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A coherent synoptic sequence, mostly over North Africa, is identified whereby an upper-level midlatitude trough (in November–March) excites several days of quasi-stationary near-surface warming across the Sahara, leading to rainfall events over northern Congo (NC), and perturbed weather more widely. Ahead of NC rainfall events, composite sequences first identify troughs for several days near Iberia, followed by relatively quick transfer to the Central Mediterranean (CMed). Iberia and CMed daily trough-strength indices reveal that both lead to warming and NC rainfall. Iberia trough linkages develop through West Africa and take longer to reach NC, while CMed linkages reach NC faster (2–3 days), with impact extent focused mostly south and east of CMed. Building up to the rainfall events, initial warming over the central Sahara migrates southeastward close to NC, ultimately with typical magnitude of about 1–2°C at 10–15°N. Such anomalies are statistically predictive for NC daily rainfall and associated nearby atmospheric features: anomalous low-level southerly wind and increased moisture; anomalous low-level westerly wind and vertical easterly shear to 600 hPa; increased mid-level moisture (600 hPa), which along with low-level moisture, connects northward into midlatitudes. A secondary route identified by which Iberia troughs can impact NC rainfall is through direct atmospheric teleconnection with precipitation to the west of NC, and subsequent migration of that convection eastward into NC. The eastern side of NC generally shows a small lag on western parts, and links more strongly to CMed troughs. Taken together, the lagged synoptic expression of Iberia and CMed troughs is widespread over several days, including much of North Africa (to equatorial latitudes), southwestern Asia, eastern Africa and the western Indian Ocean. Overall, these results can contribute to situational awareness for weather forecasters across the zones influenced by the troughs, while also providing a framework for climate timescale analyses.

Mid‐level clouds, located between 2 and 9 km height, are ubiquitous in the tropical belt. However, few studies have documented their characteristics and tried to identify the associated thermodynamic properties, particularly in West Africa. This region is characterized by a strong seasonality with precipitation occurring in the Sahel from June to September (monsoon season). This period also coincides with the annual maximum of the cloud cover. Here, we document the macro‐ and microphysical properties of mid‐level clouds, the environment in which such clouds occur, as well as their radiative properties across West Africa. To do so, we combined high‐resolution observations from two ground‐based sites (including lidar and cloud radar) in contrasted environments: one in the Sahel (Niamey, AMMA campaign, 2006) and the other in the Sahara (Bordj Badji Mokhtar, Fennec campaign, June 2011) along with the merged CloudSat‐CALIPSO satellite products. The results show that mid‐level clouds are found throughout the year with a predominance around the monsoon season early in the morning. They also are preferentially observed in the southern and western parts of West Africa. They are usually thin (most of them are less than 1000 m deep) and as observed in Niamey, mainly composed of liquid water. A clustering method applied to Niamey data allows us to distinguish three different types of cloud: one with low bases, one with high bases and another with large thicknesses. The two first cloud families are capped by an inversion. The last family is associated with a large vertical moisture transport and likely has the highest radiative effect at the Earth's surface among the three cloud types.

This study analyses the role of breeze circulations induced by a surface sensible heat flux heterogeneity on deep convection initiation. Large-eddy simulations are used to disentangle the processes at play in a typical case of daytime triggering of deep convection over a semi arid land. We show that the presence of a realistic surface sensible heat flux heterogeneity leads to an earlier triggering of convection and induces a strong determinism in the triggering location at the beta meso-scale (i.e 50 km). The transition to deep convection consists of three consecutive stages, each one corresponding to a specific mode of interaction between (i) the boundary-layer thermals (small-scale), (ii) the breeze circulation (meso-scale) and (iii) the background wind (synoptic scale). These stages are both interpreted thermodynamically and morphologically. All along the transition phase, the boundary-layer growth acts to slow down the background wind, which strengthens the breeze circulation. The breeze evolves towards a circular shape which optimizes moisture convergence and cloud formation just prior to triggering. The presence of wind shear leads with a more asymmetric shape of the breeze in the afternoon, associated with a preferential triggering on the down-shear side of the breeze circulation.

