In West Africa convective systems play a key role in the water cycle, because they provide the predominant contribution to the annual rainfall. Investigations indicate that feedbacks exist between surface, atmosphere and convective rainfall in the West Africa. Despite West Africa is identified as a "hot spot" concerning the high sensitivity of land and atmosphere to each other, little is known about the land-atmosphere feedback during the different phases of the West African monsoon (WAM). This feedback loop includes the impact of soil moisture on the energy balance of the Earth's surface, the influence of the surface fluxes of heat and moisture on the planetary boundary layer (PBL) condition and atmospheric stratification, the triggering and modification of mesoscale convective systems (MCSs) by PBL inhomogeneities and, finally, the subsequent impact of MCSs on the conditions of the soil, surface and atmosphere. At IMK-TRO some aspects of this soil - surface - atmosphere - feedback loop are investigated:
- Kohler et al. (2010) used surface observations from Dano/Burkina Faso (3°W, 11°N) to investigate the dependence of the soil moisture on the energy balance of the Earth's surface (Figure 1), boundary layer and pre-convective atmosphere.
Figure 1: Volumetric soil moisture content, Θvol, at different depth (top), precipitation (middle) and soil temperature, T, at different levels (bottom) 2006. The measurements were performed during the AMMA campaign 2006 at Bontioli, Burkina Faso (3°W, 11°N).
- In a second study Kohler et al. used measurements of soil moisture and soil temperature as well as observations of near-surface meteorological parameters and energy balance components to validate two multi-layer soil-vegetation models (SVAT).
- Abdou et al. (2010) studied the diurnal cycle of the lower part of the boundary layer during the monsoon period.
- Truckemüller (2008) used observations and Heidt (2006) and Gantner and Kalthoff (2010) model simulations to investigate the dependence of the initiation and modification of MCSs on soil moisture inhomogeneities. Precipitation of a mature system is significantly reduced approaching a dry band of soil moisture (Figure 2).
|Figure 2: 24 h accumulated precipitation in mm starting at 0600 h on 11 June 2006. (a) A band of dry soil between 1°W and 3°W is inserted into a homogenous soil moisture distribution in (b). (Taken from Gantner and Kalthoff, 2010)|
- Adler (2010) used temperature and humidity budget calculation to analyse the processes responsible for the evolution of convection-related parameters and for the triggering and evolution of convective rainfall.
|Figure 3: Vertical section of liquid water content (color coded) indicating the clouds (left) and of change in specific humidity because of phase change (color coded) and of vertical advection in g/(kg h) (Isolines without zero) (right) at 12.5°N. A positive change of humidity due to vertical advection supplies humidity that condensates in the clouds identified by a negative change of humidity due to phase change.|
- Klüpfel (2010) used soil moisture data from different sources (satellite data, model output data, surface observation) (Figure 4) to investigate their impact on the development of convective rainfall in COSMO model simulations.
|Figure 4: Soil moisture fields (volumetric soil moisture) observed from AMSR-E (left), composite for 31/07/2006 and from ALMIP Experiment 3 (Boone et al. 2009, right), performed with the ECMWF soil model HTESSEL (31/07/2006, 12 UTC).|
- Schwendike et al. (2010) analysed observations to study the impact of MCS systems on the soil, surface and boundary layer as well as their recovery time after MCS passages during the pre-onset phase of monsoon and summer monsoon.
- Abdou K., Parker D. J., Brooks B., Kalthoff N., Lebel T., 2010: The diurnal cycle of lower boundary-layer wind in the West African monsoon. Quart. J. Roy. Meteor. Soc., 136, 66–76, DOI: 10.1002/qj.536.
- Adler, B., 2010: Der Einfluss von Landoberflächeninhomogenitäten auf die Auslösung und Entwicklung eines mesoskaligen konvektiven Systems: Eine budgetbasierte Modellanalyse. Diploma thesis.
- Gantner, L., Kalthoff, N., 2010: Sensitivity of a modelled life cycle of a mesoscale convective system to soil conditions over West Africa. Quart. J. Roy. Meteor. Soc., 136, 471-482, DOI: 10.1002/qj.425.
- Hofheinz, P., 2008: Auslösebedingungen und tageszeitliche Entwicklung von Konvektion während des Westafrikanischen Monsuns 2006. Diploma thesis.
- Heidt, S., 2006: Simulation konvektiver Episoden im tropischen Westafrika mit dem Lokal-Modell des Deutschen Wetterdienstes. Diploma thesis.
- Kohler, M., Kalthoff, N., Kottmeier, Ch., 2010: The impact of soil moisture modifications on CBL characteristics in West Africa: A case-study from the AMMA campaign. Quart. J. Roy. Meteor. Soc., 136, 442–455, DOI: 10.1002/qj.430.
- Schwendike, J., Kalthoff, N., Kohler, M., 2010: The impact of mesoscale convective systems on the surface and boundary layer structure in West Africa: case studies from the AMMA campaign. Quart. J. Roy. Meteor. Soc., 136, 566-582, DOI: 10.1002/qj.599.
- Truckenmüller, M., 2008: Mesoskalige konvektive Systeme während des Westafrikanischen Monsuns: Analyse der Messdaten und Modellergebnisse der AMMA-Episode SOP2. Diploma thesis.