Some observational evidence for dry soils supporting enhanced relative humidity at the convective boundary layer top

The tendency of the relative humidity at the top of a clear convective boundary layer (RHtop) is studied as an indicator of cloud formation is studied over a semi-arid region within the conceptual framework introduced by Ek and Holtslag (2004). Typically the tendency of RHtop increases if the evaporative fraction at the land surface increases, which supports boundary layer moistening but only when boundary-layer growth is limited by atmospheric factors. This regime was supported by Cabauw observations in the original study. Here we provide new observational evidence that the tendency of RHtop can also increase as the surface becomes more dry as is consistent with another regime of the conceptual framework. The observations used are from the AMMA intensive observational campaign near Niamey, Niger, June 20-25, 2006. In addition, we evaluate whether various versions of the WRF single-column model confirm the different regimes of the conceptual framework for a typical day in the AMMA campaign. It appears that the model confirms that dryer soils can support cloud formation.