The Sahel has experienced strong climate variability in the past decades. Understanding its implications for natural and cultivated ecosystems is pivotal in a context of high population growth and mainly agriculture-based livelihoods. However, efforts to model processes at the land–atmosphere interface are hindered, particularly when the multi-decadal timescale is targeted, as climatic data are scarce, largely incomplete and often unreliable. This study presents the generation of a long-term, high-temporal resolution, multivariate local climatic data set for Niamey, Central Sahel. The continuous series spans the period 1950–2009 at a 30-min timescale and includes ground station-based meteorological variables (precipitation, air temperature, relative and specific humidity, air pressure, wind speed, downwelling long- and short-wave radiation) as well as process-modelled surface fluxes (upwelling long- and short-wave radiation, latent, sensible and soil heat fluxes and surface temperature). A combination of complementary techniques (linear/spline regressions, a multivariate analogue method, artificial neural networks and recursive gap filling) was used to reconstruct missing meteorological data. The complete surface energy budget was then obtained for two dominant land cover types, fallow bush and millet, by applying the meteorological forcing data set to a finely field-calibrated land surface model. Uncertainty in reconstructed data was expressed by means of a stochastic ensemble of plausible historical time series. Climatological statistics were computed at sub-daily to decadal timescales and compared with local, regional and global data sets such as CRU and ERA-Interim. The reconstructed precipitation statistics, ∼1 °C increase in mean annual temperature from 1950 to 2009, and mean diurnal and annual cycles for all variables were in good agreement with previous studies. The new data set, denoted NAD (Niamey Airport-derived set) and publicly available, can be used to investigate the water and energy cycles in Central Sahel, while the methodology can be applied to reconstruct series at other stations. (Resume d'auteur)

A case of daytime development of deep convection over tropical semi-arid land is used to evaluate the representation of convection in global and regional models. The case is based on observations collected during the African Monsoon Multidisciplinary Analysis (AMMA) field campaign and includes two distinct transition phases, from clear sky to shallow cumulus and from cumulus to deep convection. Different types of models, run with identical initial and boundary conditions, are intercompared: a reference large-eddy simulation (LES), single-column model (SCM) version of four different Earth system models that participated in the Coupled Model Intercomparison Project 5 exercise, the SCM version of the European Centre for Medium-range Weather Forecasts operational forecast model, the SCM version of a mesoscale model and a bulk model. Surface fluxes and radiative heating are prescribed preventing any atmosphere–surface and cloud–radiation coupling in order to simplify the analyses so that it focuses only on convective processes. New physics packages are also evaluated within this framework. As the LES correctly reproduces the observed growth of the boundary layer, the gradual development of shallow clouds, the initiation of deep convection and the development of cold pools, it provides a basis to evaluate in detail the representation of the diurnal cycle of convection by the other models and to test the hypotheses underlying convective parametrizations. Most SCMs have difficulty in representing the timing of convective initiation and rain intensity, although substantial modifications to boundary-layer and deep-convection parametrizations lead to improvements. The SCMs also fail to represent the mid-level troposphere moistening during the shallow convection phase, which we analyse further. Nevertheless, beyond differences in timing of deep convection, the SCM models reproduce the sensitivity to initial and boundary conditions simulated in the LES regarding boundary-layer characteristics, and often the timing of convection triggering.

