Evidence of lower stratosphere rehydration by convective overshooting of ice particles

Although a number of mechanisms have been proposed for 25 years, the process by which the amount of water vapour in the stratosphere is controlled is still debated. Here, from a series of backscatter/hygro-sondes in West Africa during the summer of 2006, we show evidence of the frequent presence of rehydrated layers up to 19 km (450K), well above the tropopause at 16 km, down wind or next to mesoscale convective systems. In the case of a fresh local strong local convective event, displaying weather radar echoes up to 19 km, the sonde also shows particles coincident with the rehydrated layer. These observations suggest a two-step mechanism controlling the amount of water vapour in the stratosphere: rehydration by strong land convective overshooting of ice particles across the cold tropopause, followed by a slow dehydration after cirrus cloud formation along the horizontal trajectory of the airmass, but at altitude levels where the temperature is significantly warmer than that of the cold trap. The mechanism could potentially explain a) the moisture of the lower stratosphere larger than that of the saturation ratio at the tropopause, and b) a long-term increase of stratospheric water vapour associated with an increase of convection, despite the higher and colder tropopause.