African Monsoon Multidisciplinary Analysis Research Articles

An Inverse Method for Drop Size Distribution Retrieval from Polarimetric Radar at Attenuating Frequency

A method that formulates the retrieval of drop size distribution (DSD) parameters from polarimetric radar variables at attenuating frequency as the solution of an inverse problem is presented. The DSD in each radar bin is represented by a normalized Gamma distribution defined by three parameters (Dm,N0*,μ). The direct problem that describes polarimetric radar observables—scattering and propagation terms—and their dependency on DSD parameters is analyzed based on T-matrix scattering simulations. The inverse algorithm and its application to the DSD retrieval are then presented. The inverse method is applied to an African Monsoon Multidisciplinary Analysis (AMMA) field campaign that deployed an X-band dual-polarization Doppler radar and optical disdrometers in Benin, West Africa, in 2006 and 2007. The dataset is composed of X-band polarimetric radar PPIs and disdrometer data for 15 organized convective systems observed in 2006. A priori information on DSD parameters (benchmark method) is derived from the polarimetric radar observables by applying power law relationships. The proposed retrieval method of DSD parameters leads to the following results as compared to the benchmark: (i) we found a better spatial consistency of the retrieved parameters, (ii) the reconstructed polarimetric radar observables are closer to the observations, (iii) The validation with disdrometer data confirms an improved estimation of the DSD parameters.

The use of gamma distributions to quantify the dependence of cloud particle size distributions in hurricanes on cloud and environmental conditions

AbstractMesoscale models that predict the evolution of tropical cyclones (TCs) are sensitive to the representation of cloud microphysical processes. Bulk cloud parametrizations used in such models make assumptions about the particle size distributions (PSDs) of different ice species, and their representativeness for TCs is not well known. In situ cloud probe data acquired in tropical storms, depressions and waves during the NASA African Monsoon Multidisciplinary Analyses (NAMMA) project are used to define PSDs of snow and graupel, and of all ice hydrometeors combined. These PSDs are fitted to gamma functions to determine how the intercept (N0), shape (μ), and slope (λ) vary with cloud and environmental conditions. Families of PSDs are determined for each condition (e.g. PSDs found in updraughts, downdraughts and stratiform regions, for different ranges of ice water content (IWC) and temperature (T), and for differing stages of TC development). A volume of equally plausible solutions in (N0‐μ‐λ) phase space is defined for each environmental condition sampled based on the goodness of the fits and the uncertainty in the measured PSDs due to statistical sampling. Per cent overlap between two families in each environmental and cloud condition was calculated, and results show that areas with sustained vertical velocity with a magnitude of at least 1 m·s−1 lie in a different phase space than stratiform regions, and PSDs corresponding to IWC < 0.01 g·m−3 lie in a different phase space than PSDs corresponding to IWC > 0.1 g·m−3. All other environmental and cloud conditions did not have significant impacts on either the location or uncertainty in the family of ellipsoids.

Potential Vorticity Generation by West African Squall Lines

Abstract The West African summer monsoon features multiple, complex interactions between African easterly waves (AEWs), moist convection, variable land surface properties, dust aerosols, and the diurnal cycle. One aspect of these interactions, the coupling between convection and AEWs, is explored using observations obtained during the 2006 African Monsoon Multidisciplinary Analyses (AMMA) field campaign. During AMMA, a research weather radar operated at Niamey, Niger, where it surveilled 28 squall-line systems characterized by leading convective lines and trailing stratiform regions. Nieto Ferreira et al. found that the squall lines were linked with the passage of AEWs and classified them into two tracks, northerly and southerly, based on the position of the African easterly jet (AEJ). Using AMMA sounding data, we create a composite of northerly squall lines that tracked on the cyclonic shear side of the AEJ. Latent heating within the trailing stratiform regions produced a midtropospheric positive potential vorticity (PV) anomaly centered at the melting level, as commonly observed in such systems. However, a unique aspect of these PV anomalies is that they combined with a 400–500-hPa positive PV anomaly extending southward from the Sahara. The latter feature is a consequence of the deep convective boundary layer over the hot Saharan Desert. Results provide evidence of a coupling and merging of two PV sources—one associated with the Saharan heat low and another with latent heating—that ends up creating a prominent midtropospheric positive PV maximum to the rear of West African squall lines.