Thisstudyaimstoachieveabetterunderstandingoftheinitiationofdeepconvection in the Sahel by using the African Monsoon Multidisciplinary Analyses (AMMA) dataset. Based on the Massachusetts Institute of Technology (MIT) radar, wind profiler,satellitedata,surfacefluxandmeteorologicalstations,wehavecharacterised the atmospheric convection which occurred over Niamey during the onset period of the monsoon. From 6 to 31 July, radar reflectivity fields combined with brightness temperatures were used to classify the type of convection observed each day within a 50km radius of the MIT radar location. Four types of convection have been identified: fair weather (FW) with a clear sky throughout the entire day, shallow convection (SH), afternoon locally initiated deep convection (LC), and propagating deep convection (PC). Subsequently, the mechanisms responsible for the initiation of local deep convection were investigated. Neither early morning convective available potential energy nor the convective triggering potential allowed the onset of local deep convection to be predicted correctly. In effect, they were both favourable to deep convection most of the time, while convective inhibition was typically quite large. Our results show that the daytime growth of the atmospheric boundary layer needed to be sufficient for local deep convection to occur during that period. Convergence lines, which grew within the morning clear-air roll organisation, were found to be precursors of local deep convection. Classes FW, SH and LC ultimately behaved quite similarly, with notable convergence in the lower troposphere, but FW showed smaller boundarylayer growth, and FW and SH classes revealed a significant divergence above the boundary layer. Most cases of LC generated a circular gust front. These density currents almost always generated new convective cells. Copyright c � 2013 Royal

A modelling case study designed from observations from the African Monsoon Multidisciplinary Analysis (AMMA) is presented and discussed. It aims at investigating the issue of initiation of convection in a semi-arid environment. This case corresponds to the development of local daytime convection mainly controlled by boundary layer characteristics rather than by atmospheric synoptic scales. A high-resolution three-dimensional simulation is presented and extensively evaluated against the numerous observations available for 10 July 2006 from the AMMA campaign. The simulation, run over a domain of100 × 100 km, is able to represent main boundary layer structures and processes leading to deep convection initiation as well as the formation of density currents. Sensitivity tests point to the key role of the sensible heat flux, the humidity of low to mid levels, the lapse rate at low levels and of a mesoscale ascent to initiate deep convection in those semi-arid conditions, while evaporation of precipitation is shown to play a minor role. This study thus provides a case to investigate the ability of parametrizations to handle the initiation of convection in a semi-arid environment. Copyright c � 2011 Royal Meteorological

International audience; The major advances achieved during African monsoon multidisciplinary analysis in our physical understanding of the West African monsoon (WAM) system are reviewed. Recent research provides an advanced understanding of key WAM features. The Saharan heat low, the interactions of the monsoon flow with the surface and the reversed flow on top of it, all play a more important role than previously assumed, and interact according to the phase of the diurnal cycle of convection. Recent studies also emphasise the significance of upstream conditions in Central and East Africa, as well as strong interactions between midlatitudes and the WAM.

This study investigates the synoptic variability of the monsoon flux during the establishment of the West African monsoon using observations and ECMWF analyses. It highlights variability at a 3-5-day time scale, characterized by successive northward excursions of the monsoon flux. Their characteristics and climatology prior to the monsoon onset are presented. These penetrations follow a maximum of intensity of the heat-low (extension and minimum of pressure) and are concomitant with an acceleration of the low-level meridional wind. Some penetrations are stationary whereas others propagate westward simultaneously with African easterly waves. Both types are investigated in more detail by case-studies. This enables us to distinguish the boundary-layer mechanisms involved in such penetrations. A similar conceptual model holds for both. It is argued that the heat-low dynamics is a major driver of these synoptic penetrations, pointing to the predominantly continental nature of this phenomenon. In turn, the heat-low can be partitioned by the penetrations. Horizontal advection is the main process that eventually accounts for these surges; nevertheless, turbulent mixing also plays a significant role by vertically redistributing moisture, and in more subtle ways by its contribution to the shaping of the low-level synoptic environment within which the surges take place

This study aims at assessing the consistency between different precipitable water vapour (PWV) datasets over Africa (between 35°N and 10°S). This region is characterized by large spatial and temporal variability of humidity but also by the scarcity of its operational observing network, limiting our knowledge of the hydrological cycle. We intercompare data from observing techniques such as ground-based Global Positioning System (GPS), radiosondes, AERONET sun photometers and SSM/I, as well as reanalyses from European Centre for Medium-Range Weather Forecasts (ERA-40) and National Center for Environmental Prediction (NCEP2). The GPS data, especially, are a new source of PWV observation in this region. PWV estimates from nine ground-based GPS receivers of the international GPS network data are used as a reference dataset to which the others are compared. Good agreement is found between observational techniques, though dry biases of 12–14% are evidenced in radiosonde data at three sites. Reasonable agreement is found between the observational datasets and ERA-40 (NCEP2) reanalyses with maximum bias ⩽9% (14%) and standard deviation ⩽17% (20%). Since GPS data were not assimilated in the ERA-40 and NCEP2 reanalyses, they allow for a fully independent validation of the reanalyses. They highlight limitations in the reanalyses, especially at time-scales from sub-daily to periods of a few days. This work also demonstrates the high potential of GPS PWV estimates over Africa for the analysis of the hydrological cycle, at time-scales ranging between sub-diurnal to seasonal. Such observations can help studying atmospheric processes targeted by the African Monsoon Multidisciplinary Analysis project. Copyright © 2007 Royal Meteorological Society

This study presents a comprehensive analysis of the variability of water vapour in a growing convective boundary-layer (CBL) over land, highlighting the complex links between advection, convective activity and moisture heterogeneity in the boundary layer. A Large-eddy Simulation (LES) is designed, based on observations, and validated, using an independent data-set collected during the International H2O Project (IHOP_2002) field-experiment. Ample information about the moisture distribution in space and time, as well as other important CBL parameters are acquired by mesonet stations, balloon soundings, instruments on-board two aircraft and the DLR airborne water-vapour differential-absorption lidar. Because it can deliver two-dimensional cross-sections at high spatial resolution (140 m horizontal, 200 m vertical), the airborne lidar offers valuable insights of small-scale moisture-variability throughout the CBL. The LES is able to reproduce the development of the CBL in the morning and early afternoon, as assessed by comparisons of simulated mean profiles of key meteorological variables with sounding data. Simulated profiles of the variance of water-vapour mixing-ratio were found to be in good agreement with the lidar-derived counterparts. Finally, probability-density functions of potential temperature, vertical velocity and water-vapour mixing-ratio calculated from the LES show great consistency with those derived from aircraft in situ measurements in the middle of the CBL. Downdraughts entrained from above the CBL are governing the scale of moisture variability. Characteristic length-scales are found to be larger for water-vapour mixing-ratio than for temperature The observed water-vapour variability exhibits contributions from different scales. The influence of the mesoscale (larger than LES domain size, i.e. 10 km) on the smaller-scale variability is assessed using LES and observations. The small-scale variability of water vapour is found to be important and to be driven by the dynamics of the CBL. Both lidar observations and LES evidence that dry downdraughts entrained from above the CBL are governing the scale of moisture variability. Characteristic length-scales are found to be larger for water-vapour mixing-ratio than for temperature and vertical velocity. In particular, intrusions of drier free-troposphere air from above the growing CBL impose a marked negative skewness on the water-vapour distribution within it, both as observed and in the simulation. Copyright © 2005 Royal Meteorological Society.

SUMMARY This paper reports an intercomparison study of midlatitude continental cumulus convection simulated by eight two-dimensional and twothree-dimensional cloud-resolving models (CRMs), driven by observed large-scale advective temperature and moisture tendencies, surface turbulent euxes, and radiative-heating proe les during three sub-periods of the summer 1997 Intensive Observation Period of the US Department of Energy’s Atmospheric Radiation Measurement (ARM) program. Each sub-period includes two or three precipitation events of various intensities over a span of 4 or 5 days. The results can be summarized as follows. CRMs can reasonably simulate midlatitude continental summer convection observed at the ARM Cloud and Radiation Testbed site in terms of the intensity of convective activity, and the temperature and specie c-humidity evolution. Delayed occurrences of the initial precipitation events are a common feature for all three sub-cases among the models. Cloud mass e uxes, condensate mixing ratios and hydrometeor fractions produced by all CRMs are similar. Some of the simulated cloud properties such as cloud liquid-water path and hydrometeor fraction are rather similar to available observations. All CRMs produce large downdraught mass euxes with magnitudes similar to those of updraughts, in contrast to CRM results for tropical convection. Some inter-model differences in cloud properties are likely to be related to those in the parametrizations of microphysical processes. There is generally a good agreement between the CRMs and observations with CRMs being signie cantly better than single-column models (SCMs), suggesting that current results are suitable for use in improving parametrizations in SCMs. However, improvements can still be made in the CRM simulations; these include the proper initialization of the CRMs and a more proper method of diagnosing cloud boundaries in model outputs for comparison with satellite and radar cloud observations.

