Numerical study of a Sahelian synoptic weather system: Initiation and mature stages of convection and its interactions with the large‐scale dynamics

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ABSTRACT:The propagation and diurnal cycle of organized convection in northern tropical Africa are examined using five years (1999)(2000)(2001)(2002)(2003) of digital infrared imagery for May-August. Reduced-dimension techniques are used to document the properties of cold clouds -proxies for deep convection and precipitation. Large-scale environments are diagnosed from global analyses.Organized convection in Africa consists of coherent sequences or episodes which span an average distance of about 1000 km and last about 25 h. A substantial fraction of events exhibits systematic propagation at regional to continental scales while undergoing decay and regeneration. Episodes with 36 h duration and 1472 km span recur at a one-per-day interval. Most episodes have phase speed of 10-20 m s −1 , which is faster than most African easterly waves. Convective episodes tend to initiate in the lee of high terrain, consistent with thermal forcing from elevated heat sources. Average diurnal frequency maxima result from the superposition of local diurnal maximum with the delayed-phase arrival of systems propagating from the east. Propagation occurs with moderate low-to mid-tropospheric shear, which varies with the African easterly jet migration and West African monsoon phases. Frequent deep convection occurs with local shear maxima near high terrain. For the peak monsoon period and for 10°W-10°E, where easterly waves and convective systems are frequent, 35% of cold cloud episodes occur east of the wave trough compared with about 24% to the west. Based on the coherent behaviour of organized, propagating convection, inferences may be made regarding the prediction of precipitation beyond one or two days.

Section: Introductionmentioning

confidence: 99%

The propagation and diurnal cycles of deep convection in northern tropical Africa

Laing

Carbone

Levizzani

et al. 2008

150

132

“…Diongue et al (2002) stress the importance of the quality of initial fields for the successful simulation of a real case, for convective flows in general, and especially in the Tropics where large-scale forcing is weaker than at midlatitudes. This is particularly true for humidity profiles, which are crucial for the representation of diabatic processes in the Tropical and subtropical regions.…”

Section: Introductionmentioning

confidence: 94%

A nudging scheme to assimilate satellite brightness temperature in a meteorological model: Impact on representation of African mesoscale convective systems

Orlandi

Fierli

Davolio

et al. 2010

The study presents the implementation of a nudging procedure in a mesoscale meteorological model, and its evaluation in improving the representation of mesoscale convective systems in the region of West Africa. The generation and development of convection are related to processes with different spatial scales, making it often difficult for meteorological models to reproduce their dynamics.The assimilation approach is based on the continuous assimilation of METEOSAT infrared brightness temperatures within the model in order to trigger convection, where observations show the presence of large convective systems. The nudging also inhibits convection, when the model reproduces unrealistic convective precipitation and coherently modifies the dynamical fields.Model sensitivity to the nudging scheme has been tested to evaluate the role of the forcing term. The nudging improves the geographical distribution and time evolution of mesoscale convective systems reproduced by the model; the impact of assimilation is positive up to 13 h after the end of the nudging period. It is also shown that the nudging improves the simulated amount and spatial distribution of precipitation.Finally, the assimilation is performed throughout the entire simulation, showing how the proposed approach can be used to improve meteorological analyses in terms of convective activity and rainfall.

“…A strategy to incorporate such effects is to assume a constant heating profile for the latent heating due to moist convection (as in Snyder and Lindzen, 1991), and thereby to introduce two new levels of PV anomalies comparable with the PV anomalies which are to be included in the model at the PBL top, z = z L (Section 3). This effect is not explored here in detail, partly because it rapidly makes the coupling more complex than is warranted by the simple dynamical model, and partly because the real effects of organised convection on the airflow (and PV) are still only poorly understood (Parker and Burton, 2002;Diongue et al, 2002). However, some discussion of the consequences of moist convective heating on the waves will be made in Section 4.5.…”

Section: Influence Of Lower Atmosphere Anomalies On the Surfacementioning

confidence: 99%

“…Redelsperger et al, 2002;Diongue et al, 2002;Berry and Thorncroft, 2005). The interaction of intermittent moist convection with larger-scale AEWs is a complex nonlinear problem which is still not at all well understood.…”

Section: Introductionmentioning

confidence: 99%

A simple model of coupled synoptic waves in the land surface and atmosphere of the northern Sahel

Parker

2008

A simple dynamic model is developed to describe the observed interactions between the atmosphere and the soil moisture patterns of the northern Sahel. In the model, the atmosphere follows quasi-geostrophic dynamics, while landatmosphere coupling is described by simple linear relationships. Dry surfaces heat the atmospheric boundary layer, while wet surfaces cool the boundary layer, relative to the equilibrium state of the atmosphere and land surface. In turn, cloud processes, which are assumed to maximise in the cool, humid phase of an atmospheric disturbance, cool the land surface through wetting (rainfall) and reduction of the incoming solar flux. These assumptions lead to a linear system which can be solved numerically to obtain modal solutions, and the adjoints (optimal excitation) of these. Moist convective influences on the atmospheric state are not explored in detail. The coupling with the land surface leads to the existence of unstable modes, which do not exist in the atmosphere-only part of the system. Solutions can be easterly or westerly propagating, according to wave number, with the longer waves tending to be easterly. Propagation relies on a favourable configuration between the atmospheric and soil moisture anomalies: easterly propagation requires the surface temperature pattern to be shifted to the east of the atmospheric temperature pattern. In contrast, optimal excitation of the fastest-growing mode occurs when the atmospheric pattern has a thermal anomaly lying to the east of a strong surface temperature (and moisture) anomaly. These results have value for weather prediction, and indicate the usefulness of soil moisture data for forecasters.