Estimation of planetary boundary layer height from radiosonde profiles over West Africa during the AMMA field campaign: Intercomparison of different methods

Deducing realistic planetary boundary layer heights (PBLH) is crucial for weather, climate and air quality models, despite its equivocal nature. In this paper, a comparative assessment of seven PBLH estimation methods has been performed, with radiosonde profiles taken during the African Monsoon Multidisciplinary Analyses (AMMA) project campaign from June, 2006 to July, 2007 over 18 locations in West Africa. First, PBLH was identified from the radiosonde profiles as the location of minimum gradients in mixing ratio (q), relative humidity (RH) and refractivity (N), and maximum gradient in potential temperature (θ). Other methods used to identify PBL tops were the statistical NS method and bulk Richardson (Rib) method at a critical threshold of 0.25. Next, a reference method (ho) was identified as the benchmark for PBL comparison. Visual inspection of the individual profiles allowed for assessing the ho method to yield reliable PBLH estimates. Thereafter, comparisons of the PBLH were performed for both convective and stable cases, with the stable boundary layer (SBL) height being generally below 700 m a.g.l for all methods, whereas, convective boundary layer (CBL) heights ranged between 300 m a.g.l and 1400 m a.g.l across different regions of the study area. Contrarily, the NS method failed to detect the SBL due to its inability to identify a dewpoint or virtual potential temperature discontinuity. Additionally, the Rib method, particularly in stable cases, yielded PBL tops consistent with the reference method (ho) whenever the NLLJ was clearly defined due to contribution of wind shear beneath the jet core to turbulent kinetic energy (TKE) production. Afterwards, the study domain was split into three zones and the performance of each method was assessed per each zone. Spatially, the CBL height was observed to grow thicker in the north-east direction over the dry, arid regions, where integral values of sensible heat rapidly converted by surface net radiation, has significant influence on the growth of CBL throughout daytime. Other likely reasons for this observation are advection, orography and mechanical turbulence production. However, a reversal was observed at night with the SBL being thinner in the dry, arid regions and rather relatively, higher in the coastal regions. Finally, the statistical assessment, coupled with visual inspection of the individual profiles, showed that the gradient methods (particularly N) outperformed the Rib and NS methods, yielding very low biases as well as, high and statistically significant correlation co-efficients. These results are useful for enhancing the performance of PBL models over the region. Possible limitations to the findings of this study are the different ascent times between the sites, as well as the number of ascents per site, which have potential implications for the results. Further work, based on observations from the DACCIWA (Dynamics-aerosol-chemistry-cloud interactions in West Africa) field campaign, is currently ongoing to also fully substantiate the role of radiation and energy budgets in PBL development, while further assessing their significant effect on the performance of the PBL-detection methods.

The First Forecasters’ Handbook for West Africa

AbstractBridging the gap between rapidly moving scientific research and specific forecasting tools, Meteorology of Tropical West Africa: The Forecasters’ Handbook gives unprecedented access to the latest science for the region’s forecasters, researchers, and students and combines this with pragmatic approaches to forecasting. It is set to change the way tropical meteorology is learned and will serve to drive demand for new forecasting tools. The Forecasters’ Handbook builds upon the legacy of the African Monsoon Multidisciplinary Analysis (AMMA) project, making the latest science applicable to forecasting in the region. By bringing together, at the outset, researchers and forecasters from across the region, and linking to applications, user communities, and decision-makers, The Forecasters’ Handbook provides a template for finding much needed solutions to critical issues such as building resilience to weather hazards and climate change in West Africa.

Sensitivity of GPS tropospheric estimates to mesoscale convective systems in West Africa

Abstract. This study analyzes the characteristics of GPS tropospheric estimates (zenith wet delays – ZWDs, gradients, and post-fit phase residuals) during the passage of mesoscale convective systems (MCSs) and evaluates their sensitivity to the research-level GPS data processing strategy implemented. Here, we focus on MCS events observed during the monsoon season of West Africa. This region is particularly well suited for the study of these events due to the high frequency of MCS occurrences in the contrasting climatic environments between the Guinean coast and the Sahel. This contrast is well sampled with data generated by six African Monsoon Multidisciplinary Analysis (AMMA) GPS stations. Tropospheric estimates for a 3-year period (2006–2008), processed with both the GAMIT and GIPSY-OASIS software packages, were analyzed and intercompared. First, the case of a MCS that passed over Niamey, Niger, on 11 August 2006 demonstrates a strong impact of the MCS on GPS estimates and post-fit residuals when the GPS signals propagate through the convective cells as detected on reflectivity maps from the MIT C-band Doppler radar. The estimates are also capable of detecting changes in the structure and dynamics of the MCS. However, the sensitivity is different depending on the tropospheric modeling approach adopted in the software. With GIPSY-OASIS, the high temporal sampling (5 min) of ZWDs and gradients is well suited for detecting the small-scale, short-lived, convective cells, while the post-fit residuals remain quite small. With GAMIT, the lower temporal sampling of the estimated parameters (hourly for ZWDs and daily for gradients) is not sufficient to capture the rapid delay variations associated with the passage of the MCS, but the post-fit phase residuals clearly reflect the presence of a strong refractivity anomaly. The results are generalized with a composite analysis of 414 MCS events observed over the 3-year period at the six GPS stations with the GIPSY-OASIS estimates. A systematic peak is found in the ZWDs coincident with the cold pool crossing time associated with the MCSs. The tropospheric gradients reflect the path of the MCS propagation (generally from east to west). This study concludes that ZWDs, gradients, and post-fit phase residuals provide relevant and complementary information on MCSs passing over or in the vicinity of a GPS station.

Improving surface soil moisture estimates in West Africa through GRACE data assimilation

Recent annual trends of precipitation and terrestrial water storage (TWS) in West Africa have been increasing over the past decade. Despite a significant impact of soil moisture on the TWS in West Africa, there is little research on the recent spatial and temporal behaviors of surface soil moisture (SSM) along with the hydrological trends and variability in West Africa. In this study, we assimilate TWS estimates from the Gravity Recovery and Climate Experiment (GRACE) mission into the Catchment Land Surface Model (CLSM) and evaluate its impacts on SSM simulations for the years, 2002–2017. The evaluation is performed using reference datasets: the African Monsoon Multidisciplinary Analysis (AMMA) in situ soil moisture observations, three currently available microwave satellite SSM observations from the Advanced Scatterometer (ASCAT), the Soil Moisture Ocean Salinity (SMOS), and the Soil Moisture Active Passive (SMAP) satellites and also the triple collocation analysis (TCA). Overall, modeled SSM shows good agreement with the reference datasets in terms of the anomaly SSM correlations. However, both modeled and ASCAT SSM are limited in their representation of the drying rates, as observed by ground observations, SMOS and SMAP estimates. Further, GRACE data assimilation results in improved SSM simulations in the humid regions with large annual TWS variability. This study demonstrates the utility of land data assimilation to inform hydrological conditions in West Africa, where soil moisture monitoring is necessary for water resource and livestock management.

Evaluation of Groundwater Simulations in Benin from the ALMIP2 Project

AbstractA comprehensive estimation of water budget components, particularly groundwater storage (GWS) and fluxes, is crucial. In this study, we evaluate the terrestrial water budget of the Donga basin (Benin, West Africa), as simulated by three land surface models (LSMs) used in the African Monsoon Multidisciplinary Analysis Land Surface Model Intercomparison Project, phase 2 (ALMIP2): CLM4, Catchment LSM (CLSM), and Minimal Advanced Treatments of Surface Interaction and Runoff (MATSIRO). All three models include an unconfined groundwater component and are driven by the same ALMIP2 atmospheric forcing from 2005 to 2008. Results show that all three models simulate substantially shallower water table depth (WTD) with smaller seasonal variations, approximately 1–1.5 m compared to the observed values that range between 4 and 9.6 m, while the seasonal variations of GWS are overestimated by all the models. These seemingly contradictory simulation results can be explained by the overly high specific yield prescribed in all models. All models achieve similar GWS simulations but with different fractions of precipitation partitioning into surface runoff, base flow, and evapotranspiration (ET), suggesting high uncertainty and errors in the terrestrial and groundwater budgets among models. The poor performances of models can be attributed to bias in the hydrological partitioning (base flow vs surface runoff) and sparse subsurface data. This analysis confirms the importance of subsurface hydrological processes in the current generation of LSMs and calls for substantial improvement in both surface water budget (which controls groundwater recharge) and the groundwater system (hydrodynamic parameters, vertical geometry).

Combining a Two Source Energy Balance Model Driven by MODIS and MSG-SEVIRI Products with an Aggregation Approach to Estimate Turbulent Fluxes over Sparse and Heterogeneous Vegetation in Sahel Region (Niger)

Estimates of turbulent fluxes (i.e., sensible and latent heat fluxes H and LE) over heterogeneous surfaces is not an easy task. The heterogeneity caused by the contrast in vegetation, hydric and soil conditions can generate a large spatial variability in terms of surface–atmosphere interactions. This study considered the issue of using a thermal-based two-source energy model (TSEB) driven by MODIS (Moderate resolution Imaging Spectroradiometer) and MSG (Meteosat Second Generation) observations in conjunction with an aggregation scheme to derive area-averaged H and LE over a heterogeneous watershed in Niamey, Niger (Wankama catchment). Data collected in the context of the African Monsoon Multidisciplinary Analysis (AMMA) program, including a scintillometry campaign, were used to test the proposed approach. The model predictions of area-averaged turbulent fluxes were compared to data acquired by a Large Aperture Scintillometer (LAS) set up over a transect about 3.2 km-long and spanning three vegetation types (millet, fallow and degraded shrubs). First, H and LE fluxes were estimated at the MSG-SEVIRI grid scale by neglecting explicitly the subpixel heterogeneity. Moreover, the impact of upscaling the model’s inputs was investigated using in-situ input data and three aggregation schemes of increasing complexity based on MODIS products: a simple averaging of inputs at the MODIS resolution scale, another simple averaging scheme that considers scintillometer footprint extent, and the weighted average of inputs based on the footprint weighting function. The H and LE simulated using the footprint weighted method were more accurate than for the two other aggregation rules despite the heterogeneity of the landscape. The statistical values are: correlation coefficient (R) = 0.71, root mean square error (RMSE) = 63 W/m2 and mean bias error (MBE) = −23 W/m2 for H and an R = 0.82, RMSE = 88 W/m2 and MBE = 45 W/m2 for LE. This study opens perspectives for the monitoring of convective and evaporative fluxes over heterogeneous landscape based on medium resolution satellite products.

Performance Evaluation of WRF-CHEM in Simulating Harmattan Dust over West Africa

In this study, we investigated the ability of Weather Research and Forecasting model coupled with the Chemistry option (WRF-Chem) using the GOCART dust scheme in capturing the spatial and temporal distribution of a severe dust storm which occurred in March 2006 over West Africa. The period (5th - 14th March, 2006) also coincided with the African Monsoon Multidisciplinary Analysis (AMMA) Special Observing Period (SOP) and some observed datasets from other platforms were also used. The WRF-Chem simulation was performed on a two-way nested domain in order to resolve the dust episode on high horizontal resolution dataset. From the 5th March, a dust storm raised in northern Algeria, which propagated southward spreading to the west and east on the subsequent days. Over the Gulf of Guinea, an intense dust haze was transported southward thereby generating an aerosol optical thickness (AOT) greater than 4 over Nigeria. Qualitative and quantitative characterizations between observations and WRF-Chem simulations demonstrated that the WRF-Chem model is highly efficient and can be recommended to simulate the spatial and temporal distribution of harmattan dust over West Africa. We discovered that the WRFChem modelled Aerosol Optical Depth (AOD) underestimated the AOD from AERONET as a result of all other aerosols like black carbon integrated into the observed which is not yet captured by model. The quantitative and qualitative analysis of the model output ascertain the capability of WRF-Chem to reasonably reproduce West African harmattan dust haze from the time and location of being raised to its transportation and propagation and hence its final settling or disappearance over the continental area of West Africa. Model simulation was able to partition the dust particles into five different sizes. It was discovered that dust particles of sizes 1.4 μm, 2.4 μm and 4.5 μm are more prevalent among other sizes from the raised and transported dust over West Africa.

AMMA‐CATCH, a Critical Zone Observatory in West Africa Monitoring a Region in Transition

Core Ideas AMMA‐CATCH is a long‐term critical zone observatory in West Africa. Four sites sample the sharp ecoclimatic gradient characteristic of this region. Combined measurements of meteorology, water, and vegetation dynamics began in 1990. Intensification of rainfall and hydrological cycles is observed. The strong overall re‐greening may hide contrasted changes. West Africa is a region in fast transition from climate, demography, and land use perspectives. In this context, the African Monsoon Multidisciplinary Analysis (AMMA)–Couplage de l'Atmosphère Tropicale et du Cycle eco‐Hydrologique (CATCH) long‐term regional observatory was developed to monitor the impacts of global change on the critical zone of West Africa and to better understand its current and future dynamics. The observatory is organized into three thematic axes, which drive the observation and instrumentation strategy: (i) analyze the long‐term evolution of eco‐hydrosystems from a regional perspective; (ii) better understand critical zone processes and their variability; and (iii) meet socioeconomic and development needs. To achieve these goals, the observatory has gathered data since 1990 from four densely instrumented mesoscale sites (∼104 km2 each), located at different latitudes (Benin, Niger, Mali, and Senegal) so as to sample the sharp eco‐climatic gradient that is characteristic of the region. Simultaneous monitoring of the vegetation cover and of various components of the water balance at these four sites has provided new insights into the seemingly paradoxical eco‐hydrological changes observed in the Sahel during the last decades: groundwater recharge and/or runoff intensification despite rainfall deficit and subsequent re‐greening with still increasing runoff. Hydrological processes and the role of certain key landscape features are highlighted, as well as the importance of an appropriate description of soil and subsoil characteristics. Applications of these scientific results for sustainable development issues are proposed. Finally, detecting and attributing eco‐hydrological changes and identifying possible regime shifts in the hydrologic cycle are the next challenges that need to be faced.

Evidence for a gas-flaring source of alkanes leading to elevated ozone in air above West Africa

As part of the African Monsoon Multidisciplinary Analysis (AMMA) project, the FAAM BAe-146 research aircraft sampled the lower and mid-troposphere around the West Africa sub-region. Back trajectory analysis of the air parcels sampled on-board during the entire duration of the flights showed the history and fate of the air parcels. Data from flights B228 and B231 showed strongly enhanced carbon monoxide (CO) and ozone levels attributable to emissions of anthropogenic origin from the city of Lagos and gas flaring activities in the Nigeria oil fields. The elevated levels of ozone and CO observed at about 6 km above the sea-level on flight B231 were attributed to long-range transport of biomass burning plume from the East, around Sudan. The strongly enhanced mixing ratios of short-chained alkanes and CO (> 400 ppbv) observed from measurements on flights B228 and B231 are indicative of natural gas/combustion sources. Flight B222 sampled air parcels strongly impacted by emissions from Lagos but not from the Nigeria oil field and measured relatively lower mixing ratios of ozone, CO and short-chained alkanes species. Results from this study strongly suggests gas flaring emissions in the Niger Delta area to be a prominent contributor to the enhanced levels of short-chained alkane species observed in Lagos metropolis, especially during the West Africa Monsoon (WAM) months and, hence, a significant source of atmospheric aerosol in the sub-region. Key words: African Monsoon Multidisciplinary Analysis (AMMA), gas flaring, West Africa Monsoon, alkanes, ozone, Niger Delta.

New Saharan wind observations reveal substantial biases in analysed dust‐generating winds

AbstractFor the remote Sahara, the Earth's largest dust source, there has always been a near‐absence of data for evaluating models. Here, new observations from the Fennec project are used along with Sahelian data from the African Monsoon Multidisciplinary Analysis (AMMA) to give an unprecedented evaluation of dust‐generating winds in the European Centre for Medium‐Range Weather Forecasts ERA‐Interim reanalysis (ERA‐I). Consistent with past studies, near‐surface, high‐speed winds are lacking in ERA‐I and the diurnal variability is under‐represented. During the summer monsoon season, correlations of ERA‐I with observed wind‐speed are low (∼0.35 in Sahel and 0.25–0.4 in the Sahara). Fennec data show for the first time that: (1) correlations are reduced even in the Sahara, not directly influenced by the monsoon, (2) the systematic underestimation of observed winds by ERA‐I in the summertime Sahel extends into the central Sahara: potentially explaining the failure of global models to capture the observed global dust maximum that occurs over the summertime Sahara (such as CMIP5), and demonstrates that modelled winds must be improved if they are to capture this key feature of the climatology.

Sensitivity of Convection Schemes and Microphysics in Mesoscale Simulations of Squall Line Observed During the AMMA Campaign Using the WRF Model

In this paper, a sensitivity study of convection schemes and cloud microphysics is performed using the Weather Research and Forecasting (WRF) model. It concerns the mesoscale simulation of the squall line observed during the African Monsoon Multidisciplinary Analysis campaign on September 22nd, 2006. Four convection schemes (Kain-Fritsch, Grell-Devenyi, Tietke and New Simplified Arakawa-Schubert) and five microphysics schemes (WRF Single-Moment 3-class (WSM3), WRF Single-Moment 5-class (WSM5), WRF Single-Moment 6-class (WSM6), Thompson and NSSL2-moment) are used. Each microphysics scheme is associated with a convection scheme, giving twenty combinations. The goal is to find the combination able to provide a realistic representation of the meteorological fields associated to the studied squall line, especially those of precipitation. The New Simplified Arakawa-Schubert convection scheme combined either with the WSM6 microphysics scheme, either with the WSM5 microphysics scheme or Thompson scheme was found to give a result closer to observation. The study also confirms that convection schemes have much more influence than microphysics on precipitation heights.

Modeling Surface Runoff and Water Fluxes over Contrasted Soils in the Pastoral Sahel: Evaluation of the ALMIP2 Land Surface Models over the Gourma Region in Mali

Abstract Land surface processes play an important role in the West African monsoon variability. In addition, the evolution of hydrological systems in this region, and particularly the increase of surface water and runoff coefficients observed since the 1950s, has had a strong impact on water resources and on the occurrence of floods events. This study addresses results from phase 2 of the African Monsoon Multidisciplinary Analysis (AMMA) Land Surface Model Intercomparison Project (ALMIP2), carried out to evaluate the capability of different state-of-the-art land surface models to reproduce surface processes at the mesoscale. Evaluation of runoff and water fluxes over the Mali site is carried out through comparison with runoff estimations over endorheic watersheds as well as evapotranspiration (ET) measurements. Three remote-sensing-based ET products [ALEXI, MODIS, and Global Land Evaporation Amsterdam Model (GLEAM)] are also analyzed. It is found that, over deep sandy soils, surface runoff is generally overestimated, but the ALMIP2 multimodel mean reproduces in situ measurements of ET and water stress events rather well. However, ALMIP2 models are generally unable to distinguish among the two contrasted hydrological systems typical of the study area. Employing as input a soil map that explicitly represents shallow soils improves the representation of water fluxes for the models that can account for their representation. Shallow soils are shown to be also quite challenging for remote-sensing-based ET products, even if their effect on evaporative loss was captured by the diagnostic thermal-based ALEXI. A better representation of these soils, in soil databases, model parameterizations, and remote sensing algorithms, is fundamental to improve the estimation of water fluxes in this part of the Sahel.

Streamflows over a West African Basin from the ALMIP2 Model Ensemble

Abstract Comparing streamflow simulations against observations has become a straightforward way to evaluate a land surface model’s (LSM) ability in simulating water budget within a catchment. Using a mesoscale river routing scheme (RRS), this study evaluates simulated streamflows over the upper Ouémé River basin resulting from 14 LSMs within the framework of phase 2 of the African Monsoon Multidisciplinary Analysis (AMMA) Land Surface Model Intercomparison Project (ALMIP2). The ALMIP2 RRS (ARTS) has been used to route LSM outputs. ARTS is based on the nonlinear Muskingum–Cunge method and a simple deep water infiltration formulation representing water-table recharge as previously observed in that region. Simulations are performed for the 2005–08 period during which ground observations are largely available. Experiments are designed using different ground-based rainfall datasets derived from two interpolation methods: the Thiessen technique and a combined kriging–Lagrangian methodology. LSM-based total runoff (TR) averages vary from 0.07 to 1.97 mm day−1, while optimal TR was estimated as ~0.65 mm day−1. This highly affected the RRS parameterization and streamflow simulations. Optimal Nash–Sutcliffe coefficients for LSM-averaged streamflows varied from 0.66 to 0.92, depending on the gauge station. However, individual LSM performances show a wider range. A more detailed rainfall distribution provided by the kriging–Lagrangian methodology resulted in overall better streamflow simulations. The early runoff generation related to reduced infiltration rates during early rainfall events features as one of the main reasons for poor LSM performances.

Documentation of atmospheric constants over Niamey, Niger: a theoretical aid for measuring instruments

ABSTRACTThe frequent failures of ground measuring devices, e.g. radiosondes, weather stations, in developing regions is worrisome. From the literature, established projects such as the Aerosol Robotic Network (AERONET) and the African Monsoon Multidisciplinary Analyses (AMMA) are burdened by the same challenge. At the moment, the AERONET and AMMA databases show a large volume of data loss. With only about 47% of data available to scientists, it is evident that accurate nowcasts or forecasts cannot be guaranteed. It is proposed that the challenge is not measuring device design error but systemic, i.e. the configuration of accurate calibration constants in the compact flash card of the devices. The calibration constants of most radiosonde or weather stations are not compatible with the atmospheric conditions of the West African climate. A dispersion model was developed to incorporate salient mathematical representations such as the unified number. The unified number was derived to describe the turbulence of aerosol transport in the frictional layer of the lower atmosphere. A 14 year dataset from the Multi‐angle Imaging SpectroRadiometer was tested using the dispersion model. A yearly estimation of the atmospheric constants over Niamey using the model was obtained with about 87.5% accuracy. This further revealed that the average atmospheric constants for Niamey, Niger, are a1 = 0.77975 and a2 = 0.693021 and the tuning constants are n1 = 0.140187 and n2 = 0.759236. Also, the yearly atmospheric constants confirmed that the lower atmosphere of Niamey is very dynamic. Hence, it is recommended that radiosonde and weather station manufacturers should constantly review the atmospheric constants over a geographical location to enable about 80% data retrieval.

An Improved Convective Ice Parameterization for the NASA GISS Global Climate Model and Impacts on Cloud Ice Simulation.

Partitioning of convective ice into precipitating and detrained condensate presents a challenge for GCMs since partitioning depends on the strength and microphysics of the convective updraft. It is an important issue because detrainment of ice from updrafts influences the development of stratiform anvils, impacts radiation, and can affect GCM climate sensitivity. Recent studies have shown that the CMIP5 configurations of the Goddard Institute for Space Studies (GISS) GCM simulated upper-tropospheric ice water content (IWC) that exceeded an estimated upper bound by a factor of 2. Partly in response to this bias, a new GCM parameterization of convective cloud ice has been developed that incorporates new ice particle fall speeds and convective outflow particle size distributions (PSDs) from the NASA African Monsoon Multidisciplinary Analyses (NAMMA), NASA Tropical Composition, Cloud and Climate Coupling (TC4), DOE ARM-NASA Midlatitude Continental Convective Clouds Experiment (MC3E), and DOE ARM Small Particles in Cirrus (SPARTICUS) field campaigns. The new parameterization assumes a normalized gamma PSD with two novel developments: no explicit assumption for particle habit in the calculation of mass distributions, and a formulation for translating ice particle fall speeds as a function of maximum diameter into fall speeds as a function of melted-equivalent diameter. Two parameters (particle volume- and projected area-weighted equivalent diameter) are diagnosed as a function of temperature and IWC in the convective plume, and these parameters constrain the shape and scale of the normalized gamma PSD. The diagnosed fall speeds and PSDs are combined with the GCM's parameterized convective updraft vertical velocity to partition convective updraft condensate into precipitating and detrained components. A 5-yr prescribed sea surface temperature GCM simulation shows a 30%-50% decrease in upper-tropospheric deep convective IWC, bringing the tropical and global mean ice water path into closer agreement with CloudSat best estimates.

Variability and predictability of West African monsoon on seasonal and decadal scales

Clim Dyn (2016) 47:3391–3392 DOI 10.1007/s00382-016-3429-4 EDITORIAL Variability and predictability of West African monsoon on seasonal and decadal scales Yongkang Xue 1 · Serge Janicot 2 · William K.‑M. Lau 3 Published online: 26 November 2016 © Springer-Verlag Berlin Heidelberg 2016 Recent observational evidence supports the notion that there are strong decadal climate variabilities in the Sahel and sur- rounding areas from the 1950s to the 2000s, not only in pre- cipitation, but also in sea surface temperature (SST), vegeta- tion cover, land use and land cover change (LULCC), and aerosol loading and spatial distributions (Xue et al. 2016; Boone et al. 2016; Gu et al. 2016). In the 2010 Climate Dynamics Special Issue “West African Monsoon and its Modeling,” the skills of general circulation models (GCMs) and regional climate models (RCMs) in properly simulating the West African monsoon (WAM) were extensively evalu- ated, and possible causes that contribute to model deficien- cies were investigated. While these studies provided basic understanding of the seasonal variations of WAM climatol- ogy, understanding of the complex interactions of WAM variability associated with SST, land surface, and aerosol forcings at different scales, particularly with respect to sea- sonal to decadal scale forcing, is still lacking. This special issue “Decadal variability of the West Afri- can monsoon, external surface forcings, and their mode- ling” aims to provide the latest research on the main WAM dynamic and physical processes, model predictability, and possible interactions between the WAM and external forc- ings at seasonal to decadal scales. The main themes of this * Yongkang Xue yxue@geog.ucla.edu Department of Geography, Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA, USA Laboratoire Atmospheres Milieux Observations Spatiales, Sorbonne-Universites, CNRS-UVSQ-UPMC, Paris, France Earth System Science Interdisciplinary Center, Joint Global Change Research Institute, University of Maryland, College Park, MD, USA special issue are: (1) intercomparison and evaluation of skills of GCMs and RCMs in simulating seasonal and dec- adal variability of WAM rainfall and relevant processes, and its association with SST, LULCC, and aerosol processes, as well as identification of major challenges, deficiencies, and uncertainties that remain in modeling WAM variabil- ity; (2) representations of major WAM characteristics and possible underlying physical and dynamic mechanisms; (3) application of observational and reanalyses data as a path- way for model physics evaluation and improvement, and in understanding the WAM variability; and (4) evaluation of RCM downscaling ability in terms of WAM decadal vari- ability. The papers in the special issue are mainly based on research conducted in conjunction with the second phase of the international West African Monsoon Modeling and Evaluation (WAMME II) project and the African Multidis- ciplinary Monsoon Analysis (AMMA) project. Most papers in this issue are from the WAMME II project focusing on the interactions between the WAM and external forcings. In the WAMME II experiment, realistic but ideal- ized forcings (setting to maximum forcing amplitude) are imposed to examine WAM responses in the WAMME II mod- els. Among them, Xue et al. (2016) investigate relative effects of the global and several major individual ocean basins on the WAM decadal variability and compare the SST’s role and the LULCC effect in initiating and modulating the decadal WAM variability and the 1980s Sahel drought. Boone et al. (2016) use recently available LULCC data to evaluate the sensitiv- ity of WAM seasonal variability to land surface processes and physical mechanisms involved. In addition to multi-model experiments, some of the individual models’ results are also presented. Using the aerosol data from the Goddard Chemis- try Aerosol Radiation and Transport (GOCART) model, Gu et al. (2016) compare aerosol effects over the WAM region and the Asian monsoon regions.

Intense convection over West Africa during AMMA SOP3 experiment

ERA-Interim product from the European Center for Medium-Range Weather Forecast (ECMWF) assimilation of African Monsoon Multidisciplinary Analysis (AMMA) resources, Meteosat satellite images, and synoptic observations were used to study local- and regional-scale environments associated with intense convective systems during the AMMA-SOP3 experiment over West Africa in the Northern Hemisphere of summer 2006. The convective system, from the 21st to 23rd of August 2006, was more active at 0000 and 1800UTC showing diurnal cycle of deep convection over West Africa where the African easterly waves (AEWs) are developed downstream. Downstream barotropic and baroclinic energy conversions associated with strong AEWs are important for the maintenance of AEW activity in West Africa. Barotropic energy conversions dominate south of the African easterly jet (AEJ), while baroclinic energy conversions are most important north of the AEJ. From a dynamical viewpoint, the low-level vorticity presents strong positive values over the sea and Sahara zone, indicating that exists on the cyclonic shear side of the African easterly jet, which is consistent with baroclinic growth. The 925-hPa equivalent potential temperature structure show a maximum over the Sahara which corresponds to the depression observed in this region. A mosaic of three hourly infrared (IR) satellite images, depicts a very distinct signal from an initial region of convection, developing through several stages and moving off the African coast. These observations, along with those available from the World Weather Watch, provide an opportunity to carry out numerical weather prediction (NWP) studies over West Africa utilizing high resolution limited area models.