The case chosen for this study was a boundary layer cloud observed on May 18, 1998 in the FIRE/SHEBA field experiment. The boundary layer cloud was mostly liquid in the layer 200-500 m, with an inversion height near 500 m. Winds were light easterly at the surface. On that particular day the CCN sounding exhibited concentrations ranging from 30 cm-3 below cloud base to a peak of 250 cm-3 at the inversion, lowering to 160 cm-3 at 700 m.
Two sets of simulations are conducted to investigate the entrainment of CCN from above the boundary layer into the cloudy layer, and how the entrained air affects microphysical and radiative properties of the boundary layer. The model is initialized with the sounding taken during Research Flight 5, on board the SHEBA C-130 aircraft (see online: raf.atd.ucar.edu/Projects/SHEBA/CloudPasses/rf05). In the control run, the initial CCN concentration is set to 30 cm -3, a value close to that observed in the sub-cloud layer, throughout the domain (hereafter referred to as N30). In the sensitivity run, the CCN concentration is averaged from the actual CCN data collected during FIRE-3 using an Instantaneous CCN Spectrometer [Hudson, 1989] activated at 0.8% supersaturation (hereafter referred to as N250). A 5-h long simulation is performed for both cases. For comparison purposes, both the initial CCN profile used for N250 and the observed CCN profile, active at 0.8% supersaturation, are shown in Figure 1. Note that the CCN concentration used in the N250 run has a constant value of 30 cm-3 up to 400 m from the surface, with a 220 cm-3 jump crosses the initial cloud top.
The initial total water mixing ratio rt, dry potential
temperature 
,
horizontal wind u, v, and liquid water content
(LWC) are given in Figure 2.