Dynamic response of monsoon precipitation to mineral dust radiative forcing in the West Africa region
- 376 Downloads
Mineral dust over West Africa region modulates summer monsoon through direct radiative forcing. This study examined the impact of mineral dust radiative forcing on West Africa Monsoon variability with the aid of Regional Climate Model (RegCM4) at 50 km grid resolution driven by ERA Interim re-analysis. Three experiments were performed; first with the non-dust aerosol version of the model (CONTROL), and second with the dust aerosol module (DUST) and with an increase in the dust concentration (DOUBLE DUST). The simulation was run from October, 2004 to December, 2005 over West Africa domain with the first 3 months taken as spin up for model stability. The result shows that there was no significant change with Control and Dust case experiment but as the dust AOD increases from 1.0 to 2.0, radiation flux at the Top of Atmosphere changes from − 60 to − 80 W/m2 in the Double dust experiment. The Surface Long-wave Radiation Flux of 8.0 W/m2 remains unchanged in both cases. The Outgoing Long-wave Radiation (OLR) flux changes from 2.0 to 4.0 W/m2 indicating reduction in convective formation and as well as decrease in precipitation of 2 mm/day in the Sahel, while precipitation increases from 2 to 4 mm/day in the Guinea coast region. There was also strengthening of TEJ core and weakening of AEJ above average as dust concentration increases in some parts of the region during the monsoon period. The air temperature increases from 22.5 to 38.5 °C in both cases from coastal area to Sahelian region of West Africa. It was concluded that substantial amount of dust concentration in the atmosphere could trigger and increase radiative forcing of aerosols thereby sensitive to monsoon variability and results in enhancement of precipitation amount in the Guinea coast and reduction of precipitation amount in the Sahel region of West Africa. Meanwhile, there is need to inquire more into difference aerosol concentration per specie that can trigger or increase radiative forcing in the atmosphere.
KeywordsRadiative forcing Monsoon Dynamics Enhancement
West Africa monsoon is highly variable and its variability has led to incessant increase in flood occurrence in some parts of the region, while other parts had suffered immense drought which endanger crops and animal growth. This challenge of dramatic change in the precipitation regime from wet in the early 1960s to drier condition in the late 1970s. This also includes changes in other associated rain producing system. For example, African Easterly Jet (AEJ) at 600 hpa pressure level tends to be weaker and more pole ward, while the 200 hpa Tropical Easterly Jet (TEJ) and the monsoonal flow become stronger during the wet season (Fontaine et al. 1995; Sultan and Janicot 2003; Sylla et al. 2012; Grist and Nicholson 2001. More often than not, monsoon variability has been linked to aerosol radiative forcing among other factors. Here, we will focus on the direct radiative forcing of mineral dust impact on the monsoon variability in the region. Although many works have addressed radiative forcing of mineral dust in the West Africa climate system, yet there is need for accurate quantification of aerosol concentration that can initiate forcing. Aerosol characteristics with different species and emission in the West Africa region have been widely discussed. Specific inventories of different aerosols including carbonaceous aerosol from biomass burning and mineral dust (Marticorena et al. 2010) have been developed. Aerosol emission pathway, seasonal variation, and transportation with relative to size distribution and model parameterization have also been discussed. For example (Andreae et al. 2005; Diedhiou et al. 1998; Fontaine et al. 1998; Goudie and Middleton 2001; Haywood and Shine 1997; Kaufman et al. 2002; Liao and Seinfeld 1998; Titos et al 2016; Wang and Eltahir 2000; Ming and Ramaswamy 2011; Akinyoola et al. 2018) in their intense discussion have explained different aerosol emission pathway and the seasonal variation in the West Africa region. In particular, Mineral dust ubiquitously found in the tropospheric layer of Atmosphere in the West Africa region and the maximum concentration is found in winter due to Sahara transport, while the daily concentration is extremely high during both dry and wet seasons (Marticorena et al. 2010; Eresanya et al. 2017; Ogunjobi and Awoleye 2019). The deposition pathway through which dust is being washed off from the atmosphere as a result of mesoscale convective activities had also been identified. Physico-chemical properties of mineral dust, and the radiative forcing characteristics which controls or influence precipitation processes have been extensively investigated. Single Scattering Albedo (SSA) is a critical parameter to determine the sign of the aerosol forcing. Levin and Ganor (1996), Miller and Tegen (1998), Haywood and Boucher (2000), Takemura et al. (2005), and Choobari et al. (2014) shows that mineral dust consistently scatter radiation in the visible band of radiation spectrum. They also found that SSA of the submicron dust has been estimated to be ∼ (0.99 ± 0.01) at 0.55 μm and ∼ 0.90 with the addition of the coarse mode. In line with this (Camara et al. 2010; Zakey et al. 2006a, b; Konare et al. 2008a, b; Abiodun et al. 2012; Solmon et al. 2008; Marcella and Eltahir 2014a, b; N’Datchoh et al. 2018) in their different simulation, results have concluded that dust radiative forcing influence precipitation activities in the region and were able to ascertained the forcing characteristics of Dust across the region. Yet, it is not widely agreed that climate feedback is relative to substantial amount of dust present in the atmosphere; certain mineral dust with minimum value of SSA based on seasonal variation present in the atmosphere will have little or no impact on the precipitation processes, while a substantial amount will bring significant change in the rain producing system. Substantial dust concentration will make a substantial impact on the rain producing system and processes. The sensitivity of the model to significant changes in the mineral Dust radiative forcing is essential to quantify the response of precipitation and air temperature in the region.
