Spurious effects of the deep convection parameterisation on the simulation of a Sahelian heatwave

Effets parasites de la paramétrisation de la convection profonde sur la simulation d'une vague de chaleur sahélienne

Tomasini, Mireille ; Guichard, Françoise ; Couvreux, Fleur ; Roehrig, Romain ; Barbier, Jessica

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<p align=justify>A severe heatwave occurred in April 2010 over West Africa. It was characterised by a particularly high daily minimum temperature reaching more than 35°C locally and a high water vapour content. In this study we analyse the ability of a mesoscale limited-area model to represent such an event and investigate the advantage of using an explicit representation of deep convection for such a case associated with very limited precipitation amounts. Two high-resolution simulations (5 km × 5 km horizontal grid) have been performed from 10 to 19 April 2010; they are identical except that one uses a deep convection parameterisation (simulation PARAM) and the other does not (simulation EXPL). These simulations are evaluated with different observational datasets including gridded products as well as local meteorological measurements and radiosoundings. Overall, both simulations display a negative temperature bias in the low levels but this bias is much more pronounced in PARAM, mainly due to evaporative cooling of spurious precipitation. Indeed, in PARAM, precipitation is too frequently triggered (around mid-day, i.e., several hours too early) and too strong; the intertropical discontinuity (ITD) propagates too far north during this 10-day sequence. Conversely, in EXPL, the observed northward shift of the ITD is correctly simulated and precipitation displays a better timing, variability, intensity and latitudinal extent. It thus appears that the representation of deep convection affects the atmospheric circulation associated with the heatwave event. The mechanisms involved in this humid heatwave are further investigated with thermodynamic and dynamic budgets which also underline the main differences between the two simulations. A proper representation of deep convection on sub-diurnal time-scale turns out to be necessary for the simulation of this heatwave episode, which points to the interest of convection-permitting simulations for the study of heatwaves even though they are generally characterised by very little precipitation.</p>
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