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Understanding how mesoscale convection interacts with synoptic-scale circulations over West Africa is crucial for improving regional weather forecasts and developing convection parameterizations to address biases in climate models. A 10-yr pan-African convection-permitting simulation and a corresponding parameterized simulation for current-climate conditions are used to calculate the circulation budget around a synoptic region over the diurnal cycle, splitting processes that modulate circulation tendency (vorticity accumulation and vortex tilting) into diurnal mean and anomalous contributions. Dynamical fields are composited around precipitating grid cells during afternoon and overnight convection to understand how the mesoscale convection modulates synoptic-scale processes, and the composites are compared with an observational case. The dominant process modulating circulation tendency was found to be synoptic-scale vorticity accumulation, which is similar in the two simulations. The greatest difference between the simulated budgets was the tilting term. We propose that the tilting term is affected by convective momentum transport associated with precipitating systems crossing the boundary of the region, whereas the stretching term relies on the convergence and divergence induced by storms within the region. The simulation with parameterized convection captures the heating profile similarly to the simulation with explicit convection, but there are marked differences in convective momentum transport. An accurate vertical convergence structure as well as momentum transport must be simulated in parameterizations to correctly represent the impacts of convection on circulation. Significance Statement: We used climate simulations with explicit convection and a convection parameterization to interrogate the relationship between mesoscale convection and synoptic-scale circulation over West Africa. We examined the typical behavior of mesoscale precipitating systems in both simulations and compared this with an observation of a storm. We also investigated how synoptic circulation changed over a diurnal cycle in both simulations. The biggest differences between the simulations were caused by how mesoscale systems in each simulation transport momentum when they cross the boundaries of a circulation, but the greatest impact on synoptic circulation was from the patterns of convergence and divergence induced by mesoscale systems, which are very similar in both simulations. Convection parameterizations should prioritize improving the representation of momentum transport. [ABSTRACT FROM AUTHOR]

Two field campaigns, the African Monsoon Multidisciplinary Analysis (AMMA) and the Tropical Warm Pool-International Cloud Experiment (TWP-ICE), took place in 2006 near Niamey, Niger, and Darwin, Northern Territory, Australia, providing extensive observations of mesoscale convective systems (MCSs) near a desert and a tropical coast, respectively. Under the constraint of their observations, three-dimensional cloud-resolving model simulations are carried out and presented in this paper to replicate the basic characteristics of the observed MCSs. All of the modeled MCSs exhibit a distinct structure having deep convective clouds accompanied by stratiform and anvil clouds. In contrast to the approximately 100-km-scale MCSs observed in TWP-ICE, the MCSs in AMMA have been successfully simulated with a scale of about 400 km. These modeled AMMA and TWP-ICE MCSs offer an opportunity to understand the structure and mechanism of MCSs. Comparing the water budgets between AMMA and TWP-ICE MCSs suggests that TWP-ICE convective clouds have stronger ascent while the mesoscale ascent outside convective clouds in AMMA is stronger. A case comparison, with the aid of sensitivity experiments, also suggests that vertical wind shear and ice crystal (or dust aerosol) concentration can significantly impact stratiform and anvil clouds (e.g., their areas) in MCSs. In addition, the obtained water budgets quantitatively describe the transport of water between convective, stratiform, and anvil regions as well as water sources/sinks from microphysical processes, providing information that can be used to help determine parameters in the convective and cloud parameterizations in general circulation models (GCMs). [ABSTRACT FROM AUTHOR]

The effects of Saharan dust aerosols and West African precipitation on the seasonally averaged energetics of African easterly waves (AEWs) are examined using the Weather Research and Forecasting Model coupled to an interactive dust model. Four experiments are conducted: a control for the period July–September 2008, and three other experiments in which the dust emissions and precipitation are reduced separately and in combination. An analysis of the total energy shows the relative importance of the dust and precipitation to the seasonally averaged AEW strength and AEW tracks, which straddle the African easterly jet (AEJ). Changes in the dust amount have a larger effect on the strength of the AEWs than changes in the precipitation amount. The north AEW track is more strongly affected by changes in dust, while the south AEW track is more strongly affected by changes in precipitation. An analysis of the energy conversions aids in identifying the relative importance of the wave–mean flow interaction pathways that connect the dust and precipitation fields to the AEJ–AEW system. The analysis shows that the variability of the AEWs is primarily coupled to the dustand precipitation-modified variability of the AEJ through wave–mean flow interaction. These results are discussed in light of tropical cyclone development over the eastern Atlantic Ocean. [ABSTRACT FROM AUTHOR]

