigeostrophic potential vorticity (PV), temperature, and SMD mixing ratio. A perturbation analysis yields, for a subcritical, but otherwise arbitrary, zonal-mean background state, analytical expressions for the growth rate and frequency of the AEWs. The expressions are functions of the domain-averaged wave activity, which is generated by the direct radiative effects of the SMD. The wave activity is primarily modulated by the Doppler-shifted phase speed and the background gradients in PV and SMD. Using an idealized version of the Weather Research and Forecasting (WRF) Model coupled to an interactive dust model, a linear analysis shows that, for a subcritical African easterly jet (AEJ) and a background SMD distribution that are consistent with observations, the SMD destabilizes the AEWs and slows their westward propagation, in agreement with the theoretical prediction. The SMD-induced growth rates are commensurate with, and can sometimes exceed, those obtained in previous dust-free studies in which the AEWs grow on AEJs that are supercritical with respect to the threshold for barotropic–baroclinic instability. The clarity of the theoretical framework can serve as a tool for understanding and predicting the effects of SMD aerosols on the linear instability of AEWs in subcritical, zonal-mean AEJs. Link to full paper:
  16. Effects of Saharan Dust on the Linear Dynamics of African Easterly Waves Authors: Dustin F. P. Grogan, Terrence R. Nathan and Shu-Hua Chen Published: 13th November, 2015 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. Link to full paper:
  17. Saharan Dust and the Nonlinear Evolution of the African Easterly Jet–African Easterly Wave System - Presentation and Paper A presentation at the AMS 32nd Conference on “Hurricanes and Tropical Meteorology” held at the Condado Plaza Hilton in Puerto Rico between 17th and 22nd April, 2016. Presenters: Dustin Grogan, T. Nathan and S. H. Chen  Presentation Date: 18th April, 2016 Presentation Summary: The direct radiative effects of Saharan mineral dust aerosols on the nonlinear evolution of African easterly waves (AEWs) and their interactions with the African easterly jet (AEJ) are examined using the Weather Research and Forecasting (WRF) model coupled to an online dust model. The model is initialized with zonal-mean distributions of wind, temperature and dust consistent with summertime conditions over North Africa. The dust modifies the lifecycle of the AEWs in the following way: the domain-averaged eddy kinetic energy (EKE) is enhanced during the linear and nonlinear growth phases, reaching a larger peak amplitude that subsequently decays more rapidly, eventually equilibrating at lower amplitude. The increase in EKE during the growth phases is due to local increases in barotropic energy conversions in the dust plume north of the AEJ. The dust-modified, rapidly decaying phase is primarily associated with enhanced barotropic decay that occurs near the top of the plume north of the AEJ. The timing of peak EKE depends on the initial dust concentration. Throughout the evolution of the AEJ-AEW system, the dust increases the maximum zonal-mean wind speeds. The increase is due to the dust-modified mean meridional circulation during the AEW growth phase. The dust-modified maximum wind speeds are also displaced farther southward and upward during the AEW growth phase, which is due to the enhanced wave fluxes decelerating the flow more efficiently north of the AEJ. The spatial-temporal evolution of the dust-modified critical surface is shown to play an important role in the evolution of the AEJ-AEW system. Link to conference video presentation: Please note that this presentation was withdrawn as the authors later published a full paper which was initially behind a paywall but it has just been made available on free to view basis (see below). Link to full conference agenda: Publication Date: 2nd September, 2016 Abstract (revised version for paper): The direct radiative effects of Saharan mineral dust (SMD) aerosols on the nonlinear evolution of the African easterly jet–African easterly wave (AEJ–AEW) system is examined using the Weather Research and Forecasting Model coupled to an online dust model. The SMD-modified AEW life cycles are characterized by four stages: enhanced linear growth, weakened nonlinear stabilization, larger peak amplitude, and smaller long-time amplitude. During the linear growth and nonlinear stabilization stages, the SMD increases the generation of eddy available potential energy (APE); this occurs where the maximum in the mean meridional SMD gradient is coincident with the critical surface. As the AEWs evolve beyond the nonlinear stabilization stage, the discrimination between SMD particle sizes due to sedimentation becomes more pronounced; the finer particles meridionally expand, while the coarser particles settle to the surface. The result is a reduction in the eddy APE at the base and the top of the plume. The SMD enhances the Eliassen–Palm (EP) flux divergence and residual-mean meridional circulation, which generally oppose each other throughout the AEW life cycle. The SMD-modified residual-mean meridional circulation initially dominates to accelerate the flow but quickly surrenders to the EP flux divergence, which causes an SMD-enhanced deceleration of the AEJ during the linear growth and nonlinear stabilization stages. Throughout the AEW life cycle, the SMD-modified AEJ is elevated and the peak winds are larger than without SMD. During the first (second) half of the AEW life cycle, the SMD-modified wave fluxes shift the AEJ axis farther equatorward (poleward) of its original SMD-free position. Link to full paper:
