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Multi‐scale analysis of the 25–27 July 2006 convective period over Niamey: Comparison between Doppler radar observations and simulations
Author(s) -
Barthe Christelle,
Asencio Nicole,
Lafore JeanPhilippe,
Chong Michel,
Campistron Bernard,
Cazenave Frédéric
Publication year - 2010
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.539
Subject(s) - convection , climatology , mesoscale meteorology , mesoscale convective system , trough (economics) , geology , monsoon , atmospheric sciences , squall line , doppler radar , convective available potential energy , radar , meteorology , environmental science , geography , telecommunications , computer science , economics , macroeconomics
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. Copyright © 2010 Royal Meteorological Society