The impact of mesoscale convective systems on the surface and boundary-layer structure in West Africa: Case-studies from the AMMA campaign 2006

Rainfall variability over West Africa remains a major challenge for numerical weather prediction (NWP). Due to the largely stochastic and sub-grid nature of tropical convection, current NWP models still fail to provide reliable precipitation forecasts &#8211; even for a 1-day leadtime &#8211; and are barely more skillful than climatology-based forecasts. Thus, several recent studies have investigated the presumably more predictable influence of tropical waves on environmental conditions for convection and found distinct and coherent (thermo-)dynamical patterns depending on the type and phase of the wave. Of particular interest in this context is the interaction of the wave with the lifecycle of usually westward propagating mesoscale convective systems (MCSs), which are the major providers of rain in the region and can occasionally even lead to flooding. The exact mechanisms and strength of this interaction are still not entirely known.This study combines two recent datasets in a novel way in order to systematically investigate the influence of tropical waves on MCS characteristics and lifecycle. First, MCSs are tracked within northern tropical Africa (20&#176;W-30&#176;E / 2&#176;-15&#176;N) over an 11-year period during the West African rainy season (April-October) using infrared brightness temperature fields provided by the Spinning enhanced visible and infrared imager (SEVIRI). Second, tropical waves are isolated by applying a filtering method in the wave-frequency domain to precipitation data of the Tropical Rainfall Measuring Mission (TRMM) within the 5&#176;-15&#176;N latitude band for the same target period. By combining the two datasets in space and time, the magnitude and phase of each wave is known at every timestep of the MCS tracks, which enables a systematic investigation of MCS characteristics as a function of wave properties.Preliminary results suggest that long-lived MCSs (lifetime &#8805; 12h) frequently couple with the &#8220;wet&#8221; phase of high-frequency tropical waves, in particular Kelvin, eastward inertia-gravity (EIG), and African easterly waves (AEW). Showing an enhanced occurrence frequency of MCS initiation, the wet phase of AEWs appears to have strong modulation capabilities during the genesis stage and further accompanies these long-lived MCSs during their entire lifetime. In the case of Kelvin waves and EIGs, the wet phase overlaps only with the intensification and maturity stage of these MCSs as a consequence of opposite directions of movement. Similar coupling patterns also exist for mixed Rossby gravity waves (MRGs), although to a weaker extent. Furthermore, no consistent coupling tendencies with long-lived MCSs are evident for low-frequency waves (Madden-Julian Oscillation (MJO), equatorial Rossby wave (ER)), arguably since they act on larger spatio-temporal scales. For short-lived MCSs (lifetime < 6h), the coupling with high-frequency waves is substantially weaker.In the future we will also address potential influences of wave-wave interactions on MCSs as well as potential differences in coupling mechanisms between the Guinea Coast region and the Sahel farther north. With increasing efforts in the prediction of tropical waves, this study has the potential to aid the short-term forecasting of MCS development and its lifecycle. This can be of particular importance for the anticipation of extreme rainfall events and subsequent risk assessment in West Africa.

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Review on “Sensitivity of GPS tropospheric estimates to mesoscale convective systems in West Africa“ by Samuel Nahmani, Olivier Bock, Françoise Guichard

10.5194/acp-2018-1242-rc1 ◽

2019 ◽

Author(s):

Anonymous

Keyword(s):

West Africa ◽

Mesoscale Convective Systems ◽

Convective Systems ◽

Mesoscale Convective

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Seasonality and Trends of Drivers of Mesoscale Convective Systems in Southern West Africa

Journal of Climate ◽

10.1175/jcli-d-20-0194.1 ◽

2021 ◽

Vol 34 (1) ◽

pp. 71-87

Author(s):

Cornelia Klein ◽

Francis Nkrumah ◽

Christopher M. Taylor ◽

Elijah A. Adefisan

Keyword(s):

West Africa ◽

Wind Shear ◽

Dominant Role ◽

West African Monsoon ◽

Extreme Rainfall ◽

Temperature Gradients ◽

Mesoscale Convective Systems ◽

Low Level ◽

Convective Systems ◽

Mesoscale Convective

AbstractMesoscale convective systems (MCSs) are the major source of extreme rainfall over land in the tropics and are expected to intensify with global warming. In the Sahel, changes in surface temperature gradients and associated changes in wind shear have been found to be important for MCS intensification in recent decades. Here we extend that analysis to southern West Africa (SWA) by combining 34 years of cloud-top temperatures with rainfall and reanalysis data. We identify clear trends in intense MCSs since 1983 and their associated atmospheric drivers. We also find a marked annual cycle in the drivers, linked to changes in the convective regime during the progression of the West African monsoon. Before the peak of the first rainy season, we identify a shear regime where increased temperature gradients play a crucial role for MCS intensity trends. From June onward, SWA moves into a less unstable, moist regime during which MCS trends are mainly linked to frequency increase and may be more influenced by total column water vapor. However, during both seasons we find that MCSs with the most intense convection occur in an environment with stronger wind shear, increased low-level humidity, and drier midlevels. Comparing the sensitivity of MCS intensity and peak rainfall to low-level moisture and wind shear conditions preceding events, we find a dominant role for wind shear. We conclude that MCS trends are directly linked to a strengthening of two distinct convective regimes that cause the seasonal change of SWA MCS characteristics. However, the convective environment that ultimately produces the most intense MCSs remains the same.

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Review comments on “The impact threshold of the aerosol radiation forcing on the boundary layer structure in the pollution region” by Zhao et al., 2020.

10.5194/acp-2020-553-rc2 ◽

2020 ◽

Author(s):

Anonymous

Keyword(s):

Boundary Layer ◽

Layer Structure ◽

Boundary Layer Structure ◽

The Impact

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An overview of the SCOUT-AMMA stratospheric aircraft, balloons and sondes campaign in West Africa, August 2006: rationale, roadmap and highlights

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-9-19713-2009 ◽

2009 ◽

Vol 9 (5) ◽

pp. 19713-19781 ◽

Cited By ~ 5

Author(s):

F. Cairo ◽

J. P. Pommereau ◽

K. S. Law ◽

H. Schlager ◽

A. Garnier ◽

...

Keyword(s):

West Africa ◽

Large Scale ◽

Lower Stratosphere ◽

Chemical Species ◽

Upper Troposphere ◽

Convective Systems ◽

Mesoscale Convective ◽

Aerosol Dust ◽

D 20 ◽

The Impact

Abstract. A multi-platform field measurement campaign involving aircraft and balloons took place over West Africa between 26 July and 25 August 2006, in the frame of the concomitant AMMA Special Observing Period and SCOUT-O3 African tropical activities. Specifically aiming at sampling the upper troposphere and lower stratosphere, the high-altitude research aircraft M55 Geophysica was deployed in Ouagadougou (12.3° N, 1.7° W), Burkina Faso, in conjunction with the German D-20 Falcon, while a series of stratospheric balloon and sonde flights were conducted from Niamey (13.5° N, 2.0° E), Niger. The stratospheric aircraft and balloon flights intended to gather experimental evidence for a better understanding of large scale transport, assessing the effect of lightning on NOx production, and studying the impact of intense mesoscale convective systems on water, aerosol, dust and chemical species in the upper troposphere and lower stratosphere. The M55 Geophysica carried out five local and four transfer flights between southern Europe and the Sahel and back, while eight stratospheric balloons and twenty-nine sondes were flown from Niamey. These experiments allowed a characterization of the tropopause and lower stratosphere of the region. We provide here an overview of the campaign activities together with a description of the general meteorological situation during the flights and a summary of the observations accomplished.

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