Some observational and model evidence for dry soils supporting cumulus onset

The tendency of relative humidity at the top of a clear convective boundary layer (RHtop) is studied as indicator of cumulus cloud formation over a semi-arid region. As such observations and the conceptual framework introduced by Ek and Holtslag (2004, J. Hydrometeorology, 5, p. 86-99) is utilised. The latter framework is based on a mixed layer model, coupled to the land surface via the surface energy budget. Typically in a regime of relatively high soil moisture, the tendency of RHtop increases if the evaporative fraction at the land surface increases (confirming intuition). The latter supports boundary-layer moistening but only when boundary-layer growth is limited by atmospheric factors, such as dry air entrainment and limited boundary-layer growth. This regime was confirmed earlier by Cabauw observations over relatively moist surface conditions. Here we provide the first observational evidence that the tendency of RHtop can also increase (favouring cloud formation) when the surface becomes more dry. The latter is consistent with a second regime of the conceptual framework. The observations are taken from the AMMA intensive observational campaign near Niamey, Niger, June 20-25, 2006. We also evaluate whether various versions of the WRF single-column model confirm the different regimes of the conceptual framework for typical days in the AMMA campaign. It appears that the model also confirms that dryer soils can support cloud formation.