Phenomenology of Sahelian convection observed in Niamey during the early monsoon

Dione, Cheikh; Lothon, Marie; Badiane, Daouda; Campistron, Bernard; Couvreux, Fleur; Guichard, Françoise; Sall, Saïdou Moustapha

doi:10.1002/qj.2149

Cited by 30 publications

(27 citation statements)

References 78 publications

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“…Observational evidence of cold-pool-generated convective cells is available for shallow maritime convection (Warner et al 1979;Zuidema et al 2012), maritime deep convection (Barnes and Garstang 1982;Addis et al 1984;Young et al 1995), and continental deep convection (e.g., Lima and Wilson 2008;Lothon et al 2011;Dione et al 2013). There is moreover a large amount of literature on the role of cold pools in the organization of multicell convection such as squall lines (e.g., Wakimoto 1982) or mesoscale convective systems (e.g., Fritsch and Forbes 2001;Engerer et al 2008).…”

Section: Introductionmentioning

confidence: 99%

The Formation of Wider and Deeper Clouds as a Result of Cold-Pool Dynamics

Schlemmer

Hohenegger

2014

Journal of the Atmospheric Sciences

145

192

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This study investigates how precipitation-driven cold pools aid the formation of wider clouds that are essential for a transition from shallow to deep convection. In connection with a temperature depression and a depletion of moisture inside developing cold pools, an accumulation of moisture in moist patches around the cold pools is observed. Convective clouds are formed on top of these moist patches. Larger moist patches form with time supporting more and larger clouds. Moreover, enhanced vertical lifting along the leading edges of the gravity current triggered by the cold pool is found. The interplay of moisture aggregation and lifting eventually promotes the formation of wider clouds that are less affected by entrainment and become deeper. These mechanisms are corroborated in a series of cloud-resolving model simulations representing different atmospheric environments. A positive feedback is observed in that, in an atmosphere in which cloud and rain formation is facilitated, stronger downdrafts will form. These stronger downdrafts lead to a stronger modification of the moisture field, which in turn favors further cloud development. This effect is not only observed in the transition phase but also active in prolonging the peak time of precipitation in the later stages of the diurnal cycle. These findings are used to propose a simple way for incorporating the effect of cold pools on cloud sizes and thereby entrainment rate into parameterization schemes for convection. Comparison of this parameterization to the cloud-resolving modeling output gives promising results.

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Section: Introductionmentioning

confidence: 99%

The Formation of Wider and Deeper Clouds as a Result of Cold-Pool Dynamics

Schlemmer

Hohenegger

2014

Journal of the Atmospheric Sciences

145

192

View full text Add to dashboard Cite

show abstract

“…More generally, observations (Barnes and Garstang, 1982;Zuidema et al, 2012;Dione et al, 2014;de Szoeke et al, 2017) and such simulations both point to the importance of convectively-generated cold pools which are illustrated in Fig. 14.…”

Section: Insights Into Convective Clouds Phenomenology and Process Unmentioning

confidence: 75%

A short review of numerical cloud-resolving models

Guichard

Couvreux

2017

Tellus A: Dynamic Meteorology and Oceanography

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A cloud-resolving model (CRM) allows performing numerical simulations of convective clouds, such as shallow cumulus and stratocumulus, or storms and squall-lines with a resolution on the order of a few tens of metres to a few kilometres over a limited-area 4D (time and space) domain. The development of such models over the past decades is reviewed and their specific features are presented. The latter include a non-hydrostatic dynamic and parameterizations of sub-grid turbulence, microphysical and radiative processes. The capabilities of such models are discussed based on comparisons with observations and model-intercomparison studies. CRMs are used in a variety of ways, from the exploration of cloud phenomenology and process-understanding studies to the development of algorithms for satellite products, as well as to address climate issues and to develop convective and cloud parametrizations for large-scale weather and climate models. A few results illustrating this wide utilization are presented. The continuous increase of computer power induces rapid changes in modelling perspectives and therefore, influences the developments and applications of CRMs. This is discussed together with emerging scientific questions which will further benefit from CRM simulations.

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“…Despite this complexity, observational analyses of rain event climatology suggest that the dynamical mechanism likely dominates over thermo-dynamical processes (dry or wet soil advantage) in determining soil moisture impact on rainfall formation and that the sign of this coupling is, therefore, predominantly negative [38,39]. This signal was found to be the most robust in the Sahel [29,40,41]. However, further understanding of whether this dominance could reflect a significant contribution of dry and heterogeneous soils to the properties of rainstorms in this semi-arid region has not yet been clarified.…”

Section: Introductionmentioning

confidence: 98%

“…In this case, thermal contrasts between drier and wetter surfaces form circulation patterns, with higher updrafts and moisture convergence favoring rain occurrence over the dry patch [27]. The thermally-induced circulation has proven to be an important mechanism for convection initiation, even under unfavorable atmospheric conditions [14,28,29]. Analyses of multi-year satellite data have further demonstrated the climatological relevance of this mechanism for convection triggering in soil moisture-limited regimes [30] and particularly in the Western Sahel [28].…”

Section: Introductionmentioning

confidence: 99%

Relation between Convective Rainfall Properties and Antecedent Soil Moisture Heterogeneity Conditions in North Africa

et al. 2018

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Abstract:Recent observational studies have demonstrated the relevance of soil moisture heterogeneity and the associated thermally-induced circulation on deep convection and rainfall triggering. However, whether this dynamical mechanism further influences rainfall properties-such as rain volume or timing-has yet to be confirmed by observational data. Here, we analyze 10 years of satellite-based sub-daily soil moisture and precipitation records and explore the potential of strong spatial gradients in morning soil moisture to influence the properties of afternoon rainfall in the North African region, at the 100-km scale. We find that the convective rain systems that form over locally drier soils and anomalously strong soil moisture gradients have a tendency to initiate earlier in the afternoon; they also yield lower volumes of rain, weaker intensity and lower spatial variability. The strongest sensitivity to antecedent soil conditions is identified for the timing of the rain onset; it is found to be correlated with the magnitude of the soil moisture gradient. Further analysis shows that the early initiation of rainfall over dry soils and strong surface gradients yet requires the presence of a very moist boundary layer on that day. Our findings agree well with the expected effects of thermally-induced circulation on rainfall properties suggested by theoretical studies and point to the potential of locally drier and heterogeneous soils to influence convective rainfall development. The systematic nature of the identified effect of soil moisture state on the onset time of rainstorms in the region is of particular relevance and may help foster research on rainfall predictability.

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Phenomenology of Sahelian convection observed in Niamey during the early monsoon

Cited by 30 publications

References 78 publications

The Formation of Wider and Deeper Clouds as a Result of Cold-Pool Dynamics

The Formation of Wider and Deeper Clouds as a Result of Cold-Pool Dynamics

A short review of numerical cloud-resolving models

Relation between Convective Rainfall Properties and Antecedent Soil Moisture Heterogeneity Conditions in North Africa

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