On What Scale Can We Predict the Agronomic Onset of the West African Monsoon?

Abstract Accurate prediction of the commencement of local rainfall over West Africa can provide vital information for local stakeholders and regional planners. However, in comparison with analysis of the regional onset of the West African monsoon, the spatial variability of the local monsoon onset has not been extensively explored. One of the main reasons behind the lack of local onset forecast analysis is the spatial noisiness of local rainfall. A new method that evaluates the spatial scale at which local onsets are coherent across West Africa is presented. This new method can be thought of as analogous to a regional signal against local noise analysis of onset. This method highlights regions where local onsets exhibit a quantifiable degree of spatial consistency (denoted local onset regions or LORs). It is found that local onsets exhibit a useful amount of spatial agreement, with LORs apparent across the entire studied domain; this is in contrast to previously found results. Identifying local onset regions and understanding their variability can provide important insight into the spatial limit of monsoon predictability. While local onset regions can be found over West Africa, their size is much smaller than the scale found for seasonal rainfall homogeneity. A potential use of local onset regions is presented that shows the link between the annual intertropical front progression and local agronomic onset.

Download Full-text

Time Series of InSAR-Measured Seasonal Surface Deformation Induced by the West African Monsoon in Sudanian West Africa

HydroShare Resources ◽

10.4211/hs.d03e53c0d3eb4c72ad43edc0b309755a ◽

2019 ◽

Keyword(s):

Time Series ◽

West Africa ◽

Surface Deformation ◽

West African Monsoon ◽

West African ◽

The West ◽

African Monsoon

Download Full-text

Rainfall retrievals over West Africa using SEVIRI: evaluation with TRMM-PR and monitoring of the daylight time monsoon progression

Hydrology and Earth System Sciences Discussions ◽

10.5194/hessd-7-6351-2010 ◽

2010 ◽

Vol 7 (4) ◽

pp. 6351-6380 ◽

Cited By ~ 1

Author(s):

E. L. A. Wolters ◽

B. J. J. M. van den Hurk ◽

R. A. Roebeling

Keyword(s):

West Africa ◽

Near Infrared ◽

Rain Rate ◽

West African Monsoon ◽

West African ◽

The West ◽

Cloud Particle ◽

African Monsoon ◽

Trmm Pr ◽

Rain Rates

Abstract. This paper describes the application of the KNMI cloud physical properties – precipitation properties (CPP-PP) algorithm over West Africa. The algorithm combines condensed water path (CWP), cloud phase (CPH), cloud particle effective radius (re), and cloud-top temperature (CTT) information, retrieved from visible, near-infrared and infrared observations of the Spinning Enhanced Visible and Infrared Imager (SEVIRI) onboard Meteosat-9 to estimate precipitation occurrence and intensity. It is investigated whether the CPP-PP algorithm is capable of retrieving rain occurrence and intensity over West Africa with a sufficient accuracy, using tropical monsoon measurement mission precipitation radar (TRMM-PR) and a small number of rain gauge observations as reference. As a second goal, it is assessed whether SEVIRI is capable of monitoring both the seasonal and synoptical evolution of the West African monsoon (WAM). It is shown that the SEVIRI-detected rainfall area agrees well with TRMM-PR, having a correlation coefficient of 0.86, with the areal extent of rainfall by SEVIRI being ~10% larger than TRMM-PR. The mean retrieved rain rate from CPP-PP is about 8% higher than from TRMM-PR. The frequency distributions of rain rate reveal that the median rain rates of CPP-PP and TRMM-PR are similar. However, rain rates >7 mm h−1 are retrieved more frequently by SEVIRI than by TRMM-PR, which is partly explained by known biases in TRMM-PR. Finally, it is illustrated that both the seasonal and synoptical time scale of the WAM can be well detected from SEVIRI daytime observations. It was found that the daytime westward MCS travel speed fluctuates between 50 and 60 km h−1. Furthermore, the ratio of MCS precipitation to the total precipitation was estimated to be about 27%. Our results indicate that rainfall retrievals from SEVIRI can be used to monitor the West African monsoon.

Download Full-text

Aerosol influences on low-level clouds in the West African monsoon

10.5194/acp-2019-40 ◽

2019 ◽

Cited By ~ 1

Author(s):

Jonathan W. Taylor ◽

Sophie L. Haslett ◽

Keith Bower ◽

Michael Flynn ◽

Ian Crawford ◽

...