Eastern Africa is a common region of African easterly wave (AEW) onset and AEW early life. How the large-scale environment over East Africa relates to the likelihood of an AEW subsequently undergoing tropical cyclogenesis in a climatology has not been documented. This study addresses the following hypothesis: AEWs that undergo tropical cyclogenesis (i.e., developing AEWs) initiate and propagate under a more favorable monsoon large-scale environment over eastern Africa when compared with nondeveloping AEWs. Using a 21-yr August–September (1990–2010) climatology of AEWs, differences in the large-scale environment between developers and nondevelopers are identified and are proposed to be used as key predictors of subsequent tropical cyclone (TC) formation and could inform tropical cyclogenesis prediction. TC precursors when compared with nondeveloping AEWs experience an anomalously active West African monsoon, stronger northerly flow, more intense zonal Somali jet, anomalous convergence over the Marrah Mountains (region of AEW forcing), and a more intense and elongated African easterly jet. These large-scale conditions are linked to near-trough attributes of developing AEWs that favor more moisture ingestion, vertically aligned circulation, a stronger initial 850-hPa vortex, a deeper wave pouch, and arguably more AEW and mesoscale convective systems interactions. AEWs that initiate over eastern Africa and cross the west coast of Africa are more likely to undergo tropical cyclogenesis than those initiating over central or West Africa. Developing AEWs are more likely than nondeveloping AEWs to be southern-track AEWs. [ABSTRACT FROM AUTHOR]

The dominant structural variability of African easterly waves (AEWs) is explored using an empirical orthogonal function (EOF) approach. The structure of AEWs is obtained by projecting the wind fields from reanalysis data and satellite-derived brightness temperature Tb onto the principal components associated with EOF patterns of filtered Tb (Tb EOF) and 700-hPa meridional wind (v700 EOF). The wave structure depicted by the Tb EOF has confined convection and circulation mostly south of the African easterly jet. It shares many characteristics with AEWs analyzed and discussed in the literature. In contrast, the v700 EOF exhibits less familiar characteristics and includes interactions with the equatorial and subtropical regions. The convective patterns are characterized by a "checkerboard" pattern of convection that has not been emphasized before. The most striking feature is the broad meridional extent, which depicts interactions with a mixed Rossby–gravity wave (MRG) in the equatorial region and interactions with the basic-state potential vorticity in the subtropics. The southern portion of the wave has a modified MRG structure, and this AEW–MRG hybrid cannot be separated using the EOF technique, indicating the prevalence of such structures. The subtropical interaction at mid- to lower levels establishes a vortex off the coast of Morocco that results in dry-air advection into the tropics in tandem with the northern vortex. At upper levels, a subtropical wave train is induced by the AEW-associated convective inflow and outflow. The contrasting AEW circulations are associated with differences in the precipitation rates and patterns over Africa. These results highlight the variability of AEW structures and their interactions with equatorial and subtropical waves. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Cold pool outflows, generated by downdrafts from moist convection, can generate strong winds and therefore uplift of mineral dust. These so-called haboob convective dust storms occur over all major dust source areas worldwide and contribute substantially to emissions in northern Africa, the world's largest source. Most large-scale models lack convective dust storms because they do not resolve moist convection, relying instead on convection schemes. The authors suggest a parameterization of convective dust storms to account for their contribution in such large-scale models. The parameterization is based on a simple conceptual model, in which the downdraft mass flux from the convection scheme spreads out radially in a cylindrical cold pool. The parameterization is tested with a set of Met Office Unified Model runs for June and July 2006 over West Africa. It is calibrated with a convection-permitting run and applied to a convection-parameterized run. The parameterization successfully produces the extensive area of dust-generating winds from cold pool outflows over the southern Sahara. However, this area extends farther to the east and dust-generating winds occur earlier in the day than in the convection-permitting run. These biases are caused by biases in the convection scheme. It is found that the location and timing of dust-generating winds are weakly sensitive to the parameters of the conceptual model. The results demonstrate that a simple parameterization has the potential to correct a major and long-standing limitation in global dust models. [ABSTRACT FROM AUTHOR]