SUMMARY Cloud-resolving model (CRM) simulations of organized tropical convection observed in the Tropical Ocean/Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment are used toevaluate versions of the European Centre for Medium-Range Weather Forecasts convection and cloud schemes in single-column model simulations. Emphasis is placed upon the ability of the convection scheme to represent ‘convective-scale’ processes with typically mode-1 heating structures through the troposphere, together with a cloud scheme representing the ‘stratiform (mesoscale) component’ with upper-level heating and low-level cooling due to the evaporation of precipitation. While diagnosis of convective and stratiform precipitation is sensitive to the sampling criteria applied to the CRM, vertical structures of the mass and heat budgets are robust. Using diagnostics from the CRM simulations as a guide, revisions to the convection and cloud schemes are suggested in order to enable the parametrization to represent the two scales. The study suggests that a mass-e ux convection scheme linked via detrainment to a prognostic treatment of cloud can represent organized convection, provided that the upward motion in the upper-level stratiform cloud is considered.

A bulk mass-flux convection parametrization for deep and shallow convection is presented that includes an efficient and straightforward treatment of numerics, moist thermodynamics and convective downdraughts. The scheme is evaluated in a single-column model context for a tropical deep-convective period and a trade-wind cumulus case. Preliminary applications in a global numerical weather-prediction model and a mesoscale model are also discussed. The results suggest that the present scheme provides reasonable solutions in terms of predicted rainfall, and tropical temperature and moisture structures. The application of the scheme to various scales is supported by the use of a convective available potential energy convective closure that assures a smooth interaction with the large-scale environment and efficiently suppresses conditional instability of the second kind-like spin-up processes on the grid-scale. Finally, the theoretical and practical limits of the present approach are discussed together with possible future developments.

A one-week convective period of the Coupled Ocean-Atmosphere Response Experiment (10-17 December 1992), prior to a westerly wind burst, has been simulated with a cloud-resolving model. Large-scale advection derived from observations is used to force the model, in the same way as usually done in single-column models. Our aim is to evaluate this explicit simulation against observed large-scale thermodynamic and radiative fields, and to investigate the sensitivity of model results to observational uncertainties. Precipitation, apparent heat source and moisture sink are fairly well reproduced by the model as compared to those diagnosed from observations. Temperature (T) and moisture (qv) fields are also reasonably well captured except for a moderate cold and moist bias. Simulated moist static energy is too high below 6 km and too low above, possibly because convection is slightly less active in the model than observed. In order to investigate the sensitivity of mode results to observational uncertainties, results are analysed with the moist static energy budget together with independent observational radiative datasets. This analysis suggests that the atmospheric radiative rate that is in equilibrium with the applied large-scale advection and observed surface fluxes is too weak and that its diurnal cycle is not realistic. The most likely reason for this problem is found to be related to uncertainties in the large-scale advection diagnosed from observations. This analysis also indicates that the simulated high-cloud cover is too large in the model. It is greatly improved by increasing the ice-crystal fall speed. Additional tests show a large sensitivity of the simulated moist static energy, and thus T and qv, to the range of uncertainties previously found for large-scale advection. The vertical structure of the model bias is not significantly modified by changing the intensity of these forcings, but it is most sensitive to their vertical structures. It is argued that it is crucial to get some insights into the range of uncertainties of external forcings (large-scale advection, surface fluxes and atmospheric radiative-heating rate) so as to assess the relevance of any evaluation of simulated temperature and moisture when a model, either resolving clouds or parametrizing them, is forced with large-scale advection deduced from observations.