Keyword(s):

West Africa ◽

Southern Hemisphere ◽

Biomass Burning ◽

West African Monsoon ◽

Cloud Microphysics ◽

West African ◽

The West ◽

African Monsoon ◽

Low Level ◽

Local Emissions

Abstract. Low-level clouds (LLC) cover a wide area of southern West Africa (SWA) during the summer monsoon months, and have an important cooling effect on the regional climate. Previous studies of these clouds have focused on modelling and remote sensing via satellite. We present the first comprehensive set of regional, in situ measurements of cloud microphysics, taken during June – July 2016, as part of the DACCIWA (Dynamics-Aerosol-Chemistry-Clouds Interactions in West Africa) campaign, assessing spatial and temporal variation in the properties of these clouds. LLC developed overnight and mean cloud cover peaked a few hundred kilometres inland around 10:00 local solar time (LST), before clouds began to dissipate and convection intensified in the afternoon. Additional sea breeze clouds developed near the coast in the late morning, reaching a maximum extent around 12:00 LST. Regional variation in LLC cover was largely determined by the modulation of the cool maritime inflow by the local orography, with peaks on the upwind side of hills and minima on the leeward sides. In the broad-scale cloud field, no lasting impacts related to anthropogenic aerosol were observed downwind of major population centres. The boundary layer cloud drop number concentration (CDNC) was locally variable inland, ranging from 200 to 840 cm−3 (10th and 90th percentiles at standard temperature and pressure), but showed no systematic regional variations. Enhancements were seen in pollution plumes from the coastal cities, but were not statistically significant across the region. The majority of accumulation mode aerosols, and therefore cloud condensation nuclei, were from ubiquitous biomass burning smoke transported from the southern hemisphere. Consequently, all clouds measured (inland and offshore) had significantly higher CDNC and lower effective radius than clouds over the remote south Atlantic from literature. A parcel model sensitivity analysis showed that doubling or halving local emissions only changed the calculated CDNC by 13–22 %, as the high background meant local emissions were a small fraction of total aerosol. As the population of SWA grows, local emissions are expected to rise. Biomass burning smoke transported from the southern hemisphere is likely to dampen any effect of these increased local emissions on cloud-aerosol interactions. An integrative analysis between local pollution and Central African biomass burning emissions must be considered when predicting anthropogenic impacts on the regional cloud field during the West African monsoon.

Download Full-text

Understanding the Mechanisms behind the Northward Extension of the West African Monsoon during the Mid-Holocene

Journal of Climate ◽

10.1175/jcli-d-16-0299.1 ◽

2017 ◽

Vol 30 (19) ◽

pp. 7621-7642 ◽

Cited By ~ 17

Author(s):

Marco Gaetani ◽

Gabriele Messori ◽

Qiong Zhang ◽

Cyrille Flamant ◽

Francesco S. R. Pausata

Keyword(s):

West Africa ◽

Radiative Forcing ◽

Numerical Models ◽

Substantial Reduction ◽

West African Monsoon ◽

West African ◽

Heat Fluxes ◽

Dust Concentration ◽

The West ◽

African Monsoon

Abstract Understanding the West African monsoon (WAM) dynamics in the mid-Holocene (MH) is a crucial issue in climate modeling, because numerical models typically fail to reproduce the extensive precipitation suggested by proxy evidence. This discrepancy may be largely due to the assumption of both unrealistic land surface cover and atmospheric aerosol concentration. In this study, the MH environment is simulated in numerical experiments by imposing extensive vegetation over the Sahara and the consequent reduction in airborne dust concentration. A dramatic increase in precipitation is simulated across the whole of West Africa, up to the Mediterranean coast. This precipitation response is in better agreement with proxy data, in comparison with the case in which only changes in orbital forcing are considered. Results show a substantial modification of the monsoonal circulation, characterized by an intensification of large-scale deep convection through the entire Sahara, and a weakening and northward shift (~6.5°) of the African easterly jet. The greening of the Sahara also leads to a substantial reduction in the African easterly wave activity and associated precipitation. The reorganization of the regional atmospheric circulation is driven by the vegetation effect on radiative forcing and associated heat fluxes, with the reduction in dust concentration to enhance this response. The results for the WAM in the MH present important implications for understanding future climate scenarios in the region and in teleconnected areas, in the context of projected wetter conditions in West Africa.