The southern parts of West Africa are frequently covered by an extensive deck of shallow, low (200-400 m AGL) stratus or stratocumulus clouds during the summer monsoon. These clouds usually form at night in association with a nocturnal low-level jet (NLLJ) and can persist into the early afternoon hours. Recent work suggests that the stratus deck is unsatisfactorily represented in standard satellite retrievals and state-of-the-art climate models. Here the authors use high-resolution regional simulations with the Weather Research and Forecasting Model (WRF) and observations from the African Monsoon Multidisciplinary Analysis (AMMA) 2006 campaign to investigate (i) the spatiotemporal distribution, (ii) the influence on the shortwave radiation balance, and (iii) the detailed formation and maintenance mechanisms of the stratiform clouds. At least some configurations of WRF satisfactorily reproduce the diurnal cycle of the low-cloud evolution, yielding the following main conclusions: (i) The simulated stratus deck forms after sunset along the coast, spreads inland during the course of the night, and dissipates in the early afternoon. (ii) The average surface net shortwave radiation balance in stratus-dominated regions is about 35 W m−2 lower than in those with fewer clouds. (iii) The cloud formation is related to a subtle balance between 'stratogenic' upward (downward) fluxes of latent (sensible) heat caused by shear-driven turbulence below the NLLJ, cold advection, orographic lifting, and radiative cooling on one hand, and 'stratolytic' dry advection and latent heating on the other hand. [ABSTRACT FROM AUTHOR]

Tropical, subtropical, and extratropical dynamical processes govern the synoptic-scale evolution of the subtropical jet stream(s) over Africa. However, the relative importance of the respective effects is still under debate and is the focus of this study. Interim ECMWF Re-Analysis (ERA-Interim) data are used to calculate backward trajectories from the subtropical jet over Africa during winter 2005/06. The trajectories allow for studying the jet dynamics from both a potential vorticity (PV) and an angular momentum point of view and for linking the two theoretical frameworks. Three cases of synoptic-scale Rossby wave breaking in the extratropics and subtropics are presented in detail. They illustrate basic flow configurations where (i) the subtropical jet is mainly forced by tropical dynamics, (ii) extratropical forcing contributes substantially to the jet acceleration, and (iii) strong diabatic processes in the subtropics impact the jet. The case study results are then generalized for the entire winter season. The main findings are as follows: (i) Approximately 41% of the trajectories reach the subtropical jet from the deep tropics and for these trajectories the nonconservation of angular momentum M due to eddy forcing leads to a decrease of M by about 5%. (ii) A nonnegligible fraction of roughly 18% of the trajectories reaches the subtropical jet from the extratropics. (iii) Wave breaking is instrumental for bringing extratropical, high-PV air southward. (iv) Diabatic processes in the subtropics have a negligible direct effect on the upper-level PV. This is in contrast to observations from the extratropics and might be the consequence of the small planetary vorticity in the tropics and subtropics. [ABSTRACT FROM AUTHOR]

To examine the dynamical role of convection in African easterly wave (AEW) life cycles the Weather Research and Forecasting (WRF) model is used to simulate the evolution of a single AEW from September 2004. The model simulations are validated against corresponding numerical weather prediction analyses and the mean fields closely resemble composite structures from previous studies. A potential vorticity (PV) thinking approach is used to highlight the interactions between dynamics and convection. Organized deep convection embedded within the AEW has a large contribution to the synoptic-scale mean PV and energetics of the AEW. The PV tendency is maximized in the lower troposphere, consistent with the vertical gradient in diabatic heating rates in the areas of convection. By examining terms in the Lorenz energy cycle, it is shown that diabatic heating associated with convection is as important as adiabatic energy conversion in producing eddy available potential energy of the synoptic AEW, implying that AEWs are best described as hybrid adiabatic and diabatic structures. The net effect of convection is succinctly described using a simulation whereby the parameterizations associated with convection are switched off at the midpoint of the model run. This perturbation experiment shows that, although the AEW continues to propagate westward with a similar phase speed, the net PV value continually weakens with time. This result proves that convection is vital for the maintenance of the AEW as it propagates across West Africa and suggests that without active convection the synoptic AEW cannot persist for an extended length of time. [ABSTRACT FROM AUTHOR]

