scholarly journals Characterization of the Diurnal Cycle of the West African Monsoon around the Monsoon Onset

2007 ◽  
Vol 20 (15) ◽  
pp. 4014-4032 ◽  
Author(s):  
Benjamin Sultan ◽  
Serge Janicot ◽  
Philippe Drobinski
Keyword(s):  
Diurnal Cycle ◽  
Deep Convection ◽  
West African Monsoon ◽  
Monsoon Onset ◽  
West African ◽  
Onset Date ◽  
The West ◽  
African Monsoon ◽  
Nocturnal Jet ◽  
Heat Low

Abstract This study investigates the diurnal cycle of the West African monsoon and its seasonal modulation with particular focus on the monsoon onset period. A composite analysis around the monsoon onset date is applied to the 1979–2000 NCEP–DOE reanalysis and 40-yr ECMWF Re-Analysis (ERA-40) at 0000, 0600, 1200, and 1800 UTC. This study points out two independent modes describing the space–time variability of the diurnal cycle of low-level wind and temperature. While the first mode appears to belong to a gradual and seasonal pattern linked with the northward migration of the whole monsoon system, the second mode is characterized by more rapid time variations with a peak of both temperature and wind anomalies around the monsoon onset date. This latter mode is connected with the time pattern of a nocturnal jet reaching its highest values around the onset date. The diurnal cycle of dry and deep convection is also investigated through the same method. A distinct diurnal cycle of deep convection in the ITCZ is evidenced with a peak at 1200 UTC before the monsoon onset, and at 1800 UTC after the monsoon onset. Strong ascending motions associated with deep convection may generate a gravity wave that propagates northward and reaches the Saharan heat low region 12 h later. The diurnal cycle of the dry convection in the Saharan heat low is similar during the preonset and the postonset periods with a peak at night (0000 UTC) consistent with the nocturnal jet intensification. This convection is localized at 15° and 20°N before and after the monsoon onset, respectively. Both during the first rainy season in spring and the monsoon season in summer, the nocturnal jet brings moisture in the boundary layer north of the ITCZ favoring humidification and initiation of new convective cells, helping the northward progression of the ITCZ. At the end of the summer the southward return of the ITCZ is associated with the disappearance of the core of the monsoon jet. Despite a lot of similarities between the results obtained using NCEP–DOE and ERA-40 reanalyses, giving confidence in the significance of these results, some differences are identified, especially in the diurnal cycle of deep convection, which limit the interpretation of some of these results and highlight discrepancies in the reanalyses.

scholarly journals An Idealized Two-Dimensional Framework to Study the West African Monsoon. Part II: Large-Scale Advection and the Diurnal Cycle

2007 ◽  
Vol 64 (8) ◽  
pp. 2783-2803 ◽  
Author(s):  
Philippe Peyrillé ◽  
Jean-Philippe Lafore
Keyword(s):  
Diurnal Cycle ◽  
Large Scale ◽  
West African Monsoon ◽  
West African ◽  
The West ◽  
African Monsoon ◽  
Temperature And Humidity ◽  
Low Levels ◽  
Heat Low ◽  
2D Model

The idealized 2D model developed in Part I of this study is used to study the West African monsoon sensitivity to large-scale forcing. Using ECWMF reanalyses, a large-scale forcing is introduced in the 2D model in terms of temperature and humidity advection. A coherent structure of cooling–moistening near the surface and drying–warming in the 2–4-km layer is found in the Saharan heat low region. The effect of the advective forcing is to block the monsoon propagation by strengthening the northerly flux and by an increase of convective inhibition. The heat low thus appears to play a key role in the monsoon northward penetration through its temperature and humidity budget. Ultimately, warmer low levels and/or more moist midlevels in the heat low favor a more northerly position of the ITCZ. A detailed view of the continental diurnal cycle is also presented. Potential temperature and humidity budgets are performed in the deep convective and heat low area. The moistening process to sustain deep convection is made through nocturnal advection at low levels and daytime turbulence that redistributes humidity vertically. The same mechanism occurs in the heat low except that the vertical transfers by turbulence help maintain the dryness of the low levels. A possible mechanism of interaction between the deep convective zone and the Saharan heat low is also proposed that involves gravity waves in the upper troposphere.

The diurnal cycle of the West African monsoon circulation

2005 ◽  
Vol 131 (611) ◽  
pp. 2839-2860 ◽  
Author(s):  
D. J. Parker ◽  
R. R. Burton ◽  
A. Diongue-Niang ◽  
R. J. Ellis ◽  
M. Felton ◽  
...  
Keyword(s):  
Diurnal Cycle ◽  
West African Monsoon ◽  
West African ◽  
Monsoon Circulation ◽  
The West ◽  
African Monsoon

The role of the Indian monsoon onset in the West African monsoon onset: observations and AGCM nudged simulations

2011 ◽  
Vol 38 (5-6) ◽  
pp. 965-983 ◽  
Author(s):  
Emmanouil Flaounas ◽  
Serge Janicot ◽  
Sophie Bastin ◽  
Rémy Roca ◽  
Elsa Mohino
Keyword(s):  
Indian Monsoon ◽  
West African Monsoon ◽  
Monsoon Onset ◽  
West African ◽  
The West ◽  
African Monsoon

The diurnal cycle of lower boundary-layer wind in the West African monsoon

2009 ◽  
Vol 136 (S1) ◽  
pp. 66-76 ◽  
Author(s):  
Kassimou Abdou ◽  
Douglas J. Parker ◽  
Barbara Brooks ◽  
Norbert Kalthoff ◽  
Thierry Lebel
Keyword(s):  
Boundary Layer ◽  
Diurnal Cycle ◽  
Lower Boundary ◽  
West African Monsoon ◽  
West African ◽  
The West ◽  
African Monsoon ◽  
Lower Boundary Layer

scholarly journals Stable water isotope signals in tropical ice clouds in the West African monsoon simulated with a regional convection-permitting model

2021 ◽  
Author(s):  
Andries Jan de Vries ◽  
Franziska Aemisegger ◽  
Stephan Pfahl ◽  
Heini Wernli
Keyword(s):  
Deep Convection ◽  
West African Monsoon ◽  
West African ◽  
The West ◽  
Ice Clouds ◽  
African Monsoon ◽  
Tropical Tropopause Layer ◽  
Convective Systems ◽  
Tropical Tropopause ◽  
Tropical Deep Convection

Abstract. Tropical ice clouds have an important influence on the Earth’s radiative balance. They often form as a result of tropical deep convection, which strongly affects the water budget of the tropical tropopause layer. Ice cloud formation involves complex interactions on various scales, which are not fully understood yet and lead to large uncertainties in climate predictions. In this study, we investigate the formation of tropical ice clouds related to deep convection in the West African monsoon, using stable water isotopes as tracers of moist atmospheric processes. We perform simulations using the regional isotope-enabled model COSMOiso with different resolutions and treatments of convection for the period of June–July 2016. First, we evaluate the ability of our simulations to represent the isotopic composition of monthly precipitation through comparison with GNIP observations, and the precipitation characteristics related to the monsoon evolution and convective storms based on insights from the DACCIWA field campaign in 2016. Next, a case study of a mesoscale convective system (MCS) explores the isotope signatures of tropical deep convection in atmospheric water vapour and ice. Convective updrafts within the MCS inject enriched ice into the upper troposphere leading to depletion of vapour within these updrafts due to the preferential condensation and deposition of heavy isotopes. Water vapour in downdrafts within the same MCS are enriched by non-fractionating sublimation of ice. In contrast to ice within the MCS core regions, ice in widespread cirrus shields is isotopically in approximate equilibrium with the ambient vapour, which is consistent with in situ formation of ice. These findings from the case study are supported by a statistical evaluation of isotope signals in the West African monsoon ice clouds. The following five key processes related to tropical ice clouds can be distinguished based on their characteristic isotope signatures: (1) convective lofting of enriched ice into the upper troposphere, (2) cirrus clouds that form in situ from ambient vapour under equilibrium fractionation, (3) sedimentation and sublimation of ice in the mixed-phase cloud layer in the vicinity of convective systems and underneath cirrus shields, (4) sublimation of ice in convective downdrafts that enriches the environmental vapour, and (5) the freezing of liquid water in the mixed-phase cloud layer at the base of convective updrafts. Importantly, the results show that convective systems strongly modulate the humidity budget and the isotopic composition of the lower tropical tropopause layer. They contribute to about 40 % of the total water and 60 % of HDO in the 175–125 hPa layer in the African monsoon region according to estimates based on our model simulations. Overall, this study demonstrates that isotopes can serve as useful tracers to disentangle the role of different processes in the Earth’s water cycle, including convective transport, the formation of ice clouds, and their impact on the tropical tropopause layer.

scholarly journals The West African Monsoon Onset: A Concise Comparison of Definitions

2015 ◽  
Vol 28 (22) ◽  
pp. 8673-8694 ◽  
Author(s):  
Rory G. J. Fitzpatrick ◽  
Caroline L. Bain ◽  
Peter Knippertz ◽  
John H. Marsham ◽  
Douglas J. Parker
Keyword(s):  
West African Monsoon ◽  
Monsoon Onset ◽  
West African ◽  
Seasonal Forecasting ◽  
The West ◽  
African Monsoon ◽  
Multiple Datasets ◽  
Monsoon Winds ◽  
Work Done ◽  
Selection Of

Abstract The onset of the West African monsoon (WAM) marks a vital time for local and regional stakeholders. While the seasonal progression of monsoon winds and the related migration of precipitation from the Guinea Coast toward the Sudan/Sahel is apparent, there exist contrasting man-made definitions of what the WAM onset means. Broadly speaking, onset can be analyzed regionally, locally, or over a designated intermediate scale. There are at least 18 distinct definitions of the WAM onset in publication, with little work done on comparing observed onset from different definitions or comparing onset realizations across different datasets and resolutions. Here, nine definitions have been calculated using multiple datasets of different metrics at different resolutions. It is found that mean regional onset dates are consistent across multiple datasets and different definitions. There is low interannual variability in regional onset, suggesting that regional seasonal forecasting of the onset provides few benefits over climatology. In contrast, local onsets show high spatial, interannual, and interdefinition variability. Furthermore, it is found that there is little correlation between local onset dates and regional onset dates across West Africa, implying a disharmony between regional measures of onset and the experience on a local scale. The results of this study show that evaluation of seasonal monsoon onset forecasts is far from straightforward. Given a seasonal forecasting model, it is possible to simultaneously have a good and a bad prediction of monsoon onset simply through selection of the onset definition and observational dataset used for comparison.

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