  18. Internal processes within the African Easterly Wave system Authors: D. Emmanuel Poan, Jean‐Philippe Lafore, Romain Roehrig and Fleur Couvreux Published: 7th July, 2014 Abstract: The internal processes within an African Easterly Wave (AEW) system, involving mass, dynamic and water vapour fields are investigated using ERA‐I reanalysis, in order to highlight the interactions between convection and AEWs. The budgets of heat, moisture and momentum are analysed during the different phases of AEWs detected using synoptic‐scale precipitable water anomalies as proposed by Poan et al. (2013). The strong climatological meridional gradient of moisture present in the Sahel impacts the shape of the apparent heat source and humidity sink. AEW events over the Sahel are associated with a meridional shift of the intertropical convergence zone (ITCZ). Large exchanges of momentum by small‐scale convective transport are also highlighted between the low‐ and mid‐levels, contributing to the reinforcement of the AEW circulation at 600 hPa and the damping of the monsoon flow. This also appears as a possible mechanism for the vertical tilt of the meridional wind associated with AEWs. Heat budget computation, in the southern flank of the West African Heat‐Low (HL) region where such AEWs occur, reveals that the heating anomalies are mainly driven by the horizontal advections. The vertical circulation acts as a precursor, which initiates the heat transport in the lower troposphere. However, weaker, turbulent mixing also participates in the development of these anomalies, especially in the boundary layer. These budgets ultimately allow the distinct contributions of diabatic and adiabatic processes to be determined. Link to full paper:
  19. Simulations of the West African Monsoon with a Superparameterized Climate Model. Part II: African Easterly Waves Authors: Rachel R. McCrary, David A. Randall and Cristiana Stan Published: 17th April, 2014 Abstract: The relationship between African easterly waves and convection is examined in two coupled general circulation models: the Community Climate System Model (CCSM) and the “superparameterized” CCSM (SP-CCSM). In the CCSM, the easterly waves are much weaker than observed. In the SP-CCSM, a two-dimensional cloud-resolving model replaces the conventional cloud parameterizations of CCSM. Results show that this allows for the simulation of easterly waves with realistic horizontal and vertical structures, although the model exaggerates the intensity of easterly wave activity over West Africa. The simulated waves of SP-CCSM are generated in East Africa and propagate westward at similar (although slightly slower) phase speeds to observations. The vertical structure of the waves resembles the first baroclinic mode. The coupling of the waves with convection is realistic. Evidence is provided herein that the diabatic heating associated with deep convection provides energy to the waves simulated in SP-CCSM. In contrast, horizontal and vertical structures of the weak waves in CCSM are unrealistic, and the simulated convection is decoupled from the circulation. Link to full paper:
  20. Absolute and Convective Instability of the African Easterly Jet Authors: Michael Diaz and Anantha Aiyyer Published: 28th January, 2015 Abstract: The stability of the African easterly jet (AEJ) is examined using idealized numerical simulations. It is found that a zonally homogeneous representation of the AEJ can support absolute instability in the form of African easterly waves (AEWs). This finding is verified through a local energy budget, which demonstrates the presence of both upstream and downstream energy fluxes. These energy fluxes allow unstable wave packets to spread upstream and downstream relative to their initial point of excitation. This finding is further verified by showing that the ground-relative group velocity of these wave packets has both eastward and westward components. In contrast with normal-mode instability theory, which emphasizes wave growth through energy extraction from the basic state, the life cycle of the simulated AEWs is strongly governed by energy fluxes. Convergent fluxes at the beginning of the AEW storm track generate new AEWs, whereas divergent fluxes at the end of the storm track lead to their decay. It is argued that, even with small normal-mode growth rates and a short region of instability, the presence of absolute instability allows AEWs to develop through the mixed baroclinic–barotropic instability mechanism, because upstream energy fluxes allow energy extracted through baroclinic and barotropic conversion to be recycled between successive AEWs. Link to full paper:
  21. The Genesis of African Easterly Waves by Upstream Development Authors: Michael Diaz and Anantha Aiyyer Published: 27th June, 2013 Abstract: A genesis mechanism for African easterly waves (AEWs) is proposed. In the same manner that new troughs and ridges in the midlatitudes form downstream of existing ones through a mechanism known as downstream development, it is proposed that new AEWs can be generated upstream of existing AEWs. A local eddy kinetic energy budget of the AEW that ultimately became Hurricane Alberto (2000) demonstrates that upstream development explains its genesis more convincingly than previous theories of AEW genesis. The energetics and ageostrophic secondary circulation of a composite AEW are consistent with a new AEW forming as a result of this mechanism. Some strengths and weaknesses of upstream development as a paradigm for AEW genesis are discussed with respect to other potential mechanisms. Link to full paper:
  22. Investigating The Origin of African Easterly Waves - Presentation A presentation at the AMS 32nd Conference on “Hurricanes and Tropical Meteorology” held at the Condado Plaza Hilton in Puerto Rico between 17th and 22nd April, 2016. Presenters: Joshua Dylan White and Anantha Aiyyer  Presentation Date: 18th April, 2016 Presentation Summary: African easterly waves (AEWs) are synoptic-scale disturbances that modulate west African precipitation and are precursors of a majority of Atlantic hurricanes. For decades, the proposed mechanism for generation of AEWs was the combined baroclinic-barotropic instability of the African easterly Jet (AEJ). Recent studies have revisited this issue, and two alternative mechanisms for AEW origin have emerged: (1) Triggering by mesoscale convection; and (2) Upstream energy dispersion in AEW wave packets. Our goal is to rigorously test these two mechanisms. In this study, we examine one season (July-October, 2006) in detail to determine whether clear evidence of convective triggering and/or upstream energy dispersion for each AEW can be discerned. We use European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) and satellite derived products to monitor wave activity and active convection. An eddy kinetic energy (EKE) budget will demonstrate contributions to AEW development from baroclinic and barotropic conversions, and upstream energy dispersion. Analysis of convection (size, duration and location) and its upscale impact will aid in determining its role AEW origin. Link to conference video presentation (12 minutes): Link to full conference agenda:
  23. Diabatic Rossby Waves as a Model for Convectively Coupled African Easterly Waves - Presentation A presentation at the AMS 32nd Conference on “Hurricanes and Tropical Meteorology” held at the Condado Plaza Hilton in Puerto Rico between 17th and 22nd April, 2016. Presenters: James O. H. Russell and A. Aiyyer  Presentation Date: 18th April, 2016 Presentation Summary: African easterly waves (AEWs) are synoptic-scale disturbances associated with the African Easterly Jet (AEJ) that move westward across the Sahel region of Africa during the West African Monsoon season. It has been shown that the barotropic and baroclinic extraction of energy from the AEJ is not sufficient to sustain AEWs, and that moist convection may have an important role in their maintenance. It is, however, not clear how the synoptic-scale waves and mesoscale convection interact. Furthermore the conditions under which some waves are more closely coupled with convection compared to others is also not well understood. This study examines the hypothesis that convection and AEWs may interact through the Diabatic Rossby Wave (DRW) mechanism. We examine whether the conditions for DRW genesis, a preconditioning of the low-levels ahead of a wave trough, and the generation of potential vorticity (PV) due to the vertical gradient of diabatic heating produced by convection, occur in AEWs. The impact of diabatic PV generation on the amplification, propagation, and evolution of AEWs will be examined through PV and energy budgets calculated using gridded reanalysis and satellite derived products. Results from this will motivate numerical simulations using the weather research and forecasting (WRF) model to further examine the sensitivity of AEWs to moist convection. Link to conference video presentation (14 minutes):;uniqueid=Paper293211&amp;entry_password=578878 Link to Manuscript: (pdf file via conference presentation summary) Link to full conference agenda:
  24. Instabilities of Radiative Convective Equilibrium with an Interactive Surface - Presentation A presentation at the AMS 32nd Conference on “Hurricanes and Tropical Meteorology” held at the Condado Plaza Hilton in Puerto Rico between 17th and 22nd April, 2016. Presenters: Tom Beucler, K. Emanuel Ali Asaadi, G. Brunet and P. M. K. Yau  Presentation Date: 18th April, 2016 Presentation Summary: Moist radiative-convective equilibrium (RCE) is the statistical equilibrium state the atmosphere would reach in the absence of lateral energy transport. It is arguably the simplest climatic equilibrium, where the vertically-integrated radiative cooling balances the convective heating. If the solar forcing is large enough, RCE can be unstable to water vapor perturbations, leading to a dry state with mean descent or a moist state with mean ascent. This instability is believed to correspond to the phenomenon of self aggregation of convection, where a convecting moist cluster surrounded by a broad region of dry subsiding air forms. The temperature perturbations of the surface play an important role in the organization of convection, through surface turbulent and radiative fluxes. However, it is very computationally intensive to run cloud resolving models and large eddy simulations of convection without artificially imposing a fixed surface temperature. This explains why the instabilities of RCE with an interactive surface remain an open problem. By using the MIT single column model and an idealized two-layers model of the atmosphere with a slab ocean, we are able to study the potential instabilities of the fully interactive system. We find that allowing for surface temperature perturbations adds a new air-sea potential instability to the purely atmospheric radiative-convective instability found by Emanuel et al. (2013). Furthermore, we are able to study the stability of the system for all values of solar forcing and surface heat capacities; we can then numerically compute the bifurcation diagram of the system. Finally, we show that the instability preferentially leads to a dry than a moist state, which helps explaining why self-aggregation of convection is observed to make the free-troposphere drier. Link to conference video presentation (14 minutes): Link to Manuscript: (pdf file via conference presentation summary) Link to full conference agenda:
  25. On the dynamics of the formation of the Kelvin cat's eye in tropical cyclogenesis - Presentation A presentation at the AMS 32nd Conference on “Hurricanes and Tropical Meteorology” held at the Condado Plaza Hilton in Puerto Rico between 17th and 22nd April, 2016. Presenters: Ali Asaadi, G. Brunet and P. M. K. Yau  Presentation Date: 18th April, 2016 Presentation Summary: This study brings new understanding on easterly waves that develop into named storms over the Atlantic and eastern Pacific. It has recently been shown that these storms are almost all associated with a cyclonic Kelvin cat's eye of a tropical easterly wave critical layer, located equatorward of the easterly jet axis. To better understand the dynamics of tropical cyclogenesis, the flow characteristics and the physical and dynamical mechanisms for the formation of the cat's eyes are investigated. A climatological study of 54 developing easterly waves in 1998-2001 was conducted using ECMWF reanalysis data. Time-lagged composites in a translating reference frame following the disturbances indicate a weak meridional potential vorticity (PV) gradient of the easterly jet, and a cyclonic critical layer located slightly to the south of the weak PV gradient, consistent with previous findings in the marsupial paradigm. In addition, the statistically significant pattern of developing and non-developing PV anomalies in the composite shows that the coexistence of a nonlinear critical layer and a region of weak meridional PV gradient over several days occurs in only ~25% of the easterly waves. This finding may explain why only a small fraction of easterly waves eventually develop into tropical storms. Moreover, we obtained an analytic time scale of the form τe^τQ~O(ε^(-1)) with Q being the mass sink, ε the amplitude of the initial disturbance, and τ the cat's eye formation time, that governs the onset of nonlinearity for forced disturbances on a parabolic jet critical layer. This time scale is consistent with that found in the 54 cases of easterly waves that developed into named storms, highlighting the importance of nonlinear and diabatic processes in cat's eye formation. A shallow water model is then used to study forced and unforced problems of disturbances on a parabolic jet and the observed jet in the composite. The nonlinear forced simulations, with the convective heating represented by a mass sink, produced a realistic time scale for cat's eye formation, and confirm our above-mentioned analytic solution. These results confirm the synergic role of the dynamical mechanisms, including wave breaking and PV redistribution within the nonlinear critical layer characterized by weak PV gradients, and the thermodynamical mechanisms such as convectively generated PV anomalies, in the formation of the cat's eye in tropical cyclogenesis. Link to conference video presentation (15 minutes): Link to full conference agenda: