Multi‐scale analysis of the 25–27 July 2006 convective period over Niamey: Comparison between Doppler radar observations and simulations

Abstract

ABSTRACT:The present study investigates the multi-scale processes associated with a sequence of convective events that occurred over Niamey during the period 25-26 July 2006. This period corresponds to the active stage of the first intense monsoon surge over Sahel for 2006. During this two-day period, two successive sequences of mesoscale convective systems (MCSs) were located ahead of and in phase with the trough of an African Easterly Wave (AEW). They were followed by suppressed or isolated convection behind the trough and in the vicinity of the ridge. The large AMMA-SOP dataset, in particular the UHF radar and the MIT Doppler radar in Niamey, are used in combination with a low-resolution (5 km) cloud-resolving model to understand the convection organization and its interaction with the environment. Several initial and boundary conditions have been tested, but only the simulation starting with the ECMWF AMMA reanalysis succeeds in reproducing the observed features; this emphasizes the importance of the initial state. From the simulated MCSs, the along-line component of the apparent source of momentum due to the convection is found to be up to 1 m s −1 h −1 . It seems that MCSs globally reduce the monsoon flow and generate southerlies at mid levels which can reinforce the rotation of the wind at the passage of the trough. During the afternoon of 26 July, the local convection over Niamey resulted from some favourable factors (humidity, CAPE, CIN) that triggered convection, while inhibiting factors (mid-level dry layer, weaker low-level wind shear pointing to the north, anticyclonic curvature of the streamlines at 700 hPa) prevented it organizing itself and propagating. In particular, the low-level wind shear seems of critical importance.

Radar characteristics of continental, coastal, and maritime convection observed during AMMA/NAMMA

“…There have been numerous case studies involving groundbased radar data (Houze and Cheng, 1977;Chong et al, 1987;Barthe et al, 2010), along with Doppler velocity analyses (Roux, 1988;Roux and Ju, 1990;Chong, 2009) of individual African convective events. Previous studies have also examined convective characteristics for small (Buarque and Sauvageot, 1997;Shinoda et al, 1999;Fink et al, 2006) and large (Geerts and Dejene, 2005;Fuentes et al, 2008;Laing et al, 2008) spatial regions over West Africa.…”

Regional comparison of West African convective characteristics: a TRMM‐based climatology

“…Deep convection has been observed behind AEW troughs inland and in the eastern Atlantic and ahead of and within AEW troughs near the coast (Payne and McGarry, 1977;Reed et al, 1977;Duvel, 1990;Machado et al, 1993;Diedhiou et al, 1999;Kiladis et al, 2006). Westward-propagating MCSs in West Africa often move faster than AEW troughs (Aspliden et al, 1976;Fortune, 1980;Fink et al, 2006), and interact with the larger-scale environment through the transport of momentum (Moncrieff, 1992) and moisture (Lafore et al, 1988) and can reinforce cyclonic rotation when embedded in an AEW trough (Barthe et al, 2010).…”

Anatomy of an observed African easterly wave in July 2006

“…Barthe et al (2010) gave an overview of the AEW and described the rainfall associated with the event in the Niamey region. Cuesta et al (2009) described a rainfall event in the Hoggar massif immediately prior to, and during, the case-study period.…”