The April 2010 North African heatwave: when the water vapor greenhouse effect drives nighttime temperatures
Largeron, Yann ; Guichard, Françoise ; Roehrig, Romain ; Couvreux, Fleur ; Barbier, Jessica
North Africa experienced a severe heatwave in April 2010 with daily maximum temperatures (Tmax) frequently exceeding 40?C and daily minimum temperatures (Tmin) over 27?C for more than five consecutive days in extended Saharan and Sahelian areas. Observations show that areas and periods affected by the heatwave correspond to strong positive anomalies of surface incoming longwave fluxes (LWin) and negative anomalies of incoming shortwave fluxes (SWin). The latter are explained by clouds in the Sahara, and by both clouds and dust loadings in the Sahel. However, the strong positive anomalies of LWin are hardly related to cloud or aerosol radiative effects. An analysis based on climate-model simulations (CNRM-AM) complemented by a specially-designed conceptual soil-atmospheric surface layer model (SARAWI) shows that this positive anomaly of LWin is mainly due to a water vapor greenhouse effect. SARAWI, which represents the two processes driving temperatures, namely turbulence and longwave radiative transfer between the soil and the atmospheric surface layer, points to the crucial impact of synoptic low-level advection of water vapor on Tmin. By increasing the atmospheric infrared emissivity, the advected water vapor dramatically increases the nocturnal radiative warming of the soil surface, then in turn reducing the nocturnal cooling of the atmospheric surface layer, which remains warm throughout the night. Over Western Sahel, this advection is related to an early northward incursion of the monsoon flow. Over Sahara, the anomalously high precipitable water is due to a tropical plume event. Both observations and simulations support this major influence of the low-level water vapor radiative effect on Tmin during this spring heatwave.</p>
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