The objective of this work is to investigate the influence of mineral dust radiative forcing on the monsoon dynamics and the sensitivity of model to changes induced by mineral dust radiative forcing. This paper is divided into four sections; the first section presents the introduction, second session presents the data and method including model configuration and experimental design, the third section present the results and discussion, while the fourth session discuss the Summary and conclusion.
Data and methods
In this study, three experiments with RegCM4 Model were performed and presented. The model was integrated from October, 2004 to December, 2005. However, the simulation of October–December, 2004 was considered as spin up and discarded to allow for the model stability. The first experiment which is refer to as CONTROL with Biosphere–Atmosphere Transfer Scheme (BATS) of RegCM4 model and without aerosol interference were run, the second experiment also refers to as (DUST) case with coupled dust model and run concurrently, while the third experiment represents a case when the aerosol is double and thereafter refer to as (DOUBLE DUST) case study.
Remote-sensing data sets
Multiangle Imaging Spectroradiometer (MISR)
Multiangle Imaging Spectroradiometer (MISR) was launched by the National Aeronautics and Space Administration (NASA) on December 18, 1999 and has been in operation since February 2000. The device consists of nine push broom cameras arranged to view at nominal zenith angles relative to the surface reference ellipsoid of 0.0°, ± 26.1°, ± 45.6°, 18 ± 60.0°, and ± 70.5° and measures upwelling short-wave radiance in each camera at four spectral bands, centered at 446, 558, 672, and 866 nm . With high spatial resolution and a better radio metrical and geometrical accuracy, the multiple angle-band observations of MISR allow the retrieval of a number of aerosol optical and microphysical properties over land (including a bright desert surfaces) and ocean [36–38, 100, 143–145]. Furthermore, MISR-AOD retrievals (level-3 data) have a higher grid resolution (0.5° × 0.5°) in comparison to the Moderate Resolution Imaging Spectroradiometer (MODIS) of Level-3 which has the resolution of 1° × 1°. In the present study from MISR-level-3-monthly-averaged data sets (version 31, which are available from 2000 onward), the AOD at 558 nm are utilized to evaluate the simulated results. Multiangle Imaging Spectroradiometer provides radiometrically and geometrically calibrated images in four spectral bands at each of the angles. Spatial sampling of 275 and 1100 m is provided on a global basis with a temporal resolution of 16 days, but radiances at 1.1 km resolution are processed to yield the standard Level-2 MISR aerosol product at 17.6 km × 17.6 km pixel size. Martonchik et al. (1998) created a heterogeneous land algorithm. These properties have proofed the uniqueness of MISR over other remote sensors. The MISR data information is available on https://www-misr.jpl.nasa.gov/.
Sea-viewing Wide Field-of-view Sensor (SeaWiFS)
Since September of 1997, SeaWiFS routinely provides ocean color and atmospheric products, e.g., the normalized water-leaving radiance [Lw(λ)]N for six visible wavelengths (412, 443, 490, 510, 555, and 670 nm), chlorophyll-a concentration Chl- a, AOT at 865 nm τa (865), and the aerosol Ångström exponent derived from the wavelength 510 and 865 nm α (510). SeaWiFS has gone through four major reprocessings of the entire data set. Each reprocessing has addressed the data quality issues that are related to the sensor calibration, instrument navigation, data masks and flags, and retrieval algorithms. The SeaWiFS data used in here are from the fourth data reprocessing which was carried out in July 2002 (Patt et al. 2003). Currently, the SeaWiFS data processing is optimized for the ocean color measurements (e.g., chlorophyll- a concentration). Very high AOT data such as the dust and smoke plumes are usually masked out due to large uncertainties in the ocean color products with these cases (Gordon 1997). The SeaWiFS has a reflectance threshold at 865 nm corresponding to the AOT of ∼ 0.3. Thus, the current SeaWiFS aerosol products are mostly applicable and valid in the open ocean regions where the marine aerosols are often the dominant sources. It is noted that, however, the SeaWiFS measurements can be used to derive the aerosol optical properties for very thick aerosol layers, e.g., for the dust study (Husar et al. 2001; Moulin et al. 2001a), and possibly for the ocean color data in the regional case study (Moulin et al. 2001b). Implementation is planned for an aerosol retrieval scheme in which very high AOT (e.g., dust and smoke) can still be retrieved, even though the ocean color data may have large uncertainties. In these cases, flags can be applied to the ocean color products. The choice of SeaWiFS over other sensors is as a result of its capability to capture marine aerosols around the West Africa region and also serve as tool to validate the performance of RegCM4 in capturing Aerosol Optical depth which cut across heterogeneous surfaces.
Moderate Resolution Imaging Spectroradiometer (MODIS)
Moderate Resolution Imaging Spectroradiometer is a key instrument that is carried on both the Terra (EOS AM) and Aqua (EOS PM) satellites. Terra’s orbit around the earth is timed, so that it passes from north to south across the equator in the morning, while Aqua passes south to north over the equator in the afternoon. The MODIS instrument has 36 spectral bands that provide abundant information on atmospheric, terrestrial, and oceanic environments. The instrument uses different methods for data retrieval over land (Kaufman et al. 1997) and over oceans (Tanre et al. 2001). To improve the accuracy and quality of retrieved data, the algorithms used for MODIS have been updated to make use of improved cloud-masking processes, aerosol models, and the surface reflectance database (Remer et al. 2005). The MODIS aerosol product observes the encompassing aerosol optical thickness over the oceans comprehensively and over the landmasses. Moreover, the size distribution is determined over the seas, and the type of aerosol is inferred over the land. The aerosol product incorporates the “dark blue” algorithm recently created to get aerosol optical thickness over bright land regions. The Dark Target algorithm is connected over sea and dark land (e.g., vegetation), while the Deep Blue algorithm in Collection (C6) covers the whole land regions including both dark and bright surfaces. Both results are provided on a 10 × 10 pixel scale (10 km at nadir). The DB algorithm was constrained to just bright targets; however, it has extended inclusion to incorporate both bright targets and vegetated locales, utilizing NDVI information in addition to a precalculated surface reflectance database. The algorithm details on the information are available from https://modisatmos.gsfc.nasa.gov. For this study, the Terra Collection 006 Deep blue AOD data were used because of its prefer algorithm over land.
Model input data sets
Era interim re-analysis data sets (EIN15)
The first ECMWF Re-Analysis (ERA) project has produced a new, validated 15 year data set of assimilated data. Era-interim 15 (EIN15) is a grid data set with regcminput/EIN15 as id. The data type is in netCDF dataformat. It has edu.ucar.unidata as naming authority and is freely available as NCEP model documentation. DOC/NOAA/NWS/NCEP http://www.ncep.noaa.gov/creates EIN15 as regcm data input and also published by UCAR/UNIDATA http://www.unidata.ucar.edu/. The data sets have Time step coverage. The project began in February 1993. The first phase of the work required the acquisition and preparation of the observations and forcing fields. During the 1st year, a substantial program of experimentation, closely co-ordinated with the Centre’s Research and Operational activities, was completed. This enabled the scientific components of the re-analysis system to be defined, and a strategy for production to be determined. At the same time, work was progressing on the development of both the production system and the internal validation tools. The final production system was adopted in 1994, and there followed a period of sustained production, monitoring, and validation throughout 1995 and the first 9 months of 1996. All data generated by the project likely to be of future value have been collected and preserved. These include blacklist information, radiosonde bias correction tables, TOVS bias and calibration files, and the record of which satellites have been used at different periods.
Results and discussion
JJAS seasonal Dust climatology in the West Africa region
Response of Monsoon precipitation to mineral dust radiative forcing
Monsoon is a period of inflow of moisture from the Atlantic Ocean to the inland of West Africa. The nature and variability of West Africa monsoon is being determined by evolving ocean, atmospheric constituents, and land conditions. Many studies have actually assessed the capability of RegCM in simulating precipitation climatology over West Africa region. For example, Sylla et al. (2012) assessed the capability of RegCM3 to reproduce the seasonal temperature and precipitation cycle during the period of 1981–2000 over West Africa with two sets of boundaries conditions, re-analysis data, and ECHAM5 output. These works were also in line with Solmon et al. (2008). The changes in precipitation could be linked to the fact that more short wave is being scattered in the atmosphere with higher dust concentration and this reduces the energy budget received at the surface.
Response of temperature to the experiments
Radiative forcing at the top of atmosphere
Zonal and surface wind response to the experiments
The South-westerly trade and north-easterly trade wind are the two major trade winds over West Africa associated with wet and dry seasonal period. The prevalent of each trade wind determines the position of ITD in the region as it migrates North–Southward differentiating one trade wind from another.
Summary and conclusion
In performing the sensitivity experiments, three mineral dust scenarios were used. The first scenario was Biosphere–Atmosphere–Transfer System, this is a scenario with no dust to serve as Control other aerosol radiative forcing considered includes the dust enabled scenario over West Africa to ascertain the areas that are feasible for dust emission and transport and the double dust scenario to observe the effects of increase in dust concentration on the dust Optical characteristics. There was an observation of an enhancement of monsoon precipitation in the double dust experiment. The increase in precipitation from 2 to 4 mm/day was mostly felt in the Guinea coast region and with minimal or no such increase elsewhere. The monsoon precipitation reduction noticed in the Sahel in the dust experiment was due to dust scattering ability, thereby reducing net radiation received at the surface which in turn reduces out going long-wave radiation which enhances cloud formation and another factor was the slight reduction in the speed of the AEJ. This reduction in the speed of AEJ combined with the slight reduction in the core speed of TEJ may have led to changes in the vertical wind shear between the two jets. At the Top of Atmosphere (TOA), there was negative radiative forcing noticed across the West Africa region, but with about − 15 W/m2 in the Sahel region and as the dust concentration increases in the double dust experiment, there were high negative values of radiative forcing of aerosol in the region. This means that there was more scattering of short-wave radiation at the Top of Atmosphere as the dust concentration increases.
The study has clearly demonstrated the effect of mineral dust radiative forcing on the environment which alters radiation processes and climate feedback. It can also be observed that climate change issue is strongly related to atmospheric constituent change. The study also shows how increase in mineral dust concentration can enhance rainfall in the Guinea coast, but aid drought in the Sahel region. Reduction of anthropogenic activities that increase dust emission in region must be reduced to reduce the drought occurrence in Sahel region of West Africa. With further detail study, the experiment can serve as a feasibility study for field campaign experiment as a mitigation strategy of climate change in West Africa region.
The authors sincerely appreciate International Centre for Theoretical Physics (ICTP), Italy for permission to use their model (RegCM4) for this work. The assistance of Graziano Gulaini is also greatly appreciated and every individual that have contributed immensely during the writing of the manuscript.
- Fink AH, Engel T, Ermert V, van der Linden R, Schneidewind (2016) Mean climate and seasonal cycle (Chapter 1). In: Parker DJ, Diop-Kane M (eds) Meteorology of tropical West Africa: the forecasters’ handbook. Wiley, ChichesterGoogle Scholar
- Giorgi F, Coppola E, Solmon F, Mariotti L, Sylla M, Bi X, Elguindi N, Diro GT, Nair V, Giuliani G, Cozzini S, Guettler I, O’Brien T, Tawfik A, Shalaby A, Zakey AS, Steiner A, Stordal F, Sloan L, Brankovic C (2012) RegCM4: model description and preliminary tests over multiple CORDEX domains. Clim Res. https://doi.org/10.3354/cr01018 CrossRefGoogle Scholar
- Liao H, Seinfeld J (1998) Radiative forcing by mineral dust aerosol: sensitivity to key variables. J Geophys Res 103(31):637–646Google Scholar
- Marticorena B, Chatenet B, Rajot JL, Traoré S, Coulibaly MM, Diallo A, Koné II, Maman A, NDiaye T, Zakou AA (2010) Temporal variability of mineral dust content over West Africa: analyses of a pluriannual monitoring from the AMMA Sahelian Dust Transect. Atmos Chem Phys 10:8899–8915CrossRefGoogle Scholar
- Patt FS et al (2003) Algorithm updates for the fourth SeaWiFS data reprocessing, SeaWiFS Postlaunch Tech. Rep. Ser. In: Hooker SB, Firestone ER (eds) NASA Tech. Memo. 2003–206892, vol 22. NASA Goddard Space Flight Cent, Greenbelt, p 4Google Scholar
- Prospero JM, Ginoux P, Torres O, Nicholson E, Gill TE (2002) Environmental characterization of global sources of atmospheric soil dust identified with the Nimbus 7 total ozone mapping spectrometer (TOMS) absorbing aerosol product. Rev Geophys 40(1):1002. https://doi.org/10.1029/2000RG000095 CrossRefGoogle Scholar
- Sylla MB, Gaye AT, Jenkins GS (2012) On the fine-scale topography regulating changes in atmospheric hydrological cycle and extreme rainfall over West Africa in a Regional Climate Model Projections. Int J Geophys 2012 (Article ID 981649) Google Scholar
- Titos G, Cazorla A, Zieger P, Andrews E, Lyamani H, Granados-Muñoz MJ, Olmo FJ, Alados-Arboledas L (2016) Effect of hygroscopic growth on the aerosol light-scattering coefficient: a review of measurements, techniques and error sources. Atmos Environ 1:1. https://doi.org/10.1016/j.atmosenv.2016.07.021 CrossRefGoogle Scholar
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.