This study uses mesoscale ensemble forecasts to compare the magnitude of nonaerosol effects of the Saharan air layer (SAL) with other environmental influences on the intensity of Tropical Storm Debby. Debby was a weak Cape Verde storm that dissipated over the tropical North Atlantic a few days after forming in August 2006. The system has received considerable attention because of its vicinity to the SAL as it struggled to intensify, which has led to speculation that the SAL helped lead to the storm's demise. Statistical correlation is used to better understand why some ensemble members strengthen the pre-Debby wave into a hurricane and others develop only a weak vortex. Although the results here suggest that the SAL slowed intensification during the predepression to depression stages, it was not likely responsible for Debby's dissipation. The most obvious SAL-related factor to affect long-term intensity in the ensembles is dry air above 2 km, which delays organization of the low-level vortex. Warm temperatures within the SAL and shear associated with the African easterly jet (AEJ) exhibit a weak, secondary relationship with forecast intensity variability. An important result here is that sensitivity to the dry environmental air depends considerably on cyclone strength, and it becomes insignificant once a tropical storm forms. Furthermore, Debby's most rapid period of intensification coincided with its track over somewhat higher sea surface temperatures, and intensification ended when the storm moved over cooler waters. The results herein suggest that this factor might have affected the storm's intensity more strongly than did any effect of the SAL. Even later, subsequent to the period examined by these ensembles, Debby dissipated under the influence of stronger vertical wind shear from an upper-level trough. These results show that the relationship among the SAL, AEJ, and developing tropical cyclones is not as straightforward as has been hypothesized by some recent studies. Ultimately, the nuanced relationship between storm intensity and the SAL shows that much care needs to be taken before drawing conclusions about the effect of the SAL on any particular cyclone. The authors therefore advocate more rigorous future analysis through both idealized and ensemble studies to more fully quantify the effect of the SAL on tropical cyclones in general. [ABSTRACT FROM AUTHOR]

The anvil clouds of tropical squall-line systems over West Africa have been examined using cloud radar data and divided into those that appear ahead of the leading convective line and those on the trailing side of the system. The leading anvils are generally higher in altitude than the trailing anvil, likely because the hydrometeors in the leading anvil are directly connected to the convective updraft, while the trailing anvil generally extends out of the lower-topped stratiform precipitation region. When the anvils are subdivided into thick, medium, and thin portions, the thick leading anvil is seen to have systematically higher reflectivity than the thick trailing anvil, suggesting that the leading anvil contains numerous larger ice particles owing to its direct connection to the convective region. As the leading anvil ages and thins, it retains its top. The leading anvil appears to add hydrometeors at the highest altitudes, while the trailing anvil is able to moisten a deep layer of the atmosphere. [ABSTRACT FROM AUTHOR]

A letter to the editor in response to the article which discusses a study regarding the investigation of two African easterly waves evaluated during the National Aeronautics and Space Administration (NASA) African Monsoon Multidisciplinary Analysis (NAMMA) special observation mission (SOP-3) in the 2009 issue.

The so-called “perturbation D” was a nondeveloping West African disturbance observed near Dakar (Senegal) during special observing period (SOP) 3 of the African Monsoon Multidisciplinary Analysis (AMMA) in September 2006. Its mesoscale environment is described with the dropsonde data obtained during flights on three successive days with the Service des Avions Français Instrumentés pour la Recherche en Environnement Falcon-20 aircraft. Processes involved in this evolution are studied qualitatively with ECMWF reanalyses and Meteosat-9 images. The evolution of perturbation D was the result of an interaction between processes at different scales such as the African easterly jet (AEJ), a midtropospheric African easterly wave (AEW), a series of mesoscale convective systems, the monsoon flow, dry low- to midlevel anticyclonic Saharan air, and a midlatitude upper-level trough. The interaction between these processes is further investigated through a numerical simulation conducted with the French nonhydrostatic Méso-NH model with parameterized convection. The growth of the simulated disturbance is quantified with an energy budget including barotropic and baroclinic conversions of eddy kinetic energy, proposed previously by the authors for a limited domain. The development of the simulated system is found to result from barotropic–baroclinic growth over West Africa and baroclinic growth over the tropical eastern Atlantic. It is suggested that these energy conversions were the result of an adjustment of the wind in response to the pressure decrease, presumably caused by convective activity, and other synoptic processes. A comparison with the developing case of Helene (2006) reveals that both perturbations had similar evolutions over the continent but were associated with different synoptic conditions over the ocean. For perturbation D, the anticyclonic curvature of the AEJ, caused by the intensification of the eastern ridge by a strong flow of dry Saharan air, prohibited the formation of a closed and convergent circulation. Moreover, a midlatitude upper-level trough approaching from the northwest contributed to increase the northward stretching and then weakened the perturbation. It is therefore suggested that at least as important as the intensity of the AEW trough and associated convection leaving the West African continent are synoptic conditions associated with the Saharan heat low, the subtropical high pressure zone, and even the midlatitude circulation, all of which are instrumental in the (non)cyclogenetic evolution of AEWs in the Cape Verde Islands region. [ABSTRACT FROM AUTHOR]