Download Full-text

The West African Monsoon Dynamics. Part III: The Quasi-Biweekly Zonal Dipole

Journal of Climate ◽

10.1175/2007jcli1706.1 ◽

2008 ◽

Vol 21 (9) ◽

pp. 1911-1928 ◽

Cited By ~ 57

Author(s):

Flore Mounier ◽

Serge Janicot ◽

George N. Kiladis

Keyword(s):

West Africa ◽

West African Monsoon ◽

West African ◽

Positive Pressure ◽

Kelvin Wave ◽

The West ◽

African Monsoon ◽

Low Level ◽

Surface Temperatures ◽

Moisture Advection

Abstract This paper presents an investigation of the mechanisms giving rise to the main intraseasonal mode of convection in the African monsoon during northern summer, here identified as the quasi-biweekly zonal dipole (QBZD). The QBZD is primarily characterized by a quasi-stationary zonal dipole of convection whose dimension is larger than the West African monsoon domain, with its two poles centered along the Guinean coast and between 30° and 60°W in the equatorial Atlantic. The QBZD dynamical processes within the Atlantic–Africa domain are examined in some detail. The QBZD has a dipole pattern associated with a Walker-type circulation in the near-equatorial zonal plane. It is controlled both by equatorial atmospheric dynamics through a Kelvin wave–like disturbance propagating eastward between its two poles and by land surface processes over Africa, inducing combined fluctuations in surface temperatures, surface pressure, and low-level zonal winds off the coast of West Africa. When convection is at a minimum over central and West Africa, a lack of cloud cover results in higher net shortwave flux at the surface, which increases surface temperatures and lowers surface pressures. This creates an east–west pressure gradient at the latitude of both the ITCZ (10°N) and the Saharan heat low (20°N), leading to an increase in eastward moisture advection inland. The arrival from the Atlantic of the positive pressure signal associated with a Kelvin wave pattern amplifies the low-level westerly wind component and the moisture advection inland, leading to an increase in convective activity over central and West Africa. Then the opposite phase of the dipole develops. Propagation of the QBZD convective envelope and of the associated 200 high-level velocity potential anomalies is detected from the eastern Pacific to the Indian Ocean. When the effect of the Kelvin wave propagation is removed by filtering, the stationary character of the QBZD is highlighted. The impact of the QBZD in combination with a Kelvin wave is illustrated by a case study of the monsoon onset in 1984.

Download Full-text

Mathematical Insights into the West African Monsoon

Eos ◽

10.1029/2020eo148254 ◽

2020 ◽

Vol 101 ◽

Author(s):

Kate Wheeling

Keyword(s):

West Africa ◽

Systems Theory ◽

Dynamic Systems ◽

West African Monsoon ◽

West African ◽

Dynamic Systems Theory ◽

The West ◽

African Monsoon

A tool from dynamic systems theory is helping atmospheric scientists identify how dust and moisture mix over West Africa.

Download Full-text

The Influence of the MJO on Upstream Precursors to African Easterly Waves

Journal of Climate ◽

10.1175/jcli-d-11-00232.1 ◽

2012 ◽

Vol 25 (9) ◽

pp. 3219-3236 ◽

Cited By ~ 34

Author(s):

Ghassan J. Alaka ◽

Eric D. Maloney

Keyword(s):

West Africa ◽

West African Monsoon ◽

West African ◽

Temperature Fields ◽

Boreal Summer ◽

Tropospheric Temperature ◽

The West ◽

Lake Chad ◽

African Monsoon ◽

African Easterly Jet

The Madden–Julian oscillation (MJO) produces alternating periods of increased and reduced precipitation and African easterly wave (AEW) activity in West Africa. This study documents the influence of the MJO on the West African monsoon system during boreal summer using reanalysis and brightness temperature fields. MJO-related West African convective anomalies are likely induced by equatorial Kelvin and Rossby waves generated in the Indian Ocean and West Pacific by the MJO, which is consistent with previous studies. The initial modulation of tropical African convection occurs upstream of West Africa, near the entrance of the African easterly jet (AEJ). Previous studies have hypothesized that an area to the east of Lake Chad is an initiation region for AEWs. Called the “trigger region” in this study, this area exhibits significant intraseasonal convection and wave activity anomalies prior to the wet and dry MJO phases in the West African monsoon region. In the trigger region, cold tropospheric temperature anomalies and high precipitable water, as well as an eastward extension of the African easterly jet, appear to precede and contribute to the wet MJO phase in West Africa. An anomalous stratiform heating profile is observed in advance of the wet MJO phase with anomalous PV generation maximized at the jet level. The opposite behavior occurs in advance of the dry MJO phase. The moisture budget is examined to provide further insight as to how the MJO modulates and initiates precipitation and AEW variability in this region. In particular, meridional moisture advection anomalies foster moistening in the trigger region in advance of the wet MJO phase across West Africa.

Download Full-text