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Saharan dust exerts profound impacts on the genesis and intensification of tropical cyclones (TCs). Such impacts on various stages of the TCs have yet to be explored. In this study, we utilize the Cloud‐Resolving weather research and forecasting model (WRF) to investigate the relative importance of the microphysical and radiative effects of dust on two hurricanes (Earl and Danielle) at different life stages under similar dynamical conditions in 2010. Both TCs were embedded in a dusty environment throughout their lifetime. A new dust ice nucleation scheme was implemented into the aerosol‐aware Texas A&M University two‐moment microphysical scheme in WRF. Moreover, the dust radiative effect was included in the Goddard Shortwave Scheme of WRF. Our sensitivity experiments show that the radiative effect of dust (DRAD) amplified the mid‐level ridge in the Central Atlantic Ocean through temperature perturbation, changing the tracks of Danielle and Earl. Further analyses reveal an early shift of Danielle's maximum intensity for 12 hours but a significantly suppressed Earl in DRAD. In addition, the microphysical effect of dust had little impact on the large‐scale dynamical fields and storm tracks. The inclusion of dust as ice nucleation particles results in more variations in the intensity of Danielle and Earl than in other scenarios. This is owing to the higher maximum diabatic heating rate in the rainband region that perturbs the size of the TC. This study shows the dominant dust radiative effects on both intensity and track of the storm. In addition, there is evidence that dust suppresses the early stage TC but provides favorable conditions for matured TC. Both findings have profound implications for hurricane forecast and address the importance of accounting for detailed cloud microphysics and aerosol‐TC interactions in the operational forecasting models. Plain Language Summary: This modeling study assesses how dust from the Saharan Desert affects tropical cyclones (TCs). Dust modifies the radiative transfer by reflecting and absorbing sunlight (the radiative effect) and hence the environmental temperature profile. In addition, dust also changes cloud processes (the dust microphysical effect) that perturbs the cloud distribution. A cloud‐resolving model was used to assess the dust impacts on two TCs at different development stages, that is, Danielle and Earl (2010). We found that the radiative effect of dust plays an important role in the intensity and path of both TCs. Specifically, the radiative effects of dust weakened the early stage TC Earl, resulted in an early shift of maximum intensity of the matured TC Danielle, and perturbed the path of both TCs. The dust microphysics effect, on the other hand, had little impact on the TC environment and its path but induced larger variations in the intensity of both TCs. This study reveals that dust exhibits distinct effects depending on the stage of the TC development, showing that dust suppresses the early stage TC but provides favorable conditions for the matured TC. Key Points: A dust‐related ice nucleating scheme and radiation parameterizations were implemented in the aerosol‐aware weather research and forecasting modelThe radiative effect of dust plays a larger role in the large‐scale environment and intensity of tropical cyclones (TCs) than the microphysical effectThe dust effect suppresses the early stage TC but provides favorable conditions for matured TC [ABSTRACT FROM AUTHOR]

Intense tropical cyclones (TCs) can cause catastrophic damage to coastal regions after landfall. Recent studies have linked the devastation associated with TCs to climate change, which induces favorable conditions, such as increasing sea-surface temperature, to supercharge storms. Meanwhile, environmental factors, such as atmospheric aerosols, also impact the development and intensity of TCs, but their effects remain poorly understood, particularly coupled with ocean dynamics. Here, we quantitatively assess the aerosol microphysical effects and aerosol-modified ocean feedbacks during Hurricane Katrina using a cloud-resolving atmosphere–ocean coupled model: Weather Research and Forecasting (WRF) in conjunction with the Regional Ocean Model System (ROMS). Our model simulations reveal that an enhanced storm destructive power, as reflected by larger integrated kinetic energy, heavier precipitation, and higher sea-level rise, is linked to the combined effects of aerosols and ocean feedbacks. These effects further result in an expansion of the storm circulation with a reduced intensity because of a decreasing moist static energy supply and enhancing vorticity Rossby wave outward propagation. Both accumulated precipitation and storm surge are enhanced during the mature stage of the TC with elevated aerosol concentrations, implying exacerbated flood damage over the polluted coastal region. The ocean feedback following the aerosol microphysical effects tends to mitigate the vertical mixing cooling in the ocean mixing layer and offsets the aerosol-induced storm weakening by enhancing cloud and precipitation near the eyewall region. Our results highlight the importance of accounting for the effects of aerosol microphysics and ocean-coupling feedbacks to improve the forecast of TC destructiveness, particularly near the heavily polluted coastal regions along the Gulf of Mexico. [ABSTRACT FROM AUTHOR]

Tropical cyclones (TCs) with a high Saffir-Simpson scale can cause catastrophic damages to coastal regions after landfall. Recent studies have linked the TC's devastation to climate change that induces favorable environmental conditions, such as increasing sea-surface temperature, to supercharge the storms. Also, atmospheric aerosols likely impact the development and intensity of TCs, but their effects remain poorly understood, particularly coupled with the ocean dynamics. Here we quantitatively assess the aerosol microphysical effects and aerosol-modified ocean feedbacks during Hurricane Katrina using a cloud-resolving atmosphere-ocean coupled model - Weather Research and Forecasting (WRF) in conjunction with the Regional Ocean Model System (ROMS). Our model simulations reveal that an enhanced destructive power of the storm, as reflected by larger integrated kinetic energy, heavier precipitation, and higher sea-level rise, is linked to the combined effects of aerosols and ocean feedbacks. These effects further result in an expansion of the storm circulation with a reduced intensity because of decreasing moist static energy supply and enhancing vorticity Rossby wave outward propagation. Both accumulated precipitation and storm surge are enhanced during the mature stage with elevated aerosol concentrations, implying exacerbated flooding damage over the coastal region. The ocean feedback following the aerosol microphysical effects tends to mitigate the Ekman upwelling cooling and offsets the aerosol-induced storm weakening, by invigorating cloud and precipitation near the eyewall region. Our results highlight the importance of accounting for the effects of aerosol microphysics and ocean-coupling feedbacks to improve the forecast of TC destructiveness, particularly near the heavily polluted coastal regions along the Gulf of Mexico. [ABSTRACT FROM AUTHOR]

Tropical cyclones (TCs) with a high Saffir-Simpson scale can cause catastrophic damages to coastal regions after landfall. Recent studies have linked the TC's devastation to climate change that induces favorable environmental conditions, such as increasing sea-surface temperature, to supercharge the storms. Also, atmospheric aerosols likely impact the development and intensity of TCs, but their effects remain poorly understood, particularly coupled with the ocean dynamics. Here we quantitatively assess the aerosol microphysical effects and aerosol-modified ocean feedbacks during Hurricane Katrina using a cloud-resolving atmosphere-ocean coupled model - Weather Research and Forecasting (WRF) in conjunction with the Regional Ocean Model System (ROMS). Our model simulations reveal that an enhanced destructive power of the storm, as reflected by larger integrated kinetic energy, heavier precipitation, and higher sea-level rise, is linked to the combined effects of aerosols and ocean feedbacks. These effects further result in an expansion of the storm circulation with a reduced intensity because of decreasing moist static energy supply and enhancing vorticity Rossby wave outward propagation. Both accumulated precipitation and storm surge are enhanced during the mature stage with elevated aerosol concentrations, implying exacerbated flooding damage over the coastal region. The ocean feedback following the aerosol microphysical effects tends to mitigate the Ekman upwelling cooling and offsets the aerosol-induced storm weakening, by invigorating cloud and precipitation near the eyewall region. Our results highlight the importance of accounting for the effects of aerosol microphysics and ocean-coupling feedbacks to improve the forecast of TC destructiveness, particularly near the heavily polluted coastal regions along the Gulf of Mexico. [ABSTRACT FROM AUTHOR]

The relationship between the generation of African easterly waves and instability growing in regions with reversed potential vorticity gradients is studied using a regional climate model. Results indicate that the convective generation of potential vorticity (PV) due to the meridional and vertical gradients of diabatic heating in the upper and lower troposphere causes a vertically elongated PV anomaly on the southern flank of the African easterly jet. This PV maximum at 9[degress]N in the mid-troposphere, together with a PV minimum near 15[degrees]N at lower levels because of dry convection over the Sahara, reverses the meridional PV gradient between 9[degrees] and 15[degrees]N, which suggests that the zonal flow may be unstable in this region. Analysis of the seasonal mean Eliassen-Palm flux for African waves indicates that wave energy generated convectively through baroclinic overturning in the upper troposphere propagates downward and triggers barotropic conversions south of the jet and baroclinic conversions below and north of the jet. The barotropic conversion of the jet initiates primarily outside of the region of strengthened reversed potential vorticity (q) gradients, suggesting that this barotropic conversion is a result of convectively induced eddies extracting energy from the zonal flow rather than the release of zonal kinetic energy to the waves in the unstable region. In contrast, the residual barotropic conversion occurs inside the region of reversed q gradients during the waves' decaying stage when ITCZ convection weakens. The baroclinic instability in the unstable region becomes distinguishable from that due to surface temperature gradients when the surface heat flux is weak, a condition under which the African easterly jet better acts as an internal jet. Thus, this analysis indicates that the shear instability of the jet occurs to sustain the waves at the decaying stage rather than to initiate the waves, since it does not appear strong enough to reenergize the waves.

The evolution and spatial distribution of the energetics of African waves are studied. Complete eddy energy equations for an open system are derived for the computation of energy transformations during wave generation and dissipation. It is found that baroclinic overturning is the dominant energy source, although barotropic conversions can be almost equally important when there is concentrated moist convection south of the jet or shallow cumulus convection beneath the jet. The generation of active waves usually results from the nearly in-phase evolution of baroclinic and barotropic conversions, which are associated with significant rainfall over Africa. Significant barotropic instability associated with the horizontal shear is usually induced by concentrated deep convection on the southern flank of the jet. Barotropic conversions associated with the vertical wind shear may attain even greater magnitudes than that associated with the horizontal shear when shallow cumulus convection beneath the jet is strong. The eddy available potential energy consumed by the baroclinic overturning is compensated directly by the conversion of zonal to eddy available potential energy and the generation of eddy potential energy by diabatic heating. These direct conversions of latent heat and zonal available potential energy suggest that interactions across space scales, from convective space scales to the large scales, are important for generating African waves. The convectively induced barotropic instability may enhance baroclinic overturning through the resonance between these two instabilities. This leads to the nonlinear interaction of the waves with convection, corresponding to the formation of organized precipitation migrating with the waves. A space-time spectral analysis shows that the dispersion characteristics of African easterly waves with wavelengths between 2650 and 4000 km do not follow the dispersion relation of the shallow water waves, indicating that these waves, similar to other easterly waves in the Tropics, possess significant nonlinearity, and cannot be fully explained by linear wave theory.

The destructive power of tropical cyclones is driven by latent heat released from water condensation and is inevitably linked to the abundance of aerosols as cloud condensation nuclei. However, the aerosol effects are unaccounted for in most operational hurricane forecast models. We combined multisource measurements and cloud‐resolving model simulations to show fundamentally altered cloud microphysical and thermodynamic processes by anthropogenic aerosols during Hurricane Harvey. Our observational analyses reveal intense lightning and precipitation in the proximity of Houston industrial areas, and these hot spots exhibit a striking geographic similarity to a climatological maximum of lightning flash density in the south‐central United States. Our ensemble cloud‐resolving simulations of Hurricane Harvey indicate that aerosols increase precipitation and lightning by a factor of 2 in the Houston urban area, unraveling the key anthropogenic factor in regulating flooding during this weather extreme. Plain Language Summary: The catastrophic flooding during Hurricane Harvey has received major attention, but the cause remains mysterious. The destructive power of tropical cyclones is produced by the latent heat release from phase change of water, which is linked to airborne particles emitted from vehicles and petrochemical plants. By combining observation and model simulations, our work provides microphysical and thermodynamic insights into the cause of the catastrophic flooding during Hurricane Harvey by the aerosols from industrial sources. Our discovery underscores the importance of representing the effects of anthropogenic aerosols for accurate short‐term forecast and climate projection of tropical cyclones to minimize future catastrophic destruction along the highly industrialized Gulf of Mexico region. Key Points: Aerosol increases accumulative precipitation by a factor of 2T and invigorates lightning activities in Houston during Hurricane HarveyObservations show intense lightning over Houston which exhibits geographic similarity to climatological maximum lightning flash densityTo better forecast extreme weather events, it is essential to account for aerosol effects in operational weather forecast models [ABSTRACT FROM AUTHOR]

The relationship between African easterly waves and the background climatology in which they form is studied using a regional climate model. The surface and lateral boundary conditions in the model are manipulated to modify the background climatology, especially the African easterly jet and the ITCZ, and the behavior of the waves in these different settings is evaluated. Three climate simulations are presented, with monthly mean lateral and surface boundary conditions. One has a strong jet and a strong ITCZ, the second has a strong jet and a weak ITCZ, and the third has a weak jet and a strong ITCZ. In these simulations, the presence of wave activity is more closely associated with the concentration of the ITCZ than the strength of the African easterly jet. In particular, the simulation with a strong jet accompanied by a weak ITCZ does not produce significant wave activity, but a weak jet with a strong ITCZ has realistic wave disturbances. Both the Charney–Stern and the Fjörtoft necessary conditions are satisfied in all three simulations, suggesting that combined barotropic and baroclinic instability contributes to the generation of waves. Near the origin of the waves, meridional gradient reversals of isentropic potential vorticity result from potential vorticity anomalies generated by convective heating within the ITCZ, implying that the unstable zonal flow may be caused by cumulus convection within the ITCZ and not by shear instability associated with the jet. Two additional simulations with 1988 lateral boundary conditions demonstrate that 3–5-day wave disturbances can be generated in the absence of the African easterly jet, but with unrealistically small wavelengths. These results suggest that African easterly waves are initiated by cumulus convection within the ITCZ, and not by barotropic instability associated with the jet. [ABSTRACT FROM AUTHOR]