In September 2006, NASA Goddard’s mobile ground-based laboratories were deployed to Sal Island in Cape Verde (16.73°N, 22.93°W) to support the NASA African Monsoon Multidisciplinary Analysis (NAMMA) field study. The Atmospheric Emitted Radiance Interferometer (AERI), a key instrument for spectrally characterizing the thermal IR, was used to retrieve the dust IR aerosol optical depths (AOTs) in order to examine the diurnal variability of airborne dust with emphasis on three separate dust events. AERI retrievals of dust AOT are compared with those from the coincident/collocated multifilter rotating shadowband radiometer (MFRSR), micropulse lidar (MPL), and NASA Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) sensors. The retrieved AOTs are then inputted into the Fu–Liou 1D radiative transfer model to evaluate local instantaneous direct longwave radiative effects (DRELW) of dust at the surface in cloud-free atmospheres and its sensitivity to dust microphysical parameters. The top-of-atmosphere DRELW and longwave heating rate profiles are also evaluated. Instantaneous surface DRELW ranges from 2 to 10 W m−2 and exhibits a strong linear dependence with dust AOT yielding a DRELW of 16 W m−2 per unit dust AOT. The DRELW is estimated to be ∼42% of the diurnally averaged direct shortwave radiative effect at the surface but of opposite sign, partly compensating for the shortwave losses. Certainly nonnegligible, the authors conclude that DRELW can significantly impact the atmospheric energetics, representing an important component in the study of regional climate variation. [ABSTRACT FROM AUTHOR]

The Lidar Atmospheric Sensing Experiment (LASE) on board the NASA DC-8 measured high-resolution profiles of water vapor and aerosols, and cloud distributions in 14 flights over the eastern North Atlantic during the NASA African Monsoon Multidisciplinary Analyses (NAMMA) field experiment. These measurements were used to study African easterly waves (AEWs), tropical cyclones (TCs), and the Saharan air layer (SAL). These LASE measurements represent the first simultaneous water vapor and aerosol lidar measurements to study the SAL and its interactions with AEWs and TCs. Three case studies were selected for detailed analysis: (i) a stratified SAL, with fine structure and layering (unlike a well-mixed SAL), (ii) a SAL with high relative humidity (RH), and (iii) an AEW surrounded by SAL dry air intrusions. Profile measurements of aerosol scattering ratios, aerosol extinction coefficients, aerosol optical thickness, water vapor mixing ratios, RH, and temperature are presented to illustrate their characteristics in the SAL, convection, and clear air regions. LASE extinction-to-backscatter ratios for the dust layers varied from 35 ± 5 to 45 ± 5 sr, well within the range of values determined by other lidar systems. LASE aerosol extinction and water vapor profiles are validated by comparison with onboard in situ aerosol measurements and GPS dropsonde water vapor soundings, respectively. An analysis of LASE data suggests that the SAL suppresses low-altitude convection. Midlevel convection associated with the AEW and transport are likely responsible for high water vapor content observed in the southern regions of the SAL on 20 August 2008. This interaction is responsible for the transfer of about 7 × 1015 J (or 8 × 103 J m−2) latent heat energy within a day to the SAL. Initial modeling studies that used LASE water vapor profiles show sensitivity to and improvements in model forecasts of an AEW. [ABSTRACT FROM